ipf and ka on 3ph boards

[sarge]

just been reading about measuring pfc on 3ph boards and the niceic book states that you should double the line to neutral/cpc (highest) and record it. Does this mean if the doubled result is higher than 6ka that even single phase mcb's should be rated higher than 6ka?
Any thoughts?
Thanks
Sarge...

 

[solo2771]

The way I see it is No because the 2x is to cover the root 3 thingy and it is split between the phases so the single phase PFC will be lower ie 1x ( if I am wrong I am sure someone will tell me)
Paul

 

[Guest123]

Hi.

The reason that PFC is doubled for 3 phase is because the fault current will always be higher between phases. Multiplying by 2 is an accepted rule of thumb, as solo said the actual figure is root 3 i.e 1.732 i.e 230V x 1.732......= 400V.

So, the single phase MCB's would not need to be rated at the doubled value as they are only monitoring one phase, so the PFC likely to pass through them would be the un doubled value.

 

[WayneL]

Hello,

So what happens when the phase that your single-phase MCB is protecting

short circuits with one of the other phases? - big bang and no more MCB

 

[andyb]

If you have a meter that does 400v there is no need to double it you can just measure it. The Meggaer 1552 does this and I'm sure the Fluke equilivent does. But you should only carry out this test if you know for sure that your meter handles 400v.

Also with a phase/phase fault, the mcb will take the full fault current.

 

[Guest123]

I agree with the above but......a job i completed recently when measured and multiplied at the origin had a PFC of 19.34 KA. Now i'm only aware of standard MCB's rated at upto 10 KA. The upstream devices, MCCB's, are rated at 25 KA which is more than adequate to deal with the P-P fault current.

 

[solo2771]

as the phases are 120 degrees apart the fault current will also be 120 apart so if you do the maths it will not double the fault current.
Paul

 

[sarge]

Hello,

So what happens when the phase that your single-phase MCB is protecting

short circuits with one of the other phases? - big bang and no more MCB

Kind of understand where you guys are coming from with the equations, but still not confident with the fact that even when the proper equatiosn brings the value higher than 6Ka as wayne stated above! as surely not all individual phases have there own roots.
Thanks though fo the feedback

sarge

I agree with the above but......a job i completed recently when measured and multiplied at the origin had a PFC of 19.34 KA. Now i'm only aware of standard MCB's rated at upto 10 KA. The upstream devices, MCCB's, are rated at 25 KA which is more than adequate to deal with the P-P fault current.

just read this through again, makes more sense to me now..thanks.;so should it be that all single phase mcbs in a 3ph board be rated at 10ka?

 

[Guest123]

so should it be that all single phase mcbs in a 3ph board be rated at 10ka?

Yes.

Commercial/Industrial should have 10KA MCB's, 6KA are usually only Domestic.

 

[sarge]

Yes.

Commercial/Industrial should have 10KA MCB's, 6KA are usually only Domestic.

great thanks alot, thought as much. its regarding a job i have taken on in a big garden centre, existing 3ph boards with 6ka mcbs (1ph). Colleauges couldnt realy see what i was getting at.

cheers chaps.