High PFC -

tallpault

Looking for a bit of advice going at the back of a company who carried out a periodic in a shopping centre.
In the observations they have listed as a code 1 that the Ka rating of the MCBs are unsuitable for all the circuits in the DB as they are 3Ka with the PSC 4.8Ka.
But i thought the 4.8Ka would be in reference to the supply MCCB feeding the DB rather than the individual MCBs contained within the DB.
To establish if the 3Ka mcbs were inadequate it would be the end of each cicuit fed from each MCB.
Am i enterprating that right?Cheers for any help.

Graeme Harrold

What if the fault occurs at the MCB, that is where the max fault current will apply with respect to each circuit NOT at the end. What is the PSCC and PFC at the supply to the DB?

tallpault

Not checked that,ill check it tomorrow.

pevvers

I think I've already posted this to another thread, but have a look at 536.1 in the BRB and do some research into energy let through of both your MCB at your board and the back-up MCCB.

tallpault

Had a better look today.
It's a Dorman Smith Panelboard,with 3 3 Phase DB,s on it, all the Mcbs are rated at 3Ka but each one of the DBs are protected by a 63Amp Bs88 Type G fuses.
Would that mean the rating of the mcbs would be negated by the 80Ka BS88 fuses.

zupos40

As Peever stated you will have to find out what the total thermal let through energy is for both the BS 88 Type G fuse and the mcb

If the let through energy of the fuse is less then the mcb (please note they are refered to as cb now bs767:2008) then the breaker are ok to use even when fault current is greater then stated kA, you will also have to ensure that Ksquared S squared of smallest circuit is greater then the total let through energy of fuse (sorry do not have BS7671 with me to quote reg number).

To give you an example from my own research, I found out that a 100A type II BS1361 fuse has a total let through of 27,000 A2S and that typical 6kA cb (Hager MEM, ABB etc) has a total let through of 29,500 A2S and 10kA about 34,000 A2S, as a result if the ccu/ dis-board complies with the relevant standards (read GN.3) for any fault currents upto 16Ka the fuse will disconnect the fault whether earth Pefc or short Pscc before damage can occur to the c.b. and circuit conductors.


All the main manufacturer type test their board with all combination of c.b and fuses upto 16ka to ensure compliance.

So find out if your combination comply