way a GFI works is it simply compares the current between the hot lead and the neutral and trips if there's an imbalance.
Throw a heater with a crack in the insulation (cord or housing) into a tank without a ground probe, basically the current in is the same as the current out. Now say you're grounded (touching a light fixture) and put your hand in the tank, there's an alternate path to ground, the imbalance detected and the GFIC trips. Put in a ground probe, the alternate ground path already exists if something was to fail.
Either case trip time of the device the same once the imbalance detected, just might go undetected until you put your hand in.
Last edited by mark; 02-15-2014 at 08:09 PM.
Reason: changed to heater from pump as typically heater ungrounded
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