Metal workshop in shipping container

[Tim pan]

Some friends are setting up a metal workshop inside a shipping container, on site at a commercial building that rents out studios, workshop space etc.

There is a 16mm SWA providing a three phase supply to a board inside the container, I looked and it’s connected inside the main building to a memshield 2 on a 63a MCB. They’ve already got an electrician in to connect the supply cable and wire up the workshop. Inside the shipping container they have some single and three phase commando sockets which will provide power for two welders and plasma cutter. The whole container is bonded to the board inside it, the earth is connected to the main building only using the armoured cable. I’m not sure what the earthing arrangement for the main building is.

I was there just having a nose around, and had a couple of questions. First, they plan on welding to the shipping container itself to fix a couple of leaks. I’m not a welder, I’m not sure if this would be an issue? Would there be a problem with, say, a massive current suddenly finding an unintended return earth path and melting everything in the workshop?

Shouldn’t they have an additional bonding arrangement to the main building , a separate 10mm earth cable rather than just the cable armourings?

Any other glaring issues?

I have advised them to at least get a proper EICR before they fire up the plasma cutter…

 

[ipf]

If I remember correctly, with 16mm swa cable, extra earthing is required as the armouring csa isn't sufficient.

 

[Tim pan]

Yeah - looking at table 54.7, might it be k of (line conductor/ k of cpc x csa of line conductor? (k1/k2) x S

So for 90• thermosetting cable, (143/46) x 16. Which would equal 49..mm2 ?

 

[westward10]

You need to verify the earthing system.

 

[pc1966]

If it is TN-C-S then as an extraneous conductive object it should be at least 10mm Cu equivalent, 16mm 4C is about 50mm steel so about 6mm copper so you ought to have a 10mm CPC in parallel then.

Otherwise it can be suited to the MCB's adiabatic limit. No idea what it would be for that case, but D-curve 63A Hager MCB has max of around 400k A2s so from Table 54.4 I get about 12.4mm steel which is met.

Or as above you can scale phase size using 54.7

I agree that welding from that supply to the related earth might not be a good idea. I have no idea if welders typically have the output well-isolated from the supply earth or not but I would be more inclined to use a gas welder or some separate supply like small stand-along generator.

 

[James]

welding in itself should not be a problem assuming the welders earth clamp is properly connected to the container.
however I have had to replace half the cables in an industrial unit because an earth clamp was not fitted correctly to an item being welded that was hanging from a chain hoist.

the welding current then traced back down the cpc for the chain hoist and melted like a cheese wire through many of the other cables in the trunking.

 

[ipf]

welding in itself should not be a problem assuming the welders earth clamp is properly connected to the container.
however I have had to replace half the cables in an industrial unit because an earth clamp was not fitted correctly to an item being welded that was hanging from a chain hoist.

the welding current then traced back down the cpc for the chain hoist and melted like a cheese wire through many of the other cables in the trunking.

Had basically the same a long while back at a local village. Two local blokes had an engineering works, old fashioned. even then, and pretty basic with protection. A timber building with services at one end and an open, covered area at the other, basically a welding shed. About 35m of cpc completely frazzled and lucky someone spotted the burning on a timber beam before the sheds went up.
One of the partners was that tight they stopped welding for a week before agreeing to replace and upgrade it.
No such thing as Health and Safety in such places.

 

[davesparks]

Shouldn’t they have an additional bonding arrangement to the main building , a separate 10mm earth cable rather than just the cable armourings?

From your description of the premesis I doubt that the main bonding would be as small as 10mm.

I have advised them to at least get a proper EICR before they fire up the plasma cutter…

Assuming this is a new installation then an EICR is not necessary, they should have been given an EIC for the work.

 

[Tim pan]

Thanks all. I’ll be telling them about that chain hoist incident.

Turns out that the main building is actually supplied by its own substation. Nobody from the company seem to be certain what the earthing arrangement is, but I wonder if it is more likely TNS given that fact?

 

[davesparks]

Thanks all. I’ll be telling them about that chain hoist incident.

Turns out that the main building is actually supplied by its own substation. Nobody from the company seem to be certain what the earthing arrangement is, but I wonder if it is more likely TNS given that fact?

Yes it will almost certainly be TNS if it has a direct feed from a substation.