Re: grounding question

["Tesla list" <tesla@xxxxxxxxxx>]

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