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Re: grounding question



Original poster: FIFTYGUY@xxxxxxx In a message dated 10/15/06 2:47:44 P.M. Eastern Daylight Time, tesla@xxxxxxxxxx writes:

>I would think that a 5mA fault current would cause nuisance tripping.

They do. And that's after they changed the requirements to decrease the sensitivity a few years ago. In industrial and commercial use, GFCI circuit breakers are almost a waste of time, since capacitance on long wire runs causes so much leakage current to make nuisance tripping inevitable. As an industrial electrician, whenever possible I install a separate GFCI at each receptacle, instead of "piggybacking" further receptacles downstream off the GFCI's protection. Less nuisance trips, and it's obvious where the interruption occurred when you need to reset it. Loads with large motors and reactance also cause nuisance tripping. And after a few nuisance trips at full load, the GFCI becomes unreliable and more of a liability than an asset. For that matter, nuisance trips in *big* ground-fault systems happen. The typical "ground fault pickup" current in larger systems is set around 20% of the full-load current. For example, last week we had a nuisance ground fault trip on a new 1000 kVA service at my work because the factory setting of 50 Amps was too sensitive. We set it to a correct 200 Amps. Quite a bit different than the 5 mA of residential breakers, but when you've got 200 HP induction motor loads switching in and out, it's hard not to have current surges. Same problem with AFCI (Arc-Fault Current Interrupters), which are now required in new construction. When I first heard of these, I asked an expert (one of the guys on the National Electrical Code panel) how an AFCI could tell the difference between an arcing loose connection and the arcs in a universal or DC motor in an appliance. He just said the devices were smart enough. Well, years later, it turns out they have so many nuisance trips with these devices that they have asked vacuum cleaner manufacturers to redesign their products! As you might tell, I'm not a big fan of "smart" receptacles. I just don't think it's possible to get durable electronics, robust and reliable enough breaker contacts, and cheap pricing all in one device designed to fit into about the same space as a "dumb" 40 cent receptacle (which aren't all that great to begin with!). Safety should begin with practices, not reliance on complicated technology. If you keep your appliances in good condition, don't use them in wet environments, install wiring correctly the first time, and don't do anything dumb to damage it, there wouldn't be a need for GFCI's or AFCI's. I suppose you could say the same thing about airbags in cars - we wouldn't need them if people didn't do dumb things. As far as nuisance tripping with Tesla coils, I have run my SGTCs off GFCI receptacles. Even without Terry Filters or Line Filters, I've never had a problem. I *have* tripped the breaker feeding GFCI's or the circuit breaker in the "power strip" downstream of the GFCI's (current draw). I'm curious how an AFCI would handle an unfiltered SGTC load.

-Phil LaBudde