Cable size

[uksparks]

Hi,

I am going to be connecting an out door power supply, away from the building, its all metal casing.

The supply is 10mm, I am using a 3 core SWA for part of the run, the inside part was going to be in twin and earth, but then I was just thinking, going on table 54.8 section 544.1, am I looking into this too much or does it need to have a 10mm earth as its extraneous metal which is fixed into the ground?

In which case I was just thinking of running 3x 10mm inside the building so the earth is 10mm too to them comply with min size of bonding conductor.

I am right to make it 10mm throughout the entire run aren't i?

 

[Guest125]

I really do appreciate Kate all your help and constructive feedback on this, it's an area that itself has had the regs fiddled with and also other issues like earth rods and distances to the building.

Ok so if we break it down into small chunks, let's hypothetically say we will go with the earth rod, it says connect it be k to the met. Do we have any information anywhere as to what reading is acceptable that we must achieve?

Top op of head moment:

take a 2 core SWA cable from the internal dist board using the sheath for the earth

terminate into a plastic isolator externally and RCD at this point

take another SWA from the isolator and run the cable underground to the new VCP.

Install this at least 2M from the metal building

earth the underground SWA sheath and the VCP by an earth rod and earth pit.

Test

 

[spark 68]

it is a minefield eh?

It it is unusual for DNOs to provide earths to metal framed building is it not?

This is job may need a step back and reasses. Many heads are better than one.

Quite a few years ago I was part of a crew of agency sparks working on a new agricultural building being built on a farm, we were mainly responsible for installing the services in the building (lighting and power), another crew were sorting the supply to this building including the Earthing segregation from the main intake (PME) and installing the TT parts and all ground works.

The DNO were involved due to an upgrade of the supply for the increased demand.

Anyway to cut a long story short the DNO were unhappy about the proximity of this new metal clad TT building and extraneous parts of the PME bits, I don't know what the final outcome was as we were finished our contract, but I do know there was hell to pay at the time.

 

[uksparks]

Top op of head moment:

take a 2 core SWA cable from the internal dist board using the sheath for the earth

terminate into a plastic isolator externally and RCD at this point

take another SWA from the isolator and run the cable underground to the new VCP.

Install this at least 2M from the metal building

earth the underground SWA sheath and the VCP by an earth rod and earth pit.

Test

i like the idea but there just isn't the option to do this unfortunately due to it being a car park immediately outside the building.

 

[uksparks]

Quite a few years ago I was part of a crew of agency sparks working on a new agricultural building being built on a farm, we were mainly responsible for installing the services in the building (lighting and power), another crew were sorting the supply to this building including the Earthing segregation from the main intake (PME) and installing the TT parts and all ground works.

The DNO were involved due to an upgrade of the supply for the increased demand.

Anyway to cut a long story short the DNO were unhappy about the proximity of this new metal clad TT building and extraneous parts of the PME bits, I don't know what the final outcome was as we were finished our contract, but I do know there was hell to pay at the time.

I seem to think this is going to be more problematic than planned for.

i think other than connecting the rod to the met there's probably not a lot of choice.

 

[uksparks]

If any of you have the BYB can you take a look at

Annex A722, item A722.3, gives guidance on determining the maximum resistance required for the earth electrode in (ii).

 

[Leesparkykent]

I seem to think this is going to be more problematic than planned for.

i think other than connecting the rod to the met there's probably not a lot of choice.

this is the way i prob would go

 

[spark 68]

Is it not possible to re-site it further from the building ?

 

[Guest125]

I can only go on what it says on the service head, I'll have to speak to WPD for confirmation as it may be wrong.

you should be able to see if it is PME without calling them.

 

[uksparks]

Nope not a chance in hell unfortunately.

 

[Guest125]

Even if this was a TNS we would have the same issue.

Not the same. No.

 

[uksparks]

you should be able to see if it is PME without calling them.

I wont help even if it's not anyway.

 

[uksparks]

Not the same. No.

Thats only on the basis you can get s guarantee from the DNO it will never change.

 

[spark 68]

Even if it is PME you may well find there are earth rods already fitted, a lot of newer PME industrial units have earth rods fitted, a look around the perimeter of the building often you will see the inspection pits

 

[Guest125]

i like the idea but there just isn't the option to do this unfortunately due to it being a car park immediately outside the building.

I would say tough. To much risk.

 

[uksparks]

I must say I didn't notice any having walked all round it a couple of times.