ECN Forum Forums Technical Discussion Electrical Theory and Applications 240 V GFCI

Just a little trivia here. We had a rep from UL at a meeting and GFCI's was the main topic.

GFCI protection for personnel is set to trip between 4ma and 6ma. An interesting thing mentioned was that the speed with which they operate depends upon the level of the fault. A low threshold 4ma fault could take up to something like 4.8 seconds to trip depending upon the sensitivity of the particular unit.

Bill

we probably induce more than that doing panel work....

Another piece of trivia. An Electrical product is permitted to have a loss up to .5ma and still be listed. The importance of this tidbit comes into play where there is a 'nusance' tripping going on. As the GFCI operates on a cumulative value of stray current it could seem to be tripping for no reason but it may be operating perfectly. There could be 8 items on the circuit with a .5 'loss' each (8 x .5 = 4ma) and tripping could occur and there is nothing wrong. It's something to think about when deciding on CB vs Receptacle type GFCIs' and the number of downstream devices and length of a circuit protected by a single GFCI.


Bill

If the ground fault was symetrical, it would mean that the human creating the short had three arms of identical impedance. With the current summing at zero in his chest, he wouldn't have cardiac stress, just a real bad feeling in the arms. Quite a party trick.

Bill,

Great point on the line charge Capacitance!!
A great example of when circuit charging becomes a nuisance would be a few extension cords connected together with normal NEMA 5-15 R/P connections, several power tools which use series wound commutator motors with open brushes, and a little bit of rain. This is normally followed by the quick removal of the GFCI breaker, swapped with a standard type frame

Another one would be with a whole gang load of cords running everywhere and each being a very long distance [upto 50 feet each], from there many power tools are connected to each cord. Some start and run together, some don't - most have dirty commutators, so the brushes spark like fireworks.
This stuff will trip even the most highest rated GFCI device!!
Long circuits, spark shows on brushes, cords running long distances on concrete/against bonded steel/etc.. many non-coincidental starts/runs on many circuits causes the circuit's charging Capacitance to be really high!

Just FYI. I'll return to my room now

Scott SET