My first EICR and I'm confused! I'm industrial this is domestic

[Inteificio]

All the sockets are fine.
When separating the legs I've identified which sockets are on which side.

Fortunately as only one ring socket is on one leg I'm pretty certain the fault lies between that socket and another. Finding the 'another' will be a faf.

My next step is to remove all the ring socket covers and separate the cables. A bit of long leading the N will help find the dodgy leg.

I can then remove that one cable from each end leaving (hopefully) two spurs.

What a faf.

I hope the actual exam isn't this bad or I'm going to run out of time massively!

 

[brianmoooore]

If the fault is near to one end of the RFC, you are going to end up with a spur that loops back on itself, so are going to end up with a spur that uses much more cable than one that was designed as a spur from the start.
You will need to check that loop impedance and voltage drop are all acceptable, and access the likely maximum load of the circuit, to ensure it doesn't exceed the rating of the smaller MCB.

 

[UNG]

All the sockets are fine.
When separating the legs I've identified which sockets are on which side.

Fortunately as only one ring socket is on one leg I'm pretty certain the fault lies between that socket and another. Finding the 'another' will be a faf.

My next step is to remove all the ring socket covers and separate the cables. A bit of long leading the N will help find the dodgy leg.

I can then remove that one cable from each end leaving (hopefully) two spurs.

What a faf.

I hope the actual exam isn't this bad or I'm going to run out of time massively!

Rather confused by the fact that you have identified what sockets are on each side of the fault but then you say you haven't and that finding the other socket on the ring will be a faf

I would suggest buying a cheap continuity tester https://cpc.farnell.com/tenma/ten01049/continuity-tester/dp/IN05723 rather than spending a lot of time removing all the sockets

With the continuity tester you can work along one leg of the ring quite quickly and identify where the break is and then do the same on the other leg which should bring you to a point where you can then diagnose the cause of the fault and repair the ring

 

[pc1966]

The zs/rn readings are so low that they are useless for analysis.

What are you using to measure things?

Generally speaking Zs measurements are not that precise, not when you get down to under an ohm and more so if "no trip" due to RCD action so testing at very small L-E currents, so you are unlikely to get far finding a RFC break on that alone.

However, DC low ohm measurements using a typical MFT or similar usually are good enough but you have to allow for dirty switch contacts and socket pins so sometimes you have to insert/remove the test plug adaptor (or lead) a couple of times and operated the switch a could of times to get a low stable reading.

Even then you are likely to only reliably see differences in cable distance of several meters as, for example 2.5m T&E has R1+R2 of about 0.02 ohm/meter and (I guess) most low ohm meters likely to be at hand resolve around 0.01 ohm.

 

[pc1966]

I would suggest buying a cheap continuity tester https://cpc.farnell.com/tenma/ten01049/continuity-tester/dp/IN05723 rather than spending a lot of time removing all the sockets

I don't think that is CAT rated, so I would rather not use it on anything that might be live!

Maybe a cheap voltage tester that has a continuity buzzer?

 

[UNG]

I don't think that is CAT rated, so I would rather not use it on anything that might be live!

Maybe a cheap voltage tester that has a continuity buzzer?

So is he is going to disconnect all the sockets while they are live, I think that would be a no

Given the OP is an industrial electrician I would like to think he would be able to use a simple continuity tester to locate a simple ring circuit fault

 

[Inteificio]

I'm using a Megger mft1721.
I've also got my fancy multimeter for precise testing.

You're right, the Zs is useless for this, I'll do a dc test as that may have more accuracy. Good tip, thanks.

What I'm hoping is that the failure is directly between two ring sockets. All I need to do is identify the cable between and disconnect it at both ends.
If that is the case then I know which socket is on one side. I don't know where the next one in the supposed ring is (there are 3 probables).
Unfortunately I don't know where the spurs cut in, which adds a wild card.



Hopefully the accuracy of the r1rn dc test will give clues.

As for splitting the ring, my whole house was using 14A in total. This circuit is just skirting board sockets. Unless I suddenly start loving electric room heaters the load will be limited.

Now the question is do I fit the b16 from my spares, or go buy a b20


P.s. That's a good tip about turning the sockets off before splitting them too. I wish I'd known that last night as the burns on my hands are really smarting this morning

 

[UNG]

I'm using a Megger mft1721.
I've also got my fancy multimeter for precise testing.

You're right, the Zs is useless for this, I'll do a dc test as that may have more accuracy. Good tip, thanks.

What I'm hoping is that the failure is directly between two ring sockets. All I need to do is identify the cable between and disconnect it at both ends.
If that is the case then I know which socket is on one side. I don't know where the next one in the supposed ring is (there are 3 probables).
Unfortunately I don't know where the spurs cut in, which adds a wild card.



Hopefully the accuracy of the r1rn dc test will give clues.

As for splitting the ring, my whole house was using 14A in total. This circuit is just skirting board sockets. Unless I suddenly start loving electric room heaters the load will be limited.

Now the question is do I fit the b16 from my spares, or go buy a b20


P.s. That's a good tip about turning the sockets off before splitting them too. I wish I'd known that last night as the burns on my hands are really smarting this morning

While splitting the ring is an easy fix and while it is very rare that a cable is at fault I think actually locating / repairing the fault would be more beneficial from a learning point of view for you

 

[pc1966]

So is he is going to disconnect all the sockets while they are live, I think that would be a no

Given the OP is an industrial electrician I would like to think he would be able to use a simple continuity tester to locate a simple ring circuit fault

Here, no it should be fine to use that kit.

It is more of a general dislike on my behalf of having test equipment that is not mains-fault rated in use where mains is about. Sooner or later we all make a mistake and probe the wrong circuit (or discover they are mislabelled somewhere) that is live, and then having something that does not explode on you is nice.

 

[Inteificio]

Here, no it should be fine to use that kit.

It is more of a general dislike on my behalf of having test equipment that is not mains-fault rated in use where mains is about. Sooner or later we all make a mistake and probe the wrong circuit (or discover they are mislabelled somewhere) that is live, and then having something that does not explode on you is nice.

I think we all knew what you meant.

Exploding kit aka audiovisual alert.

 

[Inteificio]

OK we have progress, but more questions.

So please humour the newbie =)

The dc measurement is more accurate so I pinned the end of the other ring to the Pc socket.
I disconnected the wires at the back of the relevant sockets. Long lead the n to confirm the link. Then r1r2 the other legs to check they were fine.

Sorted, b16 is in! I'll get the b20 if I nuisance trip. (can you guess which county I live in).

So eicr question.

What would you code the discolouration of the (original) mcb. I'd go c3 as the problem is identified and repaired.

I've also run the calcs on the single spur originally for cpc.

Now the earth fault current is 800a, which is too much for a 1.5mm mcb. However as there is rcd protection am I allowed to use the 40ms or actual trip time to calculate cable overheat?

However a l-n short will give a 1.2kA fault current. The circuit will trip in 0.1s

Unless I've got the formula wrong (or totally misunderstood) minimum CSA needs to be over 3.2....

S>{root(i²t)} /k

Have I got this wrong?

Thanks

 

[Julie.]

OK we have progress, but more questions.

So please humour the newbie =)

The dc measurement is more accurate so I pinned the end of the other ring to the Pc socket.
I disconnected the wires at the back of the relevant sockets. Long lead the n to confirm the link. Then r1r2 the other legs to check they were fine.

Sorted, b16 is in! I'll get the b20 if I nuisance trip. (can you guess which county I live in).

So eicr question.

What would you code the discolouration of the (original) mcb. I'd go c3 as the problem is identified and repaired.

I've also run the calcs on the single spur originally for cpc.

Now the earth fault current is 800a, which is too much for a 1.5mm mcb. However as there is rcd protection am I allowed to use the 40ms or actual trip time to calculate cable overheat?

However a l-n short will give a 1.2kA fault current. The circuit will trip in 0.1s

Unless I've got the formula wrong (or totally misunderstood) minimum CSA needs to be over 3.2....

S>{root(i²t)} /k

Have I got this wrong?

Thanks

What do you mean by 1.5mm^2 mcb?

Do you have a 1.5mm^2 cable in a 20A mcb, and a perspective fault current of 800A?

If so, the trip time will be in the current limiting area of the characteristic less than 0.01 sec, this should be well within the adiabatic range of 1.5mm^2

You should not code something that doesn't exist, therefore you cannot put C3 against a mcb which is no longer there. Just put a note that the damaged mcb and ring has been repaired.

 

[brianmoooore]

Are you saying that the original 32A MCB is showing signs of overheating? Why? If it is because the load of the RFC exceeds 32A for long enough periods to cause problems, then reducing the MCB to 16A or 20A is only going to cause more problems.

 

[Inteificio]

I'm not exactly sure why the MCB looked browned. My loose guess was that as the ring was split, but still with a 32A breaker on it, the wires could be getting very hot. e.g taken to the limit, one leg has 30A of kit on it, but is functionally a 20A, then that wire would get hot without tripping.

I've replaced overheating, with potentially nuisance tripping. I'm more than happy with that exchange.

Am I understanding what you are saying?

 

[brianmoooore]

You'll be replacing a C2 under one reg. with a C2 under another.

 

[Inteificio]

What do you mean by 1.5mm^2 mcb?

Do you have a 1.5mm^2 cable in a 20A mcb, and a perspective fault current of 800A?

If so, the trip time will be in the current limiting area of the characteristic less than 0.01 sec, this should be well within the adiabatic range of 1.5mm^2

You should not code something that doesn't exist, therefore you cannot put C3 against a mcb which is no longer there. Just put a note that the damaged mcb and ring has been repaired.

Yeah re-reading what I wrote is as clear as mud!

What I meant with the "1.5mm MCB":

The offending spur is 2.5 T+E on a B20, backed by an RCD.

My understanding is an MCB with sufficient fault current will disconnect in 0.1s. regs p370 (though you suggest 0.01? why?).

With my understanding, an 800A earth fault current will overheat a 1.5mm CPC in 0.1s.

However, the RCD would trip much faster than the 0.1s (can I use the actual of 17ms or the 40ms from minimum)?

So for the CPC overheating, I'm OK (please confirm).

What I'm wondering about is the potential for a short circuit at that socket.

The short circuit would be 2.5/2.5 and 1.2kA.

That would trip in the 0.1s and still cook.



The C3 was more of a theoretical. If I had repaired the ring and left the original breaker, as we have a probable cause for the browning that had been repaired, would it be a C3 for brown?
As that breaker is in the bin the question is moot, it was just a wondering?

 

[Inteificio]

question removed due to muppetness

 

[Inteificio]

You'll be replacing a C2 under one reg. with a C2 under another.

Care to expand?

 

[Julie.]

Yeah re-reading what I wrote is as clear as mud!

What I meant with the "1.5mm MCB":

The offending spur is 2.5 T+E on a B20, backed by an RCD.

My understanding is an MCB with sufficient fault current will disconnect in 0.1s. regs p370 (though you suggest 0.01? why?).

With my understanding, an 800A earth fault current will overheat a 1.5mm CPC in 0.1s.

However, the RCD would trip much faster than the 0.1s (can I use the actual of 17ms or the 40ms from minimum)?

So for the CPC overheating, I'm OK (please confirm).

What I'm wondering about is the potential for a short circuit at that socket.

The short circuit would be 2.5/2.5 and 1.2kA.

That would trip in the 0.1s and still cook.



The C3 was more of a theoretical. If I had repaired the ring and left the original breaker, as we have a probable cause for the browning that had been repaired, would it be a C3 for brown?
As that breaker is in the bin the question is moot, it was just a wondering?

Check the characteristics from a manufacturer, the 0.1s trip time is the maximum once you start to get into the "instantaneous " portion of the curve.

However 800A is around 40x mcb rating actually within the current limiting portion of the characteristic. To limit the current it has to operate within a half cycle of the ac wave - so must be less than 10ms

 

[Inteificio]

Check the characteristics from a manufacturer, the 0.1s trip time is the maximum once you start to get into the "instantaneous " portion of the curve.

However 800A is around 40x mcb rating actually within the current limiting portion of the characteristic. To limit the current it has to operate within a half cycle of the ac wave - so must be less than 10ms

Hi,

I've never actually come across this energy limiting class being used before. I only knew it existed as I wondered what the 3 on the front of the mcb was =)

Google was not kind with this. Its been discussed quite rarely and most of the time it's been abusing the guy asking the question, not helping! We've all got to learn sometime!

So Schneider puts class 3, 6k b20 as 45kA

Is it is simple as 4500=1200²xs.

S being the disconnection time?

Or have I guessed that all wrong?

If I am wrong anyone know anywhere that explains?

Thanks