This cannot be right! 0.8amp in N and 0amp in L

[CliveT]

Hi all,

This is really concerning.

Turned off all breakers in a domestic single phase consumer unit. With just the main switch on I measure a 0.8 amp current in the neutral tail into the consumer unit and virtually zero current in the Live tail. This is using a clamp meter.

One of the lighting circuit neutral conductor is showing 0.6amps with this clamp meter. And again the breaker is off!

Is this weird or have you seen this before?

Please can you offer some advice. Thanks

Clive

 

[Pat H]

Any chance the wiring in the house passes close to next doors wiring?
Could be some inductance maybe?
I'd be tempted to pull the neutral conductors out the CU bus bar while monitoring the current flow. See if it's just one.
Isolate that circuit and see what's happening then.

 

[Midwest]

What supply is it. What were the IR results?

 

[CliveT]

It's a TNS supply.

Yeah I will try ripping out the neutrals to see which circuit to troubleshoot.

The house is lived in and I do not have keys to access rooms to do insulation tests as the moment. But will get round to it once I identified which circuit

 

[Octopus]

Out of interest what are you trying to do?

 

[CliveT]

The lighting circuit needs rcd and since the Consumer unit is damaged and dangerous it was best to change the whole consumer unit. So before powering down and ripping out the old CU. some basic tests to ensure there was no leakage that may trip the rcd and therefore delay outage in the installation. This was when this weird thing happened

 

[telectrix]

fit the new CU and test each curcuit before energising. when yo come to any faulty circuits that trip thre \rcd/s, then investigate further.

 

[Octopus]

The lighting circuit needs rcd and since the Consumer unit is damaged and dangerous it was best to change the whole consumer unit. So before powering down and ripping out the old CU. some basic tests to ensure there was no leakage that may trip the rcd and therefore delay outage in the installation. This was when this weird thing happened

Well then .... time to do some IR tests then

 

[HandySparks]

Hi all,

This is really concerning.

Turned off all breakers in a domestic single phase consumer unit. With just the main switch on I measure a 0.8 amp current in the neutral tail into the consumer unit and virtually zero current in the Live tail. This is using a clamp meter.

One of the lighting circuit neutral conductor is showing 0.6amps with this clamp meter. And again the breaker is off!

Is this weird or have you seen this before?

Please can you offer some advice. Thanks

Clive

Likely to be a neutral-earth fault on one of the final circuits. On TN-S there will be a small voltage difference between neutral and earth. If there's a N-E short, it doesn't take much voltage to push 600mA through.

 

[Midwest]

I was reliable informed by a DNO engineer, that although the supply cable/head looks like a TN-S, in the road outside, or further down the street or nearby, the supply cable will have been repaired, so your TN-S is actually PME. So in reality neutral - earth should be at or near same potential? If it ain't an internal fault, could it just be the clamp meter?

What reading do you get when the live/neutral is clamped?

 

[Marvo]

Testing the neutral current on an installation that supposedly has no loads connected isn't a meaningful test as such ie it doesn't prove anything.

Are there any UPS's in the house? Any emergency lights with battery back-up maybe...is there a solar PV installation? If so isolate all of these and retest.

 

[Tomhw100]

Have you tried using the clamp meter on the lighting circuit with the Main switch off- obviously if it's not an rcbo being used as a protective device all of the neutrals will be in the main neutral terminal( along with the supply neutral) which could mean you was picking up a reading from out in the road or thurther down the line

 

[CliveT]

Thanks for the helpful suggestions folks. These all seem like possible reasons and nothing to do with insanity. I will proceed to isolate the right circuit tomorrow and do the insulation and polarity/wiring checks . Will let you know the outcome.

 

[Wilko]

Are there any UPS's in the house? Any emergency lights with battery back-up maybe...is there a solar PV installation? If so isolate all of these and retest.

Agree - and perhaps have a good look for any signs of foreign supply, as this is a couple of hundred watts (if it's really there) so something is still live.

 

[dmxtothemax]

could be that a neighbour has a reduced neutral, and so some of his returning current is using your neutral return !

 

[Deleted member 72666]

Is this a tncs/pme, if so how close is the substation to the property in question and have you clamped the main earthing terminal? Just thinking ground network fault currents.

 

[CliveT]

Traced the problem down to both the upstairs and down stairs lighting circuits. When disconnected neutral on these circuits the current meter fell to virtually zero amps.

Measured the resistance between neutral and earth conductors and got approx 6 ohms. If there is a difference in voltage between earth and neutral of say 5volts this will account for the 0.8 amps. It's a TNS supply so I guess it's possible.i forgot to measure it.

Unfortunately I couldn't pin point where the previous problem was because most of the rooms were locked by the occupants.

Thanks for your suggestions folks. This was a real eye opener for changing old fuse boards out in an old house!

 

[CliveT]

Traced the problem down to both the upstairs and down stairs lighting circuits. When disconnected neutral on these circuits the current meter fell to virtually zero amps.

Measured the resistance between neutral and earth conductors and got approx 6 ohms. If there is a difference in voltage between earth and neutral of say 5volts this will account for the 0.8 amps. It's a TNS supply so I guess it's possible.i forgot to measure it.

Unfortunately I couldn't pin point where the previous problem was because most of the rooms were locked by the occupants.

Thanks for your suggestions folks. This was a real eye opener for changing old fuse boards out in an old house!

 

[dmxtothemax]

How did you fix the problem ?
Did you get into the rooms ?

 

[tony mc]

Im at a loss as to why you are only clamping the cables to see what current is being draw!

Now you have a neutral to earth fault found that will need sorting prior the the RCD being installed!

A global IR test at the CU would of given you a good indication to where the leakage was or even clamping the tails to see how much leakage was present!!

 

[CliveT]

unfortunately i never got round to finding the location of the fault,the landlord was funny and did not want to give access and i have been called to another contract.

what started the whole thing, was that the bathroom lighting circuit was not protected by RCD. when the circuit was put on an rcd, it immediately tripped. without physically access to the rooms affected by this circuit, the basic dead test and insulation resistance was not possible.

hence a clamp meter was used to confirm that the earth leakage was tripping the rcb.

 

[Marvo]

The only way you could use a clamp meter to test the leakage is if you had clamped the live and neutral simultaneously. As I said in post 11 it wasn't a meaningful test, clamping just the Live or just the neutral or even just the CPC won't give an accurate result.

 

[Lucien Nunes]

Instead of an insulation test, a continuity test L+N -> E would have located the neutral-earth short, without any risk to connected equipment in the rooms that could not be accessed. It wouldn't find degraded or damp insulation but this is a case of copper touching copper which any multimeter will pick up.

 

[CliveT]

The only way you could use a clamp meter to test the leakage is if you had clamped the live and neutral simultaneously. As I said in post 11 it wasn't a meaningful test, clamping just the Live or just the neutral or even just the CPC won't give an accurate result.

yes that is true, and i got a figure of approximately .8 amps
0.6 amps on the downstairs lighting circuit and 0.2 amps on the upstairs lighting circuit.

 

[CliveT]

Instead of an insulation test, a continuity test L+N -> E would have located the neutral-earth short, without any risk to connected equipment in the rooms that could not be accessed. It wouldn't find degraded or damp insulation but this is a case of copper touching copper which any multimeter will pick up.

the ohm meter measured about 6 ohms between earth and neutral, which suggests that it is metal touching metal somewhere along the circuit.

 

[CliveT]

Found the faults.

First one was a neutral wire worked free and contacted the metal casing of ceiling lights.

2nd fault was actually in a ring circuit. An appliance fault must have been the reason why the earth conductor melted through the cable contacting the neutral line.

Now I need to figure out how to pull a new cable for that section of the ring.

We reached a conclusion to this thread!!!

 

[telectrix]

Now I need to figure out how to pull a new cable for that section of the ring.

if it'sgoing to be a realballache to replace that damaged section, cut it out and form 2 radials, each on a 20A MCB.

 

[CliveT]

I like this suggestion, thanks. will do it!

 

[Marvo]

Glad you found the fault(s). I like Tel's idea of making 2x 20A radial circuits if the damaged cable isn't replacable without causing devastation.