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

 

[timhoward]

@Inteificio you have actually pin pointed (or stumbled onto?!) one of the interesting areas where the regs alone don't completely answer the question.
It reminded me of an interesting thread on another forum from years ago which I've attempted to link to.

IET EngX

tinyurl.com

Plugging 800A fault current into the adiabatic with 0.1s disconnection time (the lowest the regs give us) gives a concerning answer, as you identified I think. But you need to read the bit in the box on the graph that says for larger PFCs use manufacturers data.

As above, we need a bit more info from the manufacturer to be reassured that the device will actually operate quicker than 0.1 s.
Manufactures will list a "Rated short circuit breaking capacity", i.e. it will cope with the fault current. At least 6kA usually.
And the manufacturers graphs usually go a lot lower than 0.1s. e.g. this Hager B16:


Plugging 800A into the adiabatic with 0.01s gives you a much more reassuring answer.
Hope that helps.

 

[Julie.]

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

It is probably an area that most forget, but the attachment here should help.

Ignore the "special" side of things, that is marketing, as if the other manufacturers don't do the same - they do!!

If you obtain the characteristics from the mcm manufacturer, it should both show how quick it actually trips, and the i^2t let through, which you can use to check coordination.


EDIT:
the characteristic posted by timhoward is typically of manufacturers' data.

At 40x it is going to be current limiting i.e. operates before the half cycle of 50hz completes - so actually less than 10ms.

Even if it doesn't current limit, it would clear on the first zero crossing, which is 10ms at 50hz & 8.33ms at 60hz

 

[pc1966]

Just as @Julie. says, you really need to get the manufacturer's data for more precise let-through values.

There are generic limits for this energy-limiting class of MCB and you see resulting limit values in, for example, Table B7 of the On-Site Guide. However, they are a touch pessimistic and most folks would just use the "standard circuits" of Table 7.1(i) to see if the combination of fuse or MCB, cable, and length are acceptable.

Here is an example taken from the Hager commercial catalogue technical information section for the B-curve MCB. What you see the the I2t let-through drop slowly as you go to higher thermal overload, then suddenly drop when you hit the "instant" magnetic trip region. Beyond that it increases as the PFC increases since the breaker does not open significantly faster with more overload.

But it does open a little faster, as a true "constant time" opening would be increasing at two decades of I2t for one decade of PFC (due to the I-squared part, with t=constant).

Behold:

 

[pc1966]

You can compute the equivalent "virtual opening time" from the I2t and I values. For example, taking the 6A MCB curve as easier to follow:

• At 30A PFC (Icc) the I2t is about 0.009k hence t = I2t / I^2 = (0.009 * 1000) / (30 * 30) = 0.01 = 10ms
• At 700A the I2t is about 1k hence t = (1 * 1000) / (700 * 700) = 0.002 = 2ms

In reality the opening is not instant after physical movement as it arcs, and there are other aspects to the MCB that limit peak current, but it serves to illustrate that an energy-limiting breaker opens in well under a cycle of the AC supply.

RCD are not energy-limiting in any real sense as they typically take a cycle or two of the mains to open (specification is below 40ms I think) and that is not significantly faster once you get beyond 5*In test current. Great for shock or smouldering fire prevention, but not helping much for a high current fault. In fact many domestic RCD are only rated to open 1.5kA which is below the sort of PFC you can see close to the DB. In these cases the manufacturer usually stipulates it should have a MCB that opens below 1.5kA (which is faster) so the RCD is not the one breaking a high fault current.

Even a 50A D-curve MCB meets this at 20*50A = 1kA but you will hardly see that is use domestically as it is hard to meet 5s disconnection Zs for a sub-main (compared to switched-fuse which is often better at selectivity with downstream MCBs), and even harder to meet the 0.4s for a final circuit.