MCB melted

[JohnW2]

Highest I ever measured in a domestic installation is 8.1KA

Goodness - that's going some if the service cable was the seemingly very common ~16mm² that DNOs seem to use - by my reckoning, that would require the tranny to be within about 10m of the cutout (a bit more if a little of the run were in a higher-CSA main).

Kind Regards, John

 

[JohnD]

the rated breaking current (or less) can also damage or destroy an MCB. It is only rated to break it once.

 

[Echo the husky]

It was in a flat right behind the Ryefield unit, the transformer was near the building and the supply cable was 400mm² I believe. We had to change the consumer unit to one that accepted 10KA MCBs.

The other flats were all less than 6KA.

 

[JohnW2]

It was in a flat right behind the Ryefield unit, the transformer was near the building and the supply cable was 400mm² I believe.

Ah, that's cheating I wasn't thinking about a 400mm² supply to a pile of flats (of which I have zero experience) but, rather, the 'standard' supply to 'standard' single dwelling! What's the highest PFC you've seen in such a situation?

Kind Regards, John

 

[JohnW2]

the rated breaking current (or less) can also damage or destroy an MCB. It is only rated to break it once.

I believe that's true, but it presents an almost impossible situation - I very much doubt that anyone replaces an MCB every time it operates! That's one advantage of fuses - one has to replace them if they operate

Kind Regards, John

 

[Echo the husky]

What's the highest PFC you've seen in such a situation?

Probably less than 2KA. In Devon many of the houses are some way from the transformer, less than 1KA is very common.

 

[kai]

Flats in Central London can have 20kA PFC Currents Phase-Neutral, because it is common practice in London, to have a substation at the base of the block, dedicated to that block of flats. Looks like a requirement for 20kA (M20) MCB Breakers here, unless one can install a suitable BS88 switch-fuse between the Meter and the CU, to guard against high PFC faults.

 

[JohnW2]

Probably less than 2KA. In Devon many of the houses are some way from the transformer, less than 1KA is very common.

Thanks. That's in keeping with my limited personal experience (of single standard domestic dwellings!). As I said, I have no experience of blocks of flats, or anything commercial/industrial.

Kind Regards, John

 

[JohnW2]

Looks like a requirement for 20kA (M20) MCB Breakers here, unless one can install a suitable BS88 switch-fuse between the Meter and the CU, to guard against high PFC faults.

Such a fuse would surely limit only the duration, not the magnitude, of very high fault currents. Are you suggesting that an MCB can tolerate currents considerably greater than their rated breaking current, so long as it only flows for a very short time (maybe ceasing before the contacts attempt to break the current)?

Kind Regards, John

 

[kai]

A BS88 fuse is often used to protect an incoming feed to a MCB Breaker board, if a high fault current strikes, the upstream backup fuse takes over to clear the fault. That is why consumer unit manufacturers including MK, Crabtree and Wylex state their consumer boards must be fed from a maximum BS88 or similar 100amp fuse, in order to give full performance.

 

[JohnW2]

A BS88 fuse is often used to protect an incoming feed to a MCB Breaker board, if a high fault current strikes, the upstream backup fuse takes over to clear the fault.

I'm not sure what you mean by "takes over" - as I said, the full fault current, whatever that may be, will flow through the MCB until the fuse actually operates. Could one guarantee that, say, a 100A BS88 fuse would interrupt the supply before the contacts of, say, a B6 or B16 started to open, if there were 'thousands of amps' flowing through both?

Kind Regards, John

 

[Iggifer]

A BS88 fuse is often used to protect an incoming feed to a MCB Breaker board, if a high fault current strikes, the upstream backup fuse takes over to clear the fault. That is why consumer unit manufacturers including MK, Crabtree and Wylex state their consumer boards must be fed from a maximum BS88 or similar 100amp fuse, in order to give full performance.

Nearly all supplies are already protected by a ≤100A BS88 anyway......

 

[Robin0577]

the rated breaking current (or less) can also damage or destroy an MCB. It is only rated to break it once.

That isn't quite how it's supposed to work on paper though. There are two values specified by BS EN 60898 for breaking current ratings of circuit breakers, Icn & Ics.

Icn is the rated short circuit capacity, the maximum current the device can break safely but the device may be damaged in the process. This is the rating marked on the device
Ics is the service short circuit capacity, the maximum current the device can break safely "without loss of performance"
For ratings up to 6kA Ics is supposed to equal Icn, for ratings of 10kA and above Ics is often lower than Icn.

In other words, a 6kA device should break a 6kA fault and remain serviceable, whereas a 10kA device may only break 10kA once but should break 7.5kA and remain serviceable.

(Data from IET Guidance Note 3, 17th Ed, Amd. 3 Section 2.6.17 & Table 2.8)

 

[catlad]

Well here are the pictures hopefully of the mcb. The sparky who came and changed them said he thought they were not tightened up enough and there was nothing to worry about.

 

[Iggifer]

The sparky who came and changed them said he thought they were not tightened up enough and there was nothing to worry about.

Just what I said, first reply . Although it was a fairly safe guess, I'm not gonna pat myself too hard on the back. Glad it's sorted though