Learner's question - TT supply for garage containing services from main house TN-C-S

[zempo]

I wonder if anyone could help me understand how to deal with the extraneous-conductive-parts in the following (theoretical) scenario, modified from a practice question for the C&G 2391:



The occupier of a domestic property has decided to have a new electrical supply to a steel framed garage installed. The garage has a 15mm copper water service pipe supplying fresh water to the garage.

The garage has a new all insulated consumer unit fitted, with a 100mA RCD to BS EN 61008 fitted as the main switch. The new underground 2 core SWA supply cable to the garage is 30m in length.
You are not permitted to utilise the DNO's earth as a means of earthing the garage.
The main intake to the domestic property consists of a TN-C-S earthing system.



I'm confused about whether I should maintain or interrupt any electrical continuity provided by the copper water service pipe from the main installation.


I have assumed the copper water service pipe comes straight from the main property, and it would introduce a connection to the TN-C-S system through the existing protective bonding set-up in the main installation. I thought if the garage installation had to be provided by a separate TT system using an earth electrode, then we would have to ensure total separation from the TN-C-S Earth connection.


If extraneous-conductive-parts within the garage are to be bonded to the TT system of earthing for the garage and kept separate from the TN-C-S sytem of earthing in the main property, how should we deal with the water pipe? Changing the water pipe to plastic just before it enters the garage would mean that it is no longer an extraneous-conductive-part, but somehow this doesn't seem safe to me... then again surely TT installations nationwide utilise the common Earth potential introduced from service pipes that feed a multitude of different installations, including TN-C-S ones! Doh! I did my own head in again.

Any advice on this would be greatly appreciated. Thank you!

 

[Deleted member 26818]

Re: Learner's question - TT supply for garage containing services from main house TN-

Don't worry about the pipework being connected to the earth in the TN-C-S property, just bond to the TT main earth, just the same as you would if the scenario was two adjacent properties one TT and the other TN-C-S.

 

[telectrix]

Re: Learner's question - TT supply for garage containing services from main house TN-

and that copper pipe will probably give a better earth than his TT rod ever will, unlesss he uses a 12' scaffold pole. bond the steel framework as well. should make an efficient lightning conductor.

 

[HandySparks]

Re: Learner's question - TT supply for garage containing services from main house TN-

If the copper pipe is not owned by the water utility supplier you could probably use it as the earth electrode (providing that it is suitable). See BGB 542.2.6 for conditions.

 

[clicon357]

Re: Learner's question - TT supply for garage containing services from main house TN-

This link is posted elsewhere on here:
http://www.electriciansforums.net/electrical-forum-general-electrical-forum/28159-good-guide-exported-pme-outbuildings.html

 

[zempo]

Re: Learner's question - TT supply for garage containing services from main house TN-

Thank you all for your replies.

Spinlondon, I found your answer really clear and to the point. I shan't worry about the copper pipe coming from the main property if you think it is ok - thanks for addressing my later confusion too about installations using different earthing systems sharing common services anyway. Telectrix and HandySparks, I still don't have the BGB so it is handy to know a new regulation now allows the use of metallic water supply pipework as an earth electrode. You're right about the subject matter, clicon357 and thank you for the link to the article, which specifies:

"main equipotential bonding will need to be provided at the garage connecting the metal water pipe and any other extraneous-conductive-parts to the earthing terminal in the small distribution board".

I had posted this question because it was not clear to me from the article that the metal water pipe referred to ran directly from to the main installation close by, where it was being used as an extraneous-conductive-part. (The accompanying diagram showed a water pipe in the garage going straight down but not coming from the main installation). I got confused! Think I am getting there though...

Just one more thing to make sure, if you don't mind: It is ok to bond the water pipe to the TT earth electrode, but it would not be ok to earth the SWA at both ends (should only be earthed at the main installation). Have I got this right?

Thanks again for helping me understand it better.

 

[telectrix]

Re: Learner's question - TT supply for garage containing services from main house TN-

Just one more thing to make sure, if you don't mind: It is ok to bond the water pipe to the TT earth electrode, but it would not be ok to earth the SWA at both ends (should only be earthed at the main installation). Have I got this right?

sounds right to me. SWA armour should only be earthed at the supply end if using a TT system at the garage/shed. it must however be correctly glanded at the garage end. the problem you would have there is if the garage DB were metal.

 

[HandySparks]

Re: Learner's question - TT supply for garage containing services from main house TN-

I still don't have the BGB so it is handy to know a new regulation now allows the use of metallic water supply pipework as an earth electrode.

Not a new reg, it's 542.2.4 in the BRB.
To quote: "The metallic pipe of a water utility supply shall not be used as an earth electrode. Other metallic water supply pipework shall not be used as an earth electrode unless precautions are taken against its removal and it has been considered for such a use.

So a privately owned metal water pipe (say between house and garage) can be used, with conditions.

 

[davie spark]

Re: Learner's question - TT supply for garage containing services from main house TN-

Just one more thing to make sure, if you don't mind: It is ok to bond the water pipe to the TT earth electrode, but it would not be ok to earth the SWA at both ends (should only be earthed at the main installation). Have I got this right?

sounds right to me. SWA armour should only be earthed at the supply end if using a TT system at the garage/shed. it must however be correctly glanded at the garage end. the problem you would have there is if the garage DB were metal.

Rcd also on the supply side of the supply cable. Protection from damage from impact.

 

[Deleted member 26818]

Re: Learner's question - TT supply for garage containing services from main house TN-

Rcd also on the supply side of the supply cable. Protection from damage from impact.

Why???