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

[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