Voltages between L - N - E

[iwoot]

Hi all.

First off i spent a year at collage doing c&g 1360 and 1240. this was over ten years ago so i know basic electric skills but not qualified.

i fitted a 10mm cable length probably a 40m run. its been in for 6 years no trouble. Now it keeps tripping.

Disconnected the live and neutral and the end of the run and it still trips.

Disconnected the earth as well and it no longer trips

Voltages between cables and the end are

L-E 240v
L-N 220v
N-E 20v

There shouldn't be a voltage of 20v between the Neutral and Earth should there.

I also disconnect at the CU so all three (L/N/E) were disconnected both ends. I then used y tester to test continuity between each wires but all combinations came back OK (OL)

So would this be inductive voltage and would this be acceptable ?

within the 40m run could possible be a cut/tiny nick in the wire due to other works but this was also 3-4 years ago however after heavy rain we have been having, water ingress could be a problem.

Trouble is the run was very awkward and its not easy to replicate.

What can i do next ?

 

[securespark]

You really need to megger the end.

Does the cable run underground anywhere?

Is it jointed anywhere?

 

[iwoot]

the cable run is say 30m of 10mm and the last 10m is of 6mm armored cable so it has 1 join inside.

The cable then runs out and under ground for about 2 meters.

The armored cable hasn't had any movement or disturbance so it cant be anything to do with that.

The little cut/nick was in the middle of the of the 10mm cable and this also runs outside under some fascia. It was the only route we could take.

Also if we have to replace the 10mm cable, it will have to be replaced like for like in the same place which is very tricky but can be done with a little grunting and swearing !!

 

[JohnW2]

i fitted a 10mm cable length probably a 40m run. its been in for 6 years no trouble. Now it keeps tripping.

Tripping what - an RCD or MCB? (If an MCB, what rating?)

Disconnected the live and neutral and the end of the run and it still trips. Disconnected the earth as well and it no longer trips Voltages between cables and the end are
L-E 240v L-N 220v N-E 20v

Did you also measure the voltages, particularly the N-E voltage, at the CU end of the cable?

Kind Regards, John

 

[iwoot]

MCB 32a

Should of but i didn't measure at CU no.

There doesn't seem to be any problems at the CU end just this one MCB but i will check the readings.

It doesn't trip when all three are disconnected at the end of the run but the 20volts across N-E seems suspicious to me. thats whats makes me think water ingress into the cable could have trigger it where as before now it has not been a problem.

Failing that the 20volts N-E has always been there and there is an earth fault and the end of the run.

 

[xr4x4]

How long does it take to trip?

And it's triping the 32a mcb with no load? Just the cable?

 

[JohnW2]

Disconnected the live and neutral and the end of the run and it still trips.
Disconnected the earth as well and it no longer trips

On further thought, that doesn't actually make a lot of sense. What were you disconnecting the cable from? Does this circuit/cable supply anything else?

If the earth was satisfactorily connected at the CU end then, even if there were a L-N fault in the cable causing the MCB to trip, it would have to be a very marginal situation for disconnecting the other end of the CPC to stop the tripping - and that only if it was connected to something which provided a very good path to earth.

As others have said, you need to IR test that cable.

Kind Regards, John

 

[iwoot]

sorry should have explained

it supplies a small garage CU with 2 sockets and 1 light.

Firstly i disconnected the 2 MCB's within the garage CU L + N but left the earths connected. It still tripped the MCB.

Then i disconnected the live and neutral from the RCD in the garage and left the earth connected and it stilled tripped the MCB.

Then i disconnected the earth as well in the garage so all three wires were disconnected and the MCB did not trip however the voltages are as above.

It takes about 2-3 secs for the MCB to trip.

So the CU in the garage is not connected at all yet i am getting 20v across the neutral and earth. All that is connected is the cable run.

this shouldn't be acceptable should it ?

I will check the voltages in the main CU as voltages there may not be correct.

 

[westie101]

That suggests some possibilities.

1/ There is a connection to a good earth in the garage which allows the current flow to trip the MCB

2/ There may not be a connection to earth at the house end (garage is TT)

3/ If there is a connection to earth at the house end, there is a break in the earth conductor between there and the garage end

As others have said a 250V or 500V IR test is essential

 

[iwoot]

sorry should have explained

it supplies a small garage CU with 2 sockets and 1 light.

Firstly i disconnected the 2 MCB's within the garage CU L + N but left the earths connected. It still tripped the MCB.

Then i disconnected the live and neutral from the RCD in the garage and left the earth connected and it stilled tripped the MCB.

Then i disconnected the earth as well in the garage so all three wires were disconnected and the MCB did not trip however the voltages are as above.

It takes about 2-3 secs for the MCB to trip.

So the CU in the garage is not connected at all yet i am getting 20v across the neutral and earth. All that is connected is the cable run.

this shouldn't be acceptable should it ?

I will check the voltages in the main CU as voltages there may not be correct.

 

[JohnW2]

That suggests some possibilities.
1/ There is a connection to a good earth in the garage which allows the current flow to trip the MCB
2/ There may not be a connection to earth at the house end (garage is TT)

It's all a bit odd. To trip a B32 in 2-3 seconds requires at least 160A - i.e. a Zs something like 1.4Ω. As I see it, a garage TT earth would never be that low, and its extremely unlikely that anything else connected/bonded to the garage earth would provide such a low resistance path to earth.

3/ If there is a connection to earth at the house end, there is a break in the earth conductor between there and the garage end

Again, that would require a <1.4&#937; path to earth at the garage end - again seemingly unlikely.

As others have said a 250V or 500V IR test is essential

Indeed - but if there is such a low resistance fault in the cable, then even a multimeter could probably detect it at '3V'.

I'm also confused by the 20V N-E pd and the corersponding 20V 'deficiency' in the L-N voltage. It would be easy enough to explain that in terms of a low-impedance L-N fault in the cable, but then the disconnection of the CPC would not make any difference.

Kind Regards, John

 

[westie101]

It's all a bit odd. To trip a B32 in 2-3 seconds requires at least 160A - i.e. a Zs something like 1.4&#937;. As I see it, a garage TT earth would never be that low, and its extremely unlikely that anything else connected/bonded to the garage earth would provide such a low resistance path to earth.

That is assuming a constant current flow, which may not be the case.
It is common for us to replace fuses and see no "fault" current for a short time until the fault breaks down and the we see very high currents until the protective device operates.

 

[JohnW2]

It's all a bit odd. To trip a B32 in 2-3 seconds requires at least 160A - i.e. a Zs something like 1.4&#937;. As I see it, a garage TT earth would never be that low, and its extremely unlikely that anything else connected/bonded to the garage earth would provide such a low resistance path to earth.

That is assuming a constant current flow, which may not be the case. It is common for us to replace fuses and see no "fault" current for a short time until the fault breaks down and the we see very high currents until the protective device operates.

I don't doubt any of that, but that surely doesn't alter the fact that at least 160A (i.e. a loop impedance no more than 1.4&#937 would have to flow for the OP to operate as rapidly as the OP has indicated, does it? However, as I said, I'm confused - the OP's experiments seem to suggest that the fault current is flowing to earth, but that would not explain the voltages he measured when all conductors were disconnected, would it?

Kind Regards, John

 

[westie101]

No, there could be zero current for 2 minutes 59 seconds and then fault current flow for 1 second or less as the fault breaks down which operates the MCB.

We are dealing with one that has held up with 400A fuses for 3 weeks before 2 of them operated, it can be the nature of this type of fault.

 

[JohnW2]

No, there could be zero current for 2 minutes 59 seconds and then fault current flow for 1 second or less as the fault breaks down which operates the MCB.

The OP said that the MCB operates within 2-3 seconds (presumably from energisation), not 2-3 minutes. ... but none of this alters what I'm saying about how low the 'Ze' would have to be if, as the experiments suggest, the fault current is going to earth.

Kind Regards, John