Breaking capacity of MCBs

[LR1397]

Hi all,

I have a question regarding the breaking capacity of circuit protective devices.

So, the breaking capacity rating of devices in domestic / 230v tends to be 6ka, and for commercial / 3 phase it tends to be 10ka.

Say the PFC at the origin was greater than the rating on the MCBs, and the board was right next to the origin, what circumstances would make this become the issue?

Would it need to be a fault close to or at the terminals of the MCB (e.g. a DP MCB melting due to a loose connection), so that the highest potential current would be flowing at that point? (as opposed to a short circuit fault 20m away from the board, so adding resistance and lowering PFC), or just if the conductors of the circuit were large and therefore could carry a large fault current etc?

If not, what would be an example scenario that would make:

the main PFC > MCB breaking capacity

an issue?

Thanks in advance!

 

[James]

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.

 

[LR1397]

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.

Thanks James!

 

[Julie.]

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

 

[LR1397]

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

Where is that chart from Julie?

Thanks for your help, everything days a school day!

 

[Julie.]

Where is that chart from Julie?

Thanks for your help, everything days a school day!

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

 

[LR1397]

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

Thank you very much!

 

[Julie.]

Hopefully this link will work

https://www.lawsonfuses.com/app/uploads/2021/09/ME-MF-Datasheets.pdf

 

[pc1966]

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.

 

[LR1397]

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