The cost of cutting corners

07/13/2010


With the increasing economic pressure on design engineers in the electrical construction industry today, there is a greater demand for more work in less time and a requirement to make increased profits on the time that is worked. Phrases that one hears all too often are:

  • I will have the contractor responsible for the coordination study because I do not know how to do it.
  • The project is due out this week so I am going to skip calculating fault current for this distribution system.
  • These arc fault breakers are being required by the Code just so the breaker manufacturers can make more money.
  • I don’t have the time to design this pole base, so I will just copy a detail from the last project.
  • I will just calculate the current carrying capacity of this duct bank feeder the same way that I calculate a feeder in conduit.
  • Everyone else sizes feeders using the basic table 310-16, so I am going to do it the same way.

These are just a few examples of the many design tasks that are dropping by the wayside, taking the “Engineering” out of the practice of engineering. These conflicting issues create a working environment that is the ideal breeding ground for ethical conflicts for the engineer.

The National Society of Professional Engineers keeps their view of the design engineer’s responsibility to the public short and sweet, “Engineers shall hold paramount the safety, health, and welfare of the public.”

 It is important to note that there is nothing in this statement about the project due date, how much fee is left on the job, which calculations can be skipped or whether a protective device should be left out. It only says that the public safety, health and welfare is paramount.

The American Association of Engineering Societies goes one step further by saying that, “Engineers who perceive that pursuit of their professional duties might have adverse consequences for the present or future public health and safety…. shall advise their … clients…” and, “Engineers shall perform only those services that they are qualified by training or experience to perform, ….”. Thus, the engineer must advise their client when part of their design might put the safety of the public at risk, and they must not perform design services for which they are not qualified.

All of this appears to paint a bleak picture, but a resourceful engineer must always find a way to keep public safety, health and welfare as paramount.

OVERCOMING THE CHALLENGE

Returning to the examples above, there are solutions for each of these dilemmas:

  • Coordination of overcurrent protective devices (OPD) is critical for the protection of the distribution system. Without designing the distribution at the same time as the device coordination, the design may not permit the required coordination of OPDs, so trying to foist it onto the contractor is not a solution. Failure to coordinate the OPDs could result in unnecessary outages and, potentially, could cause the destruction of key electrical equipment and expose the engineer to expensive and time consuming lawsuits.
  • If the system utilizes fuses for the main distribution equipment, some of the fuse manufacturers provide multiplying factors that will permit coordination of the main, feeders, and distribution equipment. As one example, Bussman, in their Selecting Protective Devices handbook, provides a section aptly called, Fuse Selective Coordination, which shows the multiplying factors between any two of their common fuse types. This will result in a selectively coordinated system with a very small time expenditure, virtually no learning curve, and, technically, quite simple.
  • Circuit breaker systems require somewhat greater effort but, this effort is still very small, whether the system is large or small. The cost of having a specialist coordinate the system or purchasing software to provide the design is significantly less than a costly redesign during construction or an expensive lawsuit.
  • Fault current calculations and the selection of properly rated OPDs is critical to protecting the public. An OPD that is applied in a distribution system with a fault current higher than the OPD will interrupt, is an explosion waiting to happen. Designing underrated OPDs can result in devastating lawsuits that can put many engineering firms out of business, and can end your engineering career.

The simplest fault duty calculations can take only a few minutes to complete and can insure the distribution system against all major faults. The methodology for this calculation can be found in the most basic electrical engineering handbooks, free on-line calculators, all of the basic engineering software packages, and in CSEs upcoming ebook series, The Art of Protecting Electrical Systems (revised).

  • Arc Fault Breakers have been required by the National Electrical Code (NEC) since the 1999 edition and have been mandated for installation since January 2002. These breakers were added to the NEC to reduce the number of smoky arcing faults in residential sleeping areas which had, in the past, resulted in severe injury or loss of life. In the 2008 NEC, the requirement for arc fault breakers was extended to most receptacle circuits in residential units, since these smoky arcing faults could occur on any circuit, not just the ones serving the bedroom receptacles. Preventing these arcing faults is a classic case of protecting the safety, health and welfare of the using public.
  • Pole bases for exterior lighting are sized based on soil conditions, pole height, fixture wind resistance, and several other factors. Unless the previously designed, pole base detail was for an installation having identical parameters, the base could be way too big or way too small for the new application.

By having a structural engineer design the pole base for each specific application, the electrical engineer never needs to worry about having his lighting poles knocked over when a stronger than normal wind blows in (pun intended). The design cost of having the structural engineer who is already familiar with the site, is quite small compared to the cost of replacing the foundations and having the poles reinstalled, out of the electrical engineer’s design fees.

  • Sizing conductors in a multi-conduit, duct bank is a risky shortcut. As an example, a 3000 amp, 480 V electrical service might be sized as eight sets of 500 kcmil copper conductors using 380 amps per set. However, extrapolation of the tables in appendix B of the NEC (which were derived from the work by Neher-McGrath on temperature rise for underground cables) show that an ampacity of 210 amps would be suitable for this feeder.

Therefore, there should be fourteen sets of four-500 kcmil copper conductors for an electrical service of this size, based on an earth heat conductivity of 90 and a load factor of 100. With a more heat conductive soil condition and a lower load factor, there would be fewer conductor sets required, but it would still be significantly greater than 8 sets!

While this discussion keyed on electrical engineering, engineers in all disciplines are faced with dilemmas of this type on a daily basis. The Code of Ethics noted above, are not mandated by any Code or an enforcement authority, in most locales.

However, every time an engineer is faced with these decisions, he should consider long and hard, the long term impacts which his decision might have. Skipping steps, ignoring calculations, skirting the Code and a myriad of other issues that will get the boss of your back and get the project shipped is always the easy way out.

Every time this situation arises, mentally place yourself on the witness stand and consider: “How will I respond to the prosecuting or plaintiff’s attorney when they ask if I knew that what I did was a Code violation?”. Also consider how a response like: “But everyone does it that way”, will be received by the Judge and the jury?

Finally, think about whether your PE registration, your personal integrity, and/or your freedom is worth violating the Code of Ethics, just to save a bit of time, some extra design effort, or, even, your job.

SIDEBAR

Case study: Sizing calculation errors lead to starter failure

The two-speed starter for a 40 HP roof-mounted cooling tower was installed on a formed metal, framing system near the tower. On a particularly hot day (95 degrees), the facility manager called us to investigate as to why the starter burned up and shut down the tower.

After the destroyed starter was replaced, we investigated to ascertain the reason for the failure and found that the conductors within the starter were the ignition source for the fire. Since there was no evidence of an arc that could have initiated the fire, we, then, looked into the conductor sizing and found that, based on paragraph 310-16 of the NEC, the conductors were sized for installation in a 30 degree C ambient condition using the 75 degree termination column. In addition, the conductors were sized right at 125% of the full load current of 52 amps, or #6 copper.

So, if the ambient condition for the started had actually been 30 degrees C and the terminals within the starter were rated at 75 degrees C, then the conductor would have been sized correctly. In this condition, the actual temperature of the starter interior was closer to 50 degrees C and the conductors were significantly undersized. 

The correct conductor size can be calculated as follows: 

1. Since the full load current is below 100 amperes, only the ampacities in the 60 degree C termination temperature may be used per NEC 110-14. 

2. The NEC factor of 1.25 must first be applied to the full load current, per NEC 430.22 A (“Conductors …. shall have an ampacity of not less than 125 % of the motor’s full-load current”)

3. The correction factor for installing conductors in a 50 degree C ambient instead of the 30 degree C ambient (as noted in 310-16, title), is 0.58.

4. So taking the 52 FLA and multiplying by 1.25 gives 65 amps. The ambient correction factor raised this ampacity to 112 amps and the next size above this value is 1/0 copper with a rating of 125 amps. 

Since 112 is considerably above 125% of the FLA, an argument could be made that one could round down to #1 copper which is rated at 110 amps.

However, both sizes are substantially larger than the #6 which was used, based on the erroneous assumption concerning ambient temperature and termination temperature. The result was a destroyed starter and loss of air conditioning on a very hot day. 



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