RCD's are very sensitive and will detect even minor faults very quickly.
If there is faulty appliance in the household plugged into the mains, the RCD reacts because of the power surge and disconnects the electric current.
RCD's should be installed in all homes to protect you and your family.
In 90% of cases if your RCD keeps tripping - it doesn't mean its broken, it more probably means that there is faulty appliance connected or there is an electrical fault in your electrical installation.
RCD's can sometimes be a pain but it is protecting you from an electrical injury or death.
Many people don't know that there is an electrical problem in their house until they fit new consumer unit equipped with RCD.
How to reset tripping RCD ?
1. Unplug ALL affected appliances from socket outlets and lights that are affected by the RCD tripping off , turn off all light switches as well.
Pay attention to all outdoor power outlets and lights - those must be isolated/switched OFF as well.
2. Turn on the RCD by firmly pushing toogle switch up.
On some types of RCD's you might have to flick it "ON-OFF" and then "ON" again to perform reset.
3. If RCD appears to stay in "ON" position, you can start plugging appliances back in and turning on to see if it there was any particular one that was causing tripping your RCD
4. If problem persist or RCD doesn't stay "ON" - call qualified electrician for help.
What to do if above didn't work:
Sometimes to identify the cause of RCD tripping, it is necessary to not just disconnect live, but also neutral feed to the circuit, and this can be achieved by disconnecting the neutral wires from the neutral bar in the consumer unit (fuse box). Therefore it's not really a job for someone who is not specialist.
I'd recommend to perform stuff like household appliances unplugged, and the immersion heater switched off at the double pole switch that should be near to it, but if you can't eliminate it as one of the problems, its probably best to call qualified electrician who can find the fault by doing insulation resistance tests to determine in which circuit the problem lies on.
This is a guide only, it is dangerous to perform electrical work if you are not specialist.
Always contact a qualified electrician to perform any electrical works.