Well there you go after 36 years I should have kept my mouth shut

oldtimer · Aug 20, 2011

I mentioned on this forum a while ago that I had never come across a shared neutral on a lighting circuit and thought it was an english thing so the day before I go on holday a friend of mine called at 5pm and ask if I could give him a hand as he had a problem with a lighting circuit yep lights worked ok but when the hall was switched on the RCD tripped right away I clocked a shared neutral fault and explained it to him and he said he had never heard of it anyway got it sorted.

Aye every day is a school day

ExArmy · Aug 20, 2011

can you explain what a shared neutral is and why it makes the rcd trip please?

is it just where theres more than one rcd in the house and if current is going out through one rcd and then back through another causing the rcd to see it as a fault?

Guest123 · Aug 20, 2011

We never stop learning see.:shuriken:

D Skelton · Aug 20, 2011

Essentially, if you have two lighting circuits for example, an upstairs and a downstairs, one on each RCD (assuming there's two RCD's), it's where a neutral for one of those lighting circuits, goes to the other. Normally it will be one of either the hallway or first floor landing lights that is the culprit. This means that neutral current will escape to the other circuit tricking the RCD into thinking there's an earth fault as there will be more current going through its live path than its neutral path. See the diagram below:

[ElectriciansForums.net] Well there you go after 36 years I should have kept my mouth shut

The only way to remedy this situation is to either rewire the offending circuit or combine both lighting circuits onto one breaker. The former being the proffesional way to do it, the latter being the easiest and most common way of doing it.

Hope that helps?

edward cooper · Aug 20, 2011

i did'nt think you had to put them in the same breaker as long as they are on the same rcd. it will then not register it as a fault?

telectrix · Aug 20, 2011

the reason for putting them in the same breaker is that generally, with a shared neutral, the landing light takes it's "Live" from the downstairs circuit. if you are working on the landing light and turn off the upstairs MCB,somebody then flicks the 2 way switch and you will get a nasty wake up call.

oldtimer · Aug 20, 2011

Forgot to mention this was found in a flat and how I confirmed it I just took the lamp (not the bulb as they are put in gardens son for flowers to grow out of them my old tradesman used to say) and switched the light on off no problem the 2 circuits worked ok so now they are in one CB.

Here endith the lesson

edward cooper · Aug 20, 2011

so if you put them in the same breaker what code would you give it on a pir. and on what size?

oldtimer · Aug 20, 2011

edward cooper said:
so if you put them in the same breaker what code would you give it on a pir. and on what size?

Theres always one

telectrix · Aug 20, 2011

breaker size would depend on the total load of the 2 lighting circuits,and the cable size , but would ideally be 6A or 10A. you could give it a mention in comments regarding inconvenience of losing all lights due to a fault.