GFCI

Basically, a GFCI is designed to trip when the current going through your hot wire is unequal to the current going through the return wire. If the current is not going through the return wire then the GFCI trips off, usually rather quickly and definitley more quickly than a circuit breaker will trip. I believe an ELCB is basically the same thing, only it doesn't trip quite as quickly as a GFCI. As to whether you need one or both, you will have to check with your local electrical regulations as to what is required.

Hi Shawn with the ELCB, it has a built in delay with a required MA draw, to stop spurious trips, also they are operated Hydraulically, so they are very quick faster then a small relay typically 20ms. I would guess the reason for this device, is that the earth is link to the zero volts or neutral. Or the transformer has a center tap config.

I don't think a GFCI compares the potential difference between the hot leg and the common leg.

I think the GFCI compares the potential between ground and common. There should be no potential between ground and common (afterall, they are tied together at the load center).

One GFI I looked at was little more then a Wheatstone Bridge which is built with 4 diodes.

The GFI does not care about current, it only looks at potential (voltage).

Travis

The way it was explained to me (I haven't taken one apart myself) is that there is a current sensing device between the common and hot lines. The GFCI checks for a difference in current between the two and if there is a difference then the GFCI trips. There may be other types, but I haven't taken any time to check.

The ones that I have seen look at the current. The Hot and Neutral wires are run thru the same toroid but in opposite directions. If the currents in the two wires are equal, their induced fields in the toroid cancel each other out. If a short occurs, the Neutral current will be lower and the field in the toroid is detected tripping the circuit.

sounds like someone is giving you a good sales pitch, or have they been watching some si-fi?

Nope, just the voice of experience. Trust me, I'm not out to sell anything. On a more personal note. I'm not all that in love with ELCB's. I saw someone get seriously shocked by a system that was on an ELCB. I'm talking emergency room, burns, heart fibrilation, the whole nine yards.

In most applications GFCI have replaced ELCB. You will still need to look at local electrical codes to see whats required and acceptable.

http://www.radiolocman.com/shem/shem-cache.html?di=18893

I think this article explain to what you want.

Good luck!

Best regards,

fei