Trailer is tripping a GFI in my garage....

road-runner wrote:
Most "how GFCIs work" posts on the Internet are clueless about how neutral-to-ground detection works. There is a second fault detection mechanism in the GFCI, apparently unknown to almost all of the experts. Read about it here http://www.rhtubs.com/GFCI/GFCI.htm, scroll down to "How does a GFCI work:" . If that's not convincing enough, pull the lm1851 datasheet at www.ti.com/lit/ds/symlink/lm1851.pdf, and read the same thing on page 6. There's a newer technique where the chip is somehow able to measure the impedance between the downstream neutral and ground without the additional ground-neutral coil. The ncs37000 chip is an example of this. Its datasheet www.onsemi.com/pub/Collateral/NCS37000-D.PDF mentions it on pages 4 and 6, but there's no explanation of how it is implemented.

One of the reasons there is confusion about GFCIs & neutral/ground faults is the original GFCIs did not have the feature to detect neutral/ground faults and only detected the >5ma difference between hot & neutral.

I don't remember the year it was done, but modern GFCIs have the ability to detect neutral/ground faults.

I remember watching a demo of one of the original GFIs (they didn't call them GFCIs then). The engineer wired a light bulb through the GFI, dipped his hand into a fish tank of water. When his fingers touched the water, the lamp went out. No latch, so when he took his fingers out, it came back on. Impressive, and he sure put a lot of trust in his work!