pfc

[peteludman]

today i was doing inspection and testing of a large building with 26 boards.

the part i was doing had 3 DBs about 60 m away from mains inlet and one of the boards is on a 100A supply with 25mm 4core 3ph cable which i think is to small well for VD any way because some runs from the DB are a further 50m.(seperate earth cable 16mm) what do you think?from this board im getting 11KA phase to phese (L1 -L2 -L3).
is this pfc ok the MCB are rated at 10KA or they are note effcted by pfc.

and the other DB pfc is <19.9 phase to earth which is to much i still have to double it for 3 phase.
i dont know what the mains fuse is yet but it has to be a bs 88-2.1 or a bs 88-6 .

am i right? and what could i do to solve this problem this is a grey area for me because i have never been off the scale on my tester .

thanks

 

[leokiss]

As far as i can remember your readings have to be less than your mcb's ratings. Ive changed them to higher rated ones before to solve this. As for your off the scale readings ive only had that when testing right next to main sub transformer which on some test meters prevents them been able to measure accurate. Oh and dodgy test meters.
Im interested to know myself now hopefully sumone a little wiser will know

I think the 11ka one is ok if your fuses before db have a higher ka than 11ka i remember questioning this before now and thats what the main engineer told me.

 

[controlman]

Interesting subject
As a rule of thumb - the closer to your supply transformer the higher the KA.
So if you have a bank of MCB with a lower rating, protect them with HRC (high rupturing capacity) fuse. Their ratings vary from 80KA upwards.
Heres the interesting bit
In theory if a 16amp (MCB rated 6KA) and the theory fault current is 8KA. The current is limited by the size of the conductor being protected by the 16amp breaker eg. you cannot get 40,000 amps down a a 2.5mm cable

 

[ian.settle1]

today i was doing inspection and testing of a large building with 26 boards.

the part i was doing had 3 DBs about 60 m away from mains inlet and one of the boards is on a 100A supply with 25mm 4core 3ph cable which i think is to small well for VD any way because some runs from the DB are a further 50m.(seperate earth cable 16mm) what do you think?from this board im getting 11KA phase to phese (L1 -L2 -L3).
is this pfc ok the MCB are rated at 10KA or they are note effcted by pfc.

and the other DB pfc is <19.9 phase to earth which is to much i still have to double it for 3 phase.
i dont know what the mains fuse is yet but it has to be a bs 88-2.1 or a bs 88-6 .

am i right? and what could i do to solve this problem this is a grey area for me because i have never been off the scale on my tester .

thanks

Yes because it is 11kA between 2 phases but a 3 phase breaker is made up of 3 seperate single breakers riveted together and the switch on each is joined together by a common bar and each seperate breaker is rated at 10kA.

On a single phase you would not get a fault current of 11kA as this was measured between 2 phases and not a single phase and neutral.