120 Volt short to shell - More ideas please

[Julison]

Hi,

I started a thread two days ago about a possible short that I presumed to be caused a wire on the outside light....I have since learned more about the problem - it is not caused by the outside light (12v shorts to give 50 volt vs ground!) but still need your help to track the source of the short.
See thread "Outside light as a source of shocks? " for background info.

In short "no pun intended", using a multimeter, I am seeing varying voltages vs ground...depending on how many of my breakers in the AS that I have flipped on. With all 3 of my breakers on (two 20Amp breakers and one 30Amp breaker) I will read 51 Volts from the aluminum skin to the somewhat wet ground. As I turn breakers off one at a time I will see lower and lower Voltage to ground...however even with all breakers off..tonight I had 4 to 5 volts measured from aluminum skin to ground. This voltage went to zero as I unplugged from the house. If I measure voltage to ground for each individual circuit (ie: as long as I am hooked to AC I measure Voltage from skin to ground) does this mean each circuit has a short - or does it possibly give a clue to the source of the short?

Not fully understanding how the univolt works, I then set to trying to isolate which circuit might be responsible. I sequentially remove each of the +ve leads from the univolt and then measured V vs Ground. I always measured ~50 volts. Oddly I can only read 0.2uA (micro amp) at 50volts.

What is my next step? I can't risk taking this out and hooking to AC - I was lucky last time when my daughter only felt a tingle.