Submain swa size check

[bigspark17]

Just after a bit of advice from thoose of you that deal with this regularly & want to make sure i get it right.

Swa submain to phone network mast
63Amp single phase supply
Pme supply assumed ze of 0.35
245M run

Ive been trying to calculate which size swa & cpc or 3c swa to go for, would like any experienced input.
Thanks

 

[Ian1981]

What is your protective device ?
Fuse, mccb , mcb etc?
Also installation method
May restrict your calculations as your max zs will hinder you as I would imagine your ze will probably be less than 0.35
As an example a type c 63 amp mcb has a max zs of 0.34 ohms without your 80% rule of thumb.

 

[Leesparkykent]

Ze is a major factor. Any chance you can site survey and measure it?

 

[bigspark17]

What is your protective device ?
Fuse, mccb , mcb etc?
Also installation method
May restrict your calculations as your max zs will hinder you as I would imagine your ze will probably be less than 0.35
As an example a type c 63 amp mcb has a max zs of 0.34 ohms without your 80% rule of thumb.

I was going to install switch fuse 63amp bs88.. install method is buried in ground with warning tape buried above.

Ze is a major factor. Any chance you can site survey and measure it?

Unfortunatly not as the DNO have not installed the supply into the customer built hut yet, :/

 

[bill01803]

It is unlikely to be TNCS in a hut more chance of TT

 

[Wilko]

As Bill has said above, there may be reasons to TT the installation instead of PME. The network guys (DNO and or mobile operator) should have detailed guidance for you, as its not exactly the first one. Anyway, see page 35 in this guide for some earthing info

https://library.ukpowernetworks.co....017+Customer+Installation+Earthing+Design.pdf

 

[Leesparkykent]

Without knowing Ze your a little stuffed...If there is no way of knowing then I would be looking at pricing to install a MCCB with adjustable earth leakage/fault protection.

 

[Ian1981]

It is unlikely to be TNCS in a hut more chance of TT

Why is that?
If it's a purpose built hut to house the metering and any distribution equipment why does it need to be TT?

 

[Ian1981]

 

[Ian1981]

 

[andyb]

Have you been specifically asked for a 63a supply?

 

[Leesparkykent]

Very rough calculation whilst sat on my sun lounger next to the pool.....If the earth leakage relay was set to o.8xIn then 50V/50A=1 Ohm

Gives you a bit of room to play with if your taking Ze at 0.35 Ohms.

If your calculations are using 63A as Iz then i recon your looking at using around 150mm based on volt drop.

 

[davesparks]

The DNO will be able to tell you the designed Ze for the supply, if it is a completely new supply then they will possibly be working to a maximum value of 0.25 in their calculations.
0.35 is the stated value for existing networks.

Assuming this is a 100A supply coming in that is, if it's bigger than that then the Ze requirements are lower.

 

[davesparks]

Very rough calculation whilst sat on my sun lounger next to the pool.....If the earth leakage relay was set to o.8xIn then 50V/50A=1 Ohm

Gives you a bit of room to play with if your taking Ze at 0.35 Ohms.

If your calculations are using 63A as Iz then i recon your looking at using around 150mm based on volt drop.

Can you get an earth leakage trip with a 50A tripping current? I've never yet seen them go above 10A

What are you basing your VD calculation on?
My rough calculations based on voltage drop come out at 2x50mm in parallel, based on a mobile phone mast not including any lighting requirements and not having any specified voltage limits stricter than 6%

 

[Leesparkykent]

Can you get an earth leakage trip with a 50A tripping current? I've never yet seen them go above 10A

What are you basing your VD calculation on?
My rough calculations based on voltage drop come out at 2x50mm in parallel, based on a mobile phone mast not including any lighting requirements and not having any specified voltage limits stricter than 6%

I got the mV/A/m figure from a manufacturers data sheet. Good point about to 50A setting....I just assumed they did. I've fitted one with a 30A max setting before.

 

[bigspark17]

Thanks for replies.. some good input from respected members.

Its the max disconnection time & VD that are the main influences over this length.

I was looking at putting a kmf or switch fuse between dno and metering with a bs88 gG 63amp fuse in, @ 80% value gives me a max zs of 0.62.

Using 95mm 2c swa & 35mm cpc gives me a max Zs of 0.56 assuming 0.35 @ dno.

Using the 95mm gives me a VD of 9.5v which is less than the 5% (11.5v) i need to be under.

Any thoughts on this method?

@davesparks i like the idea of parallel cables but would you do this using 2 swa or 4c in one swa?

 

[bigspark17]

Bump