Go there yourself, disconnect all the RFC conductors at the CU and lock off the CB that was feeding the RFC asking your customer to switch it off is hardly a professional approach, It's your neck on the line mate, make sure 100% all RFC conductors are ditched from the CU.
Disconnecting the circuit until it is repaired properly is the best course of action yes, however it relies on the landlord and tenant giving permission for this to happen.
If the OP was to go to the property and disconnect the circuit without permission then they could be guilty of other offences.
As it is a rented property they cannot make an alteration to the installation without the landlord's permission and they cannot enter the property without the tenant's permission.
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Got call out yesterday for a ring circuit that kept tripping rcd. I identified the faulty cable which had low 0.05 IR reading L to N. I followed this to where next point should be but on opening circuit found only one cable...not good. On closer inspection of sockets I found that most of them had c.p.c's cut out. I can only imagine previous spark had thought this good idea as a previous earth fault had been tripping rcd.
. What has surprised me is landlord has phoned me since and rather than mcb tripping what's happening is when the various appliances are on other appliances lose power. Ie washine machine on and boiler stops working, cooker on washine machine stops working. Why would this be the case rather than mcb trip out? Thanks
Assuming that is supposed to be 0.05megohms L-N then that will be passing a current of around 5mA.
So there's two problems with suggesting that this is responsible for the RCD tripping, it's not enough current to trip an RCD and RCDs don't operate on L-N faults.
I doubt that it was an electrician who disconnected the CPCs, it is more likely a diyer. It could be the tenant or landlord attempting to solve a problem themselves.
If this loss of power was not happening before you worked in the circuit then it is likely that it is caused by something you have done, this could indicate a high resistance joint somewhere.
A possibke cause of this is incorrect fitting of the MCB so that the busbar prong is behind the moving part of the terminal and not clamped within it.