thanx It looks for an imbalance between live & neutral. What goes down the live goes down the neutral. If there is a differance it means one or the other is leaking to earth. Most are set to trip at 30 mA.
Lighting circuits can play havoc with an RCD hence you will find most lighting circuits are not wired to it. A floresent light has to charge up so nothing goes down the neutral While its charging & it trips the RCD.
You will find most boards are split boards. Power is connected to the RCD via miniture circuit breakers. Lighting is direct to the main switch via miniture circuit breakers.
Excuss my spelling. I am a few beers down.