Breaking capacity of MCBs

[pc1966]

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.

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:

 

[Julie.]

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

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.

 

[Simon47]

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.

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.

 

[pc1966]

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!

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!