Power to outbuilding, possible structural steel...

[uksparks]

Hi,

I have just been to a job today to provide power to an outbuilding, 75 meters down the garden.

The supply is PME, I plan on splitting the tails into a henley block and then into a switched fuse, then 16mm SWA from there all the way down the garden, at 600mm under the lawn.

The building is a 30ft by 20ft concrete arrangement, sitting on a whopping great big concrete slab. My issue is, there's a steel frame inside, its just bolted into the floor though. Am I correct in thinking this is classed as structural steel or would it need to be more buried in the ground?

If it is classed as structural, I understand why I cannot export PME, so would use 2 core SWA, earth the armoring at the house end, and stick a rod in next to the outbuilding and ground any exposed metal.

I guess I could check to see what the IR is on the steel to a known source, but my wander lead will not stretch that far! Off the top of my head its if its less than 0.02M ohms it needs bonding correct?

Please can I have some thoughts on if its best to make it a TT? I have done quite a few sheds and the like, but never done one that's had steel all round it, always normally just wood!

Thanks

 

[Richard Burns]

Bond the structural steel to an EMT at the outbuilding (if required) use a reel of singles as wander lead if you need to.

 

[uksparks]

Is there a line you draw whether or not it's obvious to bond or not? I know it comes down to if it meets the IR readings and so on, but is something that's bolted to a concrete slab really classed as structural in the terms of if it needs bonding.

I was just thinking using singles, I'm a little cautious of this job anyway as the blokes just had the house build and it cost him £300k and everywhere is like a palace with marble floors. I'm thinking right, I need a hole there though his nice new wall...

 

[Richard Burns]

If it is an extraneous conductive part that is liable to introduce an earth potential into the installation then it will need to be bonded.

Unfortunately to check it is not providing an earth potential then this is an electrical check on the resistance to earth being >22kohms (or whatever value you use)

Steel bolted to concrete might provide a path as concrete can be surprisingly conductive at times.

 

[Badged01]

Think you will find if you got that steel bolted to concrete its going to give you an 'earth' and needs bonding if you are taking 3-core from the house down there!

 

[Deleted member 9648]

Hi,

I have just been to a job today to provide power to an outbuilding, 75 meters down the garden.

The supply is PME, I plan on splitting the tails into a henley block and then into a switched fuse, then 16mm SWA from there all the way down the garden, at 600mm under the lawn.

The building is a 30ft by 20ft concrete arrangement, sitting on a whopping great big concrete slab. My issue is, there's a steel frame inside, its just bolted into the floor though. Am I correct in thinking this is classed as structural steel or would it need to be more buried in the ground?

If it is classed as structural, I understand why I cannot export PME, so would use 2 core SWA, earth the armoring at the house end, and stick a rod in next to the outbuilding and ground any exposed metal.

I guess I could check to see what the IR is on the steel to a known source, but my wander lead will not stretch that far! Off the top of my head its if its less than 0.02M ohms it needs bonding correct?

Please can I have some thoughts on if its best to make it a TT? I have done quite a few sheds and the like, but never done one that's had steel all round it, always normally just wood!

Thanks

Why cant you 'export PME'.?...you are installing a 16mm SWA which if 3c meets the requirements for a main bond in the outbuilding,(minimum 10mm back to MET). Use the PME and bond the steel back to the DB in the outbuilding in 10mm.

 

[uksparks]

I know you are allowed to export PME, but I was under the impression that you could not do it if exporting to a location containing structural steel as you are kind of mixing TT and PME? Forgive me if I am wrong, I am not as experienced in this area as some. I just thought take earth the armour back to the house end, stick a rod in down the garden and ground the steel to the new DB in the outbuilding and keep that earth separate to the house end. I have looked at a number of other similar threads and I came to conclusion this was the right way to go from what I could tell.

 

[telectrix]

the requirement for extending PME to an outbuilding only concerns the bonding back to the MET of extraneous conductive parts. minimum of 10mm. you can use the PME and bang in a rod as well.

 

[uksparks]

I may have misunderstood then, so what you are saying is take the earth down the garden, take it into the new board, but ground the extraneous parts back to the board too and also stick a rod in? Just make sure its 10mm that connects up to the extraneous parts though. Thanks for the help.

 

[telectrix]

if you use 16mm 3 core SWA, then that 3rd core is adequate to provide bonding ( 544.1 & table 54.8 ). earth the armour at both ends as well. banging in a decent rod is optional.

 

[uksparks]

Got ya! Perfect thanks.

 

[Engineer54]

I know you are allowed to export PME, but I was under the impression that you could not do it if exporting to a location containing structural steel as you are kind of mixing TT and PME? Forgive me if I am wrong, I am not as experienced in this area as some. I just thought take earth the armour back to the house end, stick a rod in down the garden and ground the steel to the new DB in the outbuilding and keep that earth separate to the house end. I have looked at a number of other similar threads and I came to conclusion this was the right way to go from what I could tell.

You can mix, (as you put it) a TT system with either of the TN systems, If you think about it, what is a PME (TNC-S) system, ...it's an earthing system that actually relies on the neutral being grounded (via earth rod or plates) at multiple locations along it's length. So an additional earth rod(s) at the source and/or at an outbuilding of a PME supplied installation can only be a good thing. But don't think for a second that the BS 7671 200 ohm Ra max is going to be a suitable value for earthing a PME neutral, because it ain't!!

Same with TN-S, ...it was in the 50's common practice on Council estates to install an earth rod or plate, in addition to DNO TN-S sheath earthing.

Oh and i wouldn't call 75m a garden, that's more like a small field!! LOL!

 

[telectrix]

just as an aside, 75m of 16mm will give you a current capacity of 31A, allowing for a 3% volt drop, or 54A @5% VD.

 

[Rockingit]

Yep, was thinking that very same thing and you've all beaten me to it - a 16mm armoured will have more than sufficient Cu csa eq to export the PME.

And agreed - 75m is less a garden and more a paddock!!

 

[Badged01]

Have to agree with "Engineer", you can never have too many earths!

Sure you got those numbers the right way round Telectrix?

 

[telectrix]

Have to agree with "Engineer", you can never have too many earths!

Sure you got those numbers the right way round Telectrix?

if i calculated right, 2.8mV/A/m. VD of 6.5V @3%. 11.5V @ 5%. ( the 6.5 should be 6.9V though, which will allow for 33A)

 

[Badged01]

Apology Telectrix, your right, Im wrong --- I didnt read the question correctly! Luckily my exam days are over ........

 

[uksparks]

If you saw my garden you would see why 75m is rather big to me, agreed though, I have a friend with 2 acres and that's more a field than anything. Should I not be working it out on 2.4mV/A/m not 2.8mV/A/m if I am using 3 core though? Its going to have a 6A light circuit on the end a 32A ring, so at most if I said 38A max demand worst case, gives me 6.8v drop which is fine as he will never use that much unless he intends on growing something down there. I agree though, PME and an extra rod is not too different really as you say.

 

[uksparks]

I have used these a few times which are nice little units, does anyone have any other suggestions? 100A SP&N Fused Switch c/w 63A 80A & 100A fuses

 

[telectrix]

the 2.4 figure is for 3/4/ core 3 phase. for single phase, you use the 2 core column in the OSG table as the cpc core is irrelevant for the purpose of calculating VD.