Calculate the min size of cpc giving consideration to disconnection times and thermal

Baker1988 · Mar 22, 2013

Re: Calculate the min size of cpc giving consideration to disconnection times and the

Couldn't you just find your maximum demand and look in the bgb for what size cable can carry this safely and then do your volt drop calc to check volt drop if it doesn't comply then go to next size cable and do the calc again and so on, as for the 50kA fault current a type b BS EN 60898 only has a breaking capacity of 6kA or 10kA so you would have to change this for your fault current size why not use a BS 88-2 fuse as these have a breaking capacity of 80kA then do the adiabatic to find your min cpc

JuniorSparky · Mar 22, 2013

Re: Calculate the min size of cpc giving consideration to disconnection times and the

Yes, I've done all that, but I was struggling with thermal constrains because we were told to use the diagrams, but actually it was easier with adiabatic equation. I just had to use t = 0,1 s for the calculation.

About PFC I'll try to re-read again few more times. Maybe this time I'll understand it better.

I still don't know about grouping factor, why it says as a note that it can be used only for equal loads, but we use it for different loads how do we choose it exactly - by the circuits or by the cable, because three-phase is not one cable!?

johnboy6083 · Mar 22, 2013

Re: Calculate the min size of cpc giving consideration to disconnection times and the

Id like to say that it's nice to see somebody trying their homework themselves, rather than just posting a question and expecting an answer.
the PFC for a 3phase system will be roughly 1.732 x the highest PEFC on one phase. So if you divide 50 kA by 1.732 (in the absence of more reliable data) then this will give you a lower PEFC from which to size your CPC. As a general rule, the higher the fault current, the lower the disconnection times and the smaller the CPC.

as has been mentioned, if you can get a cable size and distance between the supply tx and the main intake postition of the installation, this will allow you to get a lower PFC.
in the real world calcs like this can save thousands of pounds of oversized and over engineered switchgear.

JuniorSparky · Mar 22, 2013

Re: Calculate the min size of cpc giving consideration to disconnection times and the

Thanks. I just really intend to practise that profession and I would like to know exactly what I am doing.

Well I calculated the fault current using - I[SUB]f[/SUB] = U[SUB]o[/SUB] / Z[SUB]s[/SUB] .. But I'm curious now what kind of PFC is 50 kA? If using the given formula calculated PEFC for the relevant circuit, can we say the 50 kA is for the whole installation and does that mean we have to use its value (50 kA) for calculating circuit protective conductors or it is necessary just for establishing the rating of the main protective/disconnection device?

PS: When you divide 50 kA with √3 do you mean finding the phase current from line current (delta 3-phase)? What are main areas of use for delta and star connection. I thought star is more for machines...

shanky887614 · Mar 22, 2013

Re: Calculate the min size of cpc giving consideration to disconnection times and the

JuniorSparky said:
Thanks. I just really intend to practise that profession and I would like to know exactly what I am doing.

Well I calculated the fault current using - I[SUB]f[/SUB] = U[SUB]o[/SUB] / Z[SUB]s[/SUB] .. But I'm curious now what kind of PFC is 50 kA? If using the given formula calculated PEFC for the relevant circuit, can we say the 50 kA is for the whole installation and does that mean we have to use its value (50 kA) for calculating circuit protective conductors or it is necessary just for establishing the rating of the main protective/disconnection device?

PS: When you divide 50 kA with √3 do you mean finding the phase current from line current (delta 3-phase)? What are main areas of use for delta and star connection. I thought star is more for machines...

yes and no.

a lot of machines start in star then swap to delta when they are upto speed etc.

there are different types. e.g. star motor, delta motor, star start and delta run

capacitor start, capacitor run etc (capacitor run is for single phase motors)

forgive me if im wrong but you dont normally get a neutral in delta but you do in star

this shows star and delta but not the earth

as you can see below, the neutrel and the earth come from the centre (star point of the transformer)

johnboy6083 · Mar 23, 2013

Re: Calculate the min size of cpc giving consideration to disconnection times and the

Three phase current is calculated by P/ 1.732 x V x PF

single phase = P/ V x PF

for rotating machines you also have to apply a derating for efficiency.

The above equations show why you need to divide phase to phase current by 1.732

JuniorSparky · Mar 23, 2013

Re: Calculate the min size of cpc giving consideration to disconnection times and the

Thanks guys

I found the chapter in my book for motors and generators. I'll read it first and be more prepared before asking about these systems

Back to the thread, one last thing - I'm a bit confused about applying the grouping factor in choosing the cable size. The table in BS 7671 says it is for circuits and cable running together in let's say trunking. So there are 2 things that bother me:

1 - How do we choose the number of circuit/cables when we have let's say 4 circuits, but 3 of them are 3-phase and one is single phase? Do I have to look at the column number 4 or 12 (3 x 4 + 1 x 2)?

2 - There's a note under the table saying that table is only applicable for equal loads! So what do we do in all 4 circuits have different loads?

Thanks

johnboy6083 · Mar 23, 2013

Re: Calculate the min size of cpc giving consideration to disconnection times and the

It goes by the number of circuits, it doesn't matter about number of phases.

if you have cables that are carrying less than a certain percentage of their rated current carrying capacity (after derating) then they can be ignored for grouping, but still must be accounted for when sizing the containment.