main service earths

[keenamateur]

How are the cable sizes for main water/gas ie 10mm or 6mm arrived at by regulations and what is the theory behind it?

hope this is clear


thanks in advance

 

[andy]

not really sure TBH, but its something to do with the fault currents what could occur

 

[keenamateur]

anybody else know?

 

[Lectrician]

Just fit and forget.

 

[Kilburnlad]

Where the main service bonding is associated with a PME (protective multiple earthing ) type of service the bonding cable may be required to carry load current.
This arises because the PME service cable uses a combined neutral and earth conductor; i.e. your earth is derived from the neutral. This normally provides a reliable low earth loop impedance. However, there is a very small risk that the service cable neutral (or even the external main neutral) will develop a high impedance fault or even go open-circuit.
In these circumstances normal load current of the neutral may return through the earth bonding, assuming that at least one of the other (non-electric) metallic utility services has a continuous metallic path (most now are insulated or have insulating inserts).
This is why the Regs refer you to your electricity supplier to check the minimum bonding requirments - it can depend on the local network arrangements and proximity or not of other customers using PME.

 

[Big_Spark]

Nice theory there Kilburnlad, and I am sure that somne of what you said plays a minor role, however the main reason for their size is the

POTENTIAL FAULT CURRENT THAT MAY BE PRODUCED BY THE SUPPLY CABLE IN THE EVENT OF A FAULT AND SECONDARY, IT IS TO PROVIDE AS LOW AN IMPEDANCE TO EARTH AS POSSIBLE.

There are a few other factors to take into account, but these are minor.

 

[Kilburnlad]

For the avoidance of doubt, I'm talking about the main bonds between the service earth terminal and other metallic utility services, structural steel work and lightning conductors. In addition to anything the Regs may require, for a PME supply the network operator insists on this bond, and specifies its size, to ensure among other things that equipotential conditions would be maintained under a lost neutral condition. Without it, in such circumstances the supply neutral (and thus the electrical earth) could rise in potential while other metallic services could remain at or about earth potential, creating a hazardous potential difference. In most domestic situations such bonds would connect only to gas and water (telecomms being specifcally excluded).

As for fault current, as most gas and water utility services are now plastic, where is the earth fault current going to go? If we're talking about a fault between a live conductor and a metallic water pipe (say) somewhere in the house, then the fault current would have multiple return paths through all the supplementary bonding and there certainly wouldn't be a need for a 10mm cable at the main intake to carry it back into the mains neutral, bearing in mind the sub-circuit feeding the fault will probably be no more than 4mm.

If we're talking about fault current flowing back into the main earth terminal, this will take the lowest impedance path back to the network operator's distribution transformer, which will be through the service/mains neutral conductor if the service is PME; any other path (assuming there are continuous metallic services connected to the cross-bonds) will be only secondary and of relatively high impedance. So the size of the bond is not related to this scenario.

Finally, what I said is not a theory, it's a fact, albeit a little appreciated one outside of the engineering departments in the electricity supply industry.

 

[ChrisRogers]

yeah cos what you said makes soo much sense now everyone has plastic services. if what you said was true then we'd fit rods on every PME install, somthing i think that should be done.

if the 10/16mm bond is for PME then why do we do it on other supplies?

 

[Kilburnlad]

I doubt keenamateur expected such a debate!
Earth rods aren't such a bad idea, and where more than 4 customers are supplied off a PME service the service provider should either install an external rod or some other form of alternative earth (often an additional cable).
My copy of the Regs isn't current (16th Ed) but it states:
"Except where PME conditions apply, a main equipotential bonding conductor shall have a cross-sectional area not less than half the cross sectional area required for the earthing conductor of the installation and not less than 6mm." i.e. it doesn't need to be 10mm. It then says:
"Where PME conditions apply, the main equipotential bonding conductor shall be selected in accordance with the neutral conductor of the supply and Table 54H." In this case it also says local electricity network conditions may require a larger conductor. Table 54H specifies the 10mm minimum (As did the 1988 Electricity Supply Regulations in respect of PME supplies).
So getting back to the original question, that's where the 10mm comes from. (Some supply companies ask for 16mm)
As most services are probably now PME, I expect that 10/16mm has become the norm, and it also covers the situation of the service later being upgraded to PME if it isn't already.
It should also be borne in mind that the PME bonding 'rules' came in in the 70's when plastic water and gas wasn't so commonplace, and the prospect of an 'alternative neutral path' through another metallic service was more likely. I agree this is now much less likely, meaning if you lose the neutral the current stops flowing!

 

[mapj1]

The current may stop flowing, but it also means the voltage to which your outside tap and exposed metal bits that go outside, and anything fed off an extension lead outside, becomes extremely dangerous, until all houses affected by the same fault are isolated. With overhead supplies made into PME, a tree falling through the PEN, the lower wire, but not the upper live one, can happen, and has been seen more than once. Adding earth rods is all very well, but if done all wiring to them MUST be able to handle the full load current that might flow, and the rods be positioned so as not to cause a serous step voltage.
In practice this is not done as well as it should be in many cases, as supplies are sometimes updated, with a less than thorough check.
It can be a problem for radio amateurs who's antenna systems often take earth outside.
regards M.

 

[Kilburnlad]

I quite agree mapj1.
Anything metallic that's bonded to the PME earth terminal should not be taken outside the equi-potential zone. As you say, with an open-circuit neutral, and no means for the neutral current to flow, the bonded 'earth' installation inside the zone can rise to phase potential; assuming there's nothing other than the PME neutral/earth to tie it down. This is a relatively low risk situation if the occupants can't come into contact with anything that remains at true earth potential, but 'export' the bonded metalwork outside the premises (i.e. outside the equi-potential zone) and you have a problem. The external tap presents quite a worrying prospect in such circumstances!
Outside the zone separate electrical earthing should be employed, with rcd protection on any circuits leaving the zone.
A also agree that earth rods will rarely provide a suffciently low resistance to avoid unacceptable step voltages, unless the load current is very low.

 

[plugwash]

personally i think they should mandate a voltage operated breaker with one end connected to a rod and the other end connected to the PME earth for all PME installs

 

[kai]

Isn't that going back in time - Voltage elcbs went out years ago it seems?

 

[mapj1]

problem is whatever breaker you use should also remove earth as well as N and E to give true isolation, and I don't think any do.

 

[ban-all-sheds]

problem is whatever breaker you use should also remove earth as well as N and E to give true isolation, and I don't think any do.

Fit a 3-pole?

Or use an MCB of the type where you can put an auxiliary switch module alongside?