AM2 ring fault-finding headache

[mak]

How would I pinpoint a fault on a ring circuit with very little resistances, and without any visual clues, only by measuring.
Say I have a break in neutral between db and first socket. How would I go about fault finding it?
1. I will use low resistance ohmmeter.
2. I will find that rn is not a ring.
3. I will do the Rn+R2 and whichever socket has the highest resistance the fault has to be in between the two sockets(or db) with lower reading?

Then if I have a short on neutral and earth, again between db and first socket.
1. I will do insulation resistance test between L-N, L-E, E-N on both legs and find the reading on E-N.
2. So the fault is the earth fault between neutral and cpc conductor.
3. But how will I pin point it? With the Rn+R2 test again looking for the highest resistance?

 

[pc1966]

Do you simultaneously have open N ring and N-E short?

If you don't have a wander lead, then for open ring I would short, say, N to L on one leg only, then when you go socket to scoket find L-N goes open you have found where that leg ends.

 

[Burtones]

On the AM2 the ring only has 3 points and all are close enough to the DB to test from any point to another. DB-socket1-FCU-socket2-DB

You only have one fault at a time so if you know rn-rn is open, you can disconnect neutrals at say FCU as you know that is the middle, and test each leg at DB to each leg at FCU to narrow down which side. Then split that side at the socket to see if it's between DB and socket or FCU and socket.

If you're not confident with pc1966's method above this is another way you could quickly find the fault.

 

[mak]

you can disconnect neutrals at say FCU as you know that is the middle, and test each leg at DB to each leg at FCU to narrow down which side.

Which side of what? Of the ring? As in either first point to DB or the last point to DB?

Then split that side at the socket to see if it's between DB and socket or FCU and socket.

Sorry can't follow, what socket are you talking about?

 

[Loki]

How would I pinpoint a fault on a ring circuit with very little resistances, and without any visual clues, only by measuring.
Say I have a break in neutral between db and first socket. How would I go about fault finding it?
1. I will use low resistance ohmmeter.
2. I will find that rn is not a ring.
3. I will do the Rn+R2 and whichever socket has the highest resistance the fault has to be in between the two sockets(or db) with lower reading?

Then if I have a short on neutral and earth, again between db and first socket.
1. I will do insulation resistance test between L-N, L-E, E-N on both legs and find the reading on E-N.
2. So the fault is the earth fault between neutral and cpc conductor.
3. But how will I pin point it? With the Rn+R2 test again looking for the highest resistance?

I've took my AM2S last week & i had the N-E fault on the ring.

First thing i did was took the N&E out of the load side on the fcu for the outdoor socket & then tested using continuity for the short (ir testing was not allowed). I had the N-E short between the outdoor socket & fcu. If i hadn't of found the short then i knew it would of been on the ring.

Open fault on the ring just use end to end continuity checks. Ie remove cables from rcbo the continuity test from db to 1st socket - if ok then that socket to fcu etc


I was told 2-3 weeks for the result on the AM2S

 

[pc1966]

Usually finding an open is easier than finding a short because a simple continuity check reveals the location, where as a 'short' is not always such a low or stable resistance as to be clearly different from the cable resistance between sockets, etc, such that a low-ohm measurement clearly locates it.

As this is an exam, then a wander lead is (probably) out and there are lots of real-world situations where the wander lead is impractical (length of circuit too long, risky to wander around trailing a cable as trip hazard, cables gets caught in fire-doors, etc). So you need to demonstrate that you can systematically locate a fault without having to take everything apart (which would be a time-waste in real life and risks introducing other faults). So basically you have the cable end(s) at the DB to do something (r1/r2/rn checks, etc) and to link so you can then see the open on L-N (or similar) are you go round checking each outlet.

If you have a lot of possible fault locations (say 20 sockets on a ring) and you know the order they are wired in (not always easy in practice) you can sometimes save time by doing a bisection search where you test half-way, and if that shows yes/no, you then look at 1/4 or 3/4 for the next test, etc.

Sometimes you can clearly see the cable resistance changing on something like a short, so if you check ohms on N-E at each socket it will be at a minimum at (or at least close to) the fault. Otherwise as said above, sometimes you have to deliberately open the circuit at, say, mid-way and see which side the short is located on, etc.

 

[Burtones]

Which side of what? Of the ring? As in either first point to DB or the last point to DB?

The ring yes, the fault finding bay is wired the same as the install you have to do so, DB to socket 1, then to FCU, then to socket 2, then back to DB.

Sorry can't follow, what socket are you talking about?

What I mean is, if you've split the neutrals at the FCU then one leg at the FCU to one leg at the DB will have continuity, the other will be open. Then you know that whatever side the fault is on, it can only be between the FCU and the next socket or the next socket and the DB.

Would be easier to follow with a diagram than trying to explain but either way, it's only a small circuit so easy to trace whatever method you use.

 

[nicebutdim]

3. But how will I pin point it? With the Rn+R2 test again looking for the highest resistance?

David Savery put this video out a while back, where he applied mathematics to resistance readings in order to locate a fault in a buried cable.

I wont spoil it by posting details, but it's well worth a watch. Relevant section is 15ish minutes in if you don't want to watch it all.

View: https://youtu.be/GlUlSQrneCU

 

[mak]

David Savery put this video out a while back, where he applied mathematics to resistance readings in order to locate a fault in a buried cable.

I wont spoil it by posting details, but it's well worth a watch. Relevant section is 15ish minutes in if you don't want to watch it all.

View: https://youtu.be/GlUlSQrneCU

Thanks. That's what I'm talking about. If I will test resistance from both ends the fault will be on the side of least resistance. So not only I will locate the fault on the conductor but also on which side/half.

 

[catdog1121]

If you get in trouble the easiest was in your AM2 because there are only about 3 sockets just disconnect all sockets, separating all conductors, then just test each conductor from each point until you will likely find something is open circuit.

I think with mine if I remember there was a spur and it was the neutral of the spur, it meant that the end to end was fine but when did R1+Rn there was one socket that was open circuit.

 

[mak]

If you get in trouble the easiest was in your AM2 because there are only about 3 sockets just disconnect all sockets, separating all conductors, then just test each conductor from each point until you will likely find something is open circuit.

I think with mine if I remember there was a spur and it was the neutral of the spur, it meant that the end to end was fine but when did R1+Rn there was one socket that was open circuit.

I had mine a few weeks ago and failed, unsurprisingly, but I remember the layout. I already know what I haven't done, I should have split the circuit in the middle like, ring, lighting, etc. and test from there! Need to practice more with 3-phase and have a second look at s-plan.
[automerge]1598642173[/automerge]

I've took my AM2S last week & i had the N-E fault on the ring.

First thing i did was took the N&E out of the load side on the fcu for the outdoor socket & then tested using continuity for the short (ir testing was not allowed). I had the N-E short between the outdoor socket & fcu. If i hadn't of found the short then i knew it would of been on the ring.

Open fault on the ring just use end to end continuity checks. Ie remove cables from rcbo the continuity test from db to 1st socket - if ok then that socket to fcu etc


I was told 2-3 weeks for the result on the AM2S

I did mine and failed. How was the s-plan? Is the pass really 5 out of 7 as they say?

 

[Loki]

I had mine a few weeks ago and failed, unsurprisingly, but I remember the layout. I already know what I haven't done, I should have split the circuit in the middle like, ring, lighting, etc. and test from there! Need to practice more with 3-phase and have a second look at s-plan.
[automerge]1598642173[/automerge]

I did mine and failed. How was the s-plan? Is the pass really 5 out of 7 as they say?

Its now done on a point system.

You gets points for the fault. Ie short, open circuit, high resistance, mis connection

Then same with location & rectification

But if at any point you get it wrong you don't get any more points from that point onwards (no point pun intended)

Say fault was a short & you put high resistance then you get no points but even if you got the correct location & rectification you wold still get no points for them either.

 

[mak]

Its now done on a point system.

You gets points for the fault. Ie short, open circuit, high resistance, mis connection

Then same with location & rectification

But if at any point you get it wrong you don't get any more points from that point onwards (no point pun intended)

Say fault was a short & you put high resistance then you get no points but even if you got the correct location & rectification you wold still get no points for them either.

That makes sense.

 

[Lister1987]

Its now done on a point system.

You gets points for the fault. Ie short, open circuit, high resistance, mis connection

Then same with location & rectification

But if at any point you get it wrong you don't get any more points from that point onwards (no point pun intended)

Say fault was a short & you put high resistance then you get no points but even if you got the correct location & rectification you wold still get no points for them either.

Any idea how they introduce the faults? Loose wires are easy enough, I'm thinking other faults? Stack or back to back backboxes and the fault (read; resistor) placed in circuit in the first box then the second is wired as normal

 

[Loki]

AFAIK they take the circuit to like a panel box with switches in it . Their they can simulate an on circuit with a switch, for a high resistance they would add a resistor, for a short they could use a 2 way light switch - one side clear other side shorted.

You could make up your own test rig board?