RCD problems..

[ash_s3]

Hi all

installed some downlights for a customer the other day, nice easy job (as they all start out to be)
did the usual tests, Earth loop was a high reading, checked supply and it was TT even though a lead sheathed underground supply cable.
Ze was high 290ohms, bonding to gas and water all good 10mm which was bringing the zs down a fair bit.
suggested we try and get dno to change to tn-cs or if not possible we will get a better reading on a new rod.

Heres the headscratcher..

RCD as main switch, non split load DB, although fairly new Crabtree star breaker setup.
With all loads switched off the RCD will not test, will not trip at 1x or 5x or ramp test.
Tried brand new RCD, exactly the same. Disconnected the neutral fly lead from RCD and it trips perfectly within the permitted times.
I then went through each neutral one at a time disconnecting them and testing the RCD, found that with one ring finals neutrals take out it tests fine, turns out it's not a complete ring, line is ok but high resistance on neutral end to end.

Further investigation required but couldn't figure out why this would stop the RCD from operating.

The guy that changed the DB obviously had no tester as its a right cock-up but that's another story..

 

[trev]

All due respect Ash but how come you found out it was a TT after you started the job?
Have you checked that you have your tester on the right setting? If it's a 300mA then a 30mA test current isn't going to touch it. Believe it or not I've seen it done mate.
It's pretty good when you're watching an apprentice trying to figure it out

 

[Octopus]

Sounds to me as if you have a "simple" N-E fault within the property and now you know which circuit time to find the fault.

 

[telectrix]

as above. check rthat the RCD is in fact 30mA. also, a neutral fault could be the cause of the RCD not tripping.

 

[ash_s3]

Sorry, knew it was a TT before I started, just didn't expect such a high reading.

tester is definitely set to 30mA and RCD is definitely a 30mA.

I know the circuit that's causing the fault and IR test shows that the resistance between N-CPC is 0.0M ohms

im just curious as to how a N-CPC fault can't stop the RCD from operating when it should operate in the event of a fault.

 

[telectrix]

it's one of those anomalies that crop up from time to time. your tester might read 0.00Meg. but the fault might be in the order of Kohms, too low for your tester's resolution , but not low enough to trip RCD. can just inhibit it's operation.

 

[Piratepete]

it's one of those anomalies that crop up from time to time. your tester might read 0.00Meg. but the fault might be in the order of Kohms, too low for your tester's resolution , but not low enough to trip RCD. can just inhibit it's operation.

I agree with you telectrix but this doesn't explain why the RCD won't trip at all. It should.

Pete

 

[Octopus]

I agree with you telectrix but this doesn't explain why the RCD won't trip at all. It should.

Pete

Seem it a few times - won't trip on tests, but disconnect the faulty circuit - it will!

 

[telectrix]

i don't know why, either, but i've come across it a couple of times. once was on a dual RCD CU, where neither RCD would trip, not on the test button or x1 and x5. dissed the radial with a N fault and RCDs then tested perfectly.

 

[kenny7askew]

if the cpc and N are shorted together the rcd will not detect leakage current in the cpc so it wont trip

 

[Plonker 3]

I agree with you telectrix but this doesn't explain why the RCD won't trip at all. It should.

Pete

All loads should be disconnected before doing a RCD test anyway.

 

[ash_s3]

All loads should be disconnected before doing a RCD test anyway.

There were mate

 

[Plonker 3]

There were mate

Eventually they were, you need to disconnect the whole install like you eventually did.

 

[GB.Kayak]

if the cpc and N are shorted together the rcd will not detect leakage current in the cpc so it wont trip

... did I read that correctly ... 'rcd will not detect leakage current in the CPC'?

k,

How does an RCD work?

 

[Plonker 3]

... did I read that correctly ... 'rcd will not detect leakage current in the CPC'?

k,

How does an RCD work?

It monitors the imbalance current between the live condutors. Nothing to do with the CPC.