Tripping light circuit

[Eugene Herbst]

Hi All,

I've changed a DB where all is well apart from the stairway light circuit trips the new RCD. After doing IR test found that R1-Rn>999, R2-R1 >999 But R2-Rn= 0.Mohm what could the issue be and what is the best way to fix the issue?

All loads have been removed including dimmer switches!

TIA

 

[Pete999]

Shared N between D/S and U/S lighting circuits I would hazard a guess

 

[SJD]

What actually is R2-Rn? In Ohm, rather than MOhm?
You might for example, find the value suggests a N return via another lighting circuit that is connected at the DB?

 

[Eugene Herbst]

What actually is R2-Rn? In Ohm, rather than MOhm?
You might for example, find the value suggests a N return via another lighting circuit that is connected at the DB?

There's a 0.53ohm resistance between R2 & Rn and 0.ohm between R2-R1?

 

[telectrix]

as pete999 points out. option is to put all lights on 1 circuit or do some rewiring of the 2 way light. this would usually mean replacing the strappers with a 3 core and dissing the 2 way light L from the downstairs circuit.

 

[Eugene Herbst]

as pete999 points out. option is to put all lights on 1 circuit or do some rewiring of the 2 way light. this would usually mean replacing the strappers with a 3 core and dissing the 2 way light L from the downstairs circuit.

It's a rather large house over 6 floors! I've tried putting all the lights on the same RCD (ignoring diversity) but still trips!

 

[telectrix]

It's a rather large house over 6 floors! I've tried putting all the lights on the same RCD (ignoring diversity) but still trips!

but have you transferred the N's to the correct bar?

 

[westward10]

I would have thought if this were a case of crossed neutrals the fault would be evident on more than one circuit. What you have is an insulation resistance fault and if you have a resistance of 0.53 ohm between neutral and the cpc then they are physically connected. That resistance will give some sort of indication of how far away from the board it is.

 

[Eugene Herbst]

What actually is R2-Rn? In Ohm, rather than MOhm?
You might for example, find the value suggests a N return via another lighting circuit that is connected at the DB?

Update, I have installed offending circuit onto the next RCD / neutral bar and still trips.
If I get the 0.53ohm reading on cpc to N does this not suggest a short on the cable somewhere. By my calcs and using table I1 with 1.5/1 T&E, there should be a short about 1.75mtrs away from DB. Any suggestions before i start cutting away at a cupboard?

 

[westward10]

Update, I have installed offending circuit onto the next RCD / neutral bar and still trips.
If I get the 0.53ohm reading on cpc to N does this not suggest a short on the cable somewhere. By my calcs and using table I1 with 1.5/1 T&E, there should be a short about 1.75mtrs away from DB. Any suggestions before i start cutting away at a cupboard?

As I said in #8 you would appear to have a direct connection between the neutral and cpc. Why do you need to start cutting away at the cupboard, testing will locate where in the circuit the fault is.

 

[Eugene Herbst]

As I said in #8 you would appear to have a direct connection between the neutral and cpc. Why do you need to start cutting away at the cupboard, testing will locate where in the circuit the fault is.

I would have to cut away at the cupboard because this is approximately where the testing indicates how far the direct link between CPC & N is. (0.53/30.2 x 10-3 = 1.75mtrs approximately).
As mentioned before I did a DB change and at first I thought it would be a borrowed N, but as further testing suggests a short between CPC & N I reckon I've disturbed the cables causing a short.

 

[westward10]

Do some systematic testing before you pull the place apart.

 

[Pete999]

I would have to cut away at the cupboard because this is approximately where the testing indicates how far the direct link between CPC & N is. (0.53/30.2 x 10-3 = 1.75mtrs approximately).
As mentioned before I did a DB change and at first I thought it would be a borrowed N, but as further testing suggests a short between CPC & N I reckon I've disturbed the cables causing a short.

please lel the forum know what the problem was/is.

 

[Mike Johnson]

Just a suggestion, have a look at the fixings for the cupboard, it may be that one of them has penetrated the cable and caused the short, test the fixings from cpc to any screws for continuity.?

 

[Pete999]

Just a suggestion, have a look at the fixings for the cupboard, it may be that one of them has penetrated the cable and caused the short, test the fixings from cpc to any screws for continuity.?

Carry on let us know how you get on.

 

[SJD]

Update, I have installed offending circuit onto the next RCD / neutral bar and still trips.
If I get the 0.53ohm reading on cpc to N does this not suggest a short on the cable somewhere. By my calcs and using table I1 with 1.5/1 T&E, there should be a short about 1.75mtrs away from DB. Any suggestions before i start cutting away at a cupboard?

Unless your post was a typo, I think the short is more like 15m from the DB, obviously not as the crow flies but as the wiring is routed. Though the short may have a fraction of an Ohm resistance, so this may not be totally accurate.

You can test any accessible points on the circuit where N and E are present, e.g. other light fittings. If you are lucky, you may even find a point where the resistance is so low, it has to be at that fitting.

 

[Eugene Herbst]

Update. It got to 5pm and after shoving my camera behind CU enclosure, snapped the attached image back & front of cupboard panel. Not wanting to compromise all of the circuits this I decided to call it a day. Once I return next week I shall send another update. Thank you all for the advice and comments.

 

[VWWM 1]

Frustrating especially when it’s starts getting dark!
I would just carry out continuity on the affected circuit to check where the values change. Break up the circuit to split it so you have cables isolated from other incoming cables to confirm where it is and narrow it down to a leg from point a and b.
Test from either end of that leg and you will find it pretty quickly and hopefully not have to much to put back or repair!
Good luck

 

[Eugene Herbst]

Frustrating especially when it’s starts getting dark!
I would just carry out continuity on the affected circuit to check where the values change. Break up the circuit to split it so you have cables isolated from other incoming cables to confirm where it is and narrow it down to a leg from point a and b.
Test from either end of that leg and you will find it pretty quickly and hopefully not have to much to put back or repair!
Good luck

Thanks Mark, I'm booked back at the property next week again. I started breaking it down before it got too late & thankfully it's a vacant property at the moment so I don't have people screaming at me! Will update on the thread once I get to the bottom of it!