RCD Tripping

[mike1975]

Hi everyone,

On Saturday whilst my wife was in the shower the power went off in the house because the RCD had tripped. It wouldn't allow me to turn it back on. I called a local electrician and after a couple of hours fault finding he narrowed it down to the cooker and said it's quite common.

He moved the cooker off an MCB (temporally) and everything was fine. On Sunday morning I pulled the cord for the shower and once again the RCD tripped (not the shower MCB). I disconnected the shower and pulled the cord and the little red light came on ok. I reconnected the shower and pulled the cord and the red light came on again. As soon as I pushed the shower power button the RCD tripped again.

The shower is about 2 months old and is a 10.8kw I've checked this morning and it's MCB is 40 amp but it should be 45 amp. Could this be the problem? If it is, shouldn't the MCB trip and not the RCD?

Many Thanks in advance.
Mike.

 

[MalcofArabia]

When you say you "disconnected" the shower can we take it you physically removed the large cable from inside the shower unit? or did you disconnect the large cable from the switch with the red light?

If you did the first then I hope you made that cable safe, as when you activated the switch the large cable at the shower would have been LIVE. If you did the 2nd then you would have had to have the switch down to disconnect the cable and then it would be open, be careful as that is 230V your playing with.

I take it that until Saturday for the 2 months the shower as been in the Unit worked OK? If it had that would rule out the chance of a miss wire at the CU. But 2 month old appliances have been know to fail so can not rule that out.

You may find that perhaps the shower front is not fitted properly and dampness may have got into the unit and that would trip an RCD rather than the CB.

The CB or as you call it the MCB is their for overload protection and the RCD is for fault protection, on a fault like you say that would trip first.

As you have already called a sparks out, re call him and tell him that it looks like he may have diagnosed the fault wrongly, in that 2 hours he was fault finding the shower may have dried out enough to reset the RCD.

Have you used the shower at all since the electrician moved your cooker?

When you say he moved the cooker, is the cooker still working? If it is then he may have moved that off the protected side of the board onto a non protected side, which means it may not be RCD protected now.

Is the electrician coming back to reconnect the cooker back to the original side?

What did he recommend you to do the repair the cooker? Or do you propose to leave it as is, which is not recommended really.

Quite a bit to really digest for you but the first thing is to identify what is actually tripping that RCD

 

[mike1975]

When you say you "disconnected" the shower can we take it you physically removed the large cable from inside the shower unit? or did you disconnect the large cable from the switch with the red light?

If you did the first then I hope you made that cable safe, as when you activated the switch the large cable at the shower would have been LIVE. If you did the 2nd then you would have had to have the switch down to disconnect the cable and then it would be open, be careful as that is 230V your playing with.

I take it that until Saturday for the 2 months the shower as been in the Unit worked OK? If it had that would rule out the chance of a miss wire at the CU. But 2 month old appliances have been know to fail so can not rule that out.

You may find that perhaps the shower front is not fitted properly and dampness may have got into the unit and that would trip an RCD rather than the CB.

The CB or as you call it the MCB is their for overload protection and the RCD is for fault protection, on a fault like you say that would trip first.

As you have already called a sparks out, re call him and tell him that it looks like he may have diagnosed the fault wrongly, in that 2 hours he was fault finding the shower may have dried out enough to reset the RCD.

Have you used the shower at all since the electrician moved your cooker?

When you say he moved the cooker, is the cooker still working? If it is then he may have moved that off the protected side of the board onto a non protected side, which means it may not be RCD protected now.

Is the electrician coming back to reconnect the cooker back to the original side?

What did he recommend you to do the repair the cooker? Or do you propose to leave it as is, which is not recommended really.

Quite a bit to really digest for you but the first thing is to identify what is actually tripping that RCD

Hi and thanks for your reply. I disconnected the shower inside the shower unit and I was careful about the cable, I turned it off at the mains first. It all looked fine inside and no water had been getting in. This was the first time it had been used since he moved the cooker.

He did move the cooker over to the non RCD side, however we only ever turn it on when we use it and then it's turned off again. I'll ask him to move it back over though.

It just seems strange that once he moved the cooker we were able to have all the power on fine (until I used the shower) so it did seem logical that the cooker was at fault.

I've tried to call him but I can't get a answer (so far) but I just wanted to get as much info as I could before he returns.

 

[MalcofArabia]

What made him think it was the cooker?

If the cooker and the shower is on the same RCD side then the neutrals are commoned up on the bar. So a neurtal fault on either would trip the RCD.

Did he try the shower on the non protected side?

He may when he moved the cooker onto the non RCD side moved the wrong neutral over as the cooker and shower cable maybe the same size. If he did that then you would have an inbalance as the neutrals would be reversed ie shower RCD but neutral on non RCD bar, Cooker non RCD and neutral on RCD neutral bar.

You will have to really get hold of him and get him back

 

[mike1975]

What made him think it was the cooker?

If the cooker and the shower is on the same RCD side then the neutrals are commoned up on the bar. So a neurtal fault on either would trip the RCD.

Did he try the shower on the non protected side?

He may when he moved the cooker onto the non RCD side moved the wrong neutral over as the cooker and shower cable maybe the same size. If he did that then you would have an inbalance as the neutrals would be reversed ie shower RCD but neutral on non RCD bar, Cooker non RCD and neutral on RCD neutral bar.

You will have to really get hold of him and get him back

He thought it was the fridge at first but it wasn't, then he disconnected the cooker and everything else worked fine, hence him saying it was the cooker. The shower was never mentioned to be honest. I did tell him that my wife was in the shower when the power went off.

 

[Spark123]

If the supply is TN-CS it could be a Neutral-Earth fault on any of the circuits on the RCD protected side.

 

[MalcofArabia]

Little tip in future, As he was packing away nip upstairs and switch the shower on, if you feel that was the fault then no harm seeing if you were wrong

Sparks123 surely any system either TN-C-S TN-S or TT would trip the RCD if there was a N/E fault on any circuit within the installations as the Neutrals are commoned on the bar

 

[Spark123]

TN-CS is a bit of an odd one, sometimes the RCD will hold until a largeish load is placed on the system.
This depends on influences such as length of meter tails and the impedance of the N-E fault path.
The RCD can hold until enough current is flowing in a circuit to cause a high enough voltage between N-E which causes enough trip current to flow though the N-E fault (which is in effect parallel to the N-E terminal in the service head.)

TT and TN-S are a bit different in that N and E usually have a smallish voltage between them caused by external influences.