Main bonding and new regs?

[Carlos Arruda]

Hi all.

I have purchase my new On-Site Guide (BS 7671:2008 Wiring Regulations, incorporating Amendment 3:2015) but still hasn't arrived yet.

So, maybe someone here is likely to know.

Are there any changes to main bonding conductors? Is the minimum still 6 mm CSA?

Many thanks.
Albert

 

[Richard Burns]

Main protective bonding conductors where the supply does not have PME conditions applying can have a minimum csa of 6mm².
Where the supply is PME then the minimum csa is 10mm².
No changes from previous version.

 

[Carlos Arruda]

Main protective bonding conductors where the supply does not have PME conditions applying can have a minimum csa of 6mm².
Where the supply is PME then the minimum csa is 10mm².
No changes from previous version.

Thanks Richard.

I have received my OSG today.

I shall have a good browse through it.

Regards,
Albert

 

[Carlos Arruda]

Hi Richard.

Would this (link bellow) be a good example of PME?

https://www.google.co.uk/search?q=p...%2Findex.php%3Ftitle%3DEarthing_Types;422;555

Or in this case, a bad example?

Shouldn't the earth cable coming from the 100A fuse suppliers side be 10mm in this case?

Regards,
Albert

 

[Richard Burns]

Your link is to a google search for PME electrical images so I am not sure which one you meant.
Anyway the service head can only really tell you TNCS.
PME would require you to check the earthing points along the supply cable, however it is a reasonable assumption that the DNO have made some effort in this direction is you have a TNCS head.(especially if you have PME sticker on the head!)

The earth provided by the DNO is provided by the DNO and it is up to them to decide on the size as they do not follow BS7671.
An ideal size for an earthing conductor on a 100A domestic TNCS head is 16mm², but it is possible that 10mm² would be OK, depending on the size of the line conductor of the supply.

 

[Carlos Arruda]

Hi Richard.

For some reason I thought it would show the image I had selected as an example.

https://www.dropbox.com/s/mc892iqcm065n6q/PME1.jpg?dl=0

Here is the link for the pic in question.

Thank you.

Regards,
Albert

 

[Richard Burns]

That head does have a PME sticker (even though you cannot read it), the earth does seem a bit on the thin side but who are we to query!!
It is probably only 6mm² if the green and yellow others are main earth at 16mm² and main bonding at 10mm².

 

[Carlos Arruda]

That head does have a PME sticker (even though you cannot read it), the earth does seem a bit on the thin side but who are we to query!!
It is probably only 6mm² if the green and yellow others are main earth at 16mm² and main bonding at 10mm².

I was thinking that it would make much more sense the income earth a lot thicker than all others connecting to it.

After all, a thicker cable to main bonding, would allow (due to less resistance) a lot more current, on a fault, to flow back to earth, yet the cable coming from the DNO, if thinner, could well not support such current coming back and thus would be a poor design.

I hope I made myself clear as sometimes I do get lost on explaining things.

Regards,
Albert

 

[Carlos Arruda]

What this arrangement be?

I haven't come across this yet.

https://www.dropbox.com/s/d8ai0rpnv4qt3p1/PME earth monitoring.jpg?dl=0

Still a TN-C-S correct?

Many thanks for helping.

Regards,
Albert

 

[Leesparkykent]

PME (Protective multiple earthing) – an earthing arrangement, found
in TN-C-S systems, in which the supply neutral conductor is used to connect the earthing conductor of an installation with Earth.

TN system – a system having one or more points of the source of energy directly earthed, the exposed-conductive parts of the installation being connected to that point by protective conductors

TN-C-S – a system in which neutral and protective functions are combined in a single conductor in part of the system

TN-C – a system in which neutral and protective functions are combined in a single conductor throughout the system. Where: T – Terre (from the French, meaning ‘Earth’), N – Neutral, C – Combined and S – Separate.


On the low-voltage distribution network,
the earthing and neutral functions are combined in the same conductor of the supply cable; this is known as a PEN conductor and the distribution arrangement is TN-C, it should be noted that DNOs can refer to the PEN conductor as CNE (Combined neutral and earth). Along the length of the low-voltage distribution cable the PEN conductor is earthed, using earth electrodes at regular intervals. To supply the electrical installation, the neutral and earthing functions of the PEN conductor are separated out to create neutral and earth provisions.

The term ‘protective multiple earthing’ describes the method of earthing as
used on the low-voltage distribution network. On the underground/buried network, electrodes are used to earth the neutral conductor at regular intervals, usually 25-40m apart, hence the term ‘multiple earthing’. Where the low-voltage distribution network is overhead, earth electrodes are installed at transformers and at regular intervals at distribution poles.

 

[Carlos Arruda]

Is there any truth in the fact someone said that the DNO uses light street poles as means of earth?

 

[Richard Burns]

That diagram is somewhat strange, it is not a conventional arrangement so probably not a good one to consider at the moment.

That arrangement is a TNCS head however there is also an earth rod and upfront 300mA RCD protection, this is not necessarily against the regulations but would normally be applied in very specific circumstances.
Try and ignore that one for the moment.

TNCS is where the earth for the property is derived from the neutral of the supply at the origin of the installation.
If you see a service head with an earth cable coming from the side box near the incoming neutral supply then this is probably TNCS (there are a few exceptions but not enough to worry about unless you find you have odd readings.)

What this arrangement be?

I haven't come across this yet.

https://www.dropbox.com/s/d8ai0rpnv4qt3p1/PME earth monitoring.jpg?dl=0

Still a TN-C-S correct?

Many thanks for helping.

Regards,
Albert

 

[Carlos Arruda]

Ok. Thanks.

Another thing I am wondering, is the earth, which is separate from the neutral at the customers install, taken to the PME from after or before the 100A Fuse?

In other words is it also fused in a sense or not?

My assumption is that if the earth is taken after the fuse, then it is fused (at the neutral connection).

Makes sense my question?

Thank you all for chipping in priceless info, experience and for your time.

Regards,
Albert

 

[Richard Burns]

You do need to be at least vaguely conversant with the information you are trying to assimilate.
Only the line conductor is fused not the neutral, fusing the neutral has not been permitted for 60 years or so.
Therefore asking if the earth that is derived from the neutral is before or after the fuse is not a sensible question.

 

[Simonslimline]

Also have a think why it is a bad idea to have a fused neutral.

What potential dangers could be introduced in to a system with a fused neutral!

 

[Carlos Arruda]

Also have a think why it is a bad idea to have a fused neutral.

What potential dangers could be introduced in to a system with a fused neutral!

Fused to what ratings would be one of them? There could only be 1A or 50A being drawn.

Potential dangers with a fused neutral? Current flows from the live conductor to the neutral, so no return path to the transform side?

I am a bit overwhelmed to say the least as my initial question derived from me thinking dual pole switch, which would make sense the question, rather than a fuse.

Ops...

Apologies to all.

Too late now for more questions. The chances are I'll post even more stupid, ridiculous questions.

Tomorrow, if work goes well, will be a better day for more questions.

Many thanks to all and good night.

Regards,
Albert


P.S: This will be stuck with me for the rest of my life. So, to a point, making a fool of myself makes things get stuck in forever. I shall never forget. Incoming neural is not and has not been fused for 60 years or more (to current date 2015). Sorry everybody.

 

[Richard Burns]

Good follow up response, sorry if I have made you feel as if you have been stupid, not my intention, just wanting to get the basics straight.

If you have a fused neutral and that fuse blows (but not the line) then all the line and neutral conductors in the installation will rise to the supply voltage, however nothing will be working so one could assume the power is off and so safe to work on (although electricians will test for dead!).

Keep asking questions, but also think about them and how you put them across, no need to rush to post, slow and steady wins the race.

 

[Carlos Arruda]

Richard;

3 years now and 3 to 4 months from finishing level 3.

My tutors wouldn't be impressed.

I should know better.

How did I took a 100A fuse (I know fuses are only in the live conductor) and thought dual pole switch is beyond me.

Did I really make this question? Can the thread be deleted? (kidding)

Anyway, much appreciated to yourself and Simons and all other that have helped in the past.

Always learning.

The forum is a great resource of information.

Regards,
Albert