Equipotential bonding - steel frame building

[Jim Backus]

My brother has asked me to look at some wiring in old farm buildings. I've done wiring in the past and am an electrical / electronics engineer so know I can do it safely, but I'm not clear about the requirements for bonding in steel farmed buildings. Reading the regs suggest that it would be necessary to bond across every bolted joint, but steel is a pretty good conductor so is it normal to consider the bolted metal to metal joints are sufficient? One building, a grain store, has been wired quite recently; it has a separate earth wire to the local distribution box in addition to the cable armour wired back to the earth/neutral bonding point, but there is no bonding across every bolted joint. The building I'm looking at used to be used as a cattle shed and only needs the lighting refurbishing. I'm tempted to run a single bond wire to a convenient point and rely on the bolted steel joints giving a good earth to the remainder. I've also considered testing to confirm that there is a good earth to all the extraneous metalwork.

What is considered best practice? and if it's different what is the minimum acceptable?

TIA

Jim

[R D S]

I had this issue last year when I was giving up a lease on a building. It needed a new certificate. One electrician said that each joint would need bonding, another said that one would be fine.

I checked that the council's own electricians were happy with this before having the job done and they were so we went with the latter.

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[dom]

Can't answer your question, but I'm aware there's additional wiring regs for buildings with livestock - you might want to check that section for additional info.

[Andrew Gabriel]

You really should get an electrician experienced with livestock premises if the building or location is going to have any livestock present. The danger from earth leakages is much higher than it is for humans - a leakage which you can't even feel can kill a barn or field full of livestock all too easily. You need an electrician who is familiar with designing solutions for livestock situations. ISTR someone on this newsgroup lost one or more horses this way in the distant past.

[David Hansen]

On Fri, 12 Mar 2010 11:14:35 GMT someone who may be "Jim Backus" wrote this:-

There is not a simple answer to this simple question.

If there is a reliable metal to metal joint then that is more effective than any bit of copper cable. For example where copper pipes are joined by soldered or compression fittings then only someone paid by the hour and for materials would suggest bonding across the joints.

So the first question is, is there reliable metal to metal contact? I have no idea, you will need to decide, possibly with the help of someone else, which is best done with a meter.

If the answer is yes then the second question is, is this reliable metal to metal contact likely to remain until the next inspection? Again not a question I can answer. If rain is getting in, or the use of the building is likely to cause condensation inside (for example animals, wet plants) then the answer may be no.

I agree with the point others have made about experienced firms. This is not something for house wiring bodgers, it is something for those with experience, probably a well known firm of local electrical contractors rather then a one man band. That is not to say that you may not find a one man band who has the relevant skills, but it is more likely they are in a larger and well established firm.

[Tim Lamb]

In message , David Hansen writes

Hmm..

BS 5502 barns are bolted together using plated (probably zinc) bolts.

The portal frames are bedded in concrete pads, usually 0.9m cube.

The frame can be galvanised for livestock housing but, more usually comes with a coat of primer.

Stanchion to rafter and rafter to rafter at ridge generally uses 6 bolts each.

Some barns use timber purlins but most are Z section galvanised steel bolted at each rafter. A barn with timber purlins and cement fibre roof would have no conductive path between frames other than the soil between the pads.

None of my barns have any extra strapping.

Fortunately the supply here is provided with an earth. PME might be a different kettle of fish.

regards

The bolts

[Owain]

In addition, equipment in a grain store may need additional protection against dust ingress, and some dusts can be explosive.

Owain

[James Salisbury]

Hi, I would strongly advise that you get a good set of local sparkies in. There are problems with livestock, in that figures of 30v or less have been known to kill livestock and due to the longer distances between front and back hooves compared to human feet the risks are even greater. How much is one or more dead cows worth? I would suggest that you have a glance at the 17th Ed and see how much has been written on this and also do you have all the kit, such as earth rod resistance checks?