Both RCD's trip at the same time.

[mbelectric]

Being fairly new to the profession and struggling finding work I teamed up with a friendly local Electrician to 'help' change a consumer unit.
It was a straightforward change (2 Lights, 1 Ring, 1 Cooker) and all the test results were fine.
However when doing the functional test at the end when we switched the 2 way landing light, both RCD's tripped at the same time (the upstairs lights and downstairs lights were on seperate RCD's). We double checked the Neutral connections at the CU and they were ok.
The downstairs hall switch is 2G (downstairs hall & 2 way upstairs landing) with the 2 way switch having a live feed via a link from the downstairs hall. When we disconnected the link all the lights work ok (apart from the landing). It was getting late and the customer was getting impatient so we left it disconnected and left, ready to return first thing in the morning.
Like I said, I am fairly new and am now trying to get my head around what could have happened so we can get it sorted asap. For both RCD's to trip, all I can think is that they have taken the live feed from the downstairs circuit then taken the neutral from the upstairs circuit, but even if this was the case, why would it cause the RCD's to trip?
Any pointers would be welcome.

 

[Spark123]

Sounds like you have a shared neutral i.e. the landing light is taking the phase from the downstairs circuit and neutral from the upstairs circuit.
When the circuit is made, the current will flow in the phase coil of one RCD and the neutral coil of the other RCD hence they are both imballanced.

 

[industryspark]

i would have to agree with spark123, ive seen a similar thing. If it were me id be tracing the circuits out making sure they are indeed seperate.

 

[ban-all-sheds]

I'm a bit worried that you might have teamed up with a friendly, local, not-very-experienced electrician....

 

[Taylortwocities]

 

[Pensdown]

On that note......

 

[mbelectric]

Blimey Taylortwocities, so you know him too!!!

BAS, he reckons he has 20 years experience but he his very set in his ways and won't take advice from a whippersnapper like me even though in some instances I know I am correct (eg during the CU change we had to lengthen all the circuits, I suggested crimping, he said it wasn't allowed so now there is 5 jb's clogging up the meter cupboard)

Too be fair, he gets there in the end, he does things you needn't do rather than not do things you should do, and is very thorough, honest and reliable and his customers love him for that. Plus any sort of experience for me is better than none, especially if it gets me thinking.

 

[ban-all-sheds]

BAS, he reckons he has 20 years experience

You'd think he'd have come across a borrowed neutral on a landing light by now...

but he his very set in his ways and won't take advice from a whippersnapper like me even though in some instances I know I am correct

I can understand that - just make sure you keep hold of what you know is right, even if you have to keep quiet when you're with him.

Too be fair, he gets there in the end, he does things you needn't do rather than not do things you should do,

Does he do supplementary bonding on kitchen sinks?

Plus any sort of experience for me is better than none, especially if it gets me thinking.

That's very true.

 

[Spark123]

BAS, he reckons he has 20 years experience

You'd think he'd have come across a borrowed neutral on a landing light by now...

I'd expect him to have come across this also, however the putting the two circuits on two separate RCDs is a relatively new concept.

 

[JohnD]

Doing something for 20 years doesn't mean you're any good at it.

Ask any married woman.

 

[ban-all-sheds]

I'd expect him to have come across this also, however the putting the two circuits on two separate RCDs is a relatively new concept.

Yes, but he must recognise the symptoms of an RCD trip due to a neutral being in the wrong place, and as he had only just finished putting the lighting circuits on two different RCDs I would have thought that as soon as he got them both tripping when the light was turned on he would have twigged instantly why...

 

[ricicle]

I'd expect him to have come across this also, however the putting the two circuits on two separate RCDs is a relatively new concept.

Yes, but he must recognise the symptoms of an RCD trip due to a neutral being in the wrong place, and as he had only just finished putting the lighting circuits on two different RCDs I would have thought that as soon as he got them both tripping when the light was turned on he would have twigged instantly why...

Depends whether you're the sort of Electrician who understands current paths and RCDs or one who just does things 'cos the "regs says"

 

[experience]

tell him to put both lighting circuits on the same RCD and to make a note of the borrowed neutral on his test sheets

 

[RF Lighting]

But that does not comply with BS7671, so how could he issue an EIC saying that it does?

 

[experience]

Sorry just noticed your reply, of course if dont comply but it can be noted in the departures section on the EIC.
By installing a new dual RCD CU you arnt making the installation any more dangerous, on the contrary its alot safer than what it was!

When carrying out work in various properties there will be departures that dont comply, you make a note of them on your EIC, advise the customer and take action as required depending on the work that is to be carried out. You cant be expected to rectify all faults before you carry out any electrical work.

Personally installing a new dual RCD CU, whilst upgrading the main earth, tails & bonding to the water/gas is alot safer than walking away from a job leaving the customer with on old rewirable board because of a borrowed neutral, at least if there is a departure by installing a new CU the customer has much better protection.