DIYnot
Local | Network
   DIYnot > Forums
Local | Network
DIYnot Network Local DIYnot Network Local  
  Forum IndexForum Index     RulesRules    HelpHelp     Join FREERegister Free     About CookiesCookies     SearchSearch     LoginLogin 

rationale for main bonding

Goto page Previous  1, 2
 
Post new topic   Reply to topic    DIYnot.com Forum Index > Electrics UK
Search this topic :: View previous topic :: View next topic  
Author Message
NotHimAgain

from United Kingdom

Joined: 08 Feb 2008
Posts: 435
Location: United Kingdom
Thanked: 19 times

PostPosted: Sat Aug 28, 2010 8:50 am Reply with quote

Whilst I understand and agree with the separation of the concepts of ‘earthing’ and ‘bonding’ you should not take this too far. In the example I gave the only role of the bonding in protecting against the fault under consideration is to limit touch voltage. It is definitely not to maintain or create equipotential.

Now in some other situations it will. These can include faults from outside of the installation and faults on adjacent circuits.

In most cases we are dealing with ‘faults’ that are related to the fact that our supply network is earth referenced – so earthing is very relevant. I am leaving aside special situation such as earth free locations.

With regard to the ability of those entering the industry – I left school at 15 with no qualifications at all – I now have a degree, so anything is possible icon_biggrin.gif.

For my sins I spent around 8.5 years as a full time college lecturer: 5 year with craft apprentices between 1975 and 1980; the rest with HNC students. I continue to do some part time teaching. These days it tends to be just the 17th Edition courses which I do on behalf of one of the industry bodies – all you ever wanted to know about BS 7671 in 3 days including the exam icon_eek.gif.

So I do have some idea of what is out there but as I said above – anything is possible icon_biggrin.gif.
Back to top
 Alert Moderators

If you do not want to see this advert, click here to login or if you are new click here to join free.
radweld

from United Kingdom

Joined: 10 Aug 2009
Posts: 73
Location: Sussex,
United Kingdom
Thanked: 3 times

PostPosted: Sat Aug 28, 2010 4:07 pm Reply with quote

NotHimAgain wrote:
I despair, I really do icon_cry.gif


What about? I can't see this thread is any more desperate than any others here.

NotHimAgain wrote:

If the bonding is removed

Ut = If * (R2 + Se) volts

The impedance of the ECP to 'mother earth' needs to be considered also of course which may well be the dominant factor.

plugwash wrote:
The real reason main bonding needs to be so big is that for various reasons the suppliers earth can end up at a different voltage from the local earth potential.

It's the 'various reasons' I am trying to understand. It would appear over the years the required size of the main bonding has increased. My house has a 6mm main bond which was installed according to the regulations at the time. Is this now 'unsafe' because the supply network has changed, the domestic wiring is different, our understanding of faults is better, or some other reason?

ricicle wrote:
Larger sizes of PEB are required on TNC-S systems due to the fact that there is the possibility of circulating network currents through them.

Can you explain what these currents are?

ricicle wrote:

Also there is the possibility of them carrying a proportion of load current in the event of a lost supply Neutral

plugwash wrote:

Consider for example the situation where the lead sheath of a cable has corroded through and then the sheath after the break has shorted to live!

Thanks - these are exactly what I was trying to understand! Are there any other scenarios?

NotHimAgain wrote:

Now there is far more to this subject than given above so if you seek understanding you will need to do a lot of reading.


If anybody can provide a link to the theory behind the sizing of the bonding conductors I would be grateful.

dingbat wrote:
For the general population, 'average mathematical ability' is very close to 'no mathematical ability'.

Is this an ironic set theory joke?
Back to top
 Alert Moderators
NotHimAgain

from United Kingdom

Joined: 08 Feb 2008
Posts: 435
Location: United Kingdom
Thanked: 19 times

PostPosted: Sat Aug 28, 2010 4:43 pm Reply with quote

Quote:

The impedance of the ECP to 'mother earth' needs to be considered also of course which may well be the dominant factor.


The equation stands - the impedance of ‘mother earth ‘ is not part of the earth fault loop path for TN systems, and for TT systems it is conventional taken to be zero as the impedance of the earth electrodes is much higher.

Quote:

If anybody can provide a link to the theory behind the sizing of the bonding conductors I would be grateful.


You will not get the link you require as the details appear to have been lost in the mists of time icon_eek.gif.

The original requirements for TN-C-S systems can be located in various documents produced by the now defunct Electricity Council.

Your installation may or may not be safe. The fact that the bonding conductors are 6 mm2 is not in itself a reason to say it is unsafe. However, it does depend on what earth system you have. I cannot see a reference to it in your posts.

If it is TN-C-S (PME) then it does not comply with the supply company requirements for that earthing system. If it is some other system it may still comply if the requirements of chapter 54 of BS 7671 have been met.
Back to top
 Alert Moderators
NotHimAgain

from United Kingdom

Joined: 08 Feb 2008
Posts: 435
Location: United Kingdom
Thanked: 19 times

PostPosted: Sat Aug 28, 2010 5:08 pm Reply with quote

For additional information

The Electricity Council documents I referred to are:

Engineering Recommendation G12/1 June 1975
National Code of Practice on the Application of Protective Multiple Earthing to Low Voltage Networks

Section 15(g) gave the minimum size of the earthing conductor as 16 mm2 and the bonding conductors as 6 mm2 – these sizes are minima applicable to small installations.

G12/2 August 1982 increased the size of the bonding conductors to a minimum of 10 mm2 and showed a table which is the same as that now used in BS 7671 [Table 54.8]

G12/3 1995
Did not alter the requirements of G12/2

These documents do not give any technical justification of the sizes required.

I am not aware of any changes since.
Back to top
The following user says thank you to NotHimAgain for this useful post:
radweld (29 Aug 2010)
 Alert Moderators
radweld

from United Kingdom

Joined: 10 Aug 2009
Posts: 73
Location: Sussex,
United Kingdom
Thanked: 3 times

PostPosted: Sun Aug 29, 2010 8:24 am Reply with quote

NotHimAgain wrote:
Quote:

The impedance of the ECP to 'mother earth' needs to be considered also of course which may well be the dominant factor.

The equation stands - the impedance of ‘mother earth ‘ is not part of the earth fault loop path for TN systems, and for TT systems it is conventional taken to be zero as the impedance of the earth electrodes is much higher.

If you are just talking about the normal earth fault path, then I agree the ECP would not part of the earth path. However, in this case adding the MPB does not remove the effect of supplier earth (although obviously the MPB may provide a parallel TT earth).

If you referring to the effects of bonding, then without a MPB the 'touch current' path will be through the person in contact with the faulty appliance, through the ECP and through mother earth and the impendance of the ECP to earth will clearly have an effect.

I am assuming in this instance you are considering 'touch voltage' to be the voltage across a person who is in contact with both the faulty electrical appliance and the ECP.

NotHimAgain wrote:

However, it does depend on what earth system you have. I cannot see a reference to it in your posts.

It's TN-S, although I'm interested in the topic in general.

NotHimAgain wrote:
For additional information

The Electricity Council documents I referred to are:

Engineering Recommendation G12/1 June 1975
National Code of Practice on the Application of Protective Multiple Earthing to Low Voltage Networks

Thanks - Googling brings me to ofgem, which is somewhere I hadn't looked.

NotHimAgain wrote:

These documents do not give any technical justification of the sizes required.

icon_confused.gif
Back to top
 Alert Moderators
NotHimAgain

from United Kingdom

Joined: 08 Feb 2008
Posts: 435
Location: United Kingdom
Thanked: 19 times

PostPosted: Sun Aug 29, 2010 9:30 am Reply with quote

What I should have said is that the impedance of ‘mother earth’ is not part of the earth fault loop path model. This model is of cause a simplification to concentrate the mind on the problem in question, namely shock protection.

The model has many limitations as it is generally drawn as if it is a single circuit path from the faulty appliance or other source of touch voltage to the energy source. This is, of course, not actually true as there are many other circuit paths in parallel with it – not least being all the houses in your street that share the same radial main from the sub-station icon_biggrin.gif.

If your system is TN-S then the bonding conductors should be at least half the csa of the earthing conductor, with a minimum size of 6 mm2. [BS 7671 - 544.1.1].

I did some further research and I found this comment from B D Jenkins in his book Commentary on the 15th Edition of The IEE Wiring Regulations. (This book was written at the IEE’s request – B D Jenkins was closely involved in the work on the 15th Edition)

“For protective bonding conductors, whether they are main or supplementary, it is not practicable to calculate the portion of the earth fault current which they may carry. Therefore, Section 547 in prescribing the requirements for these bonding conductors bases their minimum cross-sectional-area on an assessment of probabilities and experience.”

A similar comment is made in Paul Cook’s book on the 16th Edition. (Bottom of page 249 - this may still be in print).
Back to top
The following user says thank you to NotHimAgain for this useful post:
radweld (31 Aug 2010)
 Alert Moderators
radweld

from United Kingdom

Joined: 10 Aug 2009
Posts: 73
Location: Sussex,
United Kingdom
Thanked: 3 times

PostPosted: Tue Aug 31, 2010 6:06 pm Reply with quote

NotHimAgain wrote:
What I should have said is that the impedance of ‘mother earth’ is not part of the earth fault loop path model.

Regardless of the model, I hope you appreciate that the shock current flowing through a person in contact with an (unbonded) ECP will depend on the impedance to earth.
NotHimAgain wrote:
“Therefore, Section 547 in prescribing the requirements for these bonding conductors bases their minimum cross-sectional-area on an assessment of probabilities and experience.”


I wonder what the 'experiences' were that increased the size from 2.5mm (14th) to 10mm today - and perhaps I should install 16 or 25mm just to be futureproof...
Back to top
 Alert Moderators
NotHimAgain

from United Kingdom

Joined: 08 Feb 2008
Posts: 435
Location: United Kingdom
Thanked: 19 times

PostPosted: Tue Aug 31, 2010 8:21 pm Reply with quote

Quote:
Regardless of the model, I hope you appreciate that the shock current flowing through a person in contact with an (unbonded) ECP will depend on the impedance to earth.


Not always, but I am aware of the point you are making - however, you are introducing the touch current model and that would introduce even more complexity as it includes weighted body impedance models and the like. It is often used when assessing appliances, etc.

The touch voltage model (largely the work of Dalziel, and later Biegelmeier and Lee) is generally used for simple explanations of earthing and bonding - the work takes some account of simple body impedance models, and it gives a nice simple way to write regulations icon_biggrin.gif.

Electric shock due to faults does not have to involve 'mother earth' at all - it could be between the faulty equipment and some conductive part that links, say, to the supply earthing conductor in the house next door - we can invent all sorts of senarios icon_biggrin.gif.
Back to top
 Alert Moderators
martinxxxxxx

from United Kingdom

Joined: 24 Oct 2009
Posts: 1001
Location: Hampshire,
United Kingdom
Thanked: 56 times

PostPosted: Wed Sep 01, 2010 6:57 pm Reply with quote

Hi NHA

I would like you to write a WIKI on this subject. You present it in a a very clear and concise fashion. Which is rare anywhere, let alone on this forum.

I suspect that the effort to write on the subject may be too much to do for free, which is a shame, as you clearly have a wide range of knowledge on the subject and it's history. If it is too much to do for free, I think the market is ready for a book to rival what is out there at the moment.

Martin
Back to top
 Alert Moderators
ban-all-sheds

from United Kingdom

Joined: 27 Aug 2003
Posts: 47582
Location: London,
United Kingdom
Thanked: 1759 times

PostPosted: Wed Sep 01, 2010 10:02 pm Reply with quote

Back to top
 Alert Moderators
radweld

from United Kingdom

Joined: 10 Aug 2009
Posts: 73
Location: Sussex,
United Kingdom
Thanked: 3 times

PostPosted: Wed Sep 01, 2010 10:10 pm Reply with quote

ban-all-sheds wrote:
http://www.simplifydiy.com/electrical/basic-electrics/earth-bonding#Earthing%20domestic%20circuits


Eh?

I stopped reading after the first paragraph:

"The ground we stand on is a much better conductor of electricity than the copper wires in our domestic circuitry and electricity will always travel via the shortest and fastest route through any set of connected conductors."
Back to top
 Alert Moderators
Spark123

from United Kingdom

Joined: 02 Jun 2005
Posts: 14255
Location: Cumbria,
United Kingdom
Thanked: 408 times

PostPosted: Thu Sep 02, 2010 3:56 pm Reply with quote

I struggled to find much right with that, wonder if it was written by the same fellow as that book BAS?
Back to top
 Alert Moderators
holmslaw

from Vanuatu

Joined: 12 Dec 2006
Posts: 5844
Location: Vanuatu
Thanked: 400 times

PostPosted: Thu Sep 02, 2010 5:00 pm Reply with quote

..


Last edited by holmslaw on Mon Apr 02, 2012 4:03 pm, edited 1 time in total
Back to top
 Alert Moderators
radweld

from United Kingdom

Joined: 10 Aug 2009
Posts: 73
Location: Sussex,
United Kingdom
Thanked: 3 times

PostPosted: Thu Sep 02, 2010 5:39 pm Reply with quote

ricicle wrote:

Also there is the possibility of them carrying a proportion of load current in the event of a lost supply Neutral

plugwash wrote:

Consider for example the situation where the lead sheath of a cable has corroded through and then the sheath after the break has shorted to live!


As a follow on point, in both these cases, how would the fault ever be detected? Apart from all the metal work being live everything may still be working (In the PME scenario, I may be getting my neutral via my neighbour's MPB. Is this a case of 'circulating network currents' that has been mentioned?)

NotHimAgain wrote:
Electric shock due to faults does not have to involve 'mother earth' at all - it could be between the faulty equipment and some conductive part that links, say, to the supply earthing conductor in the house next door

Yes - that's fair comment. The point I was wanting to illustrate was your original equations overlooked the reduction the MPB may have on the ECP loop impedance - which may be undesirable. But I think this has already been discussed on other threads.
Back to top
 Alert Moderators
NotHimAgain

from United Kingdom

Joined: 08 Feb 2008
Posts: 435
Location: United Kingdom
Thanked: 19 times

PostPosted: Thu Sep 02, 2010 7:22 pm Reply with quote

A fault may only be detected when someone notices some affect – this could an electric shock if the bonding is not effective, but hopefully it might just be a change in supply voltage affecting the telly icon_biggrin.gif.

However, if a TN-S system becomes faulty there might be no indication. The same is also true for TT.

TN-C-S systems might cause ‘network currents’ to appear in the bonding conductors of an installation in various ways.

Perhaps the most often quoted case is a break in the neutral of the radial main that runs down a street. A service cable connects from each house onto this main. A broken neutral could allow the whole of the neutral current flowing in a given section of this main to ‘divert’ via bonding connections and some ‘shared’ extraneous-conductive-part.

This is potentially very dangerous as the currents and voltages involved can be substantial. However, this fault is relatively rare and often causes noticeable supply voltage problems.

Another way is if your extraneous-conductive-parts are supplying a low impedance connection to the general mass of earth. This can be a problem because the network is usually PME .

To expand that:-
The TN-C-S earthing system is usually implemented by using a PME supply network. That is Protective Multiple Earthing. This system requires that the installation earthing conductor connects to the supply neutral, but it also requires that the supply neutral be connected to earth at various points along its run back to the sub-station transformer, where it is again connected to earth.

Now all of these neutral to earth connections effectively place the general mass of earth in parallel with the supply neutral. It is not uncommon to measure several amperes flowing into an installation from the network. This current then passes to earth via an extraneous-conductive-part. This is particularly common on steel framed buildings.

A further problem occurs when a number of properties share common metallic services such as water & gas. These also end up in parallel with the neutral and will carry current. Some supply companies might ask for insulated inserts in the rising service pipes in blocks of flats. However, Coronation Street is just a block of flats lying on its side icon_biggrin.gif.
Back to top
 Alert Moderators
Search this topic :: View previous topic :: View next topic  
Post new topic   Reply to topic    DIYnot.com Forum Index > Electrics UK All times are GMT
Goto page Previous  1, 2
Page 2 of 2

 
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
Similar Topics   Replies   Views   Posted 
Main earth bonding (lack of) 11 900 Tue Feb 05, 2008 12:57 am
Main bonding installation, Cert or not?? 22 980 Tue Feb 05, 2008 7:18 pm
Main Bonding 10 840 Sun Dec 23, 2007 6:03 pm
Main Equipotential Bonding 6 760 Sat Dec 22, 2007 4:50 pm
bonding to water main 5 480 Mon Nov 05, 2007 7:56 am


 
DIYnot
Find an Expert | Find a Supplier | Search DIYnot.com
Network | Advertising | Newsletter
DIY | DIY How To | @home | DIY Wiki | DIY Forum
By using this site you agree to our Terms of Service / Disclaimer.
Please read our Privacy Policy. Copyright © 2000-2014 DIYnot Limited.