Does this need to be bonded?

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HappyHippyDad

HappyHippyDad

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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.

20220202_151137.jpg
 
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.
 
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.
 
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?
 
mattg4321 said:
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?
Click to expand...
I expect they'll be standing on the earth/dirt.
 
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.
 
ImpededLoop said:
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.
Click to expand...
Defo if accessories are fixed to the structure it will need MBPC.
Maybe the structure will be linked somehow, & thus only one MBPC point
 
cliffed said:
Defo if accessories are fixed to the structure it will need MBPC.
Maybe the structure will be linked somehow, & thus only one MBPC point
Click to expand...

That's what I was thinking - maybe all the arched poles are joined together by a central pole at the top.
 
cliffed said:
Are these hollow pipes for the metal arches for the tunnel.
Click to expand...
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 !
 
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.
 
mattg4321 said:
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.
Click to expand...
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.
 
timhoward said:
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….
Click to expand...
Yes. Would be interesting to know what the total Fli is with all metal legs tied together
timhoward said:
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.
Click to expand...
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
 
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.
 
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 said:
This may help determining an extraneous conductive part
Click to expand...
Hi cliffed,
These posts are definitely extraneous as they measure 101Ω to the MET. However, they are pretty much the same as the rod in the ground (66ohms). If anyone touched them under fault conditions the RCD would trip as they are below 1667ohms.

I've heard conflicting arguments about this.

Its clear to me why an Extraneous Conductive Part (ExtCP) needs bonding IF it measures between 1667Ω and 22KΩ to the MET, as between this range the current into the human (poorly worded) between a fault and the ExtCP will be >10mA.

The bit that is not clear to me is why do we still need to bond IF the ExtCP has a sufficiently low resistance to MET, meaning that the RCD will trip if we come into contact with it under fault conditions.

I have used 1667Ω as this is the known figure to trip a 30mA RCD in order to limit voltage to 50V. However, we use a figure of 200Ω for stability reasons AND we would have to be certain that the ExtCP is not going to be altered in any way (thus perhaps increasing its resistance to between 1667Ω and 22KΩ).

In my scenario, the posts are <200Ω to earth AND are not going to be altered, so why do they need bonding? What risk do they pose?
 
Last edited:
HappyHippyDad said:
Hi cliffed,
These posts are definitely extraneous as they measure 101Ω to the MET. However, they are pretty much the same as the rod in the ground (66ohms). If anyone touched them under fault conditions the RCD would trip as they are below 1667ohms.

I've heard conflicting arguments about this.

Its clear to me why an Extraneous Conductive Part (ExtCP) needs bonding IF it measures between 1667Ω and 22KΩ to the MET, as between this range the touch voltage between a fault and the ExtCP will be >10mA.

The bit that is not clear to me is why do we still need to bond IF the ExtCP has a sufficiently low resistance to MET, meaning that the RCD will trip if we come into contact with it under fault conditions.

I have used 1667Ω as this is the known figure to trip a 30mA RCD in order to limit voltage to 50V. However, we use a figure of 200Ω for stability reasons AND we would have to be certain that the ExtCP is not going to be altered in any way (thus perhaps increasing its resistance to between 1667Ω and 22KΩ).

In my scenario, the posts are <200Ω to earth AND are not going to be altered, so why do they need bonding? What risk do they pose?
Click to expand...
They may need bonding for a definite earth connection reference point
 
Can't see the point of connecting a metal rod in the ground to another metal rod in the ground.

Or can I?
 
pc1966 said:
Depends on the goal. Linking them lowers Ra and also under high current faults could drop the step potential gradient a bit, but really to goal is to avoid it being needed in the first place!
Click to expand...

Well, a bit. But the Ra of the proper earth rod is not going to be helped much by the resistance of the hoops I wouldn't have thought.
 
DPG said:
Well, a bit. But the Ra of the proper earth rod is not going to be helped much by the resistance of the hoops I wouldn't have thought.
Click to expand...
Probably not.

But ultimately for a TT system the issue of CPC being not being elevated comes down to reliable RCD protection, I would go for a dual RCD box (100mA incomer, few 30mA RCBO with DP switching per circuit), a decently reliable rod, and not bother with any bonding.
 
Last edited:
Pretty Mouth said:
If you use 10mA for the calculation, you get 22k ohms, if you use 30mA you get 6.67k ohms (not, i believe, 7.67k ohms)
Click to expand...
You would get 7.67k ohms with the calculation given. It's just that the calculation we generally use includes taking off 1k ohms for resistance through the body. Hence 22k rather than 23k, or in this case 6.67k rather than 7.67k.
 
HappyHippyDad said:
You would get 7.67k ohms with the calculation given. It's just that the calculation we generally use includes taking off 1k ohms for resistance through the body. Hence 22k rather than 23k, or in this case 6.67k rather than 7.67k.
Click to expand...
So which of the above figures would be regarded as the cut off point for when something is regarded as extraneous?. There is quite a gap between 7.67 and 22 Kohm.
 
LastManOnline said:
So which of the above figures would be regarded as the cut off point for when something is regarded as extraneous?. There is quite a gap between 7.67 and 22 Kohm.
Click to expand...
I'm about to sit down to a cup of earl grey and a few pieces of toblerone to watch a film, so i'm afraid I haven't got the regs to hand. However, this PDF explains it quite well.
I would always class anything below 23K ohms as extraneous.
 

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  • Extraneous-conductive-parts-17_edition_factsheet_05.pdf
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ImpededLoop said:
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.
Click to expand...

Your ECA doc gives a clear explanation and I think I would want to bond it if the hoops are metallic (they may well be plastic). It's all about keeping simultaneously accessible bits of metal at the same potential (or no potential at all if they're not extraneous - ie don't bond random metal bits and bobs if they're not extraneous!). The fact it's TT makes no difference and if the RCD fails makes it all the more important.
 
ImpededLoop said:
Your ECA doc gives a clear explanation
Click to expand...
Won't download for me. What's the doc called?. I can Google it
ImpededLoop said:
. It's all about keeping simultaneously accessible bits of metal at the same potential
Click to expand...
If simultaneousely accessible and class 1 accessories are used then that changes the scenario significantly. But I understand that previous posts established that the metal posts were not simultaneousely accessible.
 
It's a great document and one of the clearest explanations I've ever seen.
I'm not sure it helps this situation though other than establishing a helpful target let-go current.
See the diagram below and note the bit circled. That wouldn't be there in the situation this thread is discussing.

Isn't the point here that in all cases there's an 1000 ohm path straight to earth with at least 230ma current flowing, whether you add a parallel path through a unbonded pole (roughly 1000+ 110 ohms using value to MET) or a parallel path through an bonded pole (roughly 1000 + 66 ohms)?
Am I thinking about this right or losing marbles?!
1644166027646.png
 
timhoward said:
It's a great document and one of the clearest explanations I've ever seen.
I'm not sure it helps this situation though other than establishing a helpful target let-go current.
See the diagram below and note the bit circled. That wouldn't be there in the situation this thread is discussing.

Isn't the point here that in all cases there's an 1000 ohm path straight to earth with at least 230ma current flowing, whether you add a parallel path through a unbonded pole (roughly 1000+ 110 ohms using value to MET) or a parallel path through an bonded pole (roughly 1000 + 66 ohms)?
Am I thinking about this right or losing marbles?!
View attachment 94766
Click to expand...
No. Your marbles are very much where they are meant to be. But if I may offer my final comment on this post, it's this. Bonding as a topic is the "gift that keeps on giving" from the point of view that since I, ve been an apprentice no other single electrical subject has caused me more "head scratching" (along with the occasional headache.) The electrical game would be very boring without it🙂
 
By standing on the soil you'd probably be wearing shoes that provide some level of insulation. Touching a metal pole with bare skin will give a lower impedance to earth. Also it would be a case of resistors in parallel so the two paths combined would give a reduced total impedance to earth.
 

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