High PFC

[leestaf]

Hi Guys
Any advice would be much appreciated!

I have tested a property that has a PFC greater than the short circuit capacity of the mcb's protecting the final circuits, the PFC is 7.9KA and the 60898 mcb's 6KA. The property is protected at origin with a 1361 type b fuse 16.5KA. Under the regs as i understand them the protective device must have a short circuit capacity greater or at least equal to the PFC, therefore am i wrong in assuming that i advise the client that the mcb's protecting the final circuits are upgraded to 10KA or not?
Would the 1361 fuse with a higher breaking capacity and similar fusing factor to the mcb protect the installation or not, also would this mean a lot of domestic properties that are situated close to substations but with "run of the mill" 6KA mcb's require upgrading??​

 

[Sintra]

The 1361 fuse at the origin will offer sufficient protection against fault current. There is no need to change the MCB's.

 

[leestaf]

Thanks for that one of those wasnt 100% on!

 

[baldsparkies]

Another point worth considering is that your pfc has a linear relationship with your Ze and that will almost always be at the origin of the supply ie at the incomer.
In practice a fault within the instalation itself would never generate such high currents because of factors such as cable resistence ect.
It is highly likely that the mcb would disconnect the supply way before its 6k limit. And as Sintra said, the let through current would be governed by the primary device 1361 fuse at the cutout.

 

[Ladders]

Hi. I know this an old post but have just come across the same problem and the highest reading is 19.9kV. So to comply the breakers MUST be higher than the potential fault current otherwise the breakers will be damaged if they disconnect because of this. Or so I think! Anyone know where the regs say different please?

 

[tonys]

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