STATISTICAL QUALITY CONTROL WITH SAMPLING

2.1 Statistical Quality Control with Sampling

Sampling by attributes is a widely applied quality control method. The procedure is intended to determine whether or not a particular group of materials or work products is acceptable. In the literature of statistical quality control, a group of materials or work items to be tested is called a lot or batch. An assumption in the procedure is that each item in a batch can be tested and classified as either acceptable or deficient based upon mutually acceptable testing procedures and acceptance criteria. Each lot is tested to determine if it satisfies a minimum acceptable quality level (AQL) expressed as the maximum percentage of defective items in a lot or process.
In its basic form, sampling by attributes is applied by testing a pre-defined number of sample items from a lot. If the number of defective items is greater than a trigger level, then the lot is rejected as being likely to be of unacceptable quality. Otherwise, the lot is accepted. Developing this type of sampling plan requires consideration of probability, statistics and acceptable risk levels on the part of the supplier and consumer of the lot. Refinements to this basic application procedure are also possible. For example, if the number of defectives is greater than some pre-defined number, then additional sampling may be started rather than immediate rejection of the lot. In many cases, the trigger level is a single defective item in the sample. In the remainder of this section, the mathematical basis for interpreting this type of sampling plan is developed.
More formally, a lot is defined as acceptable if it contains a fraction p₁ or less defective items. Similarly, a lot is defined as unacceptable if it contains a fraction p₂ or more defective units. Generally, the acceptance fraction is less than or equal to the rejection fraction, p₁ p₂, and the two fractions are often equal so that there is no ambiguous range of lot acceptability between p₁ and p₂. Given a sample size and a trigger level for lot rejection or acceptance, we would like to determine the probabilities that acceptable lots might be incorrectly rejected (termed producer's risk) or that deficient lots might be incorrectly accepted (termed consumer's risk).
Consider a lot of finite number N, in which m items are defective (bad) and the remaining (N-m) items are non-defective (good). If a random sample of n items is taken from this lot, then we can determine the probability of having different numbers of defective items in the sample. With a pre-defined acceptable number of defective items, we can then develop the probability of accepting a lot as a function of the sample size, the allowable number of defective items, and the actual fraction of defective items. This derivation appears below.
The number of different samples of size n that can be selected from a finite population N is termed a mathematical combination .

For any combination of n and r, we can read off the value of g(p) for a given p from the corresponding OC curve. For example, n = 15 is specified in Figure 13-1. Then, for various values of r, we find:

r=0 r=0 r=1 r=1

p=24% p=4% p=24% p=4%

g(p) 2% g(p) 54% g(p) 10% g(p) 88%

The producer's and consumer's risk can be related to various points on an operating characteristic curve. Producer's risk is the chance that otherwise acceptable lots fail the sampling plan (ie. have more than the allowable number of defective items in the sample) solely due to random fluctuations in the selection of the sample. In contrast, consumer's risk is the chance that an unacceptable lot is acceptable (ie. has less than the allowable number of defective items in the sample) due to a better than average quality in the sample. For example, suppose that a sample size of 15 is chosen with a trigger level for rejection of one item. With a four percent acceptable level and a greater than four percent defective fraction, the consumer's risk is at most eighty-eight percent. In contrast, with a four percent acceptable level and a four percent defective fraction, the producer's risk is at most 1 - 0.88 = 0.12 or twelve percent.
In specifying the sampling plan implicit in the operating characteristic curve, the supplier and consumer of materials or work must agree on the levels of risk acceptable to themselves. If the lot is of acceptable quality, the supplier would like to minimize the chance or risk that a lot is rejected solely on the basis of a lower than average quality sample. Similarly, the consumer would like to minimize the risk of accepting under the sampling plan a deficient lot. In addition, both parties presumably would like to minimize the costs and delays associated with testing. Devising an acceptable sampling plan requires trade off the objectives of risk minimization among the parties involved and the cost of testing.

2.2 Safety

Construction is a relatively hazardous undertaking. As Table 13-1 illustrates, there are significantly more injuries and lost workdays due to injuries or illnesses in construction than in virtually any other industry. These work related injuries and illnesses are exceedingly costly. The Construction Industry Cost Effectiveness Project estimated that accidents cost $8.9 billion or nearly seven percent of the $137 billion (in 1979 dollars) spent annually for industrial, utility and commercial construction in the United States. [3] Included in this total are direct costs (medical costs, premiums for workers' compensation benefits, liability and property losses) as well as indirect costs (reduced worker productivity, delays in projects, administrative time, and damage to equipment and the facility). In contrast to most industrial accidents, innocent bystanders may also be injuried by construction accidents. Several crane collapses from high rise buildings under construction have resulted in fatalities to passerbys. Prudent project managers and owners would like to reduce accidents, injuries and illnesses as much as possible.

TABLE 2-1  Nonfatal Occupational Injury and Illness Incidence Rates Industry 1996 2006 Agriculture, forestry, fishing Mining Construction Manufacturing Trade,Transportation and utilities Financial activities Professional and business services 8.7 5.4 9.9 10.6 8.7 2.4 6.0 6 3.5 5.9 6 5 1.5 1.2

Note: Data represent total number of cases per 100 full-time employees

Source: U.S. Bureau of Labor Statistics, Occupational injuries and Illnesses in the United States by Industry, annual

As with all the other costs of construction, it is a mistake for owners to ignore a significant category of costs such as injury and illnesses. While contractors may pay insurance premiums directly, these costs are reflected in bid prices or contract amounts. Delays caused by injuries and illnesses can present significant opportunity costs to owners. In the long run, the owners of constructed facilities must pay all the costs of construction. For the case of injuries and illnesses, this general principle might be slightly qualified since significant costs are borne by workers themselves or society at large. However, court judgements and insurance payments compensate for individual losses and are ultimately borne by the owners.
The causes of injuries in construction are numerous. Table 13-2 lists the reported causes of accidents in the US construction industry in 1997 and 2004. A similar catalogue of causes would exist for other countries. The largest single category for both injuries and fatalities are individual falls. Handling goods and transportation are also a significant cause of injuries. From a management perspective, however, these reported causes do not really provide a useful prescription for safety policies. An individual fall may be caused by a series of coincidences: a railing might not be secure, a worker might be inattentive, the footing may be slippery, etc. Removing any one of these compound causes might serve to prevent any particular accident. However, it is clear that conditions such as unsecured railings will normally increase the risk of accidents. Table 13-3 provides a more detailed list of causes of fatalities for construction sites alone, but again each fatality may have multiple causes.

TABLE 2-2  Fatal Occupational Injuries in Construction, 1997 and 2004
Year	1997	2004
Total fatalities Falls Transportation incidents Contact with objects & equipment Exposure to harmful substances and environments	1,107 376 288 199 188	1,234 445 287 267 170

Source: Bureau of Labor Statistics

TABLE 2-3  Fatality Causes in Construction, 1996/1997 and 2006/2007 Year 96/97 06/07 Total accidents Falls from a height Struck by a moving vehicle Struck by moving/falling object Trapped by something overturning/collapsing Drowning/asphyxiation 287 88 43 57 16 9 241 45 30 40 19 16

Source: Bureau of Labor Statistics

Various measures are available to improve jobsite safety in construction. Several of the most important occur before construction is undertaken. These include design, choice of technology and education. By altering facility designs, particular structures can be safer or more hazardous to construct. For example, parapets can be designed to appropriate heights for construction worker safety, rather than the minimum height required by building codes.
Choice of technology can also be critical in determining the safety of a jobsite. Safeguards built into machinery can notify operators of problems or prevent injuries. For example, simple switches can prevent equipment from being operating when protective shields are not in place. With the availability of on-board electronics (including computer chips) and sensors, the possibilities for sophisticated machine controllers and monitors has greatly expanded for construction equipment and tools. Materials and work process choices also influence the safety of construction. For example, substitution of alternative materials for asbestos can reduce or eliminate the prospects of long term illnesses such as asbestiosis.
Educating workers and managers in proper procedures and hazards can have a direct impact on jobsite safety. The realization of the large costs involved in construction injuries and illnesses provides a considerable motivation for awareness and education. Regular safety inspections and safety meetings have become standard practices on most job sites.
Pre-qualification of contractors and sub-contractors with regard to safety is another important avenue for safety improvement. If contractors are only invitied to bid or enter negotiations if they have an acceptable record of safety (as well as quality performance), then a direct incentive is provided to insure adequate safety on the part of contractors.
During the construction process itself, the most important safety related measures are to insure vigilance and cooperation on the part of managers, inspectors and workers. Vigilance involves considering the risks of different working practices. In also involves maintaining temporary physical safeguards such as barricades, braces, guylines, railings, toeboards and the like. Sets of standard practices are also important, such as: [4]

• requiring hard hats on site.
• requiring eye protection on site.
• requiring hearing protection near loud equipment.
• insuring safety shoes for workers.
• providing first-aid supplies and trained personnel on site

While eliminating accidents and work related illnesses is a worthwhile goal, it will never be attained. Construction has a number of characteristics making it inherently hazardous. Large forces are involved in many operations. The jobsite is continually changing as construction proceeds. Workers do not have fixed worksites and must move around a structure under construction. The tenure of a worker on a site is short, so the worker's familiarity and the employer-employee relationship are less settled than in manufacturing settings. Despite these peculiarities and as a result of exactly these special problems, improving worksite safety is a very important project management concern.
Example 2-1: Trench collapse  

To replace 1,200 feet of a sewer line, a trench of between 12.5 and 18 feet deep was required down the center of a four lane street. The contractor chose to begin excavation of the trench from the shallower end, requiring a 12.5 deep trench. Initially, the contractor used a nine foot high, four foot wide steel trench box for soil support. A trench box is a rigid steel frame consisting of two walls supported by welded struts with open sides and ends. This method had the advantage that traffic could be maintained in at least two lanes during the reconstruction work.

In the shallow parts of the trench, the trench box seemed to adequately support the excavation. However, as the trench got deeper, more soil was unsupported below the trench box. Intermittent soil collapses in the trench began to occur. Eventually, an old parallel six inch water main collapsed, thereby saturating the soil and leading to massive soil collapse at the bottom of the trench. Replacement of the water main was added to the initial contract. At this point, the contractor began sloping the sides of the trench, thereby requiring the closure of the entire street.
The initial use of the trench box was convenient, but it was clearly inadequate and unsafe. Workers in the trench were in continuing danger of accidents stemming from soil collapse. Disruption to surrounding facilities such as the parallel water main was highly likely. Adoption of a tongue and groove vertical sheeting system over the full height of the trench or, alternatively, the sloping excavation eventually adopted are clearly preferable.

 References

1. Ang, A.H.S. and W.H. Tang, Probability Concepts in Engineering Planning and Design: Volume I - Basic Principles, John Wiley and Sons, Inc., New York, 1975.
2. Au, T., R.M. Shane, and L.A. Hoel, Fundamentals of Systems Engineering: Probabilistic Models, Addison-Wesley Publishing Co., Reading MA, 1972
3. Bowker, A.H. and Liebermann, G. J., Engineering Statistics, Prentice-Hall, 1972.
4. Fox, A.J. and Cornell, H.A., (eds), Quality in the Constructed Project, American Society of Civil Engineers, New York, 1984.
5. International Organization for Standardization, "Sampling Procedures and Charts for Inspection by Variables for Percent Defective, ISO 3951-1981 (E)", Statistical Methods, ISO Standard Handbook 3, International Organization for Standardization, Paris, France, 1981.
6. Skibniewski, M. and Hendrickson, C., Methods to Improve the Safety Performance of the U.S. Construction Industry, Technical Report, Department of Civil Engineering, Carnegie Mellon University, 1983.
7. United States Department of Defense, Sampling Procedures and Tables for Inspection by Variables, (Military Standard 414), Washington D.C.: U.S. Government Printing Office, 1957.
8. United States Department of Defense, Sampling Procedures and Tables for Inspection by Attributes, (Military Standard 105D), Washington D.C.: U.S. Government Printing Office, 1963.

Problems

1. Consider the following specification. Would you consider it to be a process or performance specification? Why?

"Water used in mixing or curing shall be reasonably clean and free of oil, salt, acid, alkali, sugar, vegetable, or other substance injurious to the finished product...Water known to be potable quality may be used without test. Where the source of water is relatively shallow, the intake shall be so enclosed as to exclude silt, mud, grass, or other foreign materials." [6]

2. Suppose that a sampling plan calls for a sample of size n = 50. To be acceptable, only three or fewer samples can be defective. Estimate the probability of accepting the lot if the average defective percentage is (a) 15%, (b) 5% or (c) 2%. Do not use an approximation in this calculation.
3. Repeat Problem 2 using the binomial approximation.
4. Suppose that a project manager tested the strength of one tile out of a batch of 3,000 to be used on a building. This one sample measurement was compared with the design specification and, in this case, the sampled tile's strength exceeded that of the specification. On this basis, the project manager accepted the tile shipment. If the sampled tile was defective (with a strength less than the specification), the project manager would have rejected the lot.

a. What is the probability that ninety percent of the tiles are substandard, even though the project manager's sample gave a satisfactory result?
b. Sketch out the operating characteristic curve for this sampling plan as a function of the actual fraction of defective tiles.

5. Repeat Problem 4 for sample sizes of (a) 5, (b) 10 and (c) 20.
6. Suppose that a sampling-by-attributes plan is specified in which ten samples are taken at random from a large lot (N=100) and at most one sample item is allowed to be defective for the lot to be acceptable.

a. If the actual percentage defective is five percent, what is the probability of lot acceptance? (Note: you may use relevant approximations in this calculation.)
b. What is the consumer's risk if an acceptable quality level is fifteen percent defective and the actual fraction defective is five percent?
c. What is the producer's risk with this sampling plan and an eight percent defective percentage?

7. The yield stress of a random sample of 25 pieces of steel was measured, yielding a mean of 52,800 psi. and an estimated standard deviation of s = 4,600 psi.

a. What is the probability that the population mean is less than 50,000 psi?
b. What is the estimated fraction of pieces with yield strength less than 50,000 psi?
c. Is this sampling procedure sampling-by-attributes or sampling-by-variable?

8. Suppose that a contract specifies a sampling-by-attributes plan in which ten samples are taken at random from a large lot (N=100) and at most one sample is allowed to be defective for the lot to be acceptable.

a. If the actual percentage defective is five percent, what is the probability of lot acceptance? (Note: you may use relevant approximations in this calculation).
b. What is the consumer's risk if an acceptable quality level is fifteen percent defective and the actual fraction defective is 0.05?
c. What is the producer's risk with this sampling plan and a 8% defective percentage?

9. In a random sample of 40 blocks chosen from a production line, the mean length was 10.63 inches and the estimated standard deviation was 0.4 inch. Between what lengths can it be said that 98% of block lengths will lie?

 Footnotes

1. This illustrative pay factor schedule is adapted from R.M. Weed, "Development of Multicharacteristic Acceptance Procedures for Rigid Pavement," Transportation Research Record 885, 1982, pp. 25-36. Back
2. B.A. Gilly, A. Touran, and T. Asai, "Quality Control Circles in Construction," ASCE Journal of Construction Engineering and Management, Vol. 113, No. 3, 1987, pg 432. Back
3. See Improving Construction Safety Performance, Report A-3, The Business Roundtable, New York, NY, January 1982. Back
4. Hinze, Jimmie W., Construction Safety,, Prentice-Hall, 1997. Back
5. This example was adapted from E. Elinski, External Impacts of Reconstruction and Rehabilitation Projects with Implications for Project Management, Unpublished MS Thesis, Department of Civil Engineering, Carnegie Mellon University, 1985. Back

6. American Association of State Highway and Transportation Officials, Guide Specifications for Highway Construction, Washington, D.C., Section 714.01, pg. 244. Back

QUALITY CONTROL MANAGEMENT

 Quality Control and Safety During Construction

1.1 Quality and Safety Concerns in Construction

Quality control and safety represent increasingly important concerns for project managers. Defects or failures in constructed facilities can result in very large costs. Even with minor defects, re-construction may be required and facility operations impaired. Increased costs and delays are the result. In the worst case, failures may cause personal injuries or fatalities. Accidents during the construction process can similarly result in personal injuries and large costs. Indirect costs of insurance, inspection and regulation are increasing rapidly due to these increased direct costs. Good project managers try to ensure that the job is done right the first time and that no major accidents occur on the project.
As with cost control, the most important decisions regarding the quality of a completed facility are made during the design and planning stages rather than during construction. It is during these preliminary stages that component configurations, material specifications and functional performance are decided. Quality control during construction consists largely of insuring conformance to this original design and planning decisions.
While conformance to existing design decisions is the primary focus of quality control, there are exceptions to this rule. First, unforeseen circumstances, incorrect design decisions or changes desired by an owner in the facility function may require re-evaluation of design decisions during the course of construction. While these changes may be motivated by the concern for quality, they represent occasions for re-design with all the attendant objectives and constraints. As a second case, some designs rely upon informed and appropriate decision making during the construction process itself. For example, some tunneling methods make decisions about the amount of shoring required at different locations based upon observation of soil conditions during the tunneling process. Since such decisions are based on better information concerning actual site conditions, the facility design may be more cost effective as a result. Any special case of re-design during construction requires the various considerations discussed in Chapter 3.
With the attention to conformance as the measure of quality during the construction process, the specification of quality requirements in the design and contract documentation becomes extremely important. Quality requirements should be clear and verifiable, so that all parties in the project can understand the requirements for conformance. Much of the discussion in this chapter relates to the development and the implications of different quality requirements for construction as well as the issues associated with insuring conformance.
Safety during the construction project is also influenced in large part by decisions made during the planning and design process. Some designs or construction plans are inherently difficult and dangerous to implement, whereas other, comparable plans may considerably reduce the possibility of accidents. For example, clear separation of traffic from construction zones during roadway rehabilitation can greatly reduce the possibility of accidental collisions. Beyond these design decisions, safety largely depends upon education, vigilance and cooperation during the construction process. Workers should be constantly alert to the possibilities of accidents and avoid taken unnecessary risks.

1.2 Organizing for Quality and Safety

A variety of different organizations are possible for quality and safety control during construction. One common model is to have a group responsible for quality assurance and another group primarily responsible for safety within an organization. In large organizations, departments dedicated to quality assurance and to safety might assign specific individuals to assume responsibility for these functions on particular projects. For smaller projects, the project manager or an assistant might assume these and other responsibilities. In either case, insuring safe and quality construction is a concern of the project manager in overall charge of the project in addition to the concerns of personnel, cost, time and other management issues.
Inspectors and quality assurance personnel will be involved in a project to represent a variety of different organizations. Each of the parties directly concerned with the project may have their own quality and safety inspectors, including the owner, the engineer/architect, and the various constructor firms. These inspectors may be contractors from specialized quality assurance organizations. In addition to on-site inspections, samples of materials will commonly be tested by specialized laboratories to insure compliance. Inspectors to insure compliance with regulatory requirements will also be involved. Common examples are inspectors for the local government's building department, for environmental agencies, and for occupational health and safety agencies.
The US Occupational Safety and Health Administration (OSHA) routinely conducts site visits of work places in conjunction with approved state inspection agencies. OSHA inspectors are required by law to issue citations for all standard violations observed. Safety standards prescribe a variety of mechanical safeguards and procedures; for example, ladder safety is covered by over 140 regulations. In cases of extreme non-compliance with standards, OSHA inspectors can stop work on a project. However, only a small fraction of construction sites are visited by OSHA inspectors and most construction site accidents are not caused by violations of existing standards. As a result, safety is largely the responsibility of the managers on site rather than that of public inspectors.
While the multitude of participants involved in the construction process require the services of inspectors, it cannot be emphasized too strongly that inspectors are only a formal check on quality control. Quality control should be a primary objective for all the members of a project team. Managers should take responsibility for maintaining and improving quality control. Employee participation in quality control should be sought and rewarded, including the introduction of new ideas. Most important of all, quality improvement can serve as a catalyst for improved productivity. By suggesting new work methods, by avoiding rework, and by avoiding long term problems, good quality control can pay for itself. Owners should promote good quality control and seek out contractors who maintain such standards.
In addition to the various organizational bodies involved in quality control, issues of quality control arise in virtually all the functional areas of construction activities. For example, insuring accurate and useful information is an important part of maintaining quality performance. Other aspects of quality control include document control (including changes during the construction process), procurement, field inspection and testing, and final checkout of the facility.

1.3 Work and Material Specifications

Specifications of work quality are an important feature of facility designs. Specifications of required quality and components represent part of the necessary documentation to describe a facility. Typically, this documentation includes any special provisions of the facility design as well as references to generally accepted specifications to be used during construction.
General specifications of work quality are available in numerous fields and are issued in publications of organizations such as the American Society for Testing and Materials (ASTM), the American National Standards Institute (ANSI), or the Construction Specifications Institute (CSI). Distinct specifications are formalized for particular types of construction activities, such as welding standards issued by the American Welding Society, or for particular facility types, such as the Standard Specifications for Highway Bridges issued by the American Association of State Highway and Transportation Officials. These general specifications must be modified to reflect local conditions, policies, available materials, local regulations and other special circumstances.
Construction specifications normally consist of a series of instructions or prohibitions for specific operations. For example, the following passage illustrates a typical specification, in this case for excavation for structures:

Conform to elevations and dimensions shown on plan within a tolerance of plus or minus 0.10 foot, and extending a sufficient distance from footings and foundations to permit placing and removal of concrete formwork, installation of services, other construction, and for inspection. In excavating for footings and foundations, take care not to disturb bottom of excavation. Excavate by hand to final grade just before concrete reinforcement is placed. Trim bottoms to required lines and grades to leave solid base to receive concrete.

This set of specifications requires judgment in application since some items are not precisely specified. For example, excavation must extend a "sufficient" distance to permit inspection and other activities. Obviously, the term "sufficient" in this case may be subject to varying interpretations. In contrast, a specification that tolerances are within plus or minus a tenth of a foot is subject to direct measurement. However, specific requirements of the facility or characteristics of the site may make the standard tolerance of a tenth of a foot inappropriate. Writing specifications typically requires a trade-off between assuming reasonable behavior on the part of all the parties concerned in interpreting words such as "sufficient" versus the effort and possible inaccuracy in pre-specifying all operations.
In recent years, performance specifications have been developed for many construction operations. Rather than specifying the required construction process, these specifications refer to the required performance or quality of the finished facility. The exact method by which this performance is obtained is left to the construction contractor. For example, traditional specifications for asphalt pavement specified the composition of the asphalt material, the asphalt temperature during paving, and compacting procedures. In contrast, a performance specification for asphalt would detail the desired performance of the pavement with respect to impermeability, strength, etc. How the desired performance level was attained would be up to the paving contractor. In some cases, the payment for asphalt paving might increase with better quality of asphalt beyond some minimum level of performance.
Example 1-1: Concrete Pavement Strength

Concrete pavements of superior strength result in cost savings by delaying the time at which repairs or re-construction is required. In contrast, concrete of lower quality will necessitate more frequent overlays or other repair procedures. Contract provisions with adjustments to the amount of a contractor's compensation based on pavement quality have become increasingly common in recognition of the cost savings associated with higher quality construction. Even if a pavement does not meet the "ultimate" design standard, it is still worth using the lower quality pavement and re-surfacing later rather than completely rejecting the pavement. Based on these life cycle cost considerations, a typical pay schedule might be: [1]

Load Ratio	Pay Factor
<0.50 0.50-0.69 0.70-0.89 0.90-1.09 1.10-1.29 1.30-1.49 >1.50	Reject 0.90 0.95 1.00 1.05 1.10 1.12

In this table, the Load Ratio is the ratio of the actual pavement strength to the desired design strength and the Pay Factor is a fraction by which the total pavement contract amount is multiplied to obtain the appropriate compensation to the contractor. For example, if a contractor achieves concrete strength twenty percent greater than the design specification, then the load ratio is 1.20 and the appropriate pay factor is 1.05, so the contractor receives a five percent bonus. Load factors are computed after tests on the concrete actually used in a pavement. Note that a 90% pay factor exists in this case with even pavement quality only 50% of that originally desired. This high pay factor even with weak concrete strength might exist since much of the cost of pavements are incurred in preparing the pavement foundation. Concrete strengths of less then 50% are cause for complete rejection in this case, however.

1.4 Total Quality Control

Quality control in construction typically involves insuring compliance with minimum standards of material and workmanship in order to insure the performance of the facility according to the design. These minimum standards are contained in the specifications described in the previous section. For the purpose of insuring compliance, random samples and statistical methods are commonly used as the basis for accepting or rejecting work completed and batches of materials. Rejection of a batch is based on non-conformance or violation of the relevant design specifications. Procedures for this quality control practice are described in the following sections.
An implicit assumption in these traditional quality control practices is the notion of an acceptable quality level which is a allowable fraction of defective items. Materials obtained from suppliers or work performed by an organization is inspected and passed as acceptable if the estimated defective percentage is within the acceptable quality level. Problems with materials or goods are corrected after delivery of the product.
In contrast to this traditional approach of quality control is the goal of total quality control. In this system, no defective items are allowed anywhere in the construction process. While the zero defects goal can never be permanently obtained, it provides a goal so that an organization is never satisfied with its quality control program even if defects are reduced by substantial amounts year after year. This concept and approach to quality control was first developed in manufacturing firms in Japan and Europe, but has since spread to many construction companies. The best known formal certification for quality improvement is the International Organization for Standardization's ISO 9000 standard. ISO 9000 emphasizes good documentation, quality goals and a series of cycles of planning, implementation and review.
Total quality control is a commitment to quality expressed in all parts of an organization and typically involves many elements. Design reviews to insure safe and effective construction procedures are a major element. Other elements include extensive training for personnel, shifting the responsibility for detecting defects from quality control inspectors to workers, and continually maintaining equipment. Worker involvement in improved quality control is often formalized in quality circles in which groups of workers meet regularly to make suggestions for quality improvement. Material suppliers are also required to insure zero defects in delivered goods. Initally, all materials from a supplier are inspected and batches of goods with any defective items are returned. Suppliers with good records can be certified and not subject to complete inspection subsequently.
The traditional microeconomic view of quality control is that there is an "optimum" proportion of defective items. Trying to achieve greater quality than this optimum would substantially increase costs of inspection and reduce worker productivity. However, many companies have found that commitment to total quality control has substantial economic benefits that had been unappreciated in traditional approaches. Expenses associated with inventory, rework, scrap and warranties were reduced. Worker enthusiasm and commitment improved. Customers often appreciated higher quality work and would pay a premium for good quality. As a result, improved quality control became a competitive advantage.
Of course, total quality control is difficult to apply, particular in construction. The unique nature of each facility, the variability in the workforce, the multitude of subcontractors and the cost of making necessary investments in education and procedures make programs of total quality control in construction difficult. Nevertheless, a commitment to improved quality even without endorsing the goal of zero defects can pay real dividends to organizations.
Example 1-2: Experience with Quality Circles

Quality circles represent a group of five to fifteen workers who meet on a frequent basis to identify, discuss and solve productivity and quality problems. A circle leader acts as liason between the workers in the group and upper levels of management. Appearing below are some examples of reported quality circle accomplishments in construction: [2]

1. On a highway project under construction by Taisei Corporation, it was found that the loss rate of ready-mixed concrete was too high. A quality circle composed of cement masons found out that the most important reason for this was due to an inaccurate checking method. By applying the circle's recommendations, the loss rate was reduced by 11.4%.
2. In a building project by Shimizu Construction Company, may cases of faulty reinforced concrete work were reported. The iron workers quality circle examined their work thoroughly and soon the faulty workmanship disappeared. A 10% increase in productivity was also achieved.

1.5 Quality Control by Statistical Methods

An ideal quality control program might test all materials and work on a particular facility. For example, non-destructive techniques such as x-ray inspection of welds can be used throughout a facility. An on-site inspector can witness the appropriateness and adequacy of construction methods at all times. Even better, individual craftsmen can perform continuing inspection of materials and their own work. Exhaustive or 100% testing of all materials and work by inspectors can be exceedingly expensive, however. In many instances, testing requires the destruction of a material sample, so exhaustive testing is not even possible. As a result, small samples are used to establish the basis of accepting or rejecting a particular work item or shipment of materials. Statistical methods are used to interpret the results of test on a small sample to reach a conclusion concerning the acceptability of an entire lot or batch of materials or work products.
The use of statistics is essential in interpreting the results of testing on a small sample. Without adequate interpretation, small sample testing results can be quite misleading. As an example, suppose that there are ten defective pieces of material in a lot of one hundred. In taking a sample of five pieces, the inspector might not find any defective pieces or might have all sample pieces defective. Drawing a direct inference that none or all pieces in the population are defective on the basis of these samples would be incorrect. Due to this random nature of the sample selection process, testing results can vary substantially. It is only with statistical methods that issues such as the chance of different levels of defective items in the full lot can be fully analyzed from a small sample test.
There are two types of statistical sampling which are commonly used for the purpose of quality control in batches of work or materials:

1. The acceptance or rejection of a lot is based on the number of defective (bad) or nondefective (good) items in the sample. This is referred to as sampling by attributes.
2. Instead of using defective and nondefective classifications for an item, a quantitative quality measure or the value of a measured variable is used as a quality indicator. This testing procedure is referred to as sampling by variables.

Whatever sampling plan is used in testing, it is always assumed that the samples are representative of the entire population under consideration. Samples are expected to be chosen randomly so that each member of the population is equally likely to be chosen. Convenient sampling plans such as sampling every twentieth piece, choosing a sample every two hours, or picking the top piece on a delivery truck may be adequate to insure a random sample if pieces are randomly mixed in a stack or in use. However, some convenient sampling plans can be inappropriate. For example, checking only easily accessible joints in a building component is inappropriate since joints that are hard to reach may be more likely to have erection or fabrication problems.
Another assumption implicit in statistical quality control procedures is that the quality of materials or work is expected to vary from one piece to another. This is certainly true in the field of construction. While a designer may assume that all concrete is exactly the same in a building, the variations in material properties, manufacturing, handling, pouring, and temperature during setting insure that concrete is actually heterogeneous in quality. Reducing such variations to a minimum is one aspect of quality construction. Insuring that the materials actually placed achieve some minimum quality level with respect to average properties or fraction of defectives is the task of quality control.

References

1. Ang, A.H.S. and W.H. Tang, Probability Concepts in Engineering Planning and Design: Volume I - Basic Principles, John Wiley and Sons, Inc., New York, 1975.
2. Au, T., R.M. Shane, and L.A. Hoel, Fundamentals of Systems Engineering: Probabilistic Models, Addison-Wesley Publishing Co., Reading MA, 1972
3. Bowker, A.H. and Liebermann, G. J., Engineering Statistics, Prentice-Hall, 1972.
4. Fox, A.J. and Cornell, H.A., (eds), Quality in the Constructed Project, American Society of Civil Engineers, New York, 1984.
5. International Organization for Standardization, "Sampling Procedures and Charts for Inspection by Variables for Percent Defective, ISO 3951-1981 (E)", Statistical Methods, ISO Standard Handbook 3, International Organization for Standardization, Paris, France, 1981.
6. Skibniewski, M. and Hendrickson, C., Methods to Improve the Safety Performance of the U.S. Construction Industry, Technical Report, Department of Civil Engineering, Carnegie Mellon University, 1983.
7. United States Department of Defense, Sampling Procedures and Tables for Inspection by Variables, (Military Standard 414), Washington D.C.: U.S. Government Printing Office, 1957.
8. United States Department of Defense, Sampling Procedures and Tables for Inspection by Attributes, (Military Standard 105D), Washington D.C.: U.S. Government Printing Office, 1963.

INTRODUCTION TO COAXIAL CABLES

Coaxial cables

Introduction to coaxial cables

A coaxial cable is one that consists of two conductors that share a common axis. The inner conductor is typically a straight wire, either solid or stranded and the outer conductor is typically a shield that might be braided or a foil.
Coaxial cable is a cable type used to carry radio signals, video signals, measurement signals and data signals. Coaxial cables exists because we can't run open-wire line near metallic objects (such as ducting) or bury it. We trade signal loss for convenience and flexibility. Coaxial cable consists of an insulated ceter conductor which is covered with a shield. The signal is carried between the cable shield and the center conductor. This arrangement give quite good shielding agains noise from outside cable, keeps the signal well inside the cable and keeps cable characteristics stable.
Coaxial cables and systems connected to them are not ideal. There is always some signal radiating from coaxial cable. Hence, the outer conductor also functions as a shield to reduce coupling of the signal into adjacent wiring. More shield coverage means less radiation of energy (but it does not necessarily mean less signal attenuation).
Coaxial cable are typically characterized with the impedance and cable loss. The length has nothing to do with a coaxial cable impedance. Characteristic impedance is determined by the size and spacing of the conductors and the type of dielectric used between them. For ordinary coaxial cable used at reasonable frequency, the characteristic impedance depends on the dimensions of the inner and outer conductors. The characteristic impedance of a cable (Zo) is determined by the formula 138 log b/a, where b represents the inside diameter of the outer conductor (read: shield or braid), and a represents the outside diameter of the inner conductor.
Most common coaxial cable impedances in use in various applications are 50 ohms and 75 ohms. 50 ohms cable is used in radio transmitter antenna connections, many measurement devices and in data communications (Ethernet). 75 ohms coaxial cable is used to carry video signals, TV antenna signals and digital audio signals. There are also other impedances in use in some special applications (for example 93 ohms). It is possible to build cables at other impedances, but those mentioned earlier are the standard ones that are easy to get. It is usually no point in trying to get something very little different for some marginal benefit, because standard cables are easy to get, cheap and generally very good. Different impedances have different characteristics. For maximum power handling, somewhere between 30 and 44 Ohms is the optimum. Impedance somewhere around 77 Ohms gives the lowest loss in a dielectric filled line. 93 Ohms cable gives low capacitance per foot. It is practically very hard to find any coaxial cables with impedance much higher than that.
Here is a quick overview of common coaxial cable impedances and their main uses:

• 50 ohms: 50 ohms coaxial cable is very widely used with radio transmitter applications. It is used here because it matches nicely to many common transmitter antenna types, can quite easily handle high transmitter power and is traditionally used in this type of applications (transmitters are generally matched to 50 ohms impedance). In addition to this 50 ohm coaxial cable can be found on coaxial Ethernet networks, electronics laboratory interconnection (foe example high frequency oscilloscope probe cables) and high frequency digital applications (fe example ECL and PECL logic matches nicely to 50 ohms cable). Commonly used 50 Ohm constructions include RG-8 and RG-58.
• 60 Ohms: Europe chose 60 ohms for radio applications around 1950s. It was used in both transmitting applications and antenna networks. The use of this cable has been pretty much phased out, and nowdays RF system in Europe use either 50 ohms or 75 ohms cable depending on the application.
• 75 ohms: The characteristic impedance 75 ohms is an international standard, based on optimizing the design of long distance coaxial cables. 75 ohms video cable is the coaxial cable type widely used in video, audio and telecommunications applications. Generally all baseband video applications that use coaxial cable (both analogue and digital) are matched for 75 ohm impedance cable. Also RF video signal systems like antenna signal distribution networks in houses and cable TV systems are built from 75 ohms coaxial cable (those applications use very low loss cable types). In audio world digital audio (S/PDIF and coaxial AES/EBU) uses 75 ohms coaxial cable, as well as radio receiver connections at home and in car. In addition to this some telecom applications (for example some E1 links) use 75 ohms coaxial cable. 75 Ohms is the telecommunications standard, because in a dielectric filled line, somewhere around 77 Ohms gives the lowest loss. For 75 Ohm use common cables are RG-6, RG-11 and RG-59.
• 93 Ohms: This is not much used nowadays. 93 ohms was once used for short runs such as the connection between computers and their monitors because of low capacitance per foot which would reduce the loading on circuits and allow longer cable runs. In addition thsi was used in some digital commication systems (IBM 3270 terminal networks) and some early LAN systems.

The characteristic impedance of a coaxial cable is determined by the relation of outer conductor diameter to inner conductor diameter and by the dielectric constant of the insulation. The impednage of the coaxial cable chanes soemwhat with the frequency. Impedance changes with frequency until resitance is a minor effect and until dielectric dielectric constant is table. Where it levels out is the "characteristic impedance". The freqnency where the impedance matches to the characteristic impedance varies somwehat between different cables, but this generally happens at frequency range of around 100 kHz (can vary).
Essential properties of coaxial cables are their characteristic impedance and its regularity, their attenuation as well as their behaviour concerning the electrical separation of cable and environment, i.e. their screening efficiency. In applications where the cable is used to supply voltage for active components in the cabling system, the DC resistance has significance. Also the cable velocity information is needed on some applications. The coaxial cable velocity of propagation is defined by the velocity of the dielectric. It is expressed in percents of speed of light. Here is some data of come common coaxial cable insulation materials and their velocities:

Polyethylene (PE)   66%

Teflon              70%

Foam                78..86% 

Return loss is one number which shows cable performance meaning how well it matches the nominal impedance. Poor cable return loss can show cable manufacturing defects and installation defects (cable damaged on installation). With a good quality coaxial cable in good condition you generally get better than -30 dB return loss, and you should generally not got much worse than -20 dB. Return loss is same thing as VSWR term used in radio world, only expressed differently (15 dB return loss = 1.43:1 VSWR, 23 dB return loss = 1.15:1 VSWR etc.).

Often used coaxial cable types

General data on some commonly used coaxial cables compared (most data from http://dct.draka.com.sg/coaxial_cables.htm, http://www.drakausa.com/pdfsDSC/pCOAX.pdf and http://users.viawest.net/~aloomis/coaxdat.htm):

Cable type            RG-6        RG-59 B/U   RG-11       RG-11 A/U    RG-12 A/U  RG-58 C/U  RG-213U  RG-62 A/U 

Impedance (ohms)      75          75          75          75           75         50         50       93

Conductor material    Bare        Copper      Bare        Tinned       Tinned     Tinned     Bare     Copper

                      Copper      Planted     Copper      Copper       Copper     Copper     Copper   Planted

                                  Steel                                                               Steel

Conductor strands     1           1           1           7            7          19         7        1

Conductor area (mm2)  0.95        0.58        1.63        0.40         0.40       0.18       0.75     0.64

Conductor diameter    0.028"      0.023"                  0.048"                  0.035"     0.089"   0.025"

                      21AWG       23AWG                   18AWG                   20AWG      13AWG    22AWG

Insulation material   Foam PE     PE          Foam PE     PE           PE         PE         Pe       PE (semi-solid)

Insulation diameter   4.6 mm      3.7 mm      7.24 mm     7.25 mm      9.25 mm    2.95       7.25     3.7 mm

Outer conductor       Aluminium   Bare        Aluminium   Bare         Base       Tinned     Bare     Bare

                      polyester   copper      polyester   copper       copper     copper     copper   copper

                      tape and    wire        tape and    wire         wire       wire       wire     wire

                      tin copper  braid       tin copper  braid        braid      braid      braid    braid

                      braid                   braid      

Coverage              Foil 100%   95 %        Foil 100%   95%          95%        95%        97%      95%

INTRODUCTION TO EARTHING SYSTEM DESIGN

EARTHING SYSTEM DESIGN 

Earthing is connection to earth to provide absolute zero potential. Earthing is given to the body of equipment so as to divert fault current to ground through neutral wire. operator remains safe, since current gets lower impedance path.

Purpose of earthing is to provide a low potential path to fault current which result in tripping the protective device & thus cut off the supply. Once the supply is cut off, both equipment & operator is safe. But remember earthing without the use of protective device in the circuit is of no use.

the term earthing means connection of neutral to the earth(ground).when any fault is occured then the fault current is given to earth to expose it . It is necessary to protection of operator. earthing is done by a bare simple conductor

 Earthing is connecting the neutral of the electrical

Equipment to the ground.

EARTHING AND BOND

Bonding is connecting wires together so they act as a single wire and all points along the wire have the same voltage/potential.

It all depends on the supply Normally in a domestic situation you have a combined neutral earth supply Typically you will require a 16mm main earth from the incoming mains to the consumer unit .Then a 10mm main bonding conductor from the consumer unit to the water pipe within 600 mm of the stop tap where it enters the property as well as the gas services again within 600mm I both these are located together then you can use the same wire as long as it is unbroken between clamps .In addition the pipes need to be bonded together with 6mm earth cable ie hot ,cold,gas and heating this is normally done in an airing cupboard or behind the heating unit.There are circumstances when additional bonding is required such as a room containing a bath or shower where all exposed pipework must be bonded together locally. hope it helps

These have been designated in the IEE Regulations using the letters: T, N, C and  S. These letters stand for:

 T    -  terre  (French for earth) and meaning a direct connection to earth. 

N    -  neutral

C    -  combined

S    -  separate.

When these letters are grouped, they form the classification of a type of system. 

The first letter denotes how the supply source is earthed.  

The second denotes how the metalwork of an installation is earthed.   The third and fourth indicate the functions of neutral and protective conductors.

 TT SYSTEM

 A TT system has a direct connection to the supply source to earth and a direct connection of the installation metalwork to earth.  An example is an overhead line supply with earth electrodes, and the mass of earth as a return path as

TN-S SYSTEM

 A TN-S system has the supply source directly connected to earth, the installation metalwork connected to the neutral of the supply source via the lead sheath of the supply cable, and the neutral and protective conductors throughout the whole system performing separate functions.

The resistance around the loop P-B-N-E should be no more than 0.8 ohms.

 TN-C-S SYSTEM

A TN-C-S system is as the TN-S but the supply cable sheath is also the neutral, i.e. it forms a combined earth/neutral conductor known as a PEN (protective earthed neutral) conductor.  

The installation earth and neutral are separate conductors.

This system is also known as PME (protective multiple earthing).

The resistance around the P-B-N-N loop should be less than 0.35 ohms.

SUMMARY OF EARTHING SYSTEMS

 The TT method is used mostly in country areas with overhead transmission lines. In contrast to the TN-S system there is no metallic path from the consumer's terminals back to the sub-station transformer secondary windings.  Because the earth path may be of high resistance, a residual current circuit-breaker (R.C.C.B.) is often fitted so that if a fault current flows in the earth path then a trip disconnects the phase supply.  

For protection against indirect contact in domestic premises, every socket outlet requires an RCCB with a maximum rated current of 30mA.

The TN-S system of wiring uses the incoming cable sheath as the earth return path and the phase and neutral have separate conductors.  The neutral is then connected to earth back at the transformer sub-station.

Remember in TN-S, the T stands for earth (terre), N for neutral and S denotes that the protective (earth) and neutral conductors are separate.

 The TN-C-S system has only two conductors in the incoming cable, one phase and the other neutral.  The earth is linked to the neutral at the consumer unit.  The neutral therefore is really a combined earth/neutral conductor hence the name PME.

 In order to avoid the risk of serious electric shock, it is important to provide a path for earth leakage currents to operate the circuit protection, and to endeavour to maintain all metalwork at the same potential.  This is achieved by bonding together all metalwork of electrical and non-electrical systems to earth.

The path for leakage currents would then be via the earth itself in TT systems or by a metallic return path in TN-S or TN-C-S systems.

Transformer Calculations

Before beginning to discuss the calculations involved with transformers, it is necessary to outline the most commonly available transformer sizes, in terms of KVA (kilo-volt-amps). This chart is not necessarily based on the code but is more of an industry standard held by most manufacturer's (GE, Square D, Eaton, etc.):

• 15 kva
• 30 kva
• 45 kva
• 75 kva
• 112.5 kva
• 150 kva
• 225 kva
• 300 kva

It is also important to define some terms that will be used throughout the calculations:

• connected load = total kilowatt load with no diversity taken into account.
• demand load = total kilowatt load with diversity calculated in.
• kva = kw (kilo-volt-amps = kilowatts)
• NEC = National Electrical Code (aka NFPA 70)

Calculating Transformer Size

Transformers are sized based on the total connected load on the secondary side and then selecting the next available kva size up from the connected load. Let's look at an example:

• Suppose we need to size a 480v-208v 3ø transformer. There is a 100amp panelboard on the secondary side serving power receptacles and workstations where to total connected load is 23.50kva. By reviewing the transformer chart from above, you will find that 30kva is the nect available size up from 23.50kva. Therefore a 30kva transformer is required for this system.

Calculating Overcurrent Protection on the Primary Side

Overcurrent protection for a transformer on the primary side is typically a circuit breaker. In some instances where there is not a high voltage panel, there is a fused disconnect instead. These are the two most common ways to provide overcurrent protection on the primary side.

According to NEC 450.4, "each transformer 600 volts, nominal, or less shall be protected by an individual overcurrent device installed in series with each ungrounded input conductor. Such overcurrent device shall be rated or set at not more than 125 percent of the rated full-load input current of the autotransformer." Further, according to NEC Table 450.3(B), if the primary current of the transformer is less than 9 amps, an overcurrent device rated or set at not more than 167% of the primary current shall be permitted. Where the primary current is less than 2 amps, an overcurrent device rated or set at not more than 300% shall be permitted.

From my experience, the primary current is rarely 9 amps or less, much less 2 amps. For the most part you will be using 125% to determine the primary overcurrent protection.
Let's take a look at an example:

• What size circuit breaker (overcurrent protection device) is required on the primary side to protect a 75kva 480v-208v 3ø transformer?

• 75kva x 1,000 = 75,000va
• 75,000va / (480V x √‾3) = 90.21 amps
• (Note: 480V 3ø is calculated as 480V x √‾3 or 831.38)
• The current (amps) is more than 9 amps so use 125% rating.
• 90.21 amps x 1.25 = 112.76 amps

• Therefore: Use 125amp 3-pole circuit breaker (the next highest fuse/fixed-trip circuit breaker size per NEC 240.6).

**It is important to note that the overcurrent device on the primary side must be sized based on the transformer KVA rating and not sized based on the secondary load to the transformer.

Calculating Overcurrent Protection on the Secondary Side

In order to calculate the overcurrent protection on the secondary side, most of the same principles apply as with the primary calculations, with a few exceptions.

Obviously the voltgae is different. For a 480v-208v 3ø transformer, the calculations will use 208V x √‾3 instead of 480V x √‾3.

Also, according to NEC Table 450.3(B), where the secondary current of a transformer is 9 amps or more and 125% of this current does not correspond to a standard rating of a fuse or circuit breaker, the next higher standard rating shall be required. Where the secondary current is less than 9 amps, an overcurrent device rated or set at not more than 167% of the secondary current shall be permitted.
Let's take a look back at the previous example:

• What size circuit breaker (overcurrent protection device) is required on the secondary side to protect a 75kva 480v-208v 3ø transformer?
• *Note: Calculate the secondary overcurrent protection based on the size of the transformer, not the total connected load.
• 75kva x 1,000 = 75,000va
• 75,000va / (208V x √‾3) = 208.18 amps
• (Note: 208V 3ø is calculated as 208V x √‾3 or 360.27)
• The current (amps) is more than 9 amps so use 125% rating.
• 208.18 amps x 1.25 = 260.24 amps
• Therefore: Use 300amp 3-pole circuit breaker (per NEC 240.6).

DEMAND VS DIVERSITY FACTOR

Damand Vs. Diversity Factor

There are two terms that seem to confuse designers. These terms are “diversity factor” and “demand factor.” To better understand the application of these terms when calculating the load for a service or a feeder supplying a facility, one must understand their meaning.
Diversity factor is the ratio of the sum of the individual maximum demands of the various subdivisions of a system (or part of a system) to the maximum demand of the whole system (or part of the system) under consideration. Diversity is usually more than one.
Demand factor is the ratio of the sum of the maximum demand of a system (or part of a system) to the total connected load on the system (or part of the system) under consideration. Demand factor is always less than one.
Application of diversity factor
Consider two facilities with the same maximum demand but that occur at different intervals of time. When supplied by the same feeder, the demand on such is less the sum of the two demands. In electrical design, this condition is known as diversity.
Diversity factors have been developed for main feeders supplying a number of feeders, and typically, they are 1.10 to 1.50 for lighting loads and 1.50 to 2.00 for power and lighting loads.
Diversity factor and load factor are closely related. For example, consider that a feeder supplies five users with the following load conditions: On Monday, user one reaches a maximum demand of 100 amps; on Tuesday, two reaches 95 amps; on Wednesday, three reaches 85 amps; on Thursday, four reaches 75 amps; on Friday, five reaches 65 amps. The feeder’s maximum demand is 250 amps.
The diversity factor can be determined as follows:
Diversity factor = Sum of total demands ÷ Maximum demand on feeder = 420 ÷ 250 = 1.68 × 100 = 168%
Given
Calculate the size of a main feeder from substation switchgear that is supplying five feeders with connected loads of 400, 350, 300, 250 and 200 kilovolt-amperes (kVA) with demand factors of 95, 90, 85, 80 and 75 percent respectively. Use a diversity factor of 1.5.

Solution
Calculate demand for each feeder:
400 kVA × 95% = 380 kVA •
350 kVA × 90% = 315 kVA •
300 kVA × 85% = 255 kVA •
250 kVA × 80% = 200 kVA •
200 kVA × 75% = 150 kVA •
The sum of the individual demands is equal to 1,300 kVA •
If the feeder were sized at unity diversity, then 1,300 kVA ÷ 1.00 = 1,300 kVA
However, using the diversity factor of 1.5, the kVA = 1,300 kVA ÷ 1.5 = 866 kVA for the feeder. Transformer supplying the main feeder plus wiring methods and equipment can be sized from this kilovolt-ampere rating.
Applying demand factors
Although feeder conductors should have an ampacity sufficient to carry the load, the ampacity needs not always be equal to the total of all loads on connected branch-circuits.
A study of the National Electrical Code (NEC) will show that a demand factor may be applied to the total load. Remember, the demand factor permits a feeder ampacity to be less than 100 percent of all the branch-circuit loads connected to it.
Keep in mind that demand factor is a percentage by which the total connected load on a service or feeder is multiplied to determine the greatest probable load it may be called on to carry.
When additional loads are connected to existing facilities having services and feeders as originally calculated per 220.87, the maximum kilovolt-ampere calculations in determining the load on existing services and feeders should be used if these conditions are met:
If the maximum data for the demand in kVA, such as demand meter ratings, is available for a minimum of one year •
If 125 percent of the demand ratings • for the period of one year added to the new load does not exceed the rating of the service; where demand factors are used, often the load as calculated will probably be less than the demand meter indications.
The Ex. to 220.87 contains requirements for where the maximum data for one year is not available. In such, the calculated load is permitted to be based on the maximum demand (measure of average power demand over a 15-minute period) continuously recorded over a minimum 30-day period using a recording ammeter connected to the highest loaded ungrounded (phase) of the feeder or service based on the initial loading at the start of the recording.
By referencing Parts III and IV in the NEC, designers can find other useful demand factors that are applicable to specific loads.
STALLCUP is the CEO of Grayboy Inc., which develops and authors publications for the electrical industry and specializes in classroom training on the NEC and OSHA, as well as other standards.

Equipment Load Factors, Use Factors and Diversity Factors As Well as a General Discussion of Energy Audit Procedures

To do a good job on an energy audit, the energy auditor must understand the areas of equipment load factor, use factor and diversity factor.

Definitions: First, let's define these terms.

a)         Load factor - the ratio of the load that a piece of equipment actually draws when it is in operation to the load it could draw (which we call full load).

For example, an oversized motor - 20 hp - drives a constant 15 hp load whenever it is on. The motor load factor is then 15/20 = 75%.

b)        Use (or utilization) factor - the ratio of the time that a piece of equipment is in use to the total time that it could be in use.

For example, the motor above may only be used for eight hours a day, 50 weeks a year. The hours of operation would then be 2000 hours, and the motor use factor for a base of 8760 hours per year would be 2000/8760 = 22.83%. With a base of 2000 hours per year, the motor use factor would be 100%. The bottom line is that the use factor is applied to get the correct number of hours that the motor is in use.

c)         Diversity factor - the probability that a particular piece of equipment will come on at the time of the facility's peak load.

The diversity factor is the most complicated of these factors. For example, we might have ten air conditioning units that are 20 tons each at a facility. In Florida we typically assume that the average full load equivalent operating hours for the units are 2000 hours per year. However, since the units are each thermostatically controlled, we do not know exactly when each unit turns on. If the ten units are substantially bigger than the facility's actual peak A/C load, then fewer than all ten units will likely come on at once. Thus, even though each unit runs a total of 2000 hours a year, they do not all come on at the same time to affect the facility's peak load. The diversity factor gives us a correction factor to use, which results in a lower total kW load for the ten A/C units. If the energy balance we do for this facility comes out within reason, but the demand balance shows far too many kW for the peak load, then we can use the diversity factor to bring the kW into line with the facility's true peak load. The diversity factor does not affect the kWh; it only affects the kW.

Motor load, use and diversity factors: Sometimes motor load factors that are too low are chosen because the auditor has not properly determined the hours of use of the motors - i.e. the auditor uses an incorrect use factor. For example, just because a facility has a production shift that is 2000 hours per year it does not mean that all of the production-related motors in that facility are operated for 2000 hours per year. Some motors - or machines - might only be used one day a week rather than every day. Other motors might be used every day, but for only half the day, i.e. 4 hours per day. Other motors might be in use throughout the day so that their use really is 2000 hours per year.

The auditor must collect data on the use factor - or hours of use - for every motor in the facility during site visit. For each machine, line, process or operation, ask "How many hours a day does this machine (line, process or operation) operate?" This data then needs to be entered into the energy balance. Motor load factors in many facilities are more in the range of 40% - 50%, than in the range of 80% that had been a standard assumption for many years of doing audits. Rarely do you find a motor running at 100% load factor.

However, not all motors at a facility are running at the same load factors. Ventilating fans that come from a supplier as a packaged unit with a fan and a motor are most often assumed to be operating at near full load. You should probably use a load factor of 80% here, since the manufacturer of the ventilating fans should have reasonably matched these loads. Other motors may also be in this category - some engineering judgment and common sense are required to determine which other motors these are.

Motors with variable loads are going to have the lowest load factors in general. A dust collector fan motor will normally have quite a variable load, and would often be expected to have a low load factor. Other examples are saws, presses, milling machines, sanders and grinders, waste grinders, water pumps, hydraulic pumps, etc.

If a group of motors do not all operate together all of the time, then using a diversity factor is appropriate. This is the case with a number of separate air conditioning units (considering the motors for the compressors) that are individually thermostatically controlled.   It could also be the case for a group of production motors if some of the motors are not in use all of the time. You should use a diversity factor in your motor calculations, since it is not often the case that a facility has all of the motors on at the same time.

Reconciling the energy balance: When you perform an energy balance, do not use the motor load factor as the first and only adjustment made to reconcile the estimated energy use (energy balance) with the energy bills. Making this adjustment too quickly results in failure to pick up other things that have been overlooked.

For example, if the energy use does not balance with the energy bills, the first step is to check to see that all of the equipment and uses have been accounted for.

Do the items on the energy balance spreadsheet match your recollection of the equipment you saw in the facility?

Does anything appear to be missing?

Are the utility bills for total energy use and peak kW recorded correctly?

The next step is to check the hours of use for lights and other equipment to see if it matches your knowledge of the facility's operation. Remember that each motor - as well as each other piece of equipment - does not necessarily operate the same number of hours each day or year. Finally, if some of the equipment does not come on at the same time as the facility peaks in kW use, then utilize the diversity factor to account for this.

Adjusting the motor load factors should probably be the last thing you do to reconcile the energy and demand balances. Now, if all other information and all other factors are correct to the best of your knowledge, then adjust the load factors. While motor load factors are not often in the range of 80-100%, you should be equally suspect of very low motor load factors. If you get motor load factors in the range of 20-30%, it is more likely that you have the hours of use wrong than that you have a facility which is using motors that are an average of four times too big for the job they are doing. Lumber mills and wood products facilities using lots of saws may have these low load factors. Most other places should have motors with a higher load factor.

Basic motor load measurements should be taken at the plant visit. The electrical person at the facility is generally willing to measure the current being drawn by a motor of interest. Air compressors are ones that are usually easy to do, and you should ask the plant personnel to do this for you. Let them open the motor controller or switch box and connect a clamp-on ammeter to see what the current for the motor is. You then need to know the full load current from the nameplate of the motor. The ratio of the actual current to the full load current is the approximate load factor on the motor at that time. This procedure works as long as the current is greater than or equal to about 50% of the full load current. Try to take this measurement for each of the large motors in the facility - i.e. motors of 50 hp and above; or even 20 hp or above if the facility does not have a lot of big motors. If you have not received formal electrical safety training, you should not make these electrical measurements yourself. If the facility electrician does not want to make these for you, then let it go at that.

Air handlers—use factor: Air handlers use motors and are subject to all of the comments made in the motor section. In addition, you may be able to get a better handle on the hours of use for the air handlers by knowing how the A/C system works. Ask if the air handlers run constantly when the facility is occupied. They might if the facility wants the ventilation, even though the compressors might not come on except to periodically provide some temperature reduction or moisture removal. If this is the case, then the use factor for these air handler motors should reflect an hours-of -use that matches the offices or other area that the air handlers supply. In addition, the hours-of-use must also consider the compressor run hours. Thus the total hours for the air handlers must be at least the same as the compressor hours, and may be higher if the A/C unit is left on during periods that the facility is not occupied, or if ventilation is provided.

If the air handlers only come on when the thermostat orders cooling, then the hours-of -use must be the same as the hours-of-use of the compressors.

It is important to get adequate information on the operation of the air conditioning system. To get complete data on the air handler motors for an air-conditioned facility, you will need all of the standard information - size, maker, single or three phase, etc - together with the operating basis for the air handlers discussed above.

You should also collect data on the drive belt system for air handlers. Record the number of belts, the lengths, and the types of belts. Ask about motor and drive lubrication and cleaning. Also check the A/C filters to see if they are reasonably clean.

Sometimes a visual inspection will show some real problems. Ask the maintenance person to open up one of the air handlers - or just look into it (SAFELY) if it is accessible - and see if the belt is tight, slack, or really loose. Do not stick your hand into an air handler that is off at the moment, and may come back on when the thermostat kicks in. Have the maintenance person turn the air handler motor off with the circuit breaker or control box. Do not put your finger on a moving drive belt.

Is the belt in good shape?

Is it frayed, cracked or coming apart?

Does it look like the pulleys for the motor and the fan are lined up?

Ask the electrician to measure the current that the air handler motor is drawing to see what its load factor is while driving the fan. It should be very near full load - but you never know. Maybe the original motor burned out and was replaced with a bigger one to "make sure it did not burn out again." Remember to take the full load current off the nameplate to find the load factor.