Tom Székely, P.E., LEED AP

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February 2, 2006

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The Shareholder/Unit Owner Board Member Survival Manual, or, Engineering for Dummies – Part 4


Of all facets of my engineering practice, electrical, the one I started with, is the one I’ve found most difficult for not only the lay public, but for allied design professionals, such as Architects, to grasp.  I suppose it has something to do with the fact that water, heat, cold, steam, air, etc. are able to be seen, felt or otherwise be directly sensed, and even the untrained eye can make judgments about the stability of a structure by a mere glance.  I mean, if something looks rickety, it probably is.


While humanity has been fooling around with all the rest for thousands of years, empirical knowledge of electricity is so recent that Ben Franklin did dangerous things with kites in thunderstorms in his efforts to expand the scope of human understanding of the subject.  Today’s electrical engineering majors get to play with some really scary looking mathematics in their quest for an undergraduate degree, and the kind of electrical power engineering I do occasionally deals with complex numbers, but even with all that, the extent of day to day mathematics used in designing facility electrical systems, is almost never beyond the capabilities of a four-function arithmetic calculator and some very elementary algebra.


In fact, I remember a senior engineer being somewhat surprised when I told him early in my career that all one needed to know to be a good electrical designer was how to add, subtract, multiply, divide, count, think, and read. 


This last thing of course requires that one understands what one is reading, and thereby, I believe, hangs the difficulty with things electrical.


One has to start with a clear understanding of work, power, and energy, as defined in a high school physics book, and then it requires leaps of imagination in divining things one will never be able to see.


The problem is exacerbated by the methods generally used in formal higher education to teach relatively complex subjects such as mathematics, or even something which should be so simple as learning a foreign language.


While an infant learns its native language by repeating sounds, leaving mastery of orthography and grammar until rather later on, we try to wrap our brains around the Fundamental Theorem of the calculus very early in Elementary  Calculus, and get thrown right into grammar and spelling when we first learn a foreign language.


So what terrifying concept about electricity have I been trying to get to, that I’ve felt the need to beat the concepts of understanding and education nearly to death for almost half of this piece? 


Actually there are two.  The first, which is the subject of this issue, is electrical safety with regard to overloads and short circuits vis-ŕ-vis electrical fires, and electrical shock.  The second, which I’ll discuss in the next issue, is what constitutes an adequately sized electric service to a home or dwelling unit, or other space within a multi-use building, and how that extrapolates to an electric service adequate for an entire building.


An electric wire or cable carries a flow of current much as a pipe carries water.  If a pipe breaks the surroundings get wet.  If a cable’s insulation is compromised it could result in a fire or electrocution. 


A complete electric circuit has a power source like a battery, generator, or alternator, (or the humongous alternators Con Edison uses to generate electricity for your home), analogous to a circulating pump in a piping circuit supplying baseboard fintube radiators. 


It also has supply (“hot” or “line”) and return (“neutral” or “grounded”) conductors to move high energy electricity to, and return low-energy electricity from, say, a light bulb.


While a baseboard radiator gives up heat to the surrounding space via fins on the pipe, and the circulating pump returns water to the boiler for more heat, resistance to electrical flow caused by trying to cram a ton of electrons through a skinny lamp filament makes it so hot that it gives off  visible light.


An electric utility’s alternators are so powerful that the amount of current (measured in amperes or amps) that flows through a load like a lamp is only governed by resistance of the lamp.  A 500 watt lamp with its thicker filament offers less resistance than a 100 watt lamp, and thus lets more current flow.


If someone does something stupid like connecting the “hot” terminal of a lamp socket directly to its “neutral” terminal via, say, inadvertently touching both with a screwdriver, bad things happen. The low resistance of the beefy screwdriver (when compared to the resistance of even a 500 watt lamp filament) will allow hundreds or thousands of amperes of current to flow through it, and the resulting short circuit will get the link in a fuse so hot or generate such an intense magnetic field in a circuit-breaker’s trip mechanism, that in the former that link will melt, and in the latter, resettable contacts will separate, both almost instantaneously, to prevent a fire.


Well that’s nice, but every motor draws about 6 times its normal operating current when starting from rest, and fuses and breakers are designed to ignore this inrush current for the second or two it takes to come up to speed.  If for some reason, the machinery is mechanically jammed, the continuous heating of this overload current will melt a fuse or separate a breaker’s contacts via the bimetal portion of its trip element in a very few seconds.


Unfortunately if you touch a live electrical terminal or wire, only .03 amps or less flowing through you will stop your heart.  GFCI’s (Ground-Fault Circuit Interrupters) have been required for sink, lavatory, and outdoor outlets for quite some time in order to prevent this from happening.  More recently, since 2002, the National Electric Code has required AFCI’s (Arcing-Fault Circuit Interrupters) to protect outlets in bedrooms against overload-caused fire by tripping a 15 or 20 ampere breaker at about 75 amps, rather than the 90 or 120 amps respectively that a normal breaker would pass for seconds, before it was even aware of an overload.  If the overload were caused by an arc to ground (e.g., worn insulation exposing the wire to adjacent metal), then as little as 5 amps would trip the breaker. 


Ben Franklin had it easy.

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