Borrowed neutral / RCD trips but circuits still live ?!

[Ian_Devon]

I’m just a homeowner, so please forgive me if my question sounds stupid ....

i think the lights in the hall of my small bungalow are using a borrowed neutral. I recently went to fit a new light on the ceiling in the hall using the existing connections and turned off the MCB that said “Hall lights”. However (long story short) I discovered the light also had a connection to another circuit labelled “ kitchen and lounge lights” Both the MCBs (on different sides of the board) needed to be off to remove all power. The light in the hall can be operated by two switches, one near the lounge and one just inside the front door.

From what I’ve read this is a classic borrowed neutral set up and we’ve also been getting lots of random nuisance trips with both RCDs tripping (one on each side of the board), but without any MCBs tripping.

The other day one of the RCDs tripped (on the right hand side of the board) and I then noticed that all 5 of the household circuits protected by MCBs on the same side of the board as the tripped RCD were still functioning normally ?!

This also happens if I use the RCD test feature .... the RCD trips and all the circuits protected by MCBs on that same side are all still live!!

Surely this is not right, What could be going on ?

Thanks in advance :-(

 

[SparkyChick]

When did the nuisance tripping start? After a consumer unit change perhaps?

 

[Ian_Devon]

House had the CU fitted approx 5 years beforeI I bought it. No changes to this but a lot of rewiring during major renovation work. Hall lights are connected to two different MCBs , One on each side of the board, it is my understanding that this could be a borrowed neutral case ....

Regardless of that I shouldn’t still have live circuits on the same side as an RCD that has tripped ?!

When did the nuisance tripping start? After a consumer unit change perhaps?

 

[littlespark]

I might have misunderstood, but it actually sounds like you have a borrowed live.
As in, there’s a cross connection between the 2 circuits somewhere, that may only be a problem if the switches are set a certain way.

 

[littlespark]

SC.... I’ve just replied to a duplicate of this post. You should delete the duplicate.

what I said on the other thread, I think this may be a shared live rather than neutral. Maybe with only certain switch positions, there’s a crossover between circuits.(?)

 

[Strima]

As above, sounds like a live has been swapped somewhere rather than a borrowed neutral.

 

[Ian_Devon]

When the IET certificate was completed after all the work was done, shouldn't something as dangerous as this have been picked up?

 

[SparkyChick]

Duplicate thread posts brought into this one so all the information is in one place.

 

[Wilko]

Hi - it does sound like the “Last Electrician” may’ve made a mistake and linked the two circuits. I’d ask them to fix (?).

 

[SJD]

There are various ways two circuits could have been accidentally connected. A few real examples I've found over the last few years:

• Mixing up connections in a switch with multiple circuits present, e.g. downstairs/upstairs lighting (I appreciate this issue is a bungalow, but there could still be more than one lighting circuit at a switching point).
• An outside light on a switched circuit, also back-fed with a live from a PIR on another circuit - in this case it only happened when the PIR was triggered.
• Lights in a room of a large house fed from two separate circuits that eventually got connected together during modifications because someone didn't realise this.