Dual RCD problem (interconnection between lighting circuits?)

[fire8789]

Hi guys, looking for abit of help.
Carried out a consumer unit change today, carried out all dead testing etc, all came back ok.

New board is a twin 63a RCD.

With the following configuration both RCDS tripped straight back out.

RCD 1 *upstairs lights
*Downstairs sockets
*Boiler
*Bathroom Low voltage


RCD 2 *Downstairs lights
*Upstairs sockets
*Cooker
If I swapped the upstairs lights over to RCD 2 however + then swap the downstairs lights to RCD 1 there were no problems.

I did find some kind of neutral interconnection via insulation resistance testing between upstairs and downstairs light circuits. which came back as 0.03 ohms.

Any ideas as to whether this would cause the RCDS to trip as described above and if so what the best possible way of tracking the fault would be?

Any help would be much appreciated.

Regards. Rich

 

[telectrix]

it sounds like a shared neutral. investigate that 0.03 ohms for a start, or was it 0.03 megohms?

 

[GLENNSPARK]

agree with Tel....shared neutral....
IR test between em.....that`l find it....
likely to be the up hallway lighting.....
this will be using the down lighting phase....and the upstairs neutral.....
it aint cast in stone that this is where it is.....but likely....

 

[fire8789]

Sorry telectrix yes, 0.03 Megohms was the result I meant. Do you have any idea whether this would cause the dual RCD tripping?

 

[trev]

Yes it would. Had you carried out the pre change tests you'd have known it existed.

 

[Geordie Spark]

Yes it would. Had you carried out the pre change tests you'd have known it existed.

I'll have to get you to show me how to do that sometime ....

 

[fire8789]

cheers for the replies guys,
Hi trev, I am fairly new to this. I carried out my IR tests on individual circuits so the link between the up and down lights wasn't found until the cu was connected.
Is it normal procedure to IR test between seperate circuits to rule this sort of thing out?

 

[trev]

Normally you'd do a few tests first before swapping over the CU. Global IR, test between lighting circuits to identify borrowed neutrals, EFLI etc. This is to make sure there's no nasty surprises waiting for you that will make your shiny new RCD go bang.
Didn't they tell you this?

 

[SPARTYKUS]

cheers for the replies guys,
Hi trev, I am fairly new to this. I carried out my IR tests on individual circuits so the link between the up and down lights wasn't found until the cu was connected.
Is it normal procedure to IR test between seperate circuits to rule this sort of thing out?

Yes, after its happened to you a few times!

and a global N-E IR test.

 

[GLENNSPARK]

sense from sensible people.....
glad to see its not the usual tonite....

 

[vjsmarwick]

My guess would be neutrals are looped at switches an cross linked in your stair switch box at one end or the other. I have done it before without thinking easy mistake.

 

[fire8789]

Yes I carried out what I thought to be all the neccessary checks, basically running through the schedule of test results sheet.
ZE, PFC at DB. Earthing type, Continuity, then IR on induvidual circuits.
I thought this was all that was required before a cu change...

sorry if this is frustrating for you guys, but I can't see how a global IR test would indicate that there is a problem between two neutrals as the neutal bar and earth bars are left untouched arent they?

 

[trev]

A global IR wouldn't show it but would show up anything IR related that could bite you on the ---. The test between lighting circuits would have shown the borrowed neutral though.

 

[jeffelectric]

Stick up and lights in 1 mcb not on rcd, trim the bussbar down, note on eic

 

[trev]

Just for future reference, you'd carry out a continuity test between the two circuits.

- - - Updated - - -

Stick up and lights in 1 mcb not on rcd, trim the bussbar down, note on eic

Or, spend a bit of time putting it right. By far the better option.

 

[GLENNSPARK]

Yes I carried out what I thought to be all the neccessary checks, basically running through the schedule of test results sheet.
ZE, PFC at DB. Earthing type, Continuity, then IR on induvidual circuits.
I thought this was all that was required before a cu change...

sorry if this is frustrating for you guys, but I can't see how a global IR test would indicate that there is a problem between two neutrals as the neutal bar and earth bars are left untouched arent they?

you IR between neutrals on finals......and that certainly would show up anything shared.....
thats why we do it....
its common on older installs.....

 

[Geoffsd]

There are a couple of points to raise.

The OP said that when the two lighting circuits were swapped to the other RCDs the problem disappeared.
This would indicate a link between one of the lighting circuits and another circuit which is now on the same RCD.

Also, 0.03MΩ which could actually be anything between 0.025 and 0.0349 is not enough on its own to trip a 30mA RCD.
245/25,000Ω (worst case) = 9.8mA

 

[Knobhead]

Stick up and lights in 1 mcb not on rcd, trim the bussbar down, note on eic

I take it you’re a Electrical Trainee. Only that could explain such a stupid suggestion.

I despair, I really do.

 

[Hawkmoon]

If a neutral is borrowed at the top landing light (most common I've come across), it is normally taken from a bedroom light or whatever nearby - the problem you will have identifying this is that it's only electrical connection to your downstairs circuit is through the landing lamp then down through the phase conductor/switch etc - neutral to neutral test at board may not reveal it unless you ensure that all lamps are in and all switches are closed (P to N tests across circuits also).

Another way to check for this - clamp ammeter on neutral of one circuit (dead) while other circuit is live with all lights on - then vice versa - If you have a suspect current you can turn lights off in turn till you find the culprit.

 

[telectrix]

If a neutral is borrowed at the top landing light (most common I've come across), it is normally taken from a bedroom light or whatever nearby - the problem you will have identifying this is that it's only electrical connection to your downstairs circuit is through the landing lamp then down through the phase conductor/switch etc - neutral to neutral test at board may not reveal it unless you ensure that all lamps are in and all switches are closed (P to N tests across circuits also).

Another way to check for this - clamp ammeter on neutral of one circuit (dead) while other circuit is live with all lights on - then vice versa - If you have a suspect current you can turn lights off in turn till you find the culprit.

never tried that. will have to give this method a go sometime. as they say, every day's a schoolday.

 

[Hawkmoon]

never tried that. will have to give this method a go sometime. as they say, every day's a schoolday.

I'm 52 yrs old but I still got my cap and satchel!


I could never understand why all this neutral borrowing was so common - to me best way is:

feed bottom switch (P+N), 3 core bottom switch to top switch, twin top switch to light ..... what the F can be easier?

 

[crow11]

as geoffsd as pointed out, the op hasn't swapped the 2 lighting circuits on to one rcd, he as swapped them over so they are still on different rcd's and the problem stops, problem would still be there if borrowed neutrals on lighting circuit, does sound like there is another neutral borrowed off another circuit somewhere

 

[telectrix]

I'm 52 yrs old but I still got my cap and satchel!


I could never understand why all this neutral borrowing was so common - to me best way is:

feed bottom switch (P+N), 3 core bottom switch to top switch, twin top switch to light ..... what the F can be easier?

how it was commonly done was to loop the L from the downstairs circuit from the hall light Common into the 2 way landing light Common with just a T/E for strappers to the upstairs switch. the N for the landing light would be loop-in with the upstairs lights. when a dual RCD board is fitted, the landing light then takes L from 1 RCD and N from the other, thus both RCDs will see an imbalance when the light is switched on . a quick pre_CU change would be to switch on the landing light and see which fuse/MCB killed the light.

 

[Hawkmoon]

how it was commonly done was to loop the L from the downstairs circuit from the hall light Common into the 2 way landing light Common with just a T/E for strappers to the upstairs switch. the N for the landing light would be loop-in with the upstairs lights. when a dual RCD board is fitted, the landing light then takes L from 1 RCD and N from the other, thus both RCDs will see an imbalance when the light is switched on . a quick pre_CU change would be to switch on the landing light and see which fuse/MCB killed the light.

I understand how they did it, I just don't understand why....just as easy imo to do it properly.

 

[telectrix]

one of the reasons was to avoid carting round and using 3 core/E. it was also common practice at the time to use twin with no cpc on lighting circuits.

 

[jeffelectric]

I take it you’re a Electrical Trainee. Only that could explain such a stupid suggestion.

I despair, I really do.

yeah this happened got called to a consumer unit that had gone pop from damp plaster and paper falling into it. no chance to pre-test. only found the rcds tripping on the lights . This happened in the afternoon, so now the customer is stressing that the power has been off a long time. So i advised sorting the light circuits out and the work needed he would not have it 3 times i said it is best to sort out now. I hate to think i just bodged it, but what could i do ? went to college for 3 years

 

[Guitarist]

one of the reasons was to avoid carting round and using 3 core/E. it was also common practice at the time to use twin with no cpc on lighting circuits.

Along with using 3-core for drops to 2-way switches without a cpc.