Possible ceiling light snag.

[Greenie9657]

I know, 2nd post in 24 hours. I'm like a bus!

Anyway, a sparky went round to fit a light for my gran yesterday while I was there and everything worked fine but I had a call this morning saying the freezer has stopped working. When I ran up to check, the RCCB had gone protecting the kitchen and the upstairs lights. But the MCBs were fine. On invest, I turned the 3 MCBs off that were protected and switched the RCCB back on. Obviously didn't pop. The second I put the upstairs MCB on the RCCB trips. With the lights MCB off I turn the kitchen MCB on and as soon as anything is turned on in the kitchen, the RCcB trips. (The third circuit protected is the downstairs lights, that appears fine). I reckon a wire or dodgy light fitting is causing the upstairs lights to trip but what about the kitchen? I'm going to nip up after the school run and drop the light that was fitted and see what happens. I'm competent using flukes etc so no need for concern.
Any ideas would be great though.

Many thanks

 

[Lectrician]

If it's tripping when either circuit is turned on, that usually indicates a neutral-earth fault on ANY circuit protected by that RCD.

 

[bernardgreen]

Sounds like a Neutral Earth fault in the lighting circuit.

IF you are competent to work in the CU turn of the main switch, disconnect the neutral of the lighting circuit from the neutral bar and measure insulation resistance between neutral to lights and earth to lights. ( leave the earth wire connected to earth bar ) It should be at least 2 meg ohms, better if off scale or infinite.

If less than 2 meg then likely the electrician has made an error in the light fitting.

Or get the electrician to come back and sort it out free of charge.

 

[Greenie9657]

That's brill. Many thanks. I'm guessing an earth in the ceiling light is to blame then. Ill drop the fitting and see what happens. Ill keep you posted.

 

[Greenie9657]

Sounds like a Neutral Earth fault in the lighting circuit.

IF you are competent to work in the CU turn of the main switch, disconnect the neutral of the lighting circuit from the neutral bar and measure insulation resistance between neutral to lights and earth to lights. ( leave the earth wire connected to earth bar ) It should be at least 2 meg ohms, better if off scale or infinite.

If less than 2 meg then likely the electrician has made an error in the light fitting.

Or get the electrician to come back and sort it out free of charge.

Thanks for your reply mate. I think as the only thing to change since it last worked is the light fitting ill start there before diving into the CU. I downtown a megger anyway.

 

[bernardgreen]

Thanks for your reply mate. I think as the only thing to change since it last worked is the light fitting ill start there before diving into the CU.

Remember when measuring at the light fitting that the Neutral and Earth are conected together at the main supply cable to the house ( PME system ) and so you will not be able measure the insulation resistance.

You do not need a "megger" meter to fault find, any reasonable multi-meter will find Neutral -Earth faults. The "megger" with its higher test voltage verifies there is no fault in the insulation.

 

[Greenie9657]

Hi

Im happy with fault finding between neutral and earths but i would have needed a megger if i was to go down the insulation resistance path.

Anyway, i dropped the ceiling light, completly disconnect it and put all power to the flat back on. Everything worked fine. Problem obviously in the ceiling light. On checking there are 2 terminal blocks (as its a chandeller type light) one for the neutrals and one for the lives. The neutral block had loads of strands sticking out and im guess those were earthing once the light fitting was proper installed. I sorted those out and put everything back up and so far it works fine.

Just a quick question for my ever growing knowledge on house wiring. (im hoping to turn sparky when i leave the military).
Why does a neutral touching the earth pop the RCCB but not the MCB? I understand (i think) that if the live touched then the MCB would go as it would draw max amps but i just dont get the difference between RCCB, RCD and MCB.

thanks

 

[bernardgreen]

MCB trips when the current on the Live wire exceeds the rated value, how fast it trips depends on how great the over is.

RCD trips when the difference between current on Live and current on Neutral exceeds the trip value, normally 30 mA. The difference is caused by current leaking to ground.

RCCB is similar to RCD.

 

[Taylortwocities]

Why does a neutral touching the earth pop the RCCB but not the MCB? I understand (i think) that if the live touched then the MCB would go as it would draw max amps but i just dont get the difference between RCCB, RCD and MCB.

thanks

Kirchoffs first law says that
At any junction in a circuit, the sum of the currents arriving at the junction = the sum of the currents leaving the junction.

So if there is a difference then some of the current must be going somewhere else unexpected. Usually to earth and that path to earth might be through the human body.

In domestic installation, must RCDs are 30mA. Which is deemed to be a point below that which will cause problems to the heart. So the RCD trips before you do.

In many installations, just touching the earth onto the neutral of a circuit will cause the RCD to trip. Here's why..

Neutral_Earth_trip

• bernardgreen
• 16 Jan 2011
• 48

Differences between the devices.
For most purposes an RCCB and RCD are the same thing. These operate as described above.
An MCB looks at the current flowing in the circuit and will trip if the maximum current is exceeded for a defined period of time(6amps in your lighting circuit).

Note there is also a device called an RCBO. This is a combined MCB and RCD. So a 6amp/30mA RCBO will trip if there is either a 6amp OVERLOAD, or an earth leakage current of more than 30Ma.

Hope this helps.

PS. Dear DIYNOT pedants. Yes I know the above is a general description, and I have given information about nominal values. Just so to not over-complicate things.....

 

[Taylortwocities]

Ahh Bernard, I was too slow with my wordy reply. I did steal your drawing though!

 

[bernardgreen]

I did steal your drawing though!

No problem. It was drawn to help as many people as possible.

 

[Greenie9657]

That is great knowledge. Big thanks to both of you on that one. I get the live side just trying to understand neutral. If I placed an ammeter in series (connect as you would) would see the same value on live as you would doing the same check on neutral?

 

[Taylortwocities]

Yes, the current in the live and the neutral are the same.

 

[Greenie9657]

I think I get it now. So it's a miss match in the current of live and neutral which will cause the RCD to trip? Ie neutral touch earth somewhere.

 

[Taylortwocities]

http://greenbrook.co.uk/pdf/pbcatpg21-22.pdf