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]

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.

 

[Initforthemoney]

Hi guys,

Similar situation here, except, the aluminium tunnel support hoops are bolted to a new concrete pad.

I havent started yet, but would the hoops need bonding?

The only thing that is going in is a small db, and two double sockets on a ply board mounted on a bespoke metal tube frame at one end.