Electric Fence: circuit breaker trips when ground is connected
We're having a problem with our electric fence. The charger is connected to about 1200 feet of "single line" fence (it goes out 600' from the barn & charger, near the top of the wooden fence, and then returns to the barn lower down the fence).
We have a couple small (1-foot?) ground rods grounded about 20' away from the charger. This setup worked well for a long time, but then we changed several things. First, we re-strung the fence to travel along a new piece of the wooden fence - total length and layout was the same. Then, we noticed that the electric fence wasn't working - it could have been like that for a while, as we don't check the charge often (yeah, I know...). So, we replaced the Premiere IntelliShock 53 charger w/ a ProShock L-1500 unit, but it still wasn't working. We finally noticed that every time we plugged it in, the circuit breaker tripped!
So, first off, what could make the breaker trip when we plug in the charger?
At first, we thought it was another bad charger, but then we noticed that the breaker does NOT trip if the ground is NOT connected to the charger. Stranger than that, though, is the fact that the charger appears to be working WITHOUT the ground (the light on the charge blinks, it makes the regular clicking/buzzing noise it should, and our hand-held meter shows 4000-6000 volts (I assume "volts" is the unit of measurement on the meter).
So, given that the circuit trips when the grounded charger is plugged in, I'm still pondering how the ground can be the cause of this. Additionally, how dangerous is it to run our charger like this (not grounded)?
Many, many thanks for any help anyone can offer!