Negative to earth fault on lighting circuit at consumer unit

[Murdochcat]

Odd that such a set up still exists 6 decades later.

Seen it recently on a rewire.

More lights but all in a central JB - made fault finding easier once I’d located the jb

 

[SUNRAY]

The RCD on the master switch where the supply enters the house tripped, completely out of the blue, with no work or alteration’s having taken place.

I have traced the fault to the ground floor lighting circuit, specifically the feed from the consumer unit to the lights. My multimeter registers continuity across negative and earth on the wires exiting the consumer unit at a junction box immediately after the consumer unit, but only when there is a supply from the master switch. If the master switch is off, there is no continuity...

This is exactly how it should be.
The continuity between neutral and earth is before it comes into the house and that is why you only see it on your lighting circuit with the master switch OFF.
If you check any other circuits you will find the same continuity.

 

[SUNRAY]

Why would that be odd? Very common in 1960s homes

Odd that such a set up still exists 6 decades later.

OPs picture is not of 60 year old wiring. Truthfully it doesn't look pre 2005 either.

 

[ericmark]

Two points come to mind, one to test with power off we use either 250 volt or 500 volt, these testers use DC to detect earth leakage, I paid £35 for one shown, not a 100% as it used DC not AC, but normally works, the other tester does use AC, the original one I had to the left would measure down to 0.01 amp AC only (10 mA) my new one to right goes down to 0.001 amp (1 mA) cost £35 and this one can show how much is leaking.

There will always be some leakage, due to capacitive and inductive linking, so 9 mA is considered max for each 30 mA RCD, so showing either 10 or 20 mA was not that helpful, really need the one to right to work out how close to the limit.

Both have a ohm range, but at around 8 volt rather useless. However also need to know how to use them, I have clamped the meter around the DNO tails and know overall my house has a leakage of around 26 mA, but that is spread over 14 RCBO's I have never bothered to remove the CU cover to test individual circuits.

The problem is the supply will have the earth linked to neutral somewhere, so to test earth to neutral the isolator must be off, and although some RCBO's are double pole, most are not, and MCB's are hardly ever double pole, caravans are an exception to the rule, so in the main to test means the circuit needs disconnecting.

With the clamp on you only want line and neutral through the clamp, so only place is inside the CU, and you have to test with power switched on. So there is some danger, the same using the insulation tester with 250 volt, so I am not happy telling any non electrician to start poking around inside a live CU.

 

[Thomas Lawson]

If you are convinced there exists a fault on that circuit, that JB - best thing to do, is disconnect all of those wires and test each twin and earth separately, to determine which of the four exhibits the fault. No need for any notes as to what goes where - all the reds together, all the blacks, all the earths.

The usual warning applies - of turning the power off, before touching it.

To determine which exhibits the fault, is the best way to isolate each of the four in turn and see which one causes the trip?

Or is there something I can use my multimeter on each to determine?

 

[Thomas Lawson]

Two points come to mind, one to test with power off we use either 250 volt or 500 volt, these testers View attachment 320549use DC to detect earth leakage, I paid £35 for one shown, not a 100% as it used DC not AC, but normally works, the other tester does use AC, the original one I had View attachment 320550to the left would measure down to 0.01 amp AC only (10 mA) my new one to right goes down to 0.001 amp (1 mA) cost £35 and this one can show how much is leaking.

There will always be some leakage, due to capacitive and inductive linking, so 9 mA is considered max for each 30 mA RCD, so showing either 10 or 20 mA was not that helpful, really need the one to right to work out how close to the limit.

Both have a ohm range, but at around 8 volt rather useless. However also need to know how to use them, I have clamped the meter around the DNO tails and know overall my house has a leakage of around 26 mA, but that is spread over 14 RCBO's I have never bothered to remove the CU cover to test individual circuits.

The problem is the supply will have the earth linked to neutral somewhere, so to test earth to neutral the isolator must be off, and although some RCBO's are double pole, most are not, and MCB's are hardly ever double pole, caravans are an exception to the rule, so in the main to test means the circuit needs disconnecting.

With the clamp on you only want line and neutral through the clamp, so only place is inside the CU, and you have to test with power switched on. So there is some danger, the same using the insulation tester with 250 volt, so I am not happy telling any non electrician to start poking around inside a live CU.

To clarify, I could use an earth resistance tester with the supply off to test each of the circuits from the jb and it measures in ohms
And a clamp meter is used with the supply on and it measures in miliamps

 

[Harry Bloomfield]

To clarify, I could use an earth resistance tester with the supply off to test each of the circuits from the jb and it measures in ohms
And a clamp meter is used with the supply on and it measures in miliamps

Try it for continuity/ shorted, with the meter you have - it might just identify the fault which trips the RCD, assuming that is where the fault is - but I have my doubts, that is where the problem lies.

Forget anything more than your cheap meter, the items mentioned by Eric, are expensive, and intended for the pros to use.

 

[Thomas Lawson]

Try it for continuity/ shorted, with the meter you have - it might just identify the fault which trips the RCD, assuming that is where the fault is - but I have my doubts, that is where the problem lies.

Forget anything more than your cheap meter, the items mentioned by Eric, are expensive, and intended for the pros to use.

Just wondering, a while ago, an outbuilding tripped the RCD in the consumer unit (not the main supply one that is tripping now).

Is it possible for a fault on the outbuilding circuit to cause an issue with the lighting circuit, even if the outbuilding is switched off at the consumer unit?

The outbuilding fault has been rectified and is no longer tripping, but I have switched it off at the consumer unit with the idea of eliminating it.

 

[ericmark]

Forget anything more than your cheap meter, the items mentioned by Eric, are expensive, and intended for the pros to use.

This is in some way true, there is a point where one needs to say "I need an electrician" the problem is beyond my skill set.

However I remember my friend asking me to look at a compressor blowing a fuse, I had a multi-meter to hand, so tested for an earth fault or short circuit, and no fault found, so went back for my insulation tester (Megga) and then re-tested, and it showed short circuit to earth.

It is like trying to remove a Philips screw with a flat screwdriver, you may if not too tight succeed, but really better to get right tool for the job. The two tools I show will cost you £70 and you have to consider is that £70 worth spending on the tools, or would it be better spent on getting an electrician to do the job for you?

It is easy for me, 50 plus years in the trade, what is not easy is working out other peoples ability.

 

[Harry Bloomfield]

Just wondering, a while ago, an outbuilding tripped the RCD in the consumer unit (not the main supply one that is tripping now).

If they are on separate RCD's, and correctly wired, a fault on one, should not affect another circuit.

Is it possible for a fault on the outbuilding circuit to cause an issue with the lighting circuit, even if the outbuilding is switched off at the consumer unit?

How are you switching things off? To fault find RCD trips, both poles, L and N need to be turned off/isolated. MCB's/fuses/switches etc., normally only isolate the L, leaving the N connected. A fault on the N can therefore continue to trip an RCD, despite it being switched off.

Just so you understand what an RCD is/does....

An RCD compares the current flowing through the L, against what is flowing through the N. If there is a difference greater than 30mA, then they trip. The difference can mean some leakage of current, between either L to E, or N to E. A N to E fault, or leakage, because there is almost always some voltage differential between them, is enough to trip an RCD - even if that circuit is turned off. So when trying diagnose an RCD trip, YOU MUST ISOLATE BOTH L & N.

 

[Murdochcat]

Maybe a photo of the main cu would help, showing the incoming RCD

 

[EFLImpudence]

The RCD on the master switch where the supply enters the house tripped,

And a clear picture of this one as well.

 

[Thomas Lawson]

Maybe a photo of the main cu would help, showing the incoming RCD

Maybe a photo of the main cu would help, showing the incoming RCD

 

[Thomas Lawson]

And a clear picture of this one as well.

 

[Thomas Lawson]

I thought I had posted these pictures, but apparently not.

Hopefully this time