Breaking capacity of MCBs

Any installation where the prospective fault current is higher than the protective device can handle is a danger.
generaly it will be a short circuit fault, phase to N or phase to phase that will cause the highest current.

what you are trying to avoid by having breakers of sufficient breaking capacity is either them not disconnecting quickly enough because they can’t extinguish the arc as the contacts open and/or the device literally blowing up and scattering molten copper inside and outside of the enclosure.

James said:

Any installation where the prospective fault current is higher than the protective device can handle is a danger.
generaly it will be a short circuit fault, phase to N or phase to phase that will cause the highest current.

what you are trying to avoid by having breakers of sufficient breaking capacity is either them not disconnecting quickly enough because they can’t extinguish the arc as the contacts open and/or the device literally blowing up and scattering molten copper inside and outside of the enclosure.

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Thanks James!

A PFC higher than the breaking capacity of the MCB/MCCB isn’t always an issue.

If there are fuses at the incomer, these do have the ability to limit the current.

The term perspective fault current is that level of current that would flow if nothing interrupted the current. If there is a high enough current, the fuse will blow and interrupt the current before it actually gets to the perspective value.



Take this example - for the standard incoming fuses used in residential properties.

On the left is the perspective value, take say 10kA, this would be higher than the normal MCB breaking capacity of 6kA.

However, looking across the 10kA line, it hits the 60A fuse curve at 5kA (measure on the bottom scale), thus a 6kA capacity MCB would actually be ok for a 10kA PFC if there is a 60A fuse fitted by the supplier.

However, if this was replaced with a 80A fuse, then the through fault level is just over 6kA, so a 6kA capacity MCB wouldn't be safe.

If the PFC was 9kA, then a 6kA MCB would be ok with a 80A fuse, put a 100A fuse, and again 9kA PFC would result in too much through current for a 6kA MCB (a little over 7kA) etc.

So a PFC higher than a MCB breaking capacity could be ok - but it involves some detailed investigation

Julie. said:

A PFC higher than the breaking capacity of the MCB/MCCB isn’t always an issue.

If there are fuses at the incomer, these do have the ability to limit the current.

The term perspective fault current is that level of current that would flow if nothing interrupted the current. If there is a high enough current, the fuse will blow and interrupt the current before it actually gets to the perspective value.

View attachment 107244

Take this example - for the standard incoming fuses used in residential properties.

On the left is the perspective value, take say 10kA, this would be higher than the normal MCB breaking capacity of 6kA.

However, looking across the 10kA line, it hits the 60A fuse curve at 5kA (measure on the bottom scale), thus a 6kA capacity MCB would actually be ok for a 10kA PFC if there is a 60A fuse fitted by the supplier.

However, if this was replaced with a 80A fuse, then the through fault level is just over 6kA, so a 6kA capacity MCB wouldn't be safe.

If the PFC was 9kA, then a 6kA MCB would be ok with a 80A fuse, put a 100A fuse, and again 9kA PFC would result in too much through current for a 6kA MCB (a little over 7kA) etc.

So a PFC higher than a MCB breaking capacity could be ok - but it involves some detailed investigation

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Where is that chart from Julie?

Thanks for your help, everything days a school day!

LR1397 said:

Where is that chart from Julie?

Thanks for your help, everything days a school day!

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It's from the lawson LV fuse catalogue.

It's too big to upload, but here is another catalogue showing the same sort of curves

Julie. said:

It's from the lawson LV fuse catalogue.

It's too big to upload, but here is another catalogue showing the same sort of curves

https://www.google.com/url?sa=t&source=web&rct=j&url=https://espm.co.uk/BS88%2520fuse%2520info.pdf&ved=2ahUKEwiyy4KFjYf-AhWREMAKHQCgD0c4HhAWegQIChAB&usg=AOvVaw3DCEmjYCBQDfj84K5XXUod

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Thank you very much!

Hopefully this link will work

For the domestic situation the CU is certified by the manufacturer to be safe in conjunction with the usual DNO fuse range. Basically as @Julie. has described the fuse has good current-limiting characteristics so the PFC can be higher than the MCB as under a big fault it will start to blow and so limit how much the MCB has to interrupt.

Sometimes you will see tables of "cascading values" that provide the breaking limit of a given fuse and MCB combination and they can be used for other situations to decide if a circuit is going to be safe under worst-case fault conditions.

Thanks for your help so far everyone, the more info on any given topic the better!

Julie. said:

However, if this was replaced with a 80A fuse, then the through fault level is just over 6kA, so a 6kA capacity MCB wouldn't be safe.

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I always though the MCB was rated at 6kA symmetric PFC, in which case that is close to 14kA peak, in which case a 100A fuse is limiting to that up to around 50kA symmetric, making some crude assumptions:

pc1966 said:

I always though the MCB was rated at 6kA symmetric PFC, in which case that is close to 14kA peak, in which case a 100A fuse is limiting to that up to around 50kA symmetric, making some crude assumptions:

View attachment 107249

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Yes there is a difference between the two, unfortunately the info in the MCB, and fuse manufacturer's data isn't always clear - so I just kept it simple, if I was doing the checking myself I would ensure I got the data. Especially the actual peak Asymmetrical to rms symmetrical ratio of the MCB, I haven't looked at the most recent standards for MCBs and RCBOs - last time I actually looked at the proper standard it was the old standards!

It's somewhat involved when you get into the details, and it was late last night!

I thought the post was too long as it was.

pc1966 said:

For the domestic situation the CU is certified by the manufacturer to be safe in conjunction with the usual DNO fuse range. Basically as @Julie. has described the fuse has good current-limiting characteristics so the PFC can be higher than the MCB as under a big fault it will start to blow and so limit how much the MCB has to interrupt.

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To expand on this a bit, the BS EN standard for type testing distribution boards takes this into account - so the tests are done with a fuse in the supply appropriate to the setup being tested. So basically, the manufacturer has taken account of the fuse and this is how the devices can have a breaking capacity lower than the PFC.

As the anally retentive and inquisitive person I am, I once read the standard. It covers much more than domestic CUs, and covers testing for higher current boards and cascaded boards (i.e. where you have a main board, with outgoing curcuits feeding smaller boards which feed loads).

Actually, it was regarding other things, but I found myself looking at more than I went looking for. What I was looking for was mythbusting things like "must have a cover that closes on it's own" and hence why they are all hinged at the top these days (not true), "must retain fire" so contains the ensuing fire when a hamfisted smart meter fitter doesn't re-torque the incomer terminals (not true), and a couple of other things.

Julie. said:

Yes there is a difference between the two, unfortunately the info in the MCB, and fuse manufacturer's data isn't always clear - so I just kept it simple, if I was doing the checking myself I would ensure I got the data. Especially the actual peak Asymmetrical to rms symmetrical ratio of the MCB, I haven't looked at the most recent standards for MCBs and RCBOs - last time I actually looked at the proper standard it was the old standards!

Click to expand...

Indeed the "correct" answer is probable between those two cases (assume MCB is peak-rated, assume MCB is symmetric rated but peak is in keeping with fuse's resulting peak, etc) but quite device-specific.

Which I guess is why we have the CU certification process!