Your relative should be able to open the consumer unit,disconnect all the circuit neutrals,check the resetting of the Rcd
If it now resets,the neutral conductors can be replaced one at a time to see if balance is maintained
If he is an electrician (of whatever discipline)and have test meters,the problem should be easy to identify
If it now resets,the neutral conductors can be replaced one at a time to see if balance is maintained
If he is an electrician (of whatever discipline)and have test meters,the problem should be easy to identify