RCD Tripping in outbuilding on Lights, please help

[Barnie]

Hello, i am currently doing some work on an outbuilding, i am in training at the moment, i am supervised etc but i really wanted to find out for myself why the lighting circuit keeps tripping. its only a small CCU with a mian RCD protecting an MCB for the Ring and an MCB for the lights, i am able to turn the RCD on, all ok, then the ring MCB ..ok but when i turn the Lights MCB on the RCD trips, i just wondered if you could give me a couple of steps on where to find the fault. i have all the relevent test equipment.

Thanks in advance

B

 

[Spark123]

A couple of things to look at, you may have a genuine earth fault or you may have connected the lighting circuit incorrectly so it imballances the RCD (i.e. connected the neutral to the wrong side of the RCD etc).

 

[Barnie]

when you mean a genuine earth fault do you mean just on the light circuit plus, the polarity with the RCD is fine because it stays on when the light MCB is not switched on, it only trips when i turn the light MCB on so what stepa can i take please, i know the basics but will probably do it in a different order

 

[JohnD]

One of the common installation errors that Spark123 mentioned - in the CU you will have two Neutral bars. One of them is the "input" neutral that comes from the incoming cable, and goes to the "in" side of the RCD, and for any circuits that are not to be protected by the RCD.

The other comes from the "out" side of the RCD. Connect to it the neutrals of those circuits that are protected by the RCD

(remember the RCD works by comparing the current through the Phase and the Neutral sides are in balance; so if the phase e.g. for Lighting goes through the RCD and the Neutral doesn't, the RCD will detect an imbalance and shut off.

If that isn't the prob, have another look at the switches and lampholder connections, and examine the cable for damage. You can disconnect sections of the lighting circuits if that helps.

 

[Barnie]

many thanks for the advice so far, its only an rcd protected CU dur to it bring outside. it has a 32 for sockets and a 6 for lights, only trips when i turn on the light MCB, its fine when just the sickets MCB is on, so does that suggest its a fault in the light circuit? if so what steps can you advise i take to rectify the situation, as i did say i am supervised but i really want to have a go at this myself first, i learn better that way.

 

[Barnie]

apologies for the spelling mistakes, i never wanted to be an office dude!

 

[JohnD]

If the lights MCB trips it, and you've checked the neutral bars, then you might have got an incorrect connection, or a loose wire trapped inside a fitting and touching something. Or a nail through a cable.

I'm beginning to think you should get yourself a book on doing wiring, and study it.

 

[Barnie]

yeah your right John thankyou, the light MCB only trips when i put the lamps back in at any light which would suggest to me that the neutral and earth are touching somewhere, would that be correct? and about a book, theres loads of books i have that go on about fault diagnonsis but none on fault diagnosis with what im talking about, they only talk about testing the RCD themseleves and thats it not telling you how to find the fault tripping it

 

[Pensdown]

What sort of lights are they and how many, lampholders, florescents?

 

[Qedelec]

The problem you describe sure sounds like the neutral is feeding off the supply before the RCD. This will mean the current flowing in the phase side is not equal to the current flowing in the neutral side.

 

[Barnie]

they are just your normal lampholder ceiling roses no flourescents only 8 on the circuit.

 

[Barnie]

Hello Q. thanks for your advice, all the wiring at the board is correct

 

[JohnD]

removed

 

[Spark123]

When you did the polarity check did you do it at each light fitting?
Are the insulation resistance test results for the lighting circuit satisfactory?
Is there any current using equipment plugged into the ring main? If not, does it trip when something is plugged in?

 

[Barnie]

The insulation tests are fine, the ring main doesnt trip, the lights are on a seperate breaker, i havent checked the polarity of all lights will do when i return but what i dont understand is that there are two circuits in one 6A breaker which isnt a problem as there are only 8 lights but even whe i try the circuits seperate, and try to put a lamp in they still trip. any advice would really help