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Supplementary bonding question

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Supplementary bonding question

Posted by TomLonsborough on Jan 16, 2020 1:29 pm

Alright folks,

Looking for a bit of advice with supplementary bonding. Bit of background - I'm in the process of repairing some metal bollards: I decided the best option was to rip out the old guts which comprised of some old fluorescents complete with all the control gear and no IP rating, and replace it with an IP rated LED flood. Supply is via SWA into a fused block; I've terminated the included LED flex into this block.

Question is this: do the bollards require bonding?

Initially I thought no, because they are simply a housing for the self-contained flood and the block. However upon taking a continuity reading between certain parts of some of the bollards and the MET, I was getting around 3 Ohms. R < 50A/100 (C 10A MCB, no RCD protection) is therefore not satisfied. I decided to add fly-leads connected to the block via bolts screwed into the bollard housing. Problem is the because of the condition of the bollards, i.e. rusty, painted in parts, continuity readings now vary from 0.2 Ohms to no reading at all. An insulation resistance test between MET and various parts of the bollards are also coming back as not satisfactory, i.e. <22000 Ohms. Am I potentially introducing a potential here under fault conditions with these fly leads which would not be present otherwise? Save for replacing the bollards with shiny new ones altogether, I'm not sure what I could do!

Any advice would be gratefully received!

Tom.

TL;DR Supplementary bonding of rusty metal bollards! Necessary or not?

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Re: Supplementary bonding question

Posted by Zoomup on Jan 16, 2020 1:54 pm

When I was involved with street lighting years ago for a local council, we always earthed the metal columns and bollards onto a dedicated earth stud included by the manufacturers of the column or bollard. Earthing was required. There again in those days the lights were either sodium or fluorescent with T&E cable running up to the head of the columns. On a windy day when the columns swayed back and forth in the wind you could hear the internal cable slapping against the inside of the columns, so mechanical damage was possible. If the new internal gear is not rated as double insulated (Class 2) by the manufacturers there is an argument that the metal bollards should be earthed. Also if conductors are just single insulated there is also a reason to earth the metal bollards. The metal chains that link the bollards could potentially transfer a dangerous Voltage between the metal bollards in case of a fault. Those chains could be touched by passersby or boaters.

Z.

Re: Supplementary bonding question

Posted by AJJewsbury on Jan 16, 2020 4:31 pm

I'd also want to treat them like lampposts - so unless there's segregation between the earthing system and the bollard then they'd be extraneous-conductive-parts and as they'd introduce a potential from outside the installation, by the letter of BS 7671 they'd need main bonding (rather than supplementary).  It looks like a marina situation so I'm guessing it's not a PME system, but even if it was you might be able to use 714.411.203 to justify a 6mm² bonding conductor instead of 10mm².

Is that a BS 951 clamp on SWA I see?

   - Andy.
 

Re: Supplementary bonding question

Posted by Zoomup on Jan 16, 2020 7:19 pm



Is that a BS 951 clamp on SWA?

   
 

Two in fact, one on the supply S.W.A. and another on the outgoing S.W.A.

I assumed that the location was either a riverside walk or canal side walk leading up to a lock. I hope that we find out.

I am not sure that this location falls under section 709 which applies only to circuits intended to supply craft or houseboats in marinas or similar locations.

Z.

Re: Supplementary bonding question

Posted by mapj1 on Jan 16, 2020 9:59 pm

If there is nothing to gland the SWA too, and you have no desire to make it too complex by adding glands, then removing the clamp and combing the armour strands into one or more  tails and crimping to that will be better than at present crushing onto nothing very much and perhaps damaging the inner insulation.
I have seen offcuts of steel conduit threaded like a bead onto the cable and used as a hard cylinder to crush a clamp down onto, but if you are going to the trouble of undoing the cable to thread something on, the correct thing to use is a real gland.
You are certainly doing the right thing by earthing the bollards, but clearly the forces of decay are not in your favour.  Ideally at the new earthing point the metal would be buffed to a shiny silver polish with a counter bore bit if there is a hole or a spot face cutter if there is a fixed stud, or more realistically as you are not in the factory but outside kneeling in a puddle. a wire brush or flap disk held in at a jaunty angle in the electric drill (goggles!!), but I appreciate that access is not the easiest. Stainless steel star  washers and a lot of 'graunch' may help. Once you have a decent contact,  then slobber it with vaseline using a child's paint brush.
 
regards Mike

Re: Supplementary bonding question

Posted by Alan Capon on Jan 16, 2020 10:38 pm

mapj1:
. . . I have seen offcuts of steel conduit threaded like a bead onto the cable and used as a hard cylinder to crush a clamp down onto, but if you are going to the trouble of undoing the cable to thread something on, the correct thing to use is a real gland. . . 

In the past, we have used a kit with two semicircular pieces that will slide in between the armour and the inner. This allows the clamp to be tightened down onto something rigid without damaging the inner, or having to disconnect everything. 

Regards,

Alan. 

Re: Supplementary bonding question

Posted by AJJewsbury on Jan 17, 2020 9:47 am

Or the street lighting fraternity seem to like jubilee clip versions - like this: https://www.crlighting.co.uk/shop/accessories/electrical-accessories/cr-003-10r/- again it has some support under the strands of armour for the clip to tighten onto
  - Andy.

Re: Supplementary bonding question

Posted by mapj1 on Jan 17, 2020 10:28 am

When  it is not like this here - to me the middle example is polarity crossed
regards Mike

Re: Supplementary bonding question

Posted by TomLonsborough on Jan 17, 2020 10:33 am

Thanks for all the replies folks.

Just to address a couple of points raised:
- I would suggest that if the new LED floods were class 2, there would be no need to bond the bollards... however they are class 1.
- I don't think it comes under section 709, rather 714, it's a canal-side walk way, it is not in use by any boats, no locks etc.
- I would suggest that the bollard is extraneous rather than exposed, it is simply a housing for other enclosures.
- The termination of the SWA and use of BS951 clamps is not on the scope of works, but I will raise it on the test cert. Results obtained for Zs are satisfactory in it's current state.
- Where might I find whether or not the bollard requires 6mm bonding back to the MET, or whether supplementary from the earthing terminal in the fuse block will suffice in the regs? Is there anything extra in there which isn't covered in 714.411.203? It is unclear whether a specific earth connection has been provided in the bollard or not.

Many thanks in advance!
 

Re: Supplementary bonding question

Posted by mapj1 on Jan 17, 2020 10:49 am

if the new LED floods were class 2, And all wiring with them was insulated to a standard equivalent to class 2,( i.e. insulated and over sheathed all the way into into an insulated enclosure) there would be no need to bond the bollards...

Not sure if you want regs to the letter the SWA meets that - the inner plastic over the cores is a mechanical bedding for the cores is not supposed to be relied on as insulating sheathing - though in practice it will be fine.
You could supply from an RCD, and call it TT. You may not wish to, I suspect too much tripping.
What is the rating of the circuit supplying this - the 10mm recommendation as if it was a main bond is really only required if it is credible that a large fault current can flow through it. Unless the bollards are very good electrodes, the Zs of the live and CPC loop will determine what is safe.
regards Mike

Re: Supplementary bonding question

Posted by Sparkingchip on Jan 17, 2020 1:07 pm

Does the scope of work include replacing the glazing?

Andy Betteridge 

Re: Supplementary bonding question

Posted by AJJewsbury on Jan 17, 2020 1:20 pm

Where might I find whether or not the bollard requires 6mm bonding back to the MET, or whether supplementary from the earthing terminal in the fuse block will suffice in the regs? Is there anything extra in there which isn't covered in 714.411.203?

If you've decide the bollard is an extraneous-conductive-part then 411.3.1.2 gives the requirements for main bonding. Supplementary bonding is only specified where additional protection is required (415.2) or where ADS isn't feasible (419.3). Main bonding doesn't necessarily need a separate conductor back to the MET - a c.p.c. can be utilized provided it meets the requirements for a bonding conductor as well as the requirements for a c.p.c. (Main bonds are often taken from remote CUs fed by a submain - the c.p.c. in the submain acts as part of the bonding connection between the CU and the MET.) Note that the minimum specified by reg 544.1.1 is a flat 6mm² - regardless of material - on that point there's no need to work to a 'copper equivalent' - which might give you some leeway if your system isn't PME.

That said, for items like this that will have a significant resistance to true earth, the normal c.s.a.s for main bonding conductors are way more than physics would ever require as any current (whether due to faults or diverted N currents on a PME system) would be limited by the bollard's resistance to earth - so in practice much smaller c.s.a. bonding connections may be perfectly adequate.  It's unfortunate it's a consideration that BS 7671 seems to have overlooked, but is perhaps worth considering if your aim is a safe/adequate system rather than to-the-letter BS 7671 compliance, or are willing to put your name to it as an intended departure from the regs.

   - Andy.

Re: Supplementary bonding question

Posted by TomLonsborough on Jan 17, 2020 1:31 pm

Hi Andy,

Yes it does,

Thanks T

Re: Supplementary bonding question

Posted by Sparkingchip on Jan 17, 2020 6:35 pm

Look on the bright side, I went to repair some lighting columns in a pub car park and they had plug sockets inside them for the Christmas lights and strings of lights in the trees that had been there for so long the branches had grown around the cables.

Andy B.

Re: Supplementary bonding question

Posted by Sparkingchip on Jan 17, 2020 11:46 pm

TomLonsborough:
Hi Andy,

Yes it does,

Thanks T

 
So the IP rating is being restored.

It still leaves you trying to polish a t##d.

Pipe earth clamps on SWA armour is a no, no.

Two or more pipe earth clamps interconnected using their fixing straps is also a no, no, even on adjacent water pipes.

Andy B.

Re: Supplementary bonding question

Posted by ebee on Jan 18, 2020 8:00 am

Andy,
"Two or more pipe earth clamps interconnected using their fixing straps is also a no, no, even on adjacent water pipes."
Are you refering mechanical and/or electrical connection?
I have seen the long free end of a clamp on a pipe fitted under the clamping screw of an adjacent clamp in order to acheive electrical continuity. I fact I have seen several (say 5) in a line. Looks neat but I`d rather see a cable connection to each one.
I`ve also seen two or more clamped so as to extend length therefore pipe diameter served (albeit cast iron!!!) and reasoned awh well mechanically it`s just as sound as using one clamp properly (although two points of possible failure rather than one, in that respect).,
What I will admit to is, some years ago, I got a friendly mechanical engineer to turn me some copper bobbins, internal bore to suit microbore copper tube and external diameter large enough for earth clamps to fasten, the bobbins were then machined in half so you`d put the two halves on the microbore pipe then clamp over them. Tin hat time perhaps?
 

Re: Supplementary bonding question

Posted by Sparkingchip on Jan 18, 2020 8:41 am

It used to be common practice some years ago to interconnect the adjacent pipe clamps using the fixing strap on one of them.

The message from above was don’t do it, it does however appear to have been done in these bollards.

Andy Betteridge 

 

Re: Supplementary bonding question

Posted by Chris Pearson on Jan 18, 2020 11:46 am

ebee:
What I will admit to is, some years ago, I got a friendly mechanical engineer to turn me some copper bobbins, internal bore to suit microbore copper tube and external diameter large enough for earth clamps to fasten, the bobbins were then machined in half so you`d put the two halves on the microbore pipe then clamp over them. Tin hat time perhaps?

What an excellent idea! AFAIK, there is no proper clamp for pipe which is smaller than 15 mm.

Re: Supplementary bonding question

Posted by Zoomup on Jan 18, 2020 7:34 pm

mapj1:
When  it is not like this here - to me the middle example is polarity crossed

Perhaps they only had red tails at the time and the ends out of view are correctly identified. I have come across many tails that  are just red and red or black and black where the installers apparently did not have the correct colour cables available or had run out.

Z.

Re: Supplementary bonding question

Posted by Zoomup on Jan 18, 2020 7:44 pm

I have had experience with enclosed illuminated bollards where heavy condensation built up inside the bollards under certain weather and temperature conditions. As the air in the bollards is in a dead space the condensation builds up and is unable to evaporate away into drier air. The bollard is like a closed plastic bag with just the underground bottom open a bit. Could heavy condensation inside the bollards cause electrical tracking  that could cause tingles on the metal bollards to people or animals touching the bollards under this condition? The internal rust must have been caused by moisture. If this is a possibility then earthing of the metal bollards is desirable.

Z.

Re: Supplementary bonding question

Posted by Zoomup on Jan 18, 2020 8:01 pm

 

What an excellent idea! AFAIK, there is no proper clamp for pipe which is smaller than 15 mm.

 

Bare copper "Pyro" clips do very well. They can even be soldered if required.

Z.

Re: Supplementary bonding question

Posted by Zoomup on Jan 18, 2020 8:04 pm

The bollard condensation issue has been discussed before.

https://www2.theiet.org/forums/forum/messageview.cfm?catid=205&threadid=58343

Z.

Re: Supplementary bonding question

Posted by AJJewsbury on Jan 19, 2020 2:19 pm

there is no proper clamp for pipe which is smaller than 15 mm.

There are some (if non BS 951) - I've used these in the past for supplementary bonding in a bathroom with microbore heating pipes - https://uk.rs-online.com/web/p/earthing-straps/2365264/
(admittedly not much use for a 10mm² conductor for main bonding a oil pipe say but there may be other variants out there).
  - Andy.

Re: Supplementary bonding question

Posted by Zoomup on Jan 19, 2020 5:52 pm

AJJewsbury:

there is no proper clamp for pipe which is smaller than 15 mm.

There are some (if non BS 951) - I've used these in the past for supplementary bonding in a bathroom with microbore heating pipes - https://uk.rs-online.com/web/p/earthing-straps/2365264/
(admittedly not much use for a 10mm² conductor for main bonding a oil pipe say but there may be other variants out there).
  - Andy.

 

How much? Ouch!

Z.

Re: Supplementary bonding question

Posted by Sparkingchip on Jan 19, 2020 6:30 pm

The water pipe clamps are tightened onto the steel armour wire of the SWA cable over the soft bedding material around the insulated conductors.

The bonding lead is terminated into a ring crimp secured by a bolt against a piece of wood that is between it and the metalwork of the bollard.

Spot the issues.

Andy B.

 

Re: Supplementary bonding question

Posted by Zoomup on Jan 19, 2020 7:56 pm



The bonding lead is terminated into a ring crimp secured by a bolt against a piece of wood that is between it and the metalwork of the bollard.


 

Perhaps if the threaded stud that travels through the wooden block is welded onto the inside of the bollard body, and if the eye lug is secured between two nuts and washers so that it is a very tight and permanent connection, then perhaps all will be well. Otherwise the wood may shrink in hot dry weather and the connection become loose.

Z.

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