2 RCD's on circuit

[kayloh]

Viewed a friends house where a 10mm cable from the consumer unit is feeding a Sub-consumer unit in the shed... The circuit itself is protected by a 32A MCB breaker under an 30mA RCD at the main consumer unit... The sub in the shed is a single RCD and with a single 32A MCB...

Problem is that when my friend connects any appliance in the shed sockets, the RCD in the shed trips...

Would the initial thought be that since there are 2 RCD's, we would need to separate the MCB at the main consumer unit or remove the RCD at the shed so there is only 1 RCD and thus test and everything should work?

Of course if it doesn't then it would mean the circuit itself is faulty, and is another problem to fix... either way there's dodgy work here that needs repairing...

 

[hightower]

It's not because you have two RCDs in series that's causing this - not the best setup having them like that, but wouldn't cause an issue like this. Sounds like there's a fault to earth (L-E or N-E) in the shed that needs resolving. What's your experience with electrics, how come they've called you round to investigate?

 

[kayloh]

It's not because you have two RCDs in series that's causing this - not the best setup having them like that, but wouldn't cause an issue like this. Sounds like there's a fault to earth (L-E or N-E) in the shed that needs resolving. What's your experience with electrics, how come they've called you round to investigate?

Hmm would've thought that would be the main solution but still it should be fixed either way right? Just to make sure there's only 1 RCD on the circuit? And yes I've been recently given my 2357 award but my area of expertise is new installations and not fault finding but honestly it works as practise for me... I'll check at the MCB in the shed for any cross connections between earth and neutral or phase....

 

[soms]

I wouldn't worry about removing the second RCD. It's no cause for concern and is having the unintended effect of providing additional discrimination in the case of the shed fault. Interesting that it is not tripping the RCD in the house, the shed one appears to be more sensitive and tripping first. The behaviour suggests a fault in the shed, most likely in the socket circuit as hightower suggested if any appliance is causing the problem. Visual inspection of sockets and some dead tests would go along way to finding this.
It is also possible the RCD is faulty, I have heard of this on rare occasions from colleagues. RCD tests would test the health of the device. However this is probably unlikely if there are other circuits in the shed as well, such as lights, which are working without a problem.

 

[Taylortwocities]

As above, two RCDs in series isn't going to be a problem, its just unnecessary. And the RCD in the shed is as likely to trip as the one in the house. Not the best design.

However, back to the problem.
Is this a fault that has just occured,ie it did work before??
or is this a new installation in the shed - in which case I'll bet that the registered electrician who installed it (he was registered, wasn't he/she?) has go the neutrals in the wrong place in the unnecessary shed consumer unit.

 

[Octopus]

I'm surprised that its the shed RCD tripping first!

 

[Taylortwocities]

Maybe even more reason to check the wiring of the neutrals....

 

[kayloh]

As above, two RCDs in series isn't going to be a problem, its just unnecessary. And the RCD in the shed is as likely to trip as the one in the house. Not the best design.

However, back to the problem.
Is this a fault that has just occured,ie it did work before??
or is this a new installation in the shed - in which case I'll bet that the registered electrician who installed it (he was registered, wasn't he/she?) has go the neutrals in the wrong place in the unnecessary shed consumer unit.

My friend has told me that it has been like that for a very long time and was indeed done by a DIY cowboy... she uses the shed as a massage parlour but she's having to bring extensions from the house...

she told me additionally that the sockets actually work fine if the load is small like phone chargers etc but when she uses big appliances then the RCD trips... sounds to me like an overload, she might be plugging everything into one socket... I'll have to investigate properly and do the dead test procedure on the MCB that feeds the shed sockets... and is there no point of removing an RCD from the circuit? Even if it is for the sake of making the circuit look a tiny bit more professional?

 

[Octopus]

My friend has told me that it has been like that for a very long time and was indeed done by a DIY cowboy... she uses the shed as a massage parlour but she's having to bring extensions from the house...

she told me additionally that the sockets actually work fine if the load is small like phone chargers etc but when she uses big appliances then the RCD trips... sounds to me like an overload, she might be plugging everything into one socket... I'll have to investigate properly and do the dead test procedure on the MCB that feeds the shed sockets... and is there no point of removing an RCD from the circuit? Even if it is for the sake of making the circuit look a tiny bit more professional?

I would suggest it should be removed.... as the house one will trip first in the majority of cases

 

[Taylortwocities]

Post a photo of the wiring inside the shed CU. may be able to see something.

But, really, is there really only one single 32MCB in the shed CU? No MCB for lighting ?

 

[soms]

I would have thought the RCD in question is an RCCB which would does not have overload protection?
If a DIY bodge was involved good chance circuit wiring fault is to blame. A very small load e.g. phone charger may not draw enough current for the imbalance to exceed the RCD trip value (which may be as little 16-17mA for 30mA RCD) .

 

[Andy78]

My friend has told me that it has been like that for a very long time and was indeed done by a DIY cowboy... she uses the shed as a massage parlour but she's having to bring extensions from the house...

she told me additionally that the sockets actually work fine if the load is small like phone chargers etc but when she uses big appliances then the RCD trips... sounds to me like an overload, she might be plugging everything into one socket... I'll have to investigate properly and do the dead test procedure on the MCB that feeds the shed sockets... and is there no point of removing an RCD from the circuit? Even if it is for the sake of making the circuit look a tiny bit more professional?

I'd be ruling out the appliances first, although if you suspect the RCD is tripping through an overload she might be better off having a ring round for other opinions.

 

[kingeri]

removed

 

[Lucien Nunes]

the sockets actually work fine if the load is small like phone chargers etc but when she uses big appliances then the RCD trips... sounds to me like an overload

Sounds to me like a neutral-earth fault. You might want to get your hearing checked

 

[kingeri]

I had to remove my post as it was a bit sarcastic. But why are you (the OP) getting involved in this when you don't seem to know what you are doing? I mean no disrespect, it's an honest question.

 

[kayloh]

I had to remove my post as it was a bit sarcastic. But why are you (the OP) getting involved in this when you don't seem to know what you are doing? I mean no disrespect, it's an honest question.

Like I said in a previous post, it's practice... I'm more specialised in part rewires / new rewires... never did much fault finding but this seems like a good opportunity to try... client is a long term friend so nobody loses anything since I'm doing it as a favour... it's why I want different opinions on what it could be to test out each one... I'm not here to play who's the better sparkie... I just want some guidance...

 

[Andy78]

Like I said in a previous post, it's practice... I'm more specialised in part rewires / new rewires... never did much fault finding but this seems like a good opportunity to try... client is a long term friend so nobody loses anything since I'm doing it as a favour... it's why I want different opinions on what it could be to test out each one... I'm not here to play who's the better sparkie... I just want some guidance...

Guidance:
Research RCDs, their function, and their associated testing.
Visit the property and commence fault finding in a logical manner starting with a chat with the customer about common usage patterns and appliances, and progressing to inspecting and testing the circuit.

 

[Lucien Nunes]

In the meantime, draw schematics of various possible fault conditions arising on RCD-protected circuits, calculate applicable resistances required to cause a trip (minimum can be taken as 15mA IΔN) and consider especially the reported symptom that larger loads tend to cause tripping.

 

[hightower]

Not being funny OP, but a lot of this should be very basic, fundamental knowledge, to anyone with a 2357 under their belt.

 

[kayloh]

Not being funny OP, but a lot of this should be very basic, fundamental knowledge, to anyone with a 2357 under their belt.

Mhmm most courses tend to teach about health and safety and cable calcs... lessons on fault finding and installation side tend to be broad and quick since the curriculum is more lenient towards theory (least in my college)... personally I haven't been taught how to fault find properly so it's just been picking up knowledge on the way, and your statement about having a 2357 means nothing since I know a lot students who have the same qualification and yet have no idea how to do a simple ring circuit let alone a radial... it's all about experience, which is what I'm trying to do...