Stray votlage

[mike31154]

Originally Posted by Seafan

No this was not the case at all, actually The G in gfci stands for ground which most of our equipment in our tanks does not have a ground in the plug-in therefore gfci is quite useless in half of the equipment we run, I was merely pointing out that this is why I would not run without a ground probe.

The F in GFCI stands for 'Fault'. GFCIs are designed to keep you safe irregardless of whether the equipment plugged into them has a ground prong on the plug or not. GFCIs are required by code in bathrooms and most electric shavers, hair dryers etc., only have two prong plugs. As mentioned, they monitor the current coming in and going out of the circuit they are installed in. As long as the device sees the same current on both the hot and neutral, all is well. If there is a differential of a few mA, not sure what the number is off hand, the device is designed to trip before serious harm comes to the individual using the appliance.

Just my speculation here and I could be totally wrong, but it's the only explanation that makes sense to me. I suspect that in the case of your powerhead there was some degree of deterioration of the insulation waiting to find a path to a lower potential (ground if you wish). Although you felt a shock, your body resistance was likely too high to cause enough current to flow to trip the device, i.e. it was not enough to harm you. By introducing the grounding probe (much less resistance to ground than your body), sufficient current was able to flow from the faulty powerhead to ground tripping the GFCI. It's great that you were able to isolate the faulty device by installing the ground probe before it became bad enough to trip the GFCI with your hand in the water.