Dual board rcd tripping

[macloon]

Hey there everyone, I have a question hopefully you can help with. A job where ive fitted a new circuit on to the fuse board is having problems with the RCDs. Its a split load board so has the Line and Neutral supply into the 100amp main fuse from there Neutal up to the neutral bar and Line over to a 80amp RCD, from there is a Neutral feed from the same bar as the 100amp neutral and a Neutral to a seperate neutral bar for that RCD and another Neutral and Line that goes onto a 63amp RCD which then has a Neutral going to another bar for that RCD.
When the 80amp RCD trips the 63amp RCD stays up which it should, but when the 63 amp RCD trips it takes out the 80amp 1 to. I dont understand why. Any help would be much appreciated

 

[Heisenberg]

I'm really struggling to visualize what you have described above, a picture as they say, is worth a 1000 words.

If I had to guess, I'd say the neutral from your newly installed circuit, has been connected to a different neutral bar than the one which belongs to the RCD that is protecting it.

 

[robsparky1975]

Sounds like a dual rcd consumer unit(a main switch of 100a and 2 rcd,is the neutral connected to the correct bar in relation to the circuit breaker/rcd 1 or 2.

 

[Edd]

i think his describing a duel RCD board with 100A main switch..
you say when the 80A one trips the 63A stays up but when the 63A trips it takes both out. taking both RCDs out normally indicates a crossed neutral as this will show as an imbalance across both RCD's. have you worked out what is being switched or used to cause the rcd to trip. it sounds like you may have two faults. one crossed neutral (causing both to trip) and one on the 63A RCD causing just that one to go..

Edit im asuming their both 30ma

 

[sparkie-leeds]

I agree with you guys, sounds like crossed neutral too me...:17:

 

[macloon]

yes both are 30mA, nothing is causing a fault it is when I test the Rcds that it occurs which means it will happen if a fault ever occurs.

 

[robsparky1975]

Are you testing the rcds under load?

 

[Edd]

when you say testing the RCDs you mean timer test? or testing a circuit like earth loop?
I would start by doing an IR test between Neutral bars and switch stuff on/ off etc, see if it remains open circuit ( it should) any drop in reading will indicate a resistance across the neutrals which means an imballance across the RCDs this could cause tripping problems even if its not a dead short.. apart from that without more info im stumped

 

[macloon]

yes all MCBs are connected with all neutrals going to correct neutral bar. when all are on nothing trips is only when i test the RCDS

 

[macloon]

no i mean just pressing the test button

 

[robsparky1975]

What happens when you test it at 1/2 cycle,1x and 5x.

 

[Edd]

no i mean just pressing the test button

oh ok,, the test button puts a resistive short between outgoing Line and incoming neutral. maybe, just maybe your neutral on one side of one of the RCDs is at a lower resistance than the incoming neutral, so when you press the test button the easyst path is through the other RCD..
try P-N loop on the load side of each rcd I wonder if it will show better on one than at the service head

ps this is just a theroy and would be intrested to find the outcome.