I couldn't explain it very well so I loked it up...
GFCI circuit breakers perform two functions. If it is a true circuit breaker it will prevent "over-current" conditions from overheating the wiring of your home and hopefully minimize the potential for fire. All circuit breakers and fuses are designed to 'trip' or 'blow' when an over-current condition exists. GFCI circuits have an additional function. To prevent or minimize the potential for hazardous shocks, GFCI circuits compare the current that goes across the two wires that make up the hot and common voltage lines of an outlet. If the current on the two wires does not match precisely, the assumption is that current is being lost to something outside of the circuit. This condition is called a "ground fault". In simple terms, it means that there is high potential that YOU are grounded, as in standing in a puddle of water, and YOU have touched the "hot" side of the circuit or the device somehow allows current to go outside of the normal voltage path. A ground fault can exist if an outside outlet has become wet with rainwater, or an outside extension cord is laying in water or an appliance is in the process of failing and voltage leaks to the ground wire. But as already mentioned, it can also indicate that the GFCI has failed, although that doesn't often happen.
Look at your equipment and see of you can read the current rating (amps). Add all the devices that are on that breaker and see of they exceed 75% of the breaker rating. If so then split the load up to other breakers. If not, you have a ground fault. There is something that is sourcing current to ground that shouldn't. Probably something getting wet.
A volt meter isn't going to tell you much, just that there is voltage on the ground but not where it's coming from. That is going to take some investigations.
I should mention that if you have devices that are rated only in watts, that watts divided by volts equals amps. ie. 150 watt lamp would draw 150w/120v= 1.25 A (approximately, Line voltage varies somewhat)
Peace...
John
