RCBO tripping issues

[JessJess]

Hi,
I have just replaced an old rewireable fuseboard with a 10way consumer unit with rcbo's fitted.
The issue i have is that when i connect one of the three lighting circuits it trips the other two lighting circuits together with one of the 32amp ring circuits!!!
I thought borrowed neutrals at first, but why would the 32amp ring circuit trip at the same time???

 

[littlespark]

It could be there's an interconnection of neutrals in the property, which wouldn't have been picked up on an old board without rcd.

Was any testing done prior to board change?

Temporarily connect all 3 lighting circuits to the same rcbo, and see if it holds.

Old properties where upstairs and downstairs lights shared a switch box, it was a common fault where neutrals got mixed up.

 

[Pretty Mouth]

Sometimes you come across an interconnection between a socket circuit and a lighting circuit. Usually on older properties, an example might be where an outdoor light was originally fed by an FCU on the ring, but later someone gave it a switched live from a nearby lightswitch.

You're going to have fun with this one, it sounds like you have 3 circuits connected to the suspect one. Testing 'by the book' doesn't usually pick these faults up.

 

[mainline]

I thought borrowed neutrals at first, but why would the 32amp ring circuit trip at the same time???

You may have a borrowed neutral from the ring final.

Its always a good idea to test everything when the Cu is disconnected.

 

[timhoward]

It sounds like more than one issue, but you'll have to do some testing to see where the inter-connections are, and then track them down.
Be extra careful as a disconnected lighting Live wire at the CU could be energised via the sockets RCBO.

Some things to look for:
-a DIY addition where someone tried to power something from a light switch and then realised they had no Neutral, and the nearest socket was raided
-A security light that someone wanted to add a switch to
-anything a plumber has touched regarding heating controls and immersion

 

[timhoward]

Its always a good idea to test everything when the Cu is disconnected.

To be completely fair, no one tells you at college to test for inter-connections.
It normally takes a nightmare experience to begin to appreciate the value in some interconnection tests before a shared RCD or no-RCD board change!

 

[mainline]

It sounds like more than one issue, but you'll have to do some testing to see where the inter-connections are, and then track them down.
Be extra careful as a disconnected lighting Live could be energised via the sockets RCBO.

Some things to look for:
-a DIY addition where someone tried to power something from a light switch and then realised they had no Neutral, and the nearest socket was raided
-A security light that someone wanted to add a switch to
-anything a plumber has touched regarding heating controls and immersion

Here's one for you Howard that i had the other day.

A 2 phase supply feeding 2 sub mains via 100ma time delayed rcds. (TT earthing)

L1 sub main feeds a bio mass boiler and an outbuilding.

L2 sub main feeds solar and a pump room.

Phase one had a L to Earth fault damaged cable touched the tin foil wall insulation.

Caused 3 rcbos to trip in various locations, 2 of them on the opposite phase.

The fault being all Upstream of these rcds/rcbos.

 

[pc1966]

To be completely fair, no one tells you at college to test for inter-connections.

This is one aspect where the ROI (AFIK, not worked there) has a good policy or requiring testing to show that all circuits are independent.

It normally takes a nightmare experience to begin to appreciate the value in some interconnection tests before a shared RCD or no-RCD board change!

Indeed!

Being a bit paranoid after discovering a friend's house had two crossed-connected RFC, as well as lighting borrowed neutrals, I try and IR between circuits before they go back in for use.

 

[pc1966]

Caused 3 rcbos to trip in various locations, 2 of them on the opposite phase.

The fault being all Upstream of these rcds/rcbos.

A big fault, with lots of current flowing, causes a large dV/dt (rate of change of voltage) on the supply. That can trip RCD/RCBO by virtue of the current to earth from the cable capacitance and dV/dt.

 

[JessJess]

Hi,
I have just replaced an old rewireable fuseboard with a 10way consumer unit with rcbo's fitted.
The issue i have is that when i connect one of the three lighting circuits it trips the other two lighting circuits together with one of the 32amp ring circuits!!!
I thought borrowed neutrals at first, but why would the 32amp ring circuit trip at the same time???

Thanks for all the input - looks like a day of testing tomorrow.
I have tested between all the offending line and neutrals and none show any intercionnect - just to make it more interesting

 

[mainline]

A big fault, with lots of current flowing, causes a large dV/dt (rate of change of voltage) on the supply. That can trip RCD/RCBO by virtue of the current to earth from the cable capacitance and dV/dt.

It wasn't a big current and hardly burnt the insulation.

Bear in mind, the RA is only around 120 ohms.

 

[Risteard]

This is one aspect where the ROI (AFIK, not worked there) has a good policy or requiring testing to show that all circuits are independent.

Indeed!

Being a bit paranoid after discovering a friend's house had two crossed-connected RFC, as well as lighting borrowed neutrals, I try and IR between circuits before they go back in for use.

Yes indeed. It's called an erroneous connections test.

 

[Pretty Mouth]

Thanks for all the input - looks like a day of testing tomorrow.
I have tested between all the offending line and neutrals and none show any intercionnect - just to make it more interesting

From what you've described, I expect the connections between the circuits have been made via equipment, probably lighting. ie borrowed neutrals. If the circuit conductors were directly interconnected to one another, I would expect the RCBOs to be tripping with any two or more circuits connected to the CU - this is not the case.

Keep all lamps/other equipment in circuit, light switches etc in the on position, and test between circuits at 250V. Good luck

 

[timhoward]

The issue i have is that when i connect one of the three lighting circuits it trips the other two lighting circuits together with one of the 32amp ring circuits!!!

I have tested between all the offending line and neutrals and none show any intercionnect - just to make it more interesting

Have you checked that each lighting circuit in turn 'mainly works' if connected on it's own, with the other circuits turned off?
It can help to take the bulbs out of stairs lighting during fault finding to remove the most likely path between circuits and verify the basics.

Complete speculation on my part but I'm wondering if you possibly have two neutrals the wrong way around, combined with a path from line on one to N on another via stairs lighting.

 

[pc1966]

It wasn't a big current and hardly burnt the insulation.

Bear in mind, the RA is only around 120 ohms.

Odd, that is much harder to explain.