hz1@hou2d.UUCP (H.ZARETSKY) (05/11/88)
here is the untold truth about gfci's for all those who want to know. the following is from a top engineer at Leviton Manufacturing. i happen to have worked there for a year and i am still friendly with the engineers there. for those who never heard of Leviton, they are one of the biggest manufactures of electrical supplies (outlets, switches, gfi's, home control systems, etc.) and anybody who has done any electrical work is familiar with them. there has been some articles on the network about gfci's and i want to clarify the use and operation of gfci's to all that don't know or are unsure. so here it is straight from the horse's mouth uncensored. ============================================================================== The GFCI compares current in the phase (hot) wire to the current in the neutral (return) wire. As long as the two currents are equal, the assumption is that there is no ground fault. A key requirement (and that is met for all utility supplied electrical systems) is that the neutral be grounded at the service entrance. If there should be a ground fault (current from the hot wire through the victim to ground) the GFCI detects an imbalance in current. If of sufficient magnitude , (5 ma) the GFCI will trip (the time spec is 25 ms to open the circuit) Note that a ground wire is NOT required for GFCI operation. This is one of the original purposes of GFCI's. Thus, the GFCI WILL protect two wire receptacles down stream. There are two additional points to be made. 1. The receptacle GFCI does have a ground hole for the round pin in a plug to access. This is put there for additional safety, belt and suspenders. 2. If a GFCI is used with two wire receptacles, only two wire recptacles can be used in the other branch circuit locations. The reason is so that the user won't think the additional protection is available when it isn't. The exception to the above is naturally if there indeed is a ground connection available in the boxes, but two wire receptacles were originally installed because of economic or other considerations. A very good explanation of application aspects of the GFCI is in a publication from NEMA, and can be purchased from them. NEMA (National Electrical Manufacturers Assoc.) 2101 L Street NW Washington DC 20057 202-457-8400 ask for: "Application Guide for Ground Fault Circuit Interrupters" Standards Publication No. 280-1984 ============================================================================= well folks there it is. i hope it helps. howie z.
jbn@glacier.STANFORD.EDU (John B. Nagle) (05/12/88)
We've now established that one cannot entirely rely on the protective
ground pin as a path to ground. The GFCI handles the safety aspect, but
what about the use of protective ground as part of shielding in electronic
equipment? Open grounds can cause hum, noise, and weak reception of RF signals.
They also can eliminate one of the defenses of electronic equipment vulnerable
to static charges. When you walk across the carpet and build up a big static
charge, then touch your computer, where does the charge go?
Ungrounded GFCIs are rare, fortunately. But it is worth thinking about
this if, say, you had a GFCI outlet installed in a house with older wiring
because you needed a 3-prong outlet for a computer. It is probably not a
good thing to plug elaborate electronics with interconnects to other units,
such as computers and sound systems, into such an outlet. On many such
units, logic ground is tied to protective ground and the cabinet, so
an ungrounded outlet results in a direct connection from the case and
cables to the system's ground reference.
John Naglezonker@blblbl.UUCP (Rugby Wench) (05/13/88)
First, I was wrong. NEC Art. 210-8 covers where GFIs are required. Article 210-7d covers the use of GFIs as replacements on 2 wire circuits. (and my code book is STILL in my work truck, so I can't quote it for you...) In article <2005@hou2d.UUCP>, hz1@hou2d.UUCP (H.ZARETSKY) writes: > Thus, the GFCI WILL protect two wire receptacles down stream. Obviously, this is only if you hook up the load to the load terminals of the GFI (I say this because often the old electricians would splice and take leads). These downstream plugs CANNOT be "grounded" to the box. Nor can the GFI, if there is no ground. This is so that if what SHOULD be the grounding conductor becomes live, then it will not become live throughout the circuit... > There are two additional points to be made. > 1. The receptacle GFCI does have a ground hole for the round pin in a plug > to access. This is put there for additional safety, belt and suspenders. This is put there because GFIs are primarily used on grounded circuits, and all 125v receptacles installed MUST be of the grounding type, when there is a ground available. > 2. If a GFCI is used with two wire receptacles, only two wire recptacles > can be used in the other branch circuit locations. The reason is so that > the user won't think the additional protection is available when it isn't. > The exception to the above is naturally if there indeed is a ground > connection available in the boxes, but two wire receptacles were originally > installed because of economic or other considerations. Most old receptacles, whether they are on grounded or ungrounded circuits, have only two prongs. ... > well folks there it is. i hope it helps. > howie z. And remember to test your GFI monthly. If you don't then the GFI police will swoop down on you and invade your home and take you out for public humiliation and whatnot... --Regis M. Donovan dOnOvAn eLeCtRiC sErViCe