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Topic Title: Main Protective Bond not in the right place ?
Topic Summary: Main Protective bonds, Gas and Water. 544.1.2
Created On: 31 January 2013 02:53 PM
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 31 January 2013 02:53 PM
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BILLYBOY65

Posts: 35
Joined: 11 May 2011

Hello

Could someone tell me please when carrying out the check for the Main Protective bond in a Commercial building whether there is any grey area regarding reg 544.1.2, in some instances we have located the bond but it is not within 600mm of the meter but it is accessible and correct.

Could this still be classed as satisfactory ?

What would be the continuity readings max for the check on this bond?

Cheers

Billy
 31 January 2013 03:23 PM
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AJJewsbury

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The 600mm is only a "where practicable", so I'd suggest that the distance isn't of itself a safety issue (typically the pipework will have a lot more metal than the bonding conductor, so if the bonding conductor is correspondingly shorter, the overall impedance between the MET and point of entry is probably lower (i.e. better) than if the connection had been made at the point of entry!

For me the main reason why you'd want the bond as close as possible to the point of entry is to make sure that you don't end up with a break in the continuity between the bond and the source of the introduced potential (i.e. the outside real earth). Continuity breaks in the pipework on the installation side of the bond don't normally undermine the equipotentiallity of the zone, so are less of a worry.

Obvious example is a water pipe where a plumber might insert a plastic tee fitting (or replace an existing tee with a plastic one). Gas is probably less of a worry, since (as I understand it) plastic fittings & pipe aren't permitted indoors (due to the risk of them melting in a fire).

What would be the continuity readings max for the check on this bond?

Now there has been some debate about that one! The short answer is that is should be reasonable considering the length & c.s.a. of the bonding conductor and taking into account the accuracy of the meter. If it was within the theoretical resistance of the bond plus say 0.1 Ohms, I wouldn't consider it a problem.

- Andy.
 31 January 2013 03:29 PM
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zeeper

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AJ

I thought 0.05 was the number, where you getting 0.1 from.(disregarding the resolution of meters)
 31 January 2013 03:38 PM
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OMS

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0.05 ohms is the value of resistance from the bonding conductor to the extraneous part - ie it just proves continuity across the joint.

A whole gang of you seem to have decided that it's the limit on the bonding conductor resistance and have then dreamt up all sorts of limits of conductor length for conductor size.

BS 7671 doesn't say anything like that.

The 0.1 ohm quoted by And,y is, as he pointed out, arbitary - you just need a number that's reasonable

Regards

OMS

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Failure is always an option
 31 January 2013 04:13 PM
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zeeper

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A whole gang of you


Sadly im not in a gang yet.

I think the confusion is not aided by GN3 page 35.

However I clearly understand you point, and that does sound perfectly reasonable.

regards
 31 January 2013 04:22 PM
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Parsley

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Unfortunately the new GN3 does little to dispel the myth.

Testing bonding conductors and earthing conductors To confirm the continuity of these protective conductors, test method 2 may be used.
This method can also be used to confirm a bonding connection between extraneous- conductive-parts where it is not possible to see a bonding connection, e.g. where bonding clamps have been 'built in'. The test would be done by connecting the leads of the instrument between any two points such as metallic pipes and looking for a low reading of the order of 0.05 ohm (it should be noted that not all low-resistance ohmmeters can read this low, see section 4.3).


Regards
 31 January 2013 04:38 PM
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OMS

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Originally posted by: Parsley

Unfortunately the new GN3 does little to dispel the myth.



Testing bonding conductors and earthing conductors To confirm the continuity of these protective conductors, test method 2 may be used.

Yes - that doesn't give a value does it ?

This method can also be used to confirm a bonding connection between extraneous- conductive-parts where it is not possible to see a bonding connection, e.g. where bonding clamps have been 'built in'. The test would be done by connecting the leads of the instrument between any two points such as metallic pipes and looking for a low reading of the order of 0.05 ohm (it should be noted that not all low-resistance ohmmeters can read this low, see section 4.3).

So how can you read that to be the resistance of the bonding conductor - it's quite clearly a test of the effectiveness of the connection to the extraneous conductive part where not visible





Regards


OMS

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Failure is always an option
 31 January 2013 04:52 PM
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BILLYBOY65

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Thanks OMS, makes sense.
 31 January 2013 05:23 PM
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Parsley

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Originally posted by: OMS

Originally posted by: Parsley



Unfortunately the new GN3 does little to dispel the myth.






Testing bonding conductors and earthing conductors To confirm the continuity of these protective conductors, test method 2 may be used.



Yes - that doesn't give a value does it ?



This method can also be used to confirm a bonding connection between extraneous- conductive-parts where it is not possible to see a bonding connection, e.g. where bonding clamps have been 'built in'. The test would be done by connecting the leads of the instrument between any two points such as metallic pipes and looking for a low reading of the order of 0.05 ohm (it should be noted that not all low-resistance ohmmeters can read this low, see section 4.3).



So how can you read that to be the resistance of the bonding conductor - it's quite clearly a test of the effectiveness of the connection to the extraneous conductive part where not visible

I don't! Geoff B and several others maybe your goodself educated me several years ago, but I believe this is the confusion, the old ESC guide for PIR's also states where connection of supplementary bonding cannot be verified by inspection it may be verified by a continuity test <0.05 and guess what value the NIC engineer was looking for between the MET and the incoming water pipework earlier this week when the builder had not installed an access hatch?











Regards




OMS


Regards

Parsley
 31 January 2013 05:27 PM
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OMS

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Next time you see that NIC inspector you can educate him in the error of his ways then - but only after he's given you an OK -

Regards

OMS

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 31 January 2013 10:52 PM
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AJJewsbury

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I thought 0.05 was the number, where you getting 0.1 from

The 'discussion' I referred to concluded that 0.05 was a reasonable limit for a connection (e.g. bonding conductor to pipe via a BS 951 clamp), as I expect you'd be testing between pipe and MET, there's be two connections (one at each end of the bonding conductor), hence 0.1 Ohms.

0.1 Ohms + expected c.p.c. resistance was also the limit that I was taught to look for when testing portable class 1 appliances - so 0.1 for a pair of terminations made sense when looked at from a different angle too - always reassuring!

It also seemed to be a reasonable margin for the vagaries of meter readings.

(The news that people had been increasing bonding conductor c.s.a. on long runs to ensure 0.05 Ohms was received with a degree of surprise at IET towers, and we were assured that that definitely wasn't the intention. The unfortunate final wording in GN 3 was partly due to a desire to give testers a simple test, without having to resort to calculation, for the majority of simple installations. Unfortunately committees and limited time doesn't always produce the clearest prose.)

- Andy.
 01 February 2013 07:15 AM
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zeeper

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The 'discussion' I referred to concluded that 0.05 was a reasonable limit for a connection (e.g. bonding conductor to pipe via a BS 951 clamp), as I expect you'd be testing between pipe and MET, there's be two connections (one at each end of the bonding conductor), hence 0.1 Ohms.


I must admit it does seem obvious once explained. But also interesting that there is no limit on the resistance of the bonding conductor. only an estimate of the bonding conductor connection resistance.

strange one, as this has an effect on touch voltages and disconnection times.
 01 February 2013 09:09 AM
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OMS

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strange one, as this has an effect on touch voltages and disconnection times.


Not really

Bonding has a very limited impact on disconnection times (when connected, it may lower the effective Zs and increase fault current - it doesn't form part of the fault loop though).

The touch voltage is defined by R2 and the fault level. The bonding simply limits R2 to the conductors in the installation. Without bonding, R2 extends to the erath path external to the building back to the supply transformer.

Provided the bonding resistance is "low" ie it's effectively bonded, all you have done is establish the zone reference.

It's the same concept as supplementary bonding for your compressed air line and socket outlet we discussed in another thread.

Regards

OMS

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 01 February 2013 10:14 AM
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Parsley

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Originally posted by: zeeper



The 'discussion' I referred to concluded that 0.05 was a reasonable limit for a connection (e.g. bonding conductor to pipe via a BS 951 clamp), as I expect you'd be testing between pipe and MET, there's be two connections (one at each end of the bonding conductor), hence 0.1 Ohms.




I must admit it does seem obvious once explained. But also interesting that there is no limit on the resistance of the bonding conductor. only an estimate of the bonding conductor connection resistance.



strange one, as this has an effect on touch voltages and disconnection times.


It doesn't theorotically effect disconnection times but may help via parallel paths, the resistance of the bonding conductor isn't used to calculate touch voltage.

Ut = If x R2 where bonding is present or
Ut =If x (Zpen+R2) for TNC-S if bonding isn't present.

Worst case is for Ut is TT where bonding isn't present.


I'm going to risk sounding a bit like DC but I think this is why there is a requirement to only check that the bonding is present where the pipework or extraneous part enters the building, there's no requirement in BS7671 to link across the joints of high resistance etc (not talking about supp bonding special locations) except where steel frame work has multiple entry points and bonding is only to one or two columns etc, I believe this is because the Ut value can't actually increase throughout the installation as long as the extraneous conductive part is bonded where it enters the building, although if pipework goes into the floor and becomes earthy again it should be re-bonded. I have wondered if Ut between exposed parts and extraneous parts may actually become lower due to the voltage dropped across any high resistance joints in the pipework as long as the extraneous pipework is bonded where it enters the building, sort of reverse effect of where the bonding is not present if that makes sense. You can of course re-establish a local zone like in a bathroom or if you wish. This is just my thought and I have nothing substantial to back it up, I'm sure several of you will love to correct me.

The new GN3 also includes a paragraph on bonding conductors expected test results on page 35.

"Expected test results
The results should first and foremost indicate no open circuit in the protective conductors. For lengths of conductor use Appendix B for resistance data. For joints across bonds by earth clamps and similar, the readings should approach 0.05 ohm taking into account both the resolution of the instrument and its accuracy at low values."



Regards
 01 February 2013 10:32 AM
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zeeper

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Bonding has a very limited impact on disconnection times (when connected, it may lower the effective Zs and increase fault current - it doesn't form part of the fault loop though).


+

It doesn't theorotically effect disconnection times but may help via parallel paths, the resistance of the bonding conductor isn't used to calculate touch voltage.




I was thinking line to bonded pipe, not line to cpc+bonding(parallel paths)

obviously you what disconnection if the line conductor if it comes into contacted with the bonded item. On a comercial site you could have a very long main protective bonding conductor. But there is no maximum resistance for the bonding conductor. I find that odd, I suppose the competent spark would walk around and check the touch voltages.
 01 February 2013 10:59 AM
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AJJewsbury

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I was thinking line to bonded pipe

BS 7671 doesn't consider that - the line conductor should be surrounded by either something connected to the c.p.c. or something giving the equivalent of double/reinforced insulation (or insulated+sheathed if you prefer - I'm not wanting to re-open that debate here) - so it's already deemed safe under single fault conditions.

If you had to consider line-pipe faults, you'd presumably also want to consider line-person-pipe (or line-nail-person-pipe) - which gets a bit tricky with conventional ADS (people being less than perfect conductors you don't have a fault of negligible impedance anymore).

Then you note that BS 7671's requirement for bonding conductors can in some circumstances be a lot smaller than some c.p.c.s (e.g. 16mm2 for a submain c.p.c, but 10mm2 a bond), so you've got the problem of ensuring that the bond will be adequate to carry fault currents if that's what you're planning it to do.

All in all, it ends up being half a solution to half a problem...

- Andy.
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