Why do you need a Residual Current Device if you already have trip switches?
Circuit breakers protect the wiring from overheating and the house from burning down due to electrical fire. 

The GFCI (RCD) protects you from electrocution. To put it in perspective, a 20 amp circuit breaker can pass 100-200 milliamps (0.1-0.2A) indefinitely, you cannot and are facing heart fibrillation and death if the current takes the right path through your body.

The GFCI can sense the ground fault current and shut the power off to save you from electrocution if you bridge the circuit to ground.