The two resistors represent the load in the protected zone. In normal operation, current goes through both current transformers onto the relay coil circuit. the difference across the relay coil is nothing.
close the top switch to introduce a ground fault. This takes current away from the right current transformer, and the relay coil circuit develops a voltage difference, which makes the coil activate and disconnect the supply.
article here:
http://electrical-engineering-portal.com/differential-protection-proven-technique
