Of K-factor, Ground Faults and Simplicity

Specifiers often feel compelled to simplify very complex issues for the benefit of their clients and compatriots in other professions. As they often find, however, oversimplification can be a dangerous thing. And when the issue at hand is system safety, the last thing that engineers want is imprecision.Take electrical systems, for instance.

03/01/2001


Specifiers often feel compelled to simplify very complex issues for the benefit of their clients and compatriots in other professions. As they often find, however, oversimplification can be a dangerous thing. And when the issue at hand is system safety, the last thing that engineers want is imprecision.

Take electrical systems, for instance. These are extraordinarily complex but often beg for simple answers to appease nontechnical and bean-counting types. To be on the safe side, however, long answers may be better.

One example is K-factor, a description of the heating losses a transformer can tolerate when supplying a given nonlinear load. Underwriters' Laboratories (UL) derives K-factor from a well-known standard (ANSI/IEEE C57.110) and tests transformers for coil heating and for proper sizing of the neutral conductor to handle damaging harmonic triplens.

System designers often counsel their clients that any transformers they buy should have K-factors noted on their nameplates. Yet, as any engineer will attest, the UL listing is not a guarantee that harmonics in a facility's electrical system will not ravage transformers and cause danger to occupants and operators. First of all, severe harmonic distortion can cause core heating and mechanical stresses not covered in the UL tests. Part of the design engineer's task is to address system harmonics ; a transformer with a high K-factor by no means absolves the designer of that critical task.

Another area of concern is that transformers with UL-tested K-factor ratings can have high impedance , which may even be a source of harmonic distortion. Core saturation can cause disastrous levels of distortion.

Good engineers make it a practice to explain such complexities to their clients: That transformers may have a distorted output regardless of K-factor, and that electrical system design trumps all in assuring safe facility operation.

Grounded, to a fault

Another area of concern recently has been arcing faults , which also present a safety concern to the operator-and a major design problem for the electrical engineer. Arcing-fault ground currents, for example, are often caused by equipment problems, such as insulation failure and loose component connections. But to associate the phenomenon with cheap or poorly assembled equipment has been a dodge; all consulting engineers and most experienced operators know that poor construction management, equipment mishandling and lack of system maintenance can cause arcing faults.

Preventing arcing faults starts in system design, with the specification of isolated equipment , draw-out features and proper settings for protective devices . When construction is done, the electrical contractor must assure that the system is properly cleaned and ready to be energized.

Consulting and staff engineers must effectively communicate the dangers of arcing faults to clients and operators to instill a sense of responsibility that starts in the design phase and is carried through to routine maintenance.





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