Does this need to be bonded?

[HappyHippyDad]

I have just put in a sub distribution board in the middle of a field for a new supply for some poly tunnels (soil heating mats and lighting). I have made it a TT.

I have noticed that they have concreted in a number of separate metal hollow poles (see picture) which will be part of the polytunnel. I haven't tested these yet, but they will test as extraneous. If they remain as they are (i.e accessible) do they require Main bonding?

I do get confused with bonding! Every year I re-read up on it, feel I've got it, and then either forget it or a new scenario comes along!

My thoughts are that it will need a 10mm into the nearby DB as it is an 'exposed metallic structural part of the building', which is one of the examples of an extraneous conductive part (411.3.1.2). However, each post is a separate piece of extraneous, so I need to continue this 10mm around the poly tunnel to each post. Is this right? Or, does it not require bonding for whatever reason?

PS.. there will be no livestock in the field at any time.

 

[mart spark]

I can't see why you would. They are outside already so will be at true earth potential and by the look of the photo not a part of the electricsl installation so are just pieces of metal. I could be wrong but if they aren't exposed conductive parts and aren't part of a structure then, i dont see why they would need bonding. Otherwise we will be bonding every random piece of outdoor metal.

 

[timhoward]

I get why you are asking. They pass the 3 point test for being extraneous conductive parts.
I know it sounds odd, but I think I'd loop test them!
I'd want to know whether there is any point/benefit connecting a metal pole in the ground to a metal rod in the ground, and whether it actually lowers the impedance to any degree that is meaningful.

I'm sure everything in there will be RCD protected and you'd need arms like Mr Tickle to touch two of them at once. If it proves that they have reasonable impedance anyway I currently don't see the point of bonding them.

 

[LastManOnline]

you'd need arms like Mr Tickle to touch two of them at once.

There, s your answer there. Even within the the home bonding not required if a distance greater than 2.5 mts exists between extraneous parts.

 

[mattg4321]

In the event of a faulty rcd that doesn’t disconnect on a TT system in the event of a fault, I think I’d rather as few exposed conductive/extraneous metal parts as possible.

Will there be a floor going in the poly tunnels or will you effectively be standing in the dirt?

 

[HappyHippyDad]

In the event of a faulty rcd that doesn’t disconnect on a TT system in the event of a fault, I think I’d rather as few exposed conductive/extraneous metal parts as possible.

Will there be a floor going in the poly tunnels or will you effectively be standing in the dirt?

I expect they'll be standing on the earth/dirt.

 

[mattg4321]

I expect they'll be standing on the earth/dirt.

In that case I personally don’t see the benefit of bonding, only the drawback as above.

 

[cliffed]

Are these hollow pipes for the metal arches for the tunnel.

 

[ImpededLoop]

Assuming that they are extraneous and that they will be extended with metallic hoops to for the structure of the polytunnel, I would say that they need to be bonded. You've mentioned a lighting installation, so it might be possible to touch a light fitting and a hoop simultaneously? Also, what about the controls for the heating? Where will they be located and do they have exposed conductive parts?

You may not need a 10mm bonding conductor. Have a look in GN8 as you may be able to use 6mm on your TT system.

 

[cliffed]

Assuming that they are extraneous and that they will be extended with metallic hoops to for the structure of the polytunnel, I would say that they need to be bonded. You've mentioned a lighting installation, so it might be possible to touch a light fitting and a hoop simultaneously? Also, what about the controls for the heating? Where will they be located and do they have exposed conductive parts?

You may not need a 10mm bonding conductor. Have a look in GN8 as you may be able to use 6mm on your TT system.

Defo if accessories are fixed to the structure it will need MBPC.
Maybe the structure will be linked somehow, & thus only one MBPC point

 

[DPG]

Defo if accessories are fixed to the structure it will need MBPC.
Maybe the structure will be linked somehow, & thus only one MBPC point

That's what I was thinking - maybe all the arched poles are joined together by a central pole at the top.

 

[Zerax]

Are these hollow pipes for the metal arches for the tunnel.

I suspect they'll be the hooped plastic pipe type supports... so MDPE water pipe just stuffed into those hollow pipes. The PVC sheeting, when tight, will be part of the structure... until a good gale of wind rips it and it all ends up in the next county !

 

[LastManOnline]

You may not need a 10mm bonding conductor.

A 10mm metal bond is not required in this case as the resistance of the extraneous parts in question will be much higher than the usual extraneous items we deal with such as water pipes and other metallic services. A 4mm bond will be more than adequate.

 

[timhoward]

It would still be fun to loop test them, if only
to have a good laugh if they have lower impedance than the official earth electrode….

We do have to remember that bonding in this case is joining a metal thing in the ground to another metal thing in the ground.

 

[pc1966]

In the event of a faulty rcd that doesn’t disconnect on a TT system in the event of a fault, I think I’d rather as few exposed conductive/extraneous metal parts as possible.

It might be worth using a 100mA delay incomer, while providing little in the way if direct contact shock protection that way a fault will clear even if a final circuit RCD has failed.

 

[LastManOnline]

It would still be fun to loop test them, if only
to have a good laugh if they have lower impedance than the official earth electrode….

Yes. Would be interesting to know what the total Fli is with all metal legs tied together

We do have to remember that bonding in this case is joining a metal thing in the ground to another metal thing in the ground.

Bonding in a home assumes there is little or no hand to foot fault current current flow.
This does, nt apply outside.

Bonding in the home assumes connected metalwork has a low resistance to earth.
This is unlikely to apply in OP, s case.

So what will bonding achieve?. If there is a fault in a class 1 accessory all metalwork becomes live providing multiple fault paths to "real earth".
I think I would prefer unbonded metalwork in this case and ideally use class 2 light fittings /accessories that may be attached to the metalwork

 

[timhoward]

I'm in agreement with @LastManOnline. As far as I can see, in this situation, all bonding does is give you more chances to die if the RCD fails.

If the person was reasonably insulated from real earth, e.g. standing on a giant rubber mat, then there are obvious benefits to to ensuring that a faulty appliance/light casing and the poles are held at the same potential, so under fault conditions touching either or both wouldn't cause a shock.
But in this case there is an instant ~1000 ohm path to real earth via the human to soil. Touch one thing live, never mind two, and you have a problem. So in this situation limiting the number of items you can touch at mains potential does seem preferable to me.
The only other aspect would be the poles becoming directly live themselves, e.g. a faulty class 1 heater resting against one. As it's TT then we'd be relying on the RCD anyway as limited current would flow. I like the idea of an additional up-front S type 100ma RCD and 30ma RCBOs so you get some redundancy in fault conditions.

 

[brianmoooore]

I suspect they'll be the hooped plastic pipe type supports... so MDPE water pipe just stuffed into those hollow pipes. The PVC sheeting, when tight, will be part of the structure..

This is the method of construction of the ones I've seen around here.

 

[HappyHippyDad]

Went back there today for 20 mins and tested the posts. They test 101ohms between posts and the same between post and rod.
I didn't do an EFLI test on the posts though, sorry!
I can't quite see how bonding helps in this case.
All posts will have a similar Ra to the rod. If there is a fault to earth the RCD will trip.
If a person came into contact with one of the posts whilst simultaneously touching an exposed conductive part that was somehow live, the RCD would trip as the posts are <1667ohms.

 

[cliffed]

This may help determining an extraneous conductive part