live to earth short circuit tripped mcb but not rcd?

[jim63]

Hi, I was loosening my sockets for decorating and upon loosening one of the sockets there was a loud pop and bright flash and sparks flying in my face, it turns out one of live cables of the ring had slipped out its terminal and shorted against the earth bar. This tripped the 32 amp circuit breaker but it did not trip the rcd which has got me a bit concerned, It was definitely a live to earth short circuit as the backbox had blackened and a scorch mark where the wire had touched the earth bar on the socket.

Why did the rcd not trip? I can understand an rcd not tripping on a live to neutral short as there is no imbalance but surely it should have tripped on a live to earth short as this is obviously an imbalance? Could it be that the fault current was so high that the mcb reacted first before the rcd had a chance to operate?, or could it be a more serious problem.
The consumer unit is made by hager, if that helps, the rcd is just the standard 63 amp 30ma.
thanks.

Ps yes I know i should of had the power off but I had no intention on touching the terminals, just loosening the socket plate for painting.

 

[bernardgreen]

Is that ring protected by the RCD ?

 

[Jackrae]

neutral and 'earth' crossed in the consumer unit ?
or faulty RCD

 

[jim63]

Is that ring protected by the RCD ?

Yes, all sockets circuits are protected by the rcd, its a hager split load consumer unit about 20 years old, only the lights are not on the rcd.

 

[jim63]

neutral and 'earth' crossed in the consumer unit ?
or faulty RCD

Hi,nope, polarity is all correct, no cross connections.

 

[securespark]

Faulty RCD, then?

When you press the test button, does it pop off?

 

[Taylortwocities]

Whssign

• Taylortwocities
• 16 Jul 2016

What he said

 

[JohnW2]

Whilst it's obviously possible, as has been suggested, that the RCD is faulty, has no-one considered the possibility that the L-E fault current was so high that the MCB operated very quickly, before the RCD 'had a chance' to operate?

If it were two MCBs in series, we would not be particularly surprised if one operated, but not the other, in response to an L-E fault. Cannot a similar phenomenon arise with an MCB and RCD?

Kind Regards, John

 

[jim63]

Hi all, Yes it still trips when you press the test button, Could it be age?, I'm not sure what to think? I'm thankful the mcb tripped but the rcd not tripping worries me, if it won't trip on such a huge imbalance, how will it behave if you receive an electric shock?, does it need replacing?
thanks

 

[jim63]

I'm no expert, it could may well be the fault current was so high that the mcb operated quicker, perhaps if the fault current was bit lower the rcd may have operated as well?

 

[JohnW2]

I'm no expert, it could may well be the fault current was so high that the mcb operated quicker, perhaps if the fault current was bit lower the rcd may have operated as well?

What one needs to realise is that, although their 'trip thresholds are very different, a fault current which was very much higher than the RCDs trip threshold and also very much higher than the MCB's trip threshold arose simultaneously. Which one tripped first (or whether both tripped simultaneously) therefore is simply down to how rapid is the tripping process in each case, and it's probably pot luck as to which is the quicker.

If things has happened the other way around, with just the RCD tripping, many/most people would probably say that was 'as expected'. However, a more 'suspicious' person may have questioned whether the MCB was faulty, since the fault current will inevitably have been well over it's trip threshold (but it did not trip).

Kind Regards, John

 

[EFLImpudence]

You can only have the RCD tested.

If it is satisfactory then it must be as others are saying. If not, then replace it.

Not really much point discussing it.

 

[Jackrae]

Hi,nope, polarity is all correct, no cross connections.

Have you actually verified this within the CU itself

 

[JohnW2]

Have you actually verified this within the CU itself

Is it not extraordinarily unlikely that even a complete idiot would/could manage to 'swap'/'cross' neutral and CPC connections in the CU?

Maybe I haven't thought it through adequately, but I'm currently not sure what you are thinking, since if, somehow, N and CPC were 'swapped' in the CU, the RCD would surely operate whenever there was a load drawing more than 30 mA, wouldn't it?

Kind Regards, John

 

[sparkwright]

Not really much point discussing it.

We will see.