Garage Metalwork

[Spearmint]

I'm in the middle of putting some lighting and socket outlets in my garage, nothing fancy just a few of each as you'd expect. I've run it in SWA ,which will be buried eventually, from the existing CU in the house on it's own circuit. Earthing arrangement is TT which reads 14.67 ohms, not bad I thought been in for 20+ years.

There's a cold water feed coming from the house underground and pops back up in the garage, now this is bonded as you'd expect in the house back to the MET which reads 0.54 ohms when testing against the new garage CU earth bar. So this got me thinking... it isn't extraneous as testing shows it's already connected, nor is this a separate dwelling so in effect is within the same installation.

My gut feeling is to supplementary bond it regardless due to risk of introducing a difference in potential should there be an issue with the main bond in the house, but as far as the regs are concerned???

Thoughts?

 

[Engineer54]

Think he's talking about something different, ...why would he need to bury a sub main cable to the garage if it was an integral garage??

 

[spark 68]

Well this is it, how are we supposed to advise without seeing the job, or at least a clear description ?

It doesn't help that the building regs classification does not quite marry up with BS7671, it could be classed as integral or separate depending on which way you look at it, my gut feeling would have been to make my sub-main 6mm 3 core, as it is TT, or possibly even 10mm in case they ever update it to PME, especially if it is buried.

 

[Engineer54]

Well this is it, how are we supposed to advise without seeing the job, or at least a clear description ?

It doesn't help that the building regs classification does not quite marry up with BS7671, it could be classed as integral or separate depending on which way you look at it, my gut feeling would have been to make my sub-main 6mm 3 core, as it is TT, or possibly even 10mm in case they ever update it to PME, especially if it is buried.

Let's see if he comes back to clear this point up... lol!!

 

[davesparks]

in a domestic installation with an outbuilding and TT supply I would be looking to install a submain with a cpc suitably sized to allow me to put a rod(s) in at the outbuilding which is connected back to the MET to add to the existing earth electrode(s).

If the existing Ra really is ~14ohms i'm sure it could be improved down to a very nice value with a bit more effort.

 

[Spearmint]

Think he's talking about something different, ...why would he need to bury a sub main cable to the garage if it was an integral garage??

You're correct. The garage is a separate building to the house, it's about three metres away from the main building. My use of the word dwelling was more relating to it being unoccupied, not a living space.

 

[Leesparkykent]

You're correct. The garage is a separate building to the house, it's about three metres away from the main building. My use of the word dwelling was more relating to it being unoccupied, not a living space.

Have you been back to test it then?

 

[Spearmint]

Have you been back to test it then?

Not yet, back home this afternoon so will test it.

 

[Leesparkykent]

Not yet, back home this afternoon so will test it.

Like Eng54 said it's going to be extraneous if it's metallic and buried.

 

[Spearmint]

Like Eng54 said it's going to be extraneous if it's metallic and buried.

Almost certainly. Although I have no idea how it is buried, it's been in for years well before I moved in. Could be in a duct for all I know. A quick test will verify though.

I'll report back with my findings.

 

[Engineer54]

So how do you feel about running in a suitably sized separate CPC that can take care of both the bonding and circuit protective conductor in the event of your DNO supply being PME'ed?? In the meantime it can be used as a link to an additional earth rod position at the garage via the garage EMT back to the MET in the house, and providing a suitable means of bonding to the extraneous water pipe in the garage. The TT rods can be left connected to the MET/EMT if and when a PME connection has been made available....


As davesparks has suggested, the existing Ra can be significantly improved from 14 ohms with a little thought and effort. Plus the lower the Ra achieved, will make all the difference if and when a PME earth is provided by the DNO, not to mention the existing TT system....

 

[Spearmint]

So how do you feel about running in a suitably sized separate CPC that can take care of both the bonding and circuit protective conductor in the event of your DNO supply being PME'ed?? In the meantime it can be used as a link to an additional earth rod position at the garage via the garage EMT back to the MET in the house, and providing a suitable means of bonding to the extraneous water pipe in the garage. The TT rods can be left connected to the MET/EMT if and when a PME connection has been made available....


As davesparks has suggested, the existing Ra can be significantly improved from 14 ohms with a little thought and effort. Plus the lower the Ra achieved, will make all the difference if and when a PME earth is provided by the DNO, not to mention the existing TT system....

It's definitely doable. That's a good call actually, the house next door to me was converted to PME when they had the cutout moved during a kitchen extension and they are fed from the same string of overhead lines. That was two years ago.

 

[spark 68]

Right now we have established it is a separate electrical installation, I would follow Dave and E54s advice , also as I said in post #17

"my gut feeling would have been to make my sub-main 6mm 3 core, as it is TT, or possibly even 10mm in case they ever update it to PME, especially if it is buried."

I speak from experience here lol as this happened to me a few years ago lol, luckily we had ducted the SWA cable, the DNO had updated the OH supply to ABC cabling and converted everyone in that area from TT to PME, they sent letters to every householder warning of the need to get their bonding checked, we had only laid the sub-main about 7 month previously.

Now I consider that the price difference of 10mm to 6mm SWA to be negligible on domestic installs compared to the cost of upheaval of replacing unless it is very simple to do.

Edit: and run a local MPB from the incoming water pipe (in the garage) to the garage CU EMET.

 

[Spearmint]

Just following on on the test result, it's showing 0.0M ohms when IR'ing between the MET and the pipe work in the garage.

 

[Spearmint]

...or I could utilise the armour as the MPB conductor, it has a copper equivalent to just over 6mm^2.

 

[Engineer54]

...or I could utilise the armour as the MPB conductor, it has a copper equivalent to just over 6mm^2.

NO you can't and NO it doesn't!!