Advice for providing power to a barn conversion

[The General]

I'm going to throw a swerve ball in this; if I'm reading and understanding the OP correctly then he's talking about taking the feed from the meter, splitting it, one goes to house via tails and one goes to barn via a KMF. Divorced earth at the barn end and the armour for the SWA only glanded at that end as well.

Personally, can't see any big issues with that. BGB doesn't say WHERE an armour has to be earthed, just it does. The swa becomes a 5s circuit anyway as it's not a final.

That' exactly what I'm talking about!
I was lying awake in bed last night and I couldn't think of any good reason why the armour has to be earthed at the supply side. In reality, earthing (via MET) to a rod on the supply side, or eathing (via MET) to a rod at the barn - and the two rods are less than 4ft away from each other!

 

[The General]

That was my reading of it, yes. One supply, 2 x TT's.

You'd need to have your standard 30mA RCD setup each sub.

Yes, TT for the main house, and another rod for the barn.
I was planning to use 30mA RCBOs for each circuit.

 

[Rockingit]

That' exactly the two rods are less than 4ft away from each other!

That's the only thing I'd raise an eyebrow to, as they will interfere with each other. See if you can get them further apart.

 

[The General]

That's the only thing I'd raise an eyebrow to, as they will interfere with each other. See if you can get them further apart.

Hmm. Not considered that. Clients are quite specific about what they do an don't like so the location of the new rod was decided by them. If there'd been any choice in the matter I'd have run some new armour clipped to the underside of a raised step across the path straight into the meter box - but that apparently wasn't acceptable aesthetically.....

Anyhoo.
How far apart would they need to be?

 

[Rockingit]

How far apart would they need to be?

Ooh....that's a good question! Off the top of my head I'd say greater than 3xdepth of rod, but happy to be corrected! Nowhere near a BGB all day.

 

[HandySparks]

Given the likely Ra of the TT earthing (existing or new), don't you need to put an RCD (100mA, type S?) at the supply end of the distribution circuit to provide protection in the event of an earth fault?

I'd be inclined to put a new rod in for the outbuilding, but connect the SWA armour at both ends, so that you end up with multiple rods for the whole installation. I don't see a need to separate the earthing of the two parts of the system.

 

[Richard Burns]

I would say for separation 1.5 X depth of rod would be sufficient for this installation

 

[sparks1234]

I would say for separation 1.5 X depth of rod would be sufficient for this installation

Come in no 54, your time is up!!!!

 

[Rockingit]

Given the likely Ra of the TT earthing (existing or new), don't you need to put an RCD (100mA, type S?) at the supply end of the distribution circuit to provide protection in the event of an earth fault?

I'd be inclined to put a new rod in for the outbuilding, but connect the SWA armour at both ends, so that you end up with multiple rods for the whole installation. I don't see a need to separate the earthing of the two parts of the system.

Wasn't the issue to do with it only being a 2c armoured which might not have sufficient csa to satisfy bonding requirements? And if the whole install is on RCBO I can't see what benefit is achieved by adding a 100mA at the upfront end when all it's protecting is one set of internal tails installed in a non-vulnerable method and an armoured cable.

 

[HandySparks]

Wasn't the issue to do with it only being a 2c armoured which might not have sufficient csa to satisfy bonding requirements? And if the whole install is on RCBO I can't see what benefit is achieved by adding a 100mA at the upfront end when all it's protecting is one set of internal tails installed in a non-vulnerable method and an armoured cable.

OK, maybe it's an unlikely fault, but if the SWA is damaged and the line becomes connected to the armour, what provides disconnection in the time required (1 second for a TT distribution circuit) if it's not covered by an RCD? The cartridge fuse in the KMF won't blow in the required time due to the low earth fault current on a typical TT installation. See 411.3.2.1 and 411.3.2.4

 

[Rockingit]

OK, maybe it's an unlikely fault, but if the SWA is damaged and the line becomes connected to the armour, what provides disconnection in the time required (1 second for a TT distribution circuit) if it's not covered by an RCD? The cartridge fuse in the KMF won't blow in the required time due to the low earth fault current on a typical TT installation. See 411.3.2.1 and 411.3.2.4

A valid point and one that I actually included then deleted out again on an earlier post as I didn't want to muddy the waters! We usually adhere to the convention that an armoured cable does not require RCD protection - and in this case it seems incredibly unlikely that anyone is ever going to stick a digger bucket through it, either! OK, so add another ££ onto the job and fit an upstream 100mA S type, just for peace of mind. Now, how often do you think that RCD will get routinely tested?

 

[HandySparks]

Now, how often do you think that RCD will get routinely tested?

About as often as all the other RCDs / RCBOs in the installation, i.e. never, or once every 10 years if the householder is especially diligent and gets an EICR done!

 

[sarusystems]

when we converted our barn we asked for a separate supply with its own meter to be installed
and letting it selling it etc. does not cause any issues

 

[HandySparks]

We usually adhere to the convention that an armoured cable does not require RCD protection ...

Yes, if the Zs is low enough to give proper earth fault disconnection by the MCB or fuse, but on TT I'd RCD it every time. I don't see how you can comply otherwise.

 

[HandySparks]

when we converted our barn we asked for a separate supply with its own meter to be installed
and letting it selling it etc. does not cause any issues

Indeed. Given the sorts of loads that the OP has described, this might well be the best solution, unless the load from the main house is very low.