Submain Earth/Bonding question?

[Flakey]

Hi guys/girls

Just some clarification needed. I have an house where there is sub board in the garage , and also a boiler in the garage. The water copper pipe that connects to the boiler is continuous from the house. I am aware of the exporting PME situation so plan to drop out the 3rd core of the 10mm swa that is feeding the board as well as the glanded armour currently connected to the sub board and TT the garage then bond services to the garage from MET in garage. What would be the best course of action with regards to the pipe being connected to the house as well as the garage? Would a length of plastic pipe be suffcient to to seperate both zones. Also where would be the best place to insert the plastic bit of pipe. for example before it leaves the house or anywhere?

Thanks in advance

 

[bigspark17]

Is this a new garage or wiring in an existing garage? What size swa is supplying the garage? Is there a possibility having a 10mm earthing conductor ran in along side the swa from MET to garage?

 

[Flakey]

Is this a new garage or wiring in an existing garage? What size swa is supplying the garage? Is there a possibility having a 10mm earthing conductor ran in along side the swa from MET to garage?

The garage is existing. The swa is 10mm. Im under the impression PME should not be imported due tothe unlikely event of neutral being lost. The pipe to a degree imports the earth into the garage.

Cheers

 

[DPG]

If connected with a suitably specified cable (bearing in mind bonding requirements) then the provider's earth will be preferable to knocking a rod in. Is the garage attached to the house?
Daz

 

[Flakey]

If connected with a suitably specified cable (bearing in mind bonding requirements) then the provider's earth will be preferable to knocking a rod in. Is the garage attached to the house?
Daz

Thanks for reply. Its the bonding situation that i am more concerned with as the water pipe is bonded at the house and with the pipe running all the way to the (seperate) garage. Im unsure if this arrangement is ok.

 

[Midwest]

You could clarify exactly how your metal water pipe enters 'separate' garage. Is it run on the surface, such as walls, or does it enter the garage from underground? I take it by 'separate', you mean the garage is detached from the property which has the electrical supply?

 

[Flakey]

You could clarify exactly how your metal water pipe enters 'separate' garage. Is it run on the surface, such as walls, or does it enter the garage from underground? I take it by 'separate', you mean the garage is detached from the property which has the electrical supply?

That is correct the garage is detatched from house and the metal water pipe is run under ground.

 

[bigspark17]

Is the swa 2 or 3 core?

 

[Flakey]

Is the swa 2 or 3 core?

10mm 3 core

 

[DPG]

So you need to decide on the bonding arrangements and ensure the supply cable is up to the job.

 

[Flakey]

So you need to decide on the bonding arrangements and ensure the supply cable is up to the job.

The supply cable is suitable for the job. I think my question may not be clear. 1) by sharing/exporting the suppliers earth from MET in the house to the earth bar at the garage consumer unit I am effectivley exporting the earth which would not be a problem if it was only being used for lighting and say sockets. but because I have services i.e metal pipe coming into the garage underground this could be a issue if in the unlikely event supply neutral was lost voltages could rise in earth as the earth now becomes the return path. Solution TT the seperate garage and bond the metal pipe coming into the garage from the TT side. My problem is the metal pipe is already bonded to the house MET via suppliers earth in the house. And the metal pipe continues from the house to the garage. would it be( required) to bond from the garage If the garage was a TT or is the main bond already in place at the house ok.

Cheers!!

 

[DPG]

But the supply cable to the garage can still function as combined CPC and bonding conductor (assuming suitably sized). Main bonding would also be present in the garage, creating an equipotential zone in this building.

 

[Des 56]

would it be( required) to bond from the garage If the garage was a TT or is the main bond already in place at the house ok.


It seems to me that if you TTd the garage and did not bond the pipe which is bonded at the house,there is possibility of 2 different potentials between the earthing systems at the garage end

In the event of a broken external supply neutral,the bonding cable and therefore the pipe would carry the diverted neutral current,even if the garage is TTd and bonded locally, that electrode having a higher resistance to earth, will carry only a small amount of that diverted neutral current anyway
If you were to use the 3rd core of the swa and retain a Tnc-s system and then bonded the pipe locally in the garage to that 3rd core (10mm), the diverted neutral current would be shared between the 3rd core of the swa and the Main bond in the house reducing the current on your Main bonding cables

 

[Wilko]

Hi - if I understand correctly the house is TNCS? So the size of the bonding conductor to the garage is set by the suppliers neutral and selected from Reg 544.1.1 and Table 54.8 - likely 10mm, but plse check. I would not dig up your pipe and TT etc, just do the bonding. Example of this in GN8 page 70.

 

[Flakey]

But the supply cable to the garage can still function as combined CPC and bonding conductor (assuming suitably sized). Main bonding would also be present in the garage, creating an equipotential zone in this building.

Sorry for sounding stupid I was under the impression bonding to outbuilding via the supply earth was dangerous and not allowed!

cheers!!

 

[DPG]

No. That is not correct. As someone said above, check the diagram and notes on p70 of GN8. I think you will be surprised.
Daz

 

[bigspark17]

Use the 10mm earth inside the swa to connect from house MET to EMT in garage and from there 10mm bond to pipe. Youre very lucky to find a 10mm 3core in place underground if only every outbuilding was like that. When getting telephone calls about armoured cables to garage/shed if you suggest to the customer to bung a 10mm 3c in this will usually save alot of drama when you come to connect up and future proofs the install for minimal extra cost.

 

[Ib≤In≤Iz]

Just extend the equipotential zone of the original

 

[Octopus]

I'd rather extend the TNCS than rely on a twig in the ground.....

 

[Flakey]

Thanks a lot guys for your input I think my understanding was a bit off!!