Why 10mm² for main bond cable ?

Joined
3 Nov 2006
Messages
28,027
Reaction score
3,264
Location
Bedfordshire
Country
United Kingdom
Why do regulation require a 10mm² main bond to the incoming water and gas supply pipes. ?

The reason may be to protect the CPC of a cable inside the house.

sneak on CPC.jpg


If the incoming water supply is metallic and the incoming Neutral rises above True Ground potential then current will flow through the circuit in the diagram. There is no fuse or MCB in the circuit. The current is determined by the potential difference between the incoming Neutral and the water main ( at True Ground ) and the impedance of the metallic water main. The smallest conductor in the circuit is the 1.5 mm² CPC in the 2.5 mm² ( or 4 mm² ) feed to the immersion heater.

With a water main impedance of 0.1 ohms and a potential difference of 10 volts the current will be in the order of 100 amps through the CPC.

One assumes the regulations have to cover every conceivable set of circumstance which could include a metallic supply pipe at 0.1 ohm and more than 10 volts difference.

Putting a lpw impedance cable that can safely (?) carry 200 amps from MET to Water Main would seem to be the solution that covers all eventualities in that the majority of the current flows through the 10 mm² bond cable and CPC does not have to carry the entire current.

A similar circuit exists from MET via a gas boiler and pipe work to both gas and water supply pipes
 
Sponsored Links
But surely that current, caused by the impedance and voltage can only flow if there is a closed circuit. Where do you suggest the closure coupling might be ?
 
It only has to be 10mm² for PME supplies.

What is the difference between PME and the other methods (including TN-C-S)?
 
The massive cable to bond gas and water pipes is primarily to make sure that any phase somehow touching a metal pipe will pop the fuse/mcb in a matter of milliseconds so as to avoid any risk of people using a tap or bath or shower getting toasted. When you're showering/bathin you're even more vulnerable than normal hence the ott approach.
At least that's what I was told when I did my 16th.
 
Sponsored Links
The massive cable to bond gas and water pipes is primarily to make sure that any phase somehow touching a metal pipe will pop the fuse/mcb in a matter of milliseconds
No, it isn't.
'Bond', in our usage, means join together - electrically.
Pipes are not 'earthed' for that purpose. They are 'bonded' to equalise potential.

The large conductor required in PME installations is to ensure they can carry diverted current without suffering damage, for example, with the loss of a neutral conductor.


At least that's what I was told when I did my 16th.
I'm afraid you were misled.
 
I would have thought the large bonding conductor is needed to withstand the potential high current given into accountathe csa and therefore impedance of the incoming neutral cable limiting the available diverted current.
The fact that it in turn happens to protect other parts of the electrical installation is true, due to maintaining an equipotential zone, but that wouldn't be the specific justification. More to protect people directly than cpcs.
 
It's larger on TN-C-S supplies because the neutral and earth are connected at the origin, and due to loads on the installation, there will be a voltage drop on the incoming CNE conductor, resulting in a difference in potential (voltage) between the main earthing terminal (MET) and the actual earth, which includes metallic service pipes.
This causes a current flow between the MET and other bonded items, such currents are entirely normal and inevitable on TN-C-S installations.
The magnitude of the current varies considerably and depends on the load in the particular installation, as well as loads in adjacent installations, and exactly how and where the various gas, water and other metallic services are connected and routed.

The same effect does not occur on TN-S or TT supplies, as the earth for those is not connected to the neutral at the origin of the installation, so any voltage drop in the neutral conductor does not affect the MET.

Main bonding on TN-C-S therefore carries current continually during normal operation, rather than other systems where current only flows for the typically very short duration of a fault.
 
I have a PME supply, and my supplier insisted that all bonding is in at least half the size of the metre tails. So currently on a 60amp supply with 16mm tails, so 10mm bond. At some point moving to a 100amp supply, requiring 25mm tails, so 16mm bond required.

*In Jersey we have 1 DNO who also supply the electricity, so they decide whats acceptable beyond the demark point too!
 
Main bonding on TN-C-S therefore carries current continually during normal operation, rather than other systems where current only flows for the typically very short duration of a fault.
Indeed. However, the very high currents (during normal operation) in bonding conductors that bernard talks about arise because he considers exceptionally low impedance (e.g. 0.1Ω) paths to earth via bonded extraneous-c-ps.

However, should he not be considering not the impedance of the path through the extraneous-c-p to 'true earth' but, rather, the impedance of the path back to the neutral of the transformer via the extraneous-c-p and earth? If so, that path includes the resistance of the earth rod(s) at the tranny - which I understand will often be several ohms. If that is the case, the currents would presumably be a lot lower than he suggests?

Currents during rare network fault conditions, when something approaching line potential could theoretically appear at the MET of a TN-C-S installation, are obviously a different matter.

Kind Regards, John
 
I have a PME supply, and my supplier insisted that all bonding is in at least half the size of the metre tails. So currently on a 60amp supply with 16mm tails, so 10mm bond.
Wouldn't 10mm² tails be adequate for 60A? So 6mm² (although, as here, 10mm² might be a minimum).

At some point moving to a 100amp supply, requiring 25mm tails, so 16mm bond required.
Would 80A not suffice, keeping your 16mm².
 
Wouldn't 10mm² tails be adequate for 60A?
Indeed. In fact, maybe even 6mm² (if not touching and largely horizontal) - although such figures are not given for CSAs <25mm², for 25mm² and upwards, the tabulated CCCs for singles in free air, not touching and horizontal, seem to be around 28% higher than for Method C - which, if applicable, would equate to just over 60A for 6mm² !

Kind Regards, John
 
Wouldn't 10mm² tails be adequate for 60A? So 6mm² (although, as here, 10mm² might be a minimum).

Quite possibly, I am governed by their rules!!

Would 80A not suffice, keeping your 16mm².

Yes it would, however I am only allowed 60amp or 100amp!! 80amp has been 'discontinued' apparently. I made a post on it a year ago :(
 
Quite possibly, I am governed by their rules!!



Yes it would, however I am only allowed 60amp or 100amp!! 80amp has been 'discontinued' apparently. I made a post on it a year ago :(
Do the JEC still provide Economy 20 supplies?
 

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.


Are you a trade or supplier? You can create your listing free at DIYnot Local

 
Sponsored Links
Back
Top