Open circuit on a ring main.

[Joseph11]

Hi All!
Officially losing my EF forum virginity with this one, and would greatly appreciate any help you guys could throw my way!

Through Covid-19 and no work currently being undertaken, this would seem like the best opportunity to finish the portfolio for my qualification. I am a 3rd year apprentice and a little green when it comes testing and fault finding.

My issue,
I am assigned to write about a fault, how I found it, and obviously the necessary steps to rectify it (How the exam board would like to see) but also for my own knowledge and understanding.
My circuit is a ring main wired in T+E and there are a total of 12 sockets on the circuit.

The fault I have chosen is an open circuit on the line conductor, cable has come out of terminal. So far I have explained that I found my fault by carrying out an end to end continuity test of all conductors, I received continuity results on my rn and my r2 but nothing on my r1 conductor thus indicating an open circuit...
I understand splitting the ring in half (Disconnecting all cables from socket 6) creating two radials and then testing down each leg to identify the faulty leg. This is the part im confused with, how would I physically test continuity of these two legs, Im really struggling to envision the steps from this point and how and where I would test from to break down the circuit to pin point where my fault is.
I've read so many different forums and still not getting any clarity, I appreciate any help you guys may offer!

stay safe,
Joseph

 

[telectrix]

you have a choice.

1. R1+R2..... test at each socket with L and E shorted out at the CU/DB,
2. R1 using wander lead.

 

[Andy78]

As above, measure continuity using linked conductors or a long lead. Once the faulty leg is identified then split again into half and repeat the process to cut down on fault finding time.
Also if this is used for an assignment correct terminology should be used, so ring final circuit not ring main.

 

[Joseph11]

Got it!! Thank you for your time!

 

[Lucien Nunes]

I understand splitting the ring in half (Disconnecting all cables from socket 6)

It's not necessary to disconnect anything in this scenario because the line conductor is already split into two legs at the open-circuit. If you run the wander lead from one line leg only at the CU, or bridge only one line leg and CPC at the CU (whichever of the two methods you prefer to use) and search, you will find two consecutive sockets one of which has continuity and one of which has not.

Since you have not made any mention of any of the sockets not functioning, as a householder or building user might have reported, I'm assuming that the socket from which one line leg is disconnected, still has a working connection to the other leg. Therefore, when you search, if your wander lead goes back to the leg that is still connected, the continuity will appear to stop not at this socket but the next one along, at the far end of the disconnected cable. Thus, once you have located the pair of sockets either side of the open-circuit, the fault could turn out to be in either of them.

It's necessary to split a circuit for testing when the nature of the fault is such that it shows up in 'both directions', e.g. a short or bad insulation.

 

[pc1966]

As already said, for an open on the ring you don't have to change anything, just to search for the point where it changes.

Now if you knew the order of wiring the fastest approach is a "bisection search" where you test half-way and then know is it in the first or second half, and say you find it is 1st half, so you test at 1/4 to see if it is below (i.e. 0...0.25) or above there (0.25...0.5), etc.

With 16 sockets it would take 4 tests at most, 32 sockets it would take 5, etc.

But in practice you can't because chances of knowing the actual order of wiring are pretty low! So probably you just go systematically round until you find the break point, then check from the other ring end to confirm you have really found it and not skipped a few due to some obscure route the installer took.

 

[Pretty Mouth]

This is how I like to find this kind of fault. It has the advantage of only needing to visit each socket once, and not having to use the long lead.

At the CU, on the 1st leg short L - N, and on the 2nd leg short L - E. At each socket, test continuity between L and N, and between L and E, and record the results.

For sockets on leg 1, L - E will be open circuit, while L-N will have continuity, the resistance increasing the further from the CU they are. Likewise, for sockets on leg 2, L-N will be OC, while L - E will have continuity, again resistance increasing the further down the leg they are.

Discounting spurs, your fault will be between the 2 sockets furthest along each leg.

This method won't work in all cases. If there is a parallel earth path between say socket 1 and socket 12 then this will mess up your L - E measurements. You would recognise this situation by an unexpectedly low r2 reading.

 

[telectrix]

This is how I like to find this kind of fault. It has the advantage of only needing to visit each socket once, and not having to use the long lead.

At the CU, on the 1st leg short L - N, and on the 2nd leg short L - E. At each socket, test continuity between L and N, and between L and E, and record the results.

For sockets on leg 1, L - E will be open circuit, while L-N will have continuity, the resistance increasing the further from the CU they are. Likewise, for sockets on leg 2, L-N will be OC, while L - E will have continuity, again resistance increasing the further down the leg they are.

Discounting spurs, your fault will be between the 2 sockets furthest along each leg.

This method won't work in all cases. If there is a parallel earth path between say socket 1 and socket 12 then this will mess up your L - E measurements. You would recognise this situation by an unexpectedly low r2 reading.

like it. yous not just a pretty face mouth then?

 

[G1ZQC_David]

Call me old fashioned if you like.
Disconnect Line & Neutral from the board, split the ring somewhere convenient, apply a 12v supply to the conductors, make up a gizmo with a 13a plug, a short lead to a 12v bulb.
Plug it in every socket on the ring, (having made sure there is NOT more than one ring), bulb will light till it doesn't, apply same method on the other half of the split ring, hey presto, we have a break between, or at the socket. Hope the break is not buried in a wall.

 

[James]

Call me old fashioned if you like.
Disconnect Line & Neutral from the board, split the ring somewhere convenient, apply a 12v supply to the conductors, make up a gizmo with a 13a plug, a short lead to a 12v bulb.
Plug it in every socket on the ring, (having made sure there is NOT more than one ring), bulb will light till it doesn't, apply same method on the other half of the split ring, hey presto, we have a break between, or at the socket. Hope the break is not buried in a wall.

Presumabley testing all sockets are dead before carrying out this test?
otherwise 12v lamp may well explode in your hand when you find you have plugged it into a live 230v socket fed from another breaker.

 

[G1ZQC_David]

As I said above
(having made sure there is NOT more than one ring).
But then WHO plays with a live circuit anyway, whilst "dead testing".?!

Should have gone to Specsavers.!!