Apologies for the length of this post, but please stay with me.
As a newbie I am after some advice and clarification of the OSG.
Ref 4.5 (Page 41) 'There is no requirement to main bond an incoming service where the incoming service pipe is plastic, e.g. blue for potable water.'
It goes on to say 'Where there is a plastic incoming service and a metal installation within the premises, main bonding is recommended unless it has been confirmed that any metallic pipework within the building is not introducing earth potential (see 4.3).'

The reason for the question is that one of the jobs I have been asked to quote for has the LV supply on one side of the building into the garage and the water supply on the other side of the building and NO bonding at all. The job is to replace the CU, for which bonding is one of the initial considerations.

To get a conductor from the garage to within 600mm of the incoming water supply will not be easy. Need to cross the garage, cloak,hall and kitchen), initial view is either:
up into loft from garage (not too difficult), cross to other side then chase internal wall down through bedroom into kitchen,
or as above but trunk conductor down corner of bedroom and kitchen,
or take up carpet and floors on 1st floor landing and use this space but would still need to trunk down corner of kitchen,
or chase out solid ground floor and bury cable
or go round outside of house in trunking, but this crosses back door and patio doors so not easy either.

Hence my interest in the quote from the OSG. '. . . unless it has been confirmed that any metallic pipework is not introducing an earth potential.'

Q1. Is there a test that could be used to proof the lack of potential?

Q2. If there is a test, presumably this is only accurate at the time of testing and things could change to compromise the situation in the future - so arguably it does not look like a good solution. Agreed?

Q3. Has anyone else had a similar situation and how have you approached it?
My potential customer was budgeting on the cost of a CU he'd seen in Screwfix but the cable to bond his services could turn out to cost more. Any suggestions or comments on the above would be appreciated.

Hi I have had a similar job lately

The water bond was a long awkward distance from the met, the owner was extremely difficult and fussy as to what she would accept,,in the end I used 10mm single black core clipped round the external of the house at low level and at each end where it re-entered the house I thru crimped back to 10mm earth, it actually looked really discreet and I would definitely use this method in the future as a last resort

I know it's not strictly compliant but no different than someone painting the cable, and its better bonded like that than not!

To test if you need to bond or not IR test between your MET and the house pipe work and if the reading is above 7666 ohms (thats off the top of my head, I would check that, I'll probably be corrected) then you shouldn't need to bond

-"there is no requirement to bond where the incoming service is plastic"
is this PME?
if so the requirement is to bond the service, even if the incomer is plastic.

Regards

It is not mealy earth potential, you must consider any potential that could be introduced to any special location.

You can of course test to confirm if supplementary bonding is required under the 17th.....22k ohms or greater.

Does supplementary bonding exist?

Is the Gas Bonded?

Personally I go around the outside in probably 16mm GY clipped. 10mm sounds like it might be too small for the distance.

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Regards

Martyn.

Only a mediocre person is always at their best



www.electrical contractors uk.com

Don't make too much of the 600mm bit, as close as practicable is also mentioned! If you can pick-up on the pipework somewhere more convenient do so,

Dave.

If you have less than 22 k.between the met and the water pipework it is an extraneous~conductive~part and requires bonding as 10ma or more could flow. You need to disconnect all parallel paths not easy in existing installation and test to the incoming earth conductor. If you've got copper most would bond.

Regards

And label the incoming water to that effect....

Also worth calling your registration body to ask their opinion..

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Regards

Martyn.

Only a mediocre person is always at their best



www.electrical contractors uk.com

Martyn - I take it you mean to up the size of the bonding cable to achieve a reading of <0.05 ohms, I actually took this up with the NIC the other day and they said increasing the required size of bonding cable due to length is not required, as long as the bonds are adequate through selection or calculation(of the earth conductor)

Usually the figure is derived from limiting any shock current to 10mA, allowing 1000 Ohms for body resistance - i.e. 230V/0.01 = 23k Ohms, less 1k for body resistance, gives 22k Ohms - like Martyn says. Also you may need to remove incidental connections between the pipework and MET (e.g. via c.p.c. of immersion heaters, boilers etc) to get a true reading. If no other services are extraneous, one way to achieve that might be to disconnect the MET from the means of earthing (with the power off of course) and test between the pipework and the means of earthing.


Arghhh. If you're thinking of the 0.05 Ohms test limit, members of the GN 3 committee tried very hard to dispel that particular urban myth! BS 7671 states no limit to the resistance of main bonding conductors - just their c.s.a.


Yes and no. Yes we have to cope with reasonably forseeable, but not with any possible eventuality. I've seen plumbers leave bonding clamps, complete with their 'Safety Electrical Connection - Do No Remove' labels dangling in free air on 6" of the original pipe when a pipe was replaced (well, they did what they were told didn't they!??!) Any new underground or embedded pipework would almost certainly be plastic, so the chances of adding a source of earth potential to an existing plumbing system is probably unlikely.

- Andy.

You should ignore the NIC they make up thier own rules and regs and you can be damn sure if it goes wrong and it not to the actual regs your on your own.

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Regards

Martyn.

Only a mediocre person is always at their best



www.electrical contractors uk.com

So how many properties have Gas CH ?
I would think that local supplementary bonding is a necessity regardless and would probably be achieved through main protective bonding of the metallic services within the property.

Legh

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Why do we need Vernier Calipers when we have container ships?

http://www.leghrichardson.co.uk

"Science has overcome time and space. Well, Harvey has overcome not only time and space - but any objections."

"My potential customer was budgeting on the cost of a CU he'd seen in Screwfix "

That says it all!

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Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik

Hi Guys,
thanks for all the responses I will need to go through in detail to gather concensus.

But this reply sprang out, yes the supply is PME so that means I do need to run a protective bonding conductor back to within 600mm of the water inlet pipe.

The CU cost is now a very small part of the upgrade. Wish me luck

thanks again for your replies

Before you go mental with the 10mm2, if you are refering to UKPN's response, then I'm pleased to tell you it's complete bloody nonsense.

The means of earthing has no bearing on the requirements for bonding - only the size.

If you have a plastic incomer and metal pipework, then decide if it's extraneous - ie does it go back into the ground in copper for example, or leave the building to an outbuilding - if the answer is no you can be pretty certain it's not extraneous and doesn't need bonding. Test for it if you like.

regards

OMS

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Failure is always an option

As mentioned above, can be difficult in an existing installation.

With the talk of bonding any metallic installation pipe work when the incoming service is in plastic, is there not a case to be satisfied that the main bonding to the gas will also provide main bonding to the metallic water installation pipe work through the connections at the boiler? (Forgetting about any plastic connections on the pipe work for now)

regards

OMS

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Failure is always an option

Good point!

"complete nonsense"
our poster should get out more, even when many practical forum members
are telling otherwise,he still insists the DNO are wrong.
we have been wrong for almost 50 years?

Regards

and another thing! 0.05, was a figure in a previous GN book, that for some
reason disappeared overnight, no doubt the 22k/10ma will as well soon!

Regards

I have to totally disagree with that and perhaps ease the burden a bit for you, it all went wrong 31 years ago in 1982.

Anyway stick to your Customer Installation Earthing Design manual EDS 06-0017 it is based on IEC 60364/BS 7671 and if you dont have access to the big bad books it gives you a basic understanding of electrical installations.

regards

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"Take nothing but a picture,
leave nothing but footprints!"
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"Oh! The drama of it all."
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"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
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Edited: 13 February 2013 at 11:38 AM by rocknroll