UK Main Earth Cable Source

[richard9876]

When earthing gas and water service pipes do the 10mm earthing cables HAVE to come from the main earthing terminal (first tee) or can they come from a consumer unit that is connected to the main earthing terminal by a 16mm earth cable ?

What specifiacally do the relevant regulations say about this (my house was built in 2012) ?

I ask because Cadent recently changed the gas pipes in my street to plastic and they moved my gas meter nearer the road because the gas pressure was too low with the new thin pipes.

However they refused to earth the new copper gas pipe they installed inside my garage that connects my boilers to the new meter position.

They said the earth cable for the old meter position comes from a consumer unit after the first tee which doesn't comply with regulations and so they wouldn't earth the new copper gas pipe they had just installed.

Note there are three consumer units in my house each of which is connected to the main earthing terminal by 16mm earth cables which is why I have a main earthing terminal even though it's not directly used to earth the gas and water pipes.

On a wider note do you think Cadent are being reasonable or unreasonable as regards refusing to earth the new gas copper pipe in my garage ?

 

[cliffed]

Earth needs to be where the metal gas pipe enters your building…if the DB’s have the correct main earth cable to each 16mm..then the 10mm main protective bonding conductor can be connected there.

 

[Dave OCD]

The fact that the incoming gas pipework has been replaced with plastic totally removes the need for a main protective bond to the copper gas pipe in the house. There seems to be a very poor understanding of this in the 'gas' fraternity.

 

[timhoward]

@Dave OCD is spot on.
411.3.1.2 "[....] .Metallic pipes having an insulating section at their point of entry need not be connected to the protective equipotential bonding"
You only need bonding for extraneous conductive parts. Plastic pipe isn't conductive.

 

[timhoward]

To answer the original point, in general the regs simply say you have to have a main earthing terminal or bar to connect the protective bonding conductors and the cpc's to the main earthing conductor. (542.4)
(This can be inside a consumer unit or external)
They also say that the earthing system of an installation may be subdivided as long as each divided part complies with the requirements. (541.2)
In fact if you think about it in a lot of installations the protective bonding is connected to an external MET and the cpc's are connected to a bar inside the consumer unit.
All that to say, if gas had still been conductive and need bonding, as long as the main earthing conductor is greater in csa than the protective bonding conductor I cannot see any issue at all.

 

[richard9876]

The fact that the incoming gas pipework has been replaced with plastic totally removes the need for a main protective bond to the copper gas pipe in the house.

I'm confused by this.

Surely the copper pipe between the gas meter and my boilers should be earthed as a sensible precaution (and perhaps legal requirement) in case this pipe ever becomes live due to an electrical fault somewhere.

Are you saying this pipe doesn't need to be earthed ?

 

[loz2754]

I'm confused by this.

Surely the copper pipe between the gas meter and my boilers should be earthed as a sensible precaution (and perhaps legal requirement) in case this pipe ever becomes live due to an electrical fault somewhere.

Are you saying this pipe doesn't need to be earthed ?

No it doesn't need to be bonded to the main earth terminal, if it is not in contact with the ground. (There is a test that can be done to confirm that the pipe is not likely to bring in earth potential.)

However, as the pipe is likely connected mechanically to a boiler or other equipment, it will be protected by the circuit protective conductor of the boiler.

 

[Lucien Nunes]

Leaving aside the issue of whether the pipe needs bonding (it might still be extraneous for whatever reason,) the OP's point was that they objected to the MEB being connected to an EMT in one of the three sub DBs, rather than the MET or an EMT in a DB at the origin.
Personally I consider it to be best practice to connect MEB to the MET, but provided the distribution circuit contains a CPC adequate for the purpose (in this case stated to be 16mm²) then I agree with the above comments that it is in order to use it. If the extraneous service is of very low external resistance, then avoiding a long run of MEB cable to a distant MET might have a mildly beneficial effect on touch voltage in the event of a heavy fault.

An interesting corollary is the effect that the use of a CPC as MEB has on the Iz of the distribution circuit cable. On a TN-C-S supply with low-impedance extraneous services, a just-adequate CPC might in theory experience non-trivial heat dissipation due to normal circulating currents that would theoretically impact the circuit rating. I say 'in theory' because in almost all situations the effect would indeed be trivial.

 

[richard9876]

No it doesn't need to be bonded to the main earth terminal, if it is not in contact with the ground. (There is a test that can be done to confirm that the pipe is not likely to bring in earth potential.)

However, as the pipe is likely connected mechanically to a boiler or other equipment, it will be protected by the circuit protective conductor of the boiler.

Okay, so I think you're saying the copper gas pipe will be earthed by the boiler as regards electrical faults inside the house so there is no need for it to be earthed near the meter by a dedicated 10 mm earth cable from the main earth terminal.

If I still had a metal gas pipe coming into my house and the dedicated 10 mm earth cable from the main earth terminal was needed near the meter what exactly would this earth cable be protecting me from ?

 

[Lucien Nunes]

From a potential difference between the pipe and the earthing system within your installation's equipotential zone, which could create a shock hazard. For example, in the event of a fault that elevates the potential of your MET, which might equally arise within your property or outside, the voltage present on the pipe must follow it. If the pipe is in low-resistance contact with the ground, a heavy current will flow into it, necessitating the large MEB conductor to tie it into the equipotential zone.

If the pipe is not extraneous, i.e. has a high resistance to true earth, then it will not pass a hazardous shock current to true earth even in the event of a difference in potential with respect to the earthing system, therefore does not need bonding.

 

[littlespark]

Notwithstanding the fact the pipe is plastic....

new boards still come with a earth bar inside, labelled, and there’s usually a “B” for bonding next to the numbered terminals for circuits.

saying that, if there is an MET, main earth terminal, then I’d use it.
It’s easier to test from, and you can visually check it without cover off board.

 

[timhoward]

If I still had a metal gas pipe coming into my house and the dedicated 10 mm earth cable from the main earth terminal was needed near the meter what exactly would this earth cable be protecting me from ?

In simple terms....
Firstly it prevents the pipe inside your house becoming dangerous to touch due to it becoming live from a source outside your house. The current path will be via the low resistance bond making the pipe safe to touch.
Secondly if the pipe became live inside your house, it protects someone outside your house being able to receive a shock by touching the pipe while standing on real earth as the current path would again be through the low resistance bond.
(Touching an insulated but live pipe inside your house is not something I'd volunteer to do but with dry hands wearing shoes is certainly less dangerous)

 

[Pete999]

When earthing gas and water service pipes do the 10mm earthing cables HAVE to come from the main earthing terminal (first tee) or can they come from a consumer unit that is connected to the main earthing terminal by a 16mm earth cable ?

What specifiacally do the relevant regulations say about this (my house was built in 2012) ?

I ask because Cadent recently changed the gas pipes in my street to plastic and they moved my gas meter nearer the road because the gas pressure was too low with the new thin pipes.

However they refused to earth the new copper gas pipe they installed inside my garage that connects my boilers to the new meter position.

They said the earth cable for the old meter position comes from a consumer unit after the first tee which doesn't comply with regulations and so they wouldn't earth the new copper gas pipe they had just installed.

Note there are three consumer units in my house each of which is connected to the main earthing terminal by 16mm earth cables which is why I have a main earthing terminal even though it's not directly used to earth the gas and water pipes.

On a wider note do you think Cadent are being reasonable or unreasonable as regards refusing to earth the new gas copper pipe in my garage ?

You aren't earthing the pipework, you are bonding them to ensure the pipework is at the same potential as the main earth conductor to your property, pedantic it maybe, but that is why they are called main bonding conductors. from the CU EARTH BUSBAR IMO

 

[DPG]

You aren't earthing the pipework, you are bonding them to ensure the pipework is at the same potential as the main earth conductor to your property, pedantic it maybe, but that is why they are called main bonding conductors. from the CU EARTH BUSBAR IMO

Indeed.