SWA Sub Main advice/comments

[TheCodboy]

Hi all, looking for a bit of advice and confirmation on my proposed plans for a SWA sub main to a ISO metal storage container. I've calculated the max demand in the ISO will be about 20A (300w of led lighting and 2 double sockets for chargers etc) It will be fed from a 3ph DB in another building. The cable run is 100 meters, I'm planning on using 16mm 2 core swa ( Volt drop calcs allow about 35A with 16mm) and installing a rod at the container (the vast majority of the cable run is at height, clipped to the building). The container will have a small 4 way board with RCD main switch.
The 16mm will be fed from a C32 breaker in the 3Ph board. Im installing double insulated 16mm tails and a 16mm earth from the 3ph DB to a meter (needs a meter installed) then from the meter to an adaptable box where I will terminate the SWA. Due to the fact that I'm using a rod at the other end, I'm planning on a 100ma S rcd for fault protection for the SWA (inside the adaptable box).
Can anybody see any glaring errors in my method? I have spoke to Napit and they were fine about my proposed plan, but they didn't mention the 100mA rcd.
Thanks in advance!

 

[buzzlightyear]

Looks fine to me crack on.

 

[TheCodboy]

Looks fine to me crack on.

That's the kind of answer I like!! Cheers!

 

[westward10]

Why the need for a 100ma rcd assuming the type C device will provide fault protection.

 

[buzzlightyear]

Why the need for a 100ma rcd assuming the type C device will provide fault protection

You beat me to it. 63 amp would be better.

 

[TheCodboy]

You beat me to it. 63 amp would be better.

It's an old Square D board and there a few spare 32's in the board. Thought I'd utilise that rather than trying to source another MCB.

 

[buzzlightyear]

Why the need for a 100ma rcd assuming the type C device will provide fault protection.

Westy correct.why above.

 

[westward10]

It's an old Square D board and there a few spare 32's in the board. Thought I'd utilise that rather than trying to source another MCB.

Why the 100ma rcd.

 

[buzzlightyear]

How is the SWA cable going in?

 

[TheCodboy]

Why the 100ma rcd.

Would I not need the 100mA due to the Zdb being higher than the permissible Zs value for disconnection with the C32 mcb

 

[TheCodboy]

How is the SWA cable going in?

Clipped at height to the outside of the main building, ducted in the ground for roughly 10-12 meters.

 

[westward10]

Would I not need the 100mA due to the Zdb being higher than the permissible Zs value for disconnection with the C32 mcb

So the C32 wouldn't provide fault protection.

 

[TheCodboy]

So the C32 wouldn't provide fault protection.

I would think not.

 

[buzzlightyear]

I would think not.

So what should it be?

 

[TheCodboy]

So what should it be?

Well thats why I'm including the 100ma rcd.

 

[westward10]

Well thats why I'm including the 100ma rcd.

Fair enough that would seem fine.

 

[buzzlightyear]

Now we have established what you are doing draw us a plan from start to end. If possible.

 

[pc1966]

What is the Zs/Ze of the supply board?

If you go with 16mm 2-core then you have R1+R2 = 1.15 + 3.7 = 4.85 ohm/km so 0.485 ohm and for a C32 MCB you would need no more than 0.55 ohm end of circuit Zs so supply Ze has to be 0.06 ohm or less. Unlikely.

Your VD limit would be met for 10mm, so going 3-core there your R1+R2 is 1.83 + 1.83 (ignoring armour's 6.0 in parallel for now) = 0.366 for 100m so Ze <= 0.18 ohm, which is possible.

Is this a TN-C-S supply? If so the 16mm 2-core will not meet the 10mm copper equivalent for bonding an extraneous part (armour is 42mm steel, so about 5.3mm copper equivalent), so either it has to be TT'd or you go for 10mm 3-core.

If using the 100mA RCD for avoidance of sane limits on supply Ze it will have to be at the source end, so a cable fault still meets 5s or less disconnection, and ideally a delay one so it has selectivity with the 30mA RCD for the socket outlets. No one is going to be happy walking 200m round trip to reset it!

Other options for Ze avoidance would be a 32A switched-fuse as then you would meet 5s disconnection on 1.25 ohm end-of-circuit Zs (OSG Table B4), but obviously no selectivity with the 32A C MCB, and probably little with the socket circuits (probably fine for post-13A fuse fault, not so one a 20A B RCBO or whatever and fixed wiring fault).

 

[TheCodboy]

What is the Zs/Ze of the supply board?

If you go with 16mm 2-core then you have R1+R2 = 1.15 + 3.7 = 4.85 ohm/km so 0.485 ohm and for a C32 MCB you would need no more than 0.55 ohm end of circuit Zs so supply Ze has to be 0.06 ohm or less. Unlikely.

Your VD limit would be met for 10mm, so going 3-core there your R1+R2 is 1.83 + 1.83 (ignoring armour in parallel for now) = 0.366 for 100m so Ze <= 0.18 ohm, which is possible.

Is this a TN-C-S supply? If so the 16mm 2-core will not meet the 10mm copper equivalent for bonding an extraneous part (armour is 42mm steel, so about 5.3mm copper equivalent), so either it has to be TT'd or you go for 10mm 3-core.

If using the 100mA RCD for avoidance of sane limits on supply Ze it will have to be at the source end, so a cable fault still meets 5s or less disconnection, and ideally a delay one so it has selectivity with the 30mA RCD for the socket outlets. No one is going to be happy walking 200m round trip to reset it!

Other options for Ze avoidance would be a 32A switched-fuse as then you would meet disconnection on 1.25 ohm end-of-circuit Zs, but obviously no selectivity with the 32A C MCB, and probably little with the socket circuits (probably fine for post-13A fuse, not so one a 20A B RCBO or whatever).

It will be TT

 

[pc1966]

It will be TT

OK, so you just need to meet the SWA fault protection so RCD up-front on basically any SWA choice, and a rod arrangement below 200 ohms. What is the surrounding soil/ground like?

 

[TheCodboy]

OK, so you just need to meet the SWA fault protection so RCD up-front on basically any SWA choice, and a rod arrangement below 200 ohms. What is the surrounding soil/ground like?

Haven't had a dig yet but I've been looking at bentonite if I'm struggling for a good reading.

 

[timbobelfast]

Hi all, looking for a bit of advice and confirmation on my proposed plans for a SWA sub main to a ISO metal storage container. I've calculated the max demand in the ISO will be about 20A (300w of led lighting and 2 double sockets for chargers etc) It will be fed from a 3ph DB in another building. The cable run is 100 meters, I'm planning on using 16mm 2 core swa ( Volt drop calcs allow about 35A with 16mm) and installing a rod at the container (the vast majority of the cable run is at height, clipped to the building). The container will have a small 4 way board with RCD main switch.
The 16mm will be fed from a C32 breaker in the 3Ph board. Im installing double insulated 16mm tails and a 16mm earth from the 3ph DB to a meter (needs a meter installed) then from the meter to an adaptable box where I will terminate the SWA. Due to the fact that I'm using a rod at the other end, I'm planning on a 100ma S rcd for fault protection for the SWA (inside the adaptable box).
Can anybody see any glaring errors in my method? I have spoke to Napit and they were fine about my proposed plan, but they didn't mention the 100mA rcd.
Thanks in advance!

I take it the Earthing System for the upstream supply is a TN-C-S???

 

[pc1966]

Haven't had a dig yet but I've been looking at bentonite if I'm struggling for a good reading.

Clay cat litter is cheaper and the same sort of stuff!

Obviously check for any buried services as far as you can. There are sites like this for main stream power & gas, but not everyone is on there:

LSBUD - Home

LSBUD is the free to use Safe Digging Platform providing Utility Asset Maps for over 3,9 million searches every year across the UK.

lsbud.co.uk

Also remember that the main factor is soil conductivity is moisture, so if you can go deeper (say 8' = 2.4m rod, typically in two sections) you will generally do better than two short ones in parallel as more chance of the soil remaining moist.

I have used M16 stainless studding as an earth rod driven in like a giant drill (SDS adaptor and a 1/2" drive stud runner), but obviously you get purpose made ones and matching bit like access boxes, etc. For example:

Earth Rods & Fittings Archives - Kingsmill Industries

kingsmillindustries.com

Earthing Archives

Browse our range of Earthing & Lightning protection equipment and products online at AN Wallis including earth Rods, Bars, U Bolt Clamps and more.

www.an-wallis.co.uk

 

[TheCodboy]

Clay cat litter is cheaper and the same sort of stuff!

Obviously check for any buried services as far as you can. There are sites like this for main stream power & gas, but not everyone is on there:

LSBUD - Home

LSBUD is the free to use Safe Digging Platform providing Utility Asset Maps for over 3,9 million searches every year across the UK.

lsbud.co.uk

Also remember that the main factor is soil conductivity is moisture, so if you can go deeper (say 8' = 2.4m rod, typically in two sections) you will generally do better than two short ones in parallel as more chance of the soil remaining moist.

I have used M16 stainless studding as an earth rod driven in like a giant drill (SDS adaptor and a 1/2" drive stud runner), but obviously you get purpose made ones and matching bit like access boxes, etc. For example:

Earth Rods & Fittings Archives - Kingsmill Industries

kingsmillindustries.com

Earthing Archives

Browse our range of Earthing & Lightning protection equipment and products online at AN Wallis including earth Rods, Bars, U Bolt Clamps and more.

www.an-wallis.co.uk

Just did a job today where a house had no main earth. Did a Zs at the sockets and it was 290 Ohms. Did as bit of digging out, probably a couple of feet and got the 4ft rod pretty much all the way in. Ze of 28 ohms. That'll do for me!