Rcd tripping

[mal165]

My RCD has been tripping intermittently over the past few weeks. Everything seemed normal the tcd would trip and was easily reset.
Last night it tripped and won't reset. I have tried disconnecting all appliances but no success. If mains isolator is switched off along with all MCB's the RCD can be reset but as soon as I flick the consumer unit mains switch the rcd trips.
On the off chance that the RCD is faulty I have ordered another one. But any suggestions in the meantime would be appreciated.
Thanks


T

 

[Jurassicspark]

Switch on the RCD and switch the MCBs on one by one. The faulty circuit connected to that MCB should trigger the rcd.

 

[mal165]

Hi, thanks for that. Tried it. MCB's off RCD on. Individual MCB's on RCD does not trip until consumer unit isolator switch is flicked then RCD trips

 

[ericmark]

You seem to have order wrong, RCD, then isolator, then MCB's.

However, the RCD detects an imbalance, this is normally caused by one of the live wires having a path to earth, this can be the line or the neutral, the latter is a problem as MCB's do not switch the neutral, in fact many appliances only switch the line, so step one is unplug things, not simply switch them off, but unplug, with FCU if switched often the switch is double pole, so if you can with FCU switch off, that is better than drawing the fuse.

 

[mal165]

Hi, many thanks for your reply and advice. In your first sentence are you saying this is the order they should be switched on or off? However I had gone round and unplugged everything previously but foolishly had assumed nothing was plugged in under a rarely used desk!! It seems the redundant PC speaker system was the culprit.
Problem solved and many thanks for your help

 

[ericmark]

Thank-you for telling us how you got on. The order was switch on order. In my old house one RCD did many circuits, and sometimes the way it worked seemed daft, RCD would trip supplying one fuse box (modified with MCB instead of fuses) and on re-setting it would trip the other RCD.

They were fitted around 1992 so were an old style, modern ones are better, but just before I moved here, the RCD tripped, likely due to a spike on the supply, and I lost two freezers full of food, so on moving here, there it was a no-brainer to fit all RCBO's (MCB and RCD combined) so faults limited to one circuit.

Good job too, and had a flat roof leak, took 9 months to get fixed, and had sockets in that area off all that time.

 

[mal165]

Thanks again for your help

 

[ericmark]

Thanks again for your help

There is a thanks button.

 

[Harry Bloomfield]

Turn MCB's to off, turn main switch on, then try resetting RCD[1].

If the RCD will reset, then switch MCB's on, one at a time, with a pause in between. If the then trips the RCD, you will know which MCB circuit is causing it.

[1] If the RCD trips, before any of the MCB's are turned on, then it have a neutral to earth fault, on one of the circuits.

 

[Taylortwocities]

And never order a replacement part before doing significant fault finding. RCDs do fail, but only very occasionally.
An external fault is almost always the problem.