IET logo
 
IET
Decrease font size
Increase font size
Topic Title: Incoming gas services
Topic Summary:
Created On: 17 April 2013 09:20 PM
Status: Post and Reply
Linear : Threading : Single : Branch
<< 1 2 3 4 5 Previous Next Last unread
Search Topic Search Topic
Topic Tools Topic Tools
View similar topics View similar topics
View topic in raw text format. Print this topic.
 19 April 2013 05:26 PM
User is offline View Users Profile Print this message



AJJewsbury

Posts: 11493
Joined: 13 August 2003

But that's a direct contact shock Andy - the installation has already failed.

Not necessarily - you can get close to 230V above true earth from indirect contact on TT systems. Even on TN it can be well above 115V thanks to the quaint local custom of reduced c.s.a. c.p.c.s.

Pretty low actually if it's local supplementary bonding within a bathroom - R2 even over domestic distances is now much lower for no significant increase in fault current.

Even if one end of the bonding picks up a different pipe that main bonded elsewhere? I can see corners of the 'local' bonding being easily dragged down to close to true earth - increasing the local p.d. The "50V rule" only applies to currents up to Ia - the actual fault currents (through the bonding) can exceed that substantially.
if we bring it down to less than 50V we don't need to disconnect at all

Indeed, likewise if we can keep the shock current below 10mA.

- Andy.
 19 April 2013 05:50 PM
User is offline View Users Profile Print this message



geoffsd

Posts: 347
Joined: 15 June 2010

Originally posted by: OMSI'm not saying we should do it - we were initially talking about bonding a gas main - I just raised the point in response to the idea that supplementary bonding a bath or radiator may not actually result in an increased risk

But we are not dealing with touch voltage and the reason for supplementary bonding.

It is providing an unnecessary and unwanted path to earth which will introduce a hazard should someone contact a faulty flex while, for example, vacuuming or using other tools in the room and leaning on the bath (or door handle).
Not when bathing but when working in the room at other times.
 19 April 2013 05:59 PM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19688
Joined: 23 March 2004

Originally posted by: AJJewsbury

But that's a direct contact shock Andy - the installation has already failed.


Not necessarily - you can get close to 230V above true earth from indirect contact on TT systems. Even on TN it can be well above 115V thanks to the quaint local custom of reduced c.s.a. c.p.c.s.

OK I can go with that for TT systems, but in both the TT case and TN case the local supplementary bonding re establishes the protected zone (so called equipotential) - in both cases, the touch voltage will be much lower as the effective R2 is now far smaller (in resistance terms).

Given that the shock poetntial is already there from "true" extraneous parts, it hasn't increased from bonding "non extraneous parts"



Pretty low actually if it's local supplementary bonding within a bathroom - R2 even over domestic distances is now much lower for no significant increase in fault current.


Even if one end of the bonding picks up a different pipe that main bonded elsewhere? I can see corners of the 'local' bonding being easily dragged down to close to true earth - increasing the local p.d.

But those bonded services are also connected to the MET - which has risen in potential due to the fault - so the whole installation rises in voltage rather than bits being dragged down - if we are saying the voltage experienced between parts is the product of the fault current and the effective R2 then the local PD will also be lower not higher

The "50V rule" only applies to currents up to Ia - the actual fault currents (through the bonding) can exceed that substantially.

Of course - but the person in the space isn't in series with equipotentially bonded parts - they are between the faulted system and those parts so it's the driving voltage across or through the person that matters

if we bring it down to less than 50V we don't need to disconnect at all


Indeed, likewise if we can keep the shock current below 10mA.

Agreed - so we limit the shock current to a part if we haven't bonded it and we limit the touch voltage of we have - which is back to the point about bonding a part substantially increasing danger - generally it won't.

I'm not suggesting a return to the glory days of the 15th edition, but you and I are of an age that tells us that there a millions of non extraneous parts out there that are bonded with no undue effects

regards

OMS


- Andy.


-------------------------
Failure is always an option
 20 April 2013 08:13 AM
User is offline View Users Profile Print this message


Avatar for leckie.
leckie

Posts: 1868
Joined: 21 November 2008

So, in summary regarding my op, the incoming yellow service pipe for a gas installation can be plastic, and if so you may not be required to bond the gas installation pipes if they are not extraneous, i.e. they can be treated the same as water pipes.

But if carrying out an EICR and no bonding was in place to gas or water with incoming plastic services would you code it? You would have to check if it was an extraneous conductive part, how would you prove that the pipes were not? You could try disconnecting the earthing conductor and testing between the earthing conductor and the MET. If over 23kOhms then you may consider that none of the pipes, etc are extraneous conductive parts. Gets a bit trickier in say an industrial shed with bonded structural steel. If pipes to heating systems, water supplies, etc, are fixed via metal fixing to the structural steel they would all test below 23kOhms even if you disconnected the steelwork bond as the steel would introduce an earth via the ground. You could have equipment such as metal clad sockets, etc, fixed to the steelwork introducing further connections to the MET via the cpc's.
But that doesn't prove that the water pipes, etc are extraneous, just that they are connected to the MET. So how do you code it? The original electrical installer may have designed it as not requiring bonding as the service pipes were plastic, but this can no longer be proven to be the case without dismantling vast amount of the installation.
 20 April 2013 01:50 PM
User is offline View Users Profile Print this message


Avatar for sparkingchip.
sparkingchip

Posts: 6076
Joined: 18 January 2003

When I had my last NAPIT assessment I had my first, indeed my one and only non-compliance. I did not have the latest edition of the BS7671 On Site Guide, I had the big book itself, but not the guide. So using this website and the links above I bought myself a copy of the Onsite Guide and emailed the invoice from the IET to NAPIT thus resolving the issue.

I did skim through it when it arrived before putting it in the glove box of the van, I suggest you all do the same!

Onsite Guide page 41- 4.5 Main protective bonding of plastic services.

There is no requirement to main bond an incoming service where the incoming service is plastic, for example, where yellow is used for natural gas and blue for potable water.

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 premises is not introducing a earth potential (see 4.3)

Now then, 4.3 tells us that we need to consider parts of the installation pipe work such as central heating pipes buried in concrete or floor screed.

It has been common practice to bury internal gas pipework in floor screeds, so this is not as cut and dried as it first appears with the statement "There is no requirement to main bond an incoming service where the incoming service is plastic" due diligence is required in making an assessment of the gas and water installations.

Andy
 20 April 2013 09:58 PM
User is offline View Users Profile Print this message


Avatar for leckie.
leckie

Posts: 1868
Joined: 21 November 2008

Well we are back to the start! IMO if in any doubt, which you probably will be, bond it. That's why people have done it for many years. Whilst I understand and agree with the counter argument, look at my previous post, last but one, at a later date it is virtually impossible in some cases to prove that pipe installations are not extraneous during an EICR , so just bond services and be done with it. Even thought his is "following the herd" as OMS says, I personally think this is the way to go. So I'm not dumping all my 10mm green yellow after all.
 21 April 2013 09:53 PM
User is offline View Users Profile Print this message



briandoherty

Posts: 303
Joined: 08 May 2004

Originally posted by: Martynduerden
Bs6891 requires a gas supply to have a minimum of a 10mm bond, however more generally it requires compliance with bs7671, essentially this as I read it means if the pipe is extraneous then it must be bonded with 10mm, if its not extraneous it does not require a bond.


I'd put the emphasis the other way, Martyn. BS6891 really just requires compliance with BS7671, stating "8.16.4.1 All domestic gas installations shall have main equipotential bonding of the gas installation pipework conforming to BS 7671." The bit in BS6891 about the size of the Main Equipotential Bond is just an informative commentary, i.e. "COMMENTARY AND RECOMMENDATIONS ON 8.16.4.2
Main equipotential bonding of the gas installation pipework should be made using cable with minimum cross-sectional area of 10 mm2 cable with green and yellow insulation, construction reference 6491X conforming to BS 6004.
"

For compliance with a British Standard such as BS6891, all that is needed is to meet the criteria in the normative 'provision' elements, typically including the word "shall" (as in 8.16.4.1 above - "shall...conforming to BS 7671"). Other 'informative' elements, such as commentary, are just additional useful information (sometimes not much more than the musings of the drafting committee...); there's no need at all to 'comply' with these informative elements.

-------------------------
Regards,

Brian
 21 April 2013 11:26 PM
User is offline View Users Profile Print this message


Avatar for Martynduerden.
Martynduerden

Posts: 3211
Joined: 13 July 2008

Originally posted by: briandoherty

Originally posted by: Martynduerden

Bs6891 requires a gas supply to have a minimum of a 10mm bond, however more generally it requires compliance with bs7671, essentially this as I read it means if the pipe is extraneous then it must be bonded with 10mm, if its not extraneous it does not require a bond.


I'd put the emphasis the other way, Martyn. BS6891 really just requires compliance with BS7671, stating "8.16.4.1 All domestic gas installations shall have main equipotential bonding of the gas installation pipework conforming to BS 7671." The bit in BS6891 about the size of the Main Equipotential Bond is just an informative commentary, i.e. "COMMENTARY AND RECOMMENDATIONS ON 8.16.4.2

Main equipotential bonding of the gas installation pipework should be made using cable with minimum cross-sectional area of 10 mm2 cable with green and yellow insulation, construction reference 6491X conforming to BS 6004.
"

For compliance with a British Standard such as BS6891, all that is needed is to meet the criteria in the normative 'provision' elements, typically including the word "shall" (as in 8.16.4.1 above - "shall...conforming to BS 7671"). Other 'informative' elements, such as commentary, are just additional useful information (sometimes not much more than the musings of the drafting committee...); there's no need at all to 'comply' with these informative elements.


I don't disagree generally but not complying with the 10mm bit rightly or wrongly fails a GSR inspection, nor would I like to be on the wrong side of an HSE investigation. I'm darn sure I could argue a 6mm if I'd installed it but for what?

-------------------------
Regards

Martyn.

Only a mediocre person is always at their best



www.electrical contractors uk.com
 21 April 2013 11:39 PM
User is offline View Users Profile Print this message


Avatar for leckie.
leckie

Posts: 1868
Joined: 21 November 2008

If you do not bond gas pipe installations to a new build it will fail a NHBC inspection. You can argue all you want and give all the technical reasons but..., your client will go mad at the delay, and if you want an easy life - just bond it. It doesn't make it right, it's just how it is.
 22 April 2013 10:20 AM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19688
Joined: 23 March 2004

As I said earlier(in relation to main equipotential bonding):

Given that for "small" installations, the "designer" is usually the electrician doing the install, and that even within the domestic sector there are all sorts of variations of gas and water pipework, including re entrant sections etc and there is broadly no control of systems anyway, the "industry" has generally accepted that for avoidance of doubt (or avoidance of thinking by a competent person in reality) then regardless of the conductive or insulating nature of the service, if the internal pipework is conductive, then we bond it without question - it's the comparatively least risky option - better to bond what isn't actually extraneous rather than not bond what is potentially extraneous


Think back to whan you were trainees - we told you a few white lies about such things as resistance and impedance for example to make it easy to understand - some of you found out the lies - some of you didn't - but basically this is what it's all about:

Guidance that says "just do it" is basically trying to avoid the cost of thinking about it by creating a simple rule - which in reality is what installers want to hear ie "We always do it this way"


Regards

OMS

-------------------------
Failure is always an option
 22 April 2013 11:42 AM
User is online View Users Profile Print this message


Avatar for ebee.
ebee

Posts: 5707
Joined: 02 December 2004

"White Lies"
Never!
Are they?
No surely not!

Go on then.

Let`s start a list of such white lies/urban myths



-------------------------
Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 22 April 2013 11:51 AM
User is offline View Users Profile Print this message


Avatar for Parsley.
Parsley

Posts: 1020
Joined: 04 November 2004

Originally posted by: ebee

"White Lies"

Never!

Are they?

No surely not!



Go on then.



Let`s start a list of such white lies/urban myths





You're not allowed to use steel conduit as a CPC

Regards
 22 April 2013 11:57 AM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19688
Joined: 23 March 2004

Originally posted by: geoffsd

Originally posted by: OMSI'm not saying we should do it - we were initially talking about bonding a gas main - I just raised the point in response to the idea that supplementary bonding a bath or radiator may not actually result in an increased risk


But we are not dealing with touch voltage and the reason for supplementary bonding.

It is providing an unnecessary and unwanted path to earth which will introduce a hazard should someone contact a faulty flex while, for example, vacuuming or using other tools in the room and leaning on the bath (or door handle).

Not when bathing but when working in the room at other times.


Wrong equipment being used in the wrong location or incorrectly or both then ?


Regards

OMS

-------------------------
Failure is always an option
 22 April 2013 12:01 PM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19688
Joined: 23 March 2004

Originally posted by: ebee

"White Lies"

Never!

Are they?

No surely not!

Go on then.


Let`s start a list of such white lies/urban myths



I was thinking more along the lines of Ohms law and it's application in single and polyphase AC circuits as one example

So I does not equal V divided by R (as we lied to you about the resistance as it is effectively impedance)

There are more !!

regards

OMS

-------------------------
Failure is always an option
 22 April 2013 12:24 PM
User is offline View Users Profile Print this message


Avatar for Parsley.
Parsley

Posts: 1020
Joined: 04 November 2004

Originally posted by: OMS

Originally posted by: ebee



"White Lies"



Never!


.

Are they?



No surely not!



Go on then.





Let`s start a list of such white lies/urban myths



[IMG][/IMG]




I was thinking more along the lines of Ohms law and it's application in single and polyphase AC circuits as one example



So I does not equal V divided by R (as we lied to you about the resistance as it is effectively impedance)



There are more !!



regards



OMS


The highest fault current on a TP&N installation is double the L-N value.

Regards
 22 April 2013 12:53 PM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19688
Joined: 23 March 2004

Yup - that's another one

I think suffice to say that we tell simplifications to craft level trainees - bonding internal metallic pipework in a building served by plastic services is one of those - better to than to not - just in case it's re-enrant and becomes "earthy"

Even UKPN still believes

Regards

OMS

-------------------------
Failure is always an option
 22 April 2013 01:05 PM
User is online View Users Profile Print this message


Avatar for ebee.
ebee

Posts: 5707
Joined: 02 December 2004

To correctly tighten a terminal use a torque terminal driver

or

Tighten as hard as you can, say candy man candy man candy man candy man then waggle conductors and tighten again then say candy man candy man candy man candy man

-------------------------
Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 22 April 2013 01:14 PM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19688
Joined: 23 March 2004

and don't blow in the pyro pots - you'll make the cable damp !!

regards

OMS

-------------------------
Failure is always an option
 22 April 2013 01:56 PM
User is offline View Users Profile Print this message


Avatar for sparkingchip.
sparkingchip

Posts: 6076
Joined: 18 January 2003

It levels the playing field if everyone quotes the same work rather than charging extra for main bonding conductors.

Andy
 22 April 2013 02:00 PM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19688
Joined: 23 March 2004

But the economic advantage is for those contractors that understand the issue - as I said, we use a simple rule to make it easy for those who don't want to inccur the effort (and cost) of thinking - if you are satisfied that you don't have extraneous parts then a clever contractor will save on the G/Y cable and bonding clamps

regards

OMS

-------------------------
Failure is always an option
IET » Wiring and the regulations » Incoming gas services

<< 1 2 3 4 5 Previous Next Last unread
Topic Tools Topic Tools
Statistics

See Also:



FuseTalk Standard Edition v3.2 - © 1999-2014 FuseTalk Inc. All rights reserved.