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.

 

[LastManOnline]

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.

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.

 

[cliffed]

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.

Good point thinking the 7.67 would be the obvious @ 30ma

 

[HappyHippyDad]

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.

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.

 

[LastManOnline]

I'm about to sit down to a cup of earl grey and a few pieces of toblerone to watch a film,

Thanks. And sorry to hear you are having such a stressful afternoon

 

[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.

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.

 

[LastManOnline]

Your ECA doc gives a clear explanation

Won't download for me. What's the doc called?. I can Google it

. It's all about keeping simultaneously accessible bits of metal at the same potential

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.

 

[loz2754]

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.

A useful reference

 

[ImpededLoop]

Won't download for me. What's the doc called?. I can Google it

key factsheet: bs 7671: 2008 – extraneous-conductive

Free essays, homework help, flashcards, research papers, book reports, term papers, history, science, politics

studylib.net

 

[LastManOnline]

key factsheet: bs 7671: 2008 – extraneous-conductive

Free essays, homework help, flashcards, research papers, book reports, term papers, history, science, politics

studylib.net

10ma been the threshold for "no let go" is new for me. I had always being led to believe it was 16milli amp.A very Interesting read indeed.

 

[timhoward]

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?!

 

[LastManOnline]

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

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

 

[ImpededLoop]

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.