HIE on a radial circuit.

[RF Lighting]

I've been to look at a job. It's a new radial circuit which requires high integrity earthing.

The circuit is to be supplied from a DB which is a submains fed in 2 core SWA.

This DB supplies some circuits in an existing 3x3 steel trunking, and there are some other circuits supplied from another DB about 6m away.

As the trunking links the earth from the two DBs together, I'm thinking this DB should be ok to supply a circuit requiring HIE as even if one of the earths was to fail, the circuit will still be earthed.

Does this sound right? I could run an additional earth from the main DB to the sub DB but the incommer is in the cellar and the sub DB is on the second floor and there is no riser, so I'd prefer not to.

 

[JohnD]

is this the sort of trunking made of steel channel and held together with nuts and bolts? that people sometimes undo?

 

[RF Lighting]

I would hope that people wouldn't undo the nuts and bolts as trunking is an exposed conductive part, and is required to be earthed. This is done through said bolts.

Also trunking is permitted for use as a CPC, so people really shouldn't be tampering with the nuts and bolts.

I was thinking more along the lines of whether using the earth from a different DB to supply this final circuit would be allowed?

 

[ricicle]

Isn't it only final circuits that require HIE earthing ?

How far do you want to go back upstream to ensure fail-safe earthing ?

 

[RF Lighting]

You might be onto something there actually. I've just been re-reading the regs, and it looks as though as long as the DB has an earth supply of 10.0mm² or larger then it is ok to supply final circuits that require HIE.

It's all in the book

 

[Adam_151]

Regs are in the van, but pretty sure if your PC is greater than 10mm² , which your amourings will be (and there is none of this of equal conductance to copper stuff, this is a purely mechanical consideration) then HIE is met automatically. Of course you'd want to do a quick risk assessment, most eviroments its simply not forseeable that the armour should get pulled out... but if its industrial with fork lift trucks about, then that might not be the case.

Anyway, you've got the trunking between the DB, as long as that is sound then I cant see any problem with taking it as a second earth path. You will then have met and exceeded the requirements for HIE. if you doubt the soundness of the trunking in future as john does you could always put a suitable sized 6491x green/yellow in the trunking between the two boards which are bolted it it but its a bit overkill

 

[17thman]

how many items of equipent are you supplying

 

[17thman]

and is therea similar radial circuit adjacent

 

[RF Lighting]

I think it is highly unlikely that any mechanical damage could happen to the SWA or the trunking. It's only in a commercial building.

The circuit will supply between one and ten items of equipment depending on who is using the circuit.

 

[Adam_151]

I cant see a proble with what you propose rob, your armouring CPC is greater than 10mm (so already meets regs requirement for HIE), unlikely to suffer damage, and you have got a dual path through the trunking anyway. Really cant see an issue

 

[RF Lighting]

No, I'm happy with it all. I'd just got it into my head that the DB required two earths to it such as a 3 core SWA, an extra earth or steel containment for it to be used for HIE.

Having re-read the regs, and read the bits on here, I'm happy that the installation will actually go beyond the requirements of BS7671.