Circuit breaker selection that can be tripped with a microcontroller
If you want to simulate some ground leakage for a GFI/RCD device, then a standard mains rated relay with resistor in series should do. The relay coil can be driven from ta microcontroller output + transistor (or directly if the coil current/voltage is low enough) The typical threshold is around 5-30mA according to the link. So if your line voltage is e.g 115V, you should guarantee a trip with 50mA. 

Then a 115V / 0.050A = 2.3k&Omega;. A beefy resistor would be required, as you are looking at 115V * 0.05A = 5.75W. You could either use a 10W 2.3k&Omega; resistor, or use e.g. 4 * 2.5W 10k resistors in parallel (for 46mA)    

Alternatively, use a ready made solenoid voltmeter, which (according to the above link) are apparently used for testing RCDs quite commonly due to their relatively high current draw. This would have to be used manually though.