RCD Tripping When Light Switched On

[RyanM]

Issue....

A bayonet light fitting was replaced like-for-like, as it got damaged and everything worked fine for a week.

Then all of a sudden, when the pull cord was pulled, it would trip out the RCD (100A, 30mA).

Inside the bayonet fitting was a red, black and earth wire. If these are disconnected and put in to connector block, the RCD will still trip. So this can eliminate the new fitting.

The pull cord switch was replaced with two others, know to work fine on the circuit. This didn't solve the problem either.

Turning all the other lights on the circuit off, then trying said pull cord still trips the RCD.

The pull cord circuit has an extractor fan on it, which when disconnected and put in to connector block doesn't help things.

Nothing has changed in the building. I.e, no screws or nails have been hammered in to the wall.


Any ideas when the next step would be, and what to check?


Thanks

 

[sparkwright]

Possibly a damaged cable when screwing the new fitting to the ceiling?

Is the light fitting metal? Perhaps you have just nicked a wire against it, possibly at the cable entry hole.

 

[ColJack]

View media item 9882
either that or the screw through the cable is the prime suspect.. takes time to break down that last minute sliver of insulation..

 

[willelectrician]

maybe its a ghost

 

[RyanM]

Thanks for the replies.

I don't think it will be the screws (could be wrong though), as the light fitting is screwed on to a beam, and the T+E comes out of the side.

The fitting is all plastic. Like this

The T+E is fairly old (black and red), and painted.....and nothing looks damaged. I cannot see the cable for the pull cord switch though, as that is in the rood (sloped roof).

There is the permanent live to the switch, so I guess the main issue is from the Switch Live (black cable in this case, with no sleeving) to the fitting? Although I cannot see if the switched live goes to a junction box or not. As the only cables in the light fitting are Live and Neutral (and Earth), but in theory if the switched live was coming straight from the switch to the light fitting, the live would be Black, and Neutral Red.....which I cannot see happening. Although I cannot test this as the RCD trips out.


Any suggestions I could try? Trace the Switched live back to somewhere? Although this will be difficult as it is inside the wall/roof eve.


Thanks a lot

 

[sparkwright]

You could check no wires are pinched against the earth terminal in both the light fitting and the switch.

 

[RyanM]

I will double check again the wires in the fitting/switch, but I'm sure they are ok.

Do I need to really go down the route of tracing the switched live wire to the fitting? As it only trips the RCD when the switch is switched.

 

[riveralt]

Do I need to really go down the route of tracing the switched live wire to the fitting? As it only trips the RCD when the switch is switched.

Well think about it for a second - the RCD is okay (we will assume there is no issue with the RCD) then you pull the switch and the RCD trips.
When you pull the switch the switch live is energised at the switch and at the batten light and at the extractor.
You have already told us that the batten holder is a bayonet type and not edison, so it seems probable that somewhere along the switch live cable route the lies the problem.

 

[STI]

Hi reading your posts i dont think you have tried disconnecting the switched live at the switch. From your descriptions youve change the switch so this suggestion is a real belt and braces approach to eliminate everything before and at the switch. If you do this and switch on, if it doesnt trip then it pretty much confirms that your problem is between the switch and the light fitting.

 

[RyanM]

Hi,

I did try disconnecting the switched live from the switch, and put it in connector block, then switched the switch. It didn't trip the RCD.

Is this what you mean?


Thanks a lot

 

[RyanM]

Do I need to really go down the route of tracing the switched live wire to the fitting? As it only trips the RCD when the switch is switched.

Well think about it for a second - the RCD is okay (we will assume there is no issue with the RCD) then you pull the switch and the RCD trips.
When you pull the switch the switch live is energised at the switch and at the batten light and at the extractor.
You have already told us that the batten holder is a bayonet type and not edison, so it seems probable that somewhere along the switch live cable route the lies the problem.

Apologise, I didn't mean for that to read how I actually wrote it! (I'm tired )

I did mean that I agree it has to be on the switched live side of the switch, and between the light fitting an extractor fan.

So I guess it will be a matter of tracing it back, and hopefully find the fault or completely remove the cable and replace it?


Thanks for all the help

 

[STI]

Hi,

I did try disconnecting the switched live from the switch, and put it in connector block, then switched the switch. It didn't trip the RCD.

Is this what you mean?


Thanks a lot

Sorry,I misunderstood your post about the connector block i thought you had disconnected at the light fitting. However if you have effectively isolated the switch from the light fitting switched on and it didnt trip then its got to be between the switch and fitting the fault is occurring

 

[riveralt]

[So I guess it will be a matter of tracing it back, and hopefully find the fault or completely remove the cable and replace it?

Any photographs of the wires in situ at the switch, ceiling rose (jb) and extractor fan would be helpful.

Do you have a multi-meter?

 

[RyanM]

Thanks STI (do you own an impreza?)

riveralt, I can get some photo's at the weekend.

Yeah, I have a multi-meter and Megga.

 

[riveralt]

riveralt, I can get some photo's at the weekend.

Yeah, I have a multi-meter and Megga.

So first make sure the terminal connections are correct and secure. If they are then you should be able to test for continuity along the switch live and hopefully find the fault.