Sub-mains earthing question

[Lionscourt]

I’ve recently run a supply to a garage and I had a question concerning the earthing. Here is the situation:

TT supply to house.
Supply goes into a time-delay 100mA RCD before it splits.
This then goes through a sub-mains isolator at the house.
3-core SWA runs about 60 metres to garage.
17[SUP]th[/SUP] edition consumer unit in the garage.
New earth spike installed for supply at the garage.

So, my question: Should the SWA section be connected to earth at both ends, or only at one? Is there a reason I would NOT want to connect the earth at both ends?

 

[telectrix]

i would connect at both ends. that way the connection between the 2 rods should bring the Ze of both down.

 

[Amp David]

Is there not any overcurrent protection for the sub main then?

 

[telectrix]

didn't spot that. isolator need to be fused to protect the SWA.

 

[Lionscourt]

didn't spot that. isolator need to be fused to protect the SWA.

The sub-mains isolation is currently a BS 60947-3 along with a BS 60898 MCB. Are you saying that it must be something like a BS 88 then?

 

[telectrix]

no, the 60898is fine. it's just in your post 1 you didn't mention overcurrent protection for the SWA.

 

[clive dobbs]

Hi
As it's a TT system at origin no problem connecting SWA earth at both ends, as mentioned before would help your earthing value. But if your origin earth was PME for example, then the SWA wire from origin should be earthed but not be connected to the consumer unit in the Garage, that should be earth free of PME, then the earthing arrangment for the Garage would become TT from the earth electrode allready installed.

 

[telectrix]

Hi
As it's a TT system at origin no problem connecting SWA earth at both ends, as mentioned before would help your earthing value. But if your origin earth was PME for example, then the SWA wire from origin should be earthed but not be connected to the consumer unit in the Garage, that should be earth free of PME, then the earthing arrangment for the Garage would become TT from the earth electrode allready installed.

why??? what's wrong with the PME in the garage?

 

[clive dobbs]

Nothing in princible, but the example above, running 60 meters and a new consumer unit to a garage could require a bond to a water supply or any other extraneous conductive parts therfore that could introduce a potential back into the PME zone. If the cable was 3 core 10mm then bonding requirments for 100amp PME supply would be ok

 

[Lionscourt]

Thanks for the comments everyone, they're appreciated.