RCBO tripping issues

[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.

 

[mainline]

Odd, that is much harder to explain.

The only thing I can think of is there would have been a rise on the cpc which caused some down stream rcds/rcbos to trip.

 

[timhoward]

The only thing I can think of is there would have been a rise on the cpc which caused some down stream rcds/rcbos to trip.

A bonded pipe touching the foil?

 

[mainline]

A bonded pipe touching the foil?

The cause was someone putting a screw into the stud wall going through the tin foil insulation and catching the tail of phase one, the tin foil insulation must be getting earth through some of the screws that various Dbs are screwed in.

The location is basically a pump house wooden shed that has had insulation and ply skin.

 

[timhoward]

The cause was someone putting a screw into the stud wall going through the tin foil insulation and catching the tail of phase one, the tin foil insulation must be getting earth through some of the screws that various Dbs are screwed in.

Well that's a nice case study .... tails behind insulation in a stud wall.
It sounds like someone could have once done a bit better!

 

[mainline]

Well that's a nice case study .... tails behind insulation in a stud wall.
It sounds like someone could have once done a bit better!

I’ve altered it since and run both phases + N in surface mounted in some trunking.

Bit untidy looking but a bit safer