RCD stands for Residual Current Device, as Hero999 said. And usually have 2 poles/contacts. If you want something that will protect an electronic circuit, a Single Pole MCB will suitable. You just need to find one close to the current of the supply. Lowest i've seen are 2 amps. usually they will have the amperage printed onto them, with a C in front. for a 2 amp unit, C2 would be there. is that it?