sizing SWA armour as cpc using adiabatic......

[brman]

I'm doing a mod for a 3 phase circuit already installed. The only trouble is I can't work out why the existing cables are as they are so i am doubting my sums.

Existing final circuit.
3 pole memshield 2, D32 rated feeding 10mm[SUP]2[/SUP] 4 core swa with a seperate 16mm[SUP]2[/SUP] G/Y as cpc.
Measurements at the DB are max 5.6kA single phase. Zs 0.04

Cable run is about 20m so I don't think voltage drop is an issue. So why has someone fitted an additional 16mm[SUP]2[/SUP] cpc?

If I use table 54.7 I need a cpc of 10mm[SUP]2[/SUP].

If I use the adiabatic if find:
1) max Ipf across phases is 1.73x5.6=9.7kA
2) let through energy for the mem D32 MCB is 43000 at 10kA according to their specs.
3) 43000 A[SUP]2[/SUP]s and a k of 143 (assuming xlpe) gives me a minimum cpc csa of 1.45mm[SUP]2
[/SUP]
So if I wanted to use a seperate cpc I would have gone for either 10mm[SUP]2[/SUP] if using the table values or 4mm[SUP]2[/SUP] (min unprotected) if using the adiabatic.

In fact I would probably have used the pyranha nuts and the armour of the swa as 10mm2 4 core has a armour csa of 43mm[SUP]2[/SUP] which is much more than 8 times 1.45mm[SUP]2[/SUP]

Have I got my sums right as someone spent good money putting in 16mm2 so I am thinking there must have been a reason?

The only thing I can think is that using the adiabatic simply with a 5.6kA ipf and t=0.1 gives 15.4mm[SUP]2[/SUP]
but a) which select a size bigger than table 54.7 requires and b) the 5.6kA is the single phase fault current.

Plus, why 10mm[SUP]2 [/SUP]SWA? Voltage drop at 32A would be about 2.5% I think even if I used 4mm[SUP]2[/SUP]and Zs should not be a problem either, even with a type D breaker.

My job is to put a spur on this circuit but am not keep to match the cable sizes already used for no good reason.........

 

[telectrix]

you can't use the phase/phase fault (PSCC) current in the adiabatic, as you are looking at a fault of negligible impedance between phase and Earth. (PEFC).

 

[brman]

you can't use the phase/phase fault (PSCC) current in the adiabatic, as you are looking at a fault of negligible impedance between phase and Earth. (PEFC).

A good point, one which I should have realised given I am looking at sizing the cpc......:rolleyes4:

apart from that brain fart is there anything else wrong with my sums?

 

[Richard Burns]

Your calculations look right using the figures you have given.
Possibilities are that it is sized as a bonding conductor or that someone had some 10mm and decided to use it or that it was originally employed for a greater load.

 

[telectrix]

you have to use manufacturers data for the calculation as the trip time is out of range of the time/current graphs in BS7671. using that figure of 43000, your 1.45mm is correct. however, as richard said, if used as a bonding conductor, 10mm would be required. maybe they only had 16mm to hand.

 

[snowhead]

Have you allowed for the "We always fit 16mm earth" factor.?

 

[telectrix]

" to boldly go where other sausages have gone before"

 

[brman]

Thanks Guys, you have confirmed I am not going mad at least

I did think it might have been sized for bonding but it isn't used for it. I think the "what we had in the van" might have some truth.....

I've gone back and quoted for a new circuit on a new breaker as it is only a few quid more than messing with the old one when I take my time into account. Plus it has the bonus that I am not trying to guess what the original design intent was!

 

[Engineer54]

Rule of thumb. ....All SWA cables ((be they of PVC or XLPE insulation) from 1.5mm 2 Core, up to 95 mm 4 Core, the SWA is of a suitable CSA to be used as that cables/circuit CPC!!

 

[rmc86uk]

E54 one of the lads at work tried to tell me that depending on the length of run you cannot use the SWA as the cpc. I thought he was talking rubbish and from your reply above it sounds like he was! This was a 1.5 4 core.