When you go to the shop to buy the 10mA to 30mA RCCB, this is designed for water heater. A single jolt of up to 50mA can actually kill us when there's a leakage from the water heater. When the 10mA or 30mA trip, it's designed to check for residual current so that anything over 10mA / 30mA will trip it to save your life.
Hence why only 10mA will be used for heater in case of water heater leakage. Mind you, installing a good RCCB would not save you if your house grounding wire is not good. You must remove the grounding cable from the electrode / grounding rod and  test to get less than 10 ohm. This is dictated by ST. Less or equal to zero is the best and this is done for factories. This can be tested with a grounding meter. Please make sure to connect the cable back to grounding rod after opening it. Also, grounding cable must be at least 10mm cable or best is 16mm cable. Why this is needed to be this thick is because the grounding cable will have lower resistance and all voltage when hits your house during a lightning will travel via this cable to the grounding rod to be absorbed by earth. If you have a 1.5mm or 2.5mm or even 4mm, it will not work as good as a 10mm cable as the resistance is high thus the voltage will not use it as the main cable to travel to earth and will flow elsewhere into your house circuit.
A lot of people don't understand and says that this will trip easily is because they do not know what it's designed for. If your house grounding is good and the 10mA RCCB trips, means there's issue with the circuit and require your attention. Either there's a lightning strike or the cable been damaged by mouse etc. You have to test the cables to make sure it's good.
For house, the main RCCB should be at 100mA. Not 300mA. Those electrician who tells you your house need a 300mA, you better ask them to go and fly kite. 100mA is the maximum dictated by ST. Best is 30mA but it will always trip if there's lightning but if you have a 65kV Surge Arrestor installed, you should be alright without any RCCB trip cause the surge arrestor will absorb the voltage from the lightning. 45k is around RM500 plus. 65kV is around RM1000. Go figure. Buy only good MCB or RCCB with Sirim. There's a lot of fakes out there. EPS is low cost range. Buy Hager / Schneider. Hager is around RM8 for a MCB while Schneider is around RM 13 each. More expensive means it's more sensitive. EPS only cost RM4 dollars. I won't put my life in danger by using a MCB costing dollars....