RCBO trips different RCBO

[Richard Booty]

I've just change a CU and installed RCBO's. There is a 32a SP RCBO suppling a ring final circuit. There is a 6a SP RCBO radial lighting circuit. When the 6a radial circuit is energised the 32a RCBO trips. With the 6a dead the 32a stays in.
I expected to find a shared neutral but there is no continuity between the neutrals and no continuity between the lives.

 

[Charlie_]

Have you tested the cables?

 

[Richard Booty]

Have you tested the cables?

Yes, from memory (I haven't got the figures in front of me)insulation resistance is about 350 M Ohms
Resistance of the ring line and neutral 0.74
Resistance of radial line and neutral 0.43

 

[Charlie_]

Did you test between circuits?

 

[Wilko]

Hi - as per Charlie, I suspect the two circuits are connected .

 

[Richard Booty]

Did you test between circuits?

Yes

 

[Richard Booty]

Hi - as per Charlie, I suspect the two circuits are connected .

Thats what I thought but I get infinate continuity between line of 6a and line of 32a. Infinite continuity between neutral of 6a and neutral of 32a. I also tested between line of 6a and neutral of 32a, and between line of 32a and neutral of 6a with same results.

 

[Strima]

Did you test with light switches in different positions?

 

[buzzlightyear]

Answer is borrowed neutral some where.

 

[Pete999]

Answer is borrowed neutral some where.

Agree ,ore than likely a borrowed N

 

[Dustydazzler]

Shared neutral

(Don’t you just hate it when this happens)

 

[Richard Booty]

Hi guys, well I'm back looking at this issue again. I cannot prove a shared neutral no matter what test I run but have found some weird ---- going on. With all light switches off both circuits will stay in but switch a light on and sometimes the 32a will trip sometimes it stays in, it will usually trip when I switch on a second but it doesn'tseem to matter which lights I pick. If I press the test button on any of the other 8 RCBO'S the 32a RCBO in question trips as well. Also when I arrived the 32a had tripped over night.

 

[Pete999]

Hi guys, well I'm back looking at this issue again. I cannot prove a shared neutral no matter what test I run but have found some weird **** going on. With all light switches off both circuits will stay in but switch a light on and sometimes the 32a will trip sometimes it stays in, it will usually trip when I switch on a second but it doesn'tseem to matter which lights I pick. If I press the test button on any of the other 8 RCBO'S the 32a RCBO in question trips as well. Also when I arrived the 32a had tripped over night.

What tests have you conducted? I cannot access your profile, so we cant ascertain what if any experience or qualifications in the electrical field you hold, so it makes it doubly difficult to tailor any advice or answers, telling you to get an electrician in would be crass unless we know your limitations.

 

[Strima]

IR test results?

 

[Richard Booty]

What tests have you conducted? I cannot access your profile, so we cant ascertain what if any experience or qualifications in the electrical field you hold, so it makes it doubly difficult to tailor any advice or answers, telling you to get an electrician in would be crass unless we know your limitations.

I'm got my inspection and testing tickets. I've done all the standard tests as well as loads of continuity test between the different circuits (neutral to neutral, etc) I also did a insulation resistance between the neutrals, achieving a reading of 185M ohms.
It did just trip when I energised a third circuit.

 

[Strima]

Check for loose neutrals on the supply side.

 

[Andrew Hunter]

As Strima said check for loose neutrals,
Another favorite when people change boards for split phase CUs are having the neautrals on the wrong neutral links as they only trip when start adding load to them. The number of my sparky freinds that call me with this problem
If not this then Have you done IR test between the neutrals and the CPCs of the affected circiuts, i have had it before where a voltage exceeding 50 volts has been present on the CPC of a circuit that wasnt even connected and when all connected in to new RCD loaded CU has identified the problem thet as the custer tries to say "There wasnt a problem before you touched it" errm "yes it was its just RCDs will find issues no previously apparent"