Fault Finding Brain Teaser

[SparkyChick]

Trainees... need some help...

I got called out the other night by a lady complaining she had no power through out the house. I attended and this is what I found...

The circuit in question supplied a number of sockets, both upstairs and downstairs. With the power on, some sockets tested were showing L-N 230v, L-E 230v, N-E continuity but some were showing L-N continuity, L-E 230v, N-E 230v.

The circuit was a ring circuit, mixed imperial/metric cable on a 32A breaker. I couldn't find the issue but I did leave it safer (there were three circuits, a ring in the kitchen on a 32A breaker, the circuit with the fault and a single double socket outlet on a 16A breaker). How could I adjust things to leave it safer than it was before I arrived?

I need to know how many faults I'm looking for and what tests I should do to help me narrow the fault down and assuming I fix it, what tests I should do to ensure it's safe for continued use.

Please put your answers in a spoiler section so people can choose to read them or not. But, I need to know how to proceed before I get back there next week

 

[Jay 1987]

Trainees... need some help...

I got called out the other night by a lady complaining she had no power through out the house. I attended and this is what I found...

The circuit in question supplied a number of sockets, both upstairs and downstairs. With the power on, some sockets tested were showing L-N 230v, L-E 230v, N-E continuity but some were showing L-N continuity, L-E 230v, N-E 230v.

The circuit was a ring circuit, mixed imperial/metric cable on a 32A breaker. I couldn't find the issue but I did leave it safer (there were three circuits, a ring in the kitchen on a 32A breaker, the circuit with the fault and a single double socket outlet on a 16A breaker). How could I adjust things to leave it safer than it was before I arrived?

I need to know how many faults I'm looking for and what tests I should do to help me narrow the fault down and assuming I fix it, what tests I should do to ensure it's safe for continued use.

Please put your answers in a spoiler section so people can choose to read them or not. But, I need to know how to proceed before I get back there next week

Spoiler: attempt 1

Hey Sparky Chick,

here's my attempt.....

I think the voltage indicator is measuring continuity between line and neutral because they are at the same potential, 230v. Therefore the voltage indicator will not be able to read any differences in voltage, except for continuity.

If this is the case, it would then make sense that you were getting a voltage potential of 230 v between neutral and earth. Neutral being connected to 230 v and earth being connected to 0 v.

In regards to what tests to carry out, I would say Continuity of ring main conductors connecting at the board and testing at every socket, to narrow down the faults via the readings, i would also do insulation resistance to check the integrity of the insulation perhaps that may indicate a low reading and show a interconnection through some form of interconnection or thermal damage, not 100% sure though.

To leave it safer than when you arrived.... hmm not sure about that. Assuming the ring is continuous I'm not sure if it would be safe to isolate the problematic sockets and keep the rest of the ring in service. So with the limited knowledge i have now, i would err on the side of caution and isolate the whole circuit, you mentioned that you would re arrange things around but I'm not sure tbh.


In regards to tests to complete before re-commissioning the circuit i would say same tests as for the fault finding. In addition i would do earth fault loop impedance to make sure my safety devices will have a sufficient current to trip and that the loop impedance zs = ze + (R1+R2) is compliant for the installations earthing system.

Edited to make the spoiler work (SC)

 

[FatAlan]

Bit confused SC. Initially you say no power throughout the house. Then three socket circuits, one of which has the fault. How is the fault manifesting? One RCD tripping and knocking out all the circuits or just the OCPD for the faulty circuit?

 

[SparkyChick]

Bit confused SC. Initially you say no power throughout the house. Then three socket circuits, one of which has the fault. How is the fault manifesting? One RCD tripping and knocking out all the circuits or just the OCPD for the faulty circuit?

Re-reading it, it could be a little unclear.

There were three socket circuits... a ring circuit serving the kitchen on a 32A MCB, a ring circuit serving the rest of the house on a 32A MCB (this is the circuit with the fault on it) and a radial socket circuit supplying a single socket outlet in the kitchen, used by the tumble drier, on a 16A MCB.

I left the installation safer than it was when I arrived, whilst maintaining power to all socket outlets that had it previously by rearranging these circuits with respect to which OCPD they were connected to. How did I do it?

 

[King84]

Trainees... need some help...

I got called out the other night by a lady complaining she had no power through out the house. I attended and this is what I found...

The circuit in question supplied a number of sockets, both upstairs and downstairs. With the power on, some sockets tested were showing L-N 230v, L-E 230v, N-E continuity but some were showing L-N continuity, L-E 230v, N-E 230v.

The circuit was a ring circuit, mixed imperial/metric cable on a 32A breaker. I couldn't find the issue but I did leave it safer (there were three circuits, a ring in the kitchen on a 32A breaker, the circuit with the fault and a single double socket outlet on a 16A breaker). How could I adjust things to leave it safer than it was before I arrived?

I need to know how many faults I'm looking for and what tests I should do to help me narrow the fault down and assuming I fix it, what tests I should do to ensure it's safe for continued use.

Please put your answers in a spoiler section so people can choose to read them or not. But, I need to know how to proceed before I get back there next week

As you stated on some sockets you were getting reading of N-E230V which suggests there is L-N reverse polarity thats why the voltage appeared which should not have appeared.
To narrow down the fault, on the same circuit breaker, I would take R1+Rn or R1+R2 . Where the fault persist as reverse polarity, The readings wont be shown so the point of fault will be located and can be rectified.
In terms of making the place safer before you left, Once the reverse polarity has been recitified, Zs values should be taken and ensure they are under 80% to comply with Bs 7671.

 

[SparkyChick]

230v N-E could also indicate a broken neutral with at least 2 breaks... you just happen to be measuring the voltage within the break with something plugged in and turned on.

One of the key points about this question was 'How could I make the installation safer temporarily whilst it was waiting for a fix'. What I did in this case was move the broken ring final circuit onto the 16A circuit breaker and I made the single socket supplied by the 16A breaker a spur from the other ring circuit.

On the testing front... insulation resistance and continuity were the weapons of choice. Continuity was applied in numerous ways (end to end ring continuity, conductor to conductor continuity, modified figure 8 check - with only one end of the ring linked).