Pros...cons...preferences..

[Baddegg]

Oh god! Here goes......

If = 230x0.95 (c min) / 0.36 (Zs) = 606 a

t = 0.1s (40a 60898 mcb)

K = 115 (copper line conductor) / 51 (steel armour) x 10 size of line conductor = 22.54

Sooooooo......

S = (sq root symbol ?) 606a 2 x 0.1/22.54 = 8.5mm

CSA of 2 core 10mm2 swa is 41mm2 so all is good?

Kill me now!

 

[Deleted member 26818]

Why does t = 0.1s?
It’s a distribution circuit, so t = 5s.

 

[Baddegg]

Ahhhhh yes.....

 

[Deleted member 26818]

Ahhhhh yes.....

Also K for steel armour is 51 For 70 degrees C and 46 for 90 degrees C.

 

[Deleted member 26818]

square root of 606 x 606 x 5 divided by 51.
26.56mm2.
CSA of SWA for 2 core 10mm2 is 40mm2

 

[Baddegg]

Did I not need to make the calculation for using different materials, copper and steel?
Thanks for taking the time spin I do appreciate it

 

[Deleted member 26818]

No, you don’t need to make a calculation for the different materials, unless you wanted to know what the copper equivalence of 40mm2 SWA would be. 17.7mm2.

 

[Baddegg]

Bugger! So close.....ish

 

[Risteard]

Why does t = 0.1s?
It’s a distribution circuit, so t = 5s.

Actually t is the actual disconnection time. So in this instance it won't be anything like 5 seconds, but probably between 0.01 and 0.1 seconds.

 

[TJ Anderson]

Yeah, 0.1s is as low as BS7671 goes for your calcs, any Zs =/< max Zs for that 60898 would achieve that (as needs at least that fault current). As Zs drops, fault current increases and disconnection will get quicker. Would require manufacturer's data if you needed to go lower than 0.1s to satisfy thermal constraints but you definitely don't need to in the case of this job.

 

[Wilko]

Hi - just FYI, from GN8 I read that 2 core swa 10mm has actual steel of 26mm (table B2). Since you only need 22mm (ish) steel to equal 10mm Cu (for thermal effects for 70C cable) then it’s fine.
Not sure, but your 41mm figure could be from 4 core 10mm cable?

 

[Deleted member 26818]

Hi - just FYI, from GN8 I read that 2 core swa 10mm has actual steel of 26mm (table B2). Since you only need 22mm (ish) steel to equal 10mm Cu (for thermal effects for 70C cable) then it’s fine.
Not sure, but your 41mm figure could be from 4 core 10mm cable?

There are 3 different types of SWA cable.
70 degree C PVC 2 core 10mm2 BS6346 SWA has a CSA of 40mm2, and a value for K of 51.
90 degree C PVC 2 core 10mm2 BS5467 SWA has a CSA of 26mm2 and a value for K of 46.
90 degree C XLPE 2 core 10mm2 BS6724 SWA has a CSA of 26mm2 and a value for K 46.

 

[Deleted member 26818]

When calculating the CSA of a protective conductor, using tabulated values, you use the tabulated values.
If you were calculating using manufacture’s data, you would use their values.

 

[Wilko]

There are 3 different types of SWA cable.
70 degree C PVC 2 core 10mm2 BS6346 SWA has a CSA of 40mm2, and a value for K of 51.
90 degree C PVC 2 core 10mm2 BS5467 SWA has a CSA of 26mm2 and a value for K of 46.
90 degree C XLPE 2 core 10mm2 BS6724 SWA has a CSA of 26mm2 and a value for K 46.

Hi - so are you saying that GN8 is wrong or that I’ve read it wrong? .

 

[andyb]

Am I missing something here?
If you know that the cpc is 26mm, why do you need to do the adiabatic?
Apologies if I'm being stupid.

 

[Pretty Mouth]

Baddegg, genuine question - why use Cmin when calculating the fault current for adiabatic? Would this not give a best-case scenario lower fault current, where we would want a worst-case high fault current?

 

[Baddegg]

Good question pretty mouth, havnt used the adiabatic in real life scenario before so just went through my notes, this was a good opportunity to do some calcs and get the brain working agian as it’s my own property, I will be redoing it all this evening but as Andy points out it’s all a bit academic as we nlkniw the csa anyway.......possibly maybe

 

[Charlie_]

The nicknames/profile names on this forum, are you boys also know as this in the real world?

Would feel a bit uneasy shouting over to pretty mouth on a building site

 

[Wilko]

Hi - no questions can be stupid .

My thought is - the actual steel in 2x10mm swa is at least 26mm which when converted with the appropriate temperature rise k factors is equivalent to more than 10mm copper so it’s going to be ok for CPC. As you say, no real need to look at the adiabatic equation for that circuit.

Happy to have feedback if I’ve got this wrong .

 

[Edtwozeronine]

You think the cable is amusing....I’m off to do the adiabatic! Wait till I post them maths up ha ha ha

Isn't the adiabatic shore somewhere near Albania?