Cable length

[Keith the beef]

Can anybody please simplify some calculations for me please i have a 100A PME supply that is supplying a house roughly 100-150 metres away. it is run in 50mm 4 core SWA three phase and neutral they have run a ten mm cpc seperate alongside it, No continuity from the armourings as they have glanded it into Bakelite on the fuse board, as there has been a square hole knocked out in the bottom of the board.
Didn't see this at first as there was that much muck/dust inside the board.
Now what i'm after is can anyone remind me of the calcs i need t find out whether the armourings are suitably sized if they were connected?
Personally i don't know why they ran a 10mm alongside it as i don't believe this will carry anywhere near the fault current.
I have looked through my regs book but am having a proper no brain day (in fact year) too much on my plate at the minute.
I am wondering if they have thought that a shortfall in the calcs for the armourings can be bought up to regs with just a 10mm.
Not done any calcs like this for a while need brain refreshing

 

[Mac]

There was a discussion on this a little while ago.CPC should be 50% the size of the conductor.So it looks like 25mm needed.

 

[Notsosmart]

Section 543 to 547 of the regs may help. The rule of thumb is, as Mac has stated, half conductor size.

It sounds like who-ever installed the cable may have got confused with reg 547: equipotential bonding conductors (even then 16mm should have been installed)

 

[sparks1234]

Hi.
Dont think the 2 previous posts understood the question correctly,
As I understand it you are asking if the armour of the SWA is large enough to be the earth, there is a very usefull chart on the web and you will have to google it, type in SWA as earth conductor, you should be able to find a chart which gives you the CSA of most cable sizes, the figures in brackets are the ones that require an additional earth.

Hope that helps, if you cant find it then let me know and I will get it for you

 

[Keith the beef]

Thanks for the reply chaps, i have been and had a closer look at this today,
Then 10mm doesn't run all the way with the armoured, so it is not a seperate cpc along with the supply, i put my meters on a socket outlet this morning to get a loop reading of 8.89, a tad high me thinks!!!!!!
so i probe a little further measure earth terminal on the board again 8. something,
i walked down to the supply end had a look around, measured the PME 0.28, thats ok then.
Back up to the house did a loop test on the gland nut 0.41 now this cable is on 100A bs 88 fuses and to comply with maximum earth loop readings for protective devices that'll need dropping to 80A fuses allowing me a loop reading of 0.46 (80% of value given in bs7671 as given on back of front cover of test sheets)
Now the cable has a 40 Bw gland on it it, for the life of me i can't decide whether it is a 35mm, 50mm, ?

Measured the cable accross outer sheathing etc it's 32mm accross.
I paced it out at roughly 110m long

Now as i mentioned earlier the gland has no continuity to the board which must be sorted.
However just to clarify my readings a little later i did another loop test on the socket outlet and got 0.81 on a cooker switch socket (same one as before) and 0.62 on the earth bar in the board?????
I checked and checked my leads, connections, my sanity.
Whats going on here??????
Keith

Sparks just read your post thanks buddy , i've had a look and seen a chart like this, wasn't sure whether it was accurate or not as it wasn't from the bible, bs7671.
I may have been looking at a different one to you though do you have a link pleae?
that is my question though is the armour of this swa sufficient?
How can i confirm the cable size?
Thanks again
Keith

 

[sparks1234]

steel wire armour as the earth conductor

 

[Keith the beef]

Ta very much my friend

Well according to that all should be good whether it be 35, or 50mm.
I would like to confirm the size 100% though any ideas?

 

[Mac]

My understanding from regulation 543.11 is that you can use regulation 543.1.3 or 543.1.4 for calculation of the cpc size. Regulation 543.1.4 uses table 54.7 which is the one you are referring to and shows 16mm2 (for cable sizes between 16 and 35 mm2).There is a tool witch calculates cpc in terms of table 54.7 at:
myElectrical - Electrical Engineering - cpc Calculations

 

[Keith the beef]

so if i took a worst case scenario and sais it was 35mm 4 core swa , i could do a volt drop calculation to see if it's acceptable on an 80 fuse?

 

[Mac]

Yes mate.Where is the supply being fed from.How big is the house,must be big if it need's a 3 phase supply.

 

[Keith the beef]

the supply is in a litlle purpose built hut down the bottom of the garden, the house is of one phase of it,
Then it goes out of the board again 3p+n to the swimming pool which is in a seperate building behind the house.
Has an electric pool heater, three phase 9kw per phase

 

[Mac]

the supply is in a litlle purpose built hut down the bottom of the garden, the house is of one phase of it,
Then it goes out of the board again 3p+n to the swimming pool which is in a seperate building behind the house.
Has an electric pool heater, three phase 9kw per phase

Sorry mate,getting confused here.Supply is in the hut in the garden.3 phase to the pool and 1 phase to the house

 

[Keith the beef]

Apologies 6KW per phase

at the entrance to the house, ie at the bottom of the garden bottom of the drive there is a small hut containg the incomers = 3p+n pme.
This travels up to the house in a 4 core swa using the armour as earth this is the 35/50mm cable in question.
it terminates into a three phase board,
House is on one phase jumped of the board 63A breaker with a 17th edition board
then another cable 3p+n goes from this board to the swimming pool enclosure
Keith

 

[Mac]

Sorry mate,really going brain dead at the mo.What are you trying to clarify

 

[Keith the beef]

Mac if your going brain dead, then i'm stood right next to you lol.
My query was, is the 4 core swa from the supply down the garden, the incomer...
Suitable for the task, ie it is using armours for the cpc, i can't decide whether it is 35 or 50mm, it was on 100 Amps fuses
Cheers Mac

 

[Mac]

Personally i wouldn't rely on the armour sheathing as an earth.I would run a separate earth from the hut to the house.

 

[Keith the beef]

i feel the same way, however it would mean a trench digging across patio's and beautiful lawns, i am a little uncertain as what to do for the best, ie if i drop the supply fuses down to 80A i am in my earth loop for overprotective device tolerances whearas at the minute it doesn't comply

 

[Mac]

i feel the same way, however it would mean a trench digging across patio's and beautiful lawns, i am a little uncertain as what to do for the best, ie if i drop the supply fuses down to 80A i am in my earth loop for overprotective device tolerances whearas at the minute it doesn't comply

Is the armour not in a duct.Can you not put a spike in mate,saves all the hassle of digging the garden up.

 

[Keith the beef]

No duct as far as i can see,
Do you mean drop a spike in the ground near the house distribution board to lower the Ze? thus making 100A acceptable?

 

[Mac]

No duct as far as i can see,
Do you mean drop a spike in the ground near the house distribution board to lower the Ze? thus making 100A acceptable?

Yes mate

 

[Keith the beef]

it's one of the options i'm considering.
Mac i must thank you, you've stuck with me one this to the end, not a very exciting subject i know, we've suffered confusion, trying to explain via typing it's not easy sometimes.
Thanks again
Keith

Now lets talk about cheer leaders and motorbikes!

 

[Mac]

it's one of the options i'm considering.
Mac i must thank you, you've stuck with me one this to the end, not a very exciting subject i know, we've suffered confusion, trying to explain via typing it's not easy sometimes.
Thanks again
Keith

Now lets talk about cheer leaders and motorbikes!

Definitely mate

 

[Keith the beef]

Re this subject i have emailed the company who made the cable, they reckon as it's external diameter is 32mm it will be a 50mm 4 core swa, now the calcs begin!

 

[crabapple]

50mm2 = 0.387 mohms/m

length 100-150 m (assume its 150)

max conductor op temp for XLPE = 1.28

total R1+R2 = (0.387*0.387) * 150m * 1.28/1000 =28.756/1000 = 0.0287 ohms

Zs = Ze + total R1 + R2

Ze = 0.28 and 100A bs 88
Zs = 0.28 + 0.0287 = 0.3087

Ia=Uo/Zs

fault current is;

240/0.3087 =777.5 A

t = 1s (BRB pp248 bs 88 100A fuses in 1s with Ia of 790A !)

S = sqr rt(Ia^2*t)/k

k = 143
t =1s

sq rt(778*778*1)/143 = sq rt(605284)/143 =778/143 =5.44mm2



sorry, just couldnt rsist, sorry keith ignore me