Tt Earth Question

[MeFil]

I've got a client whose property has a 100A TT supply. He is building a detached garage about 15 metres away from the house. I've got to install sockets and lighting in the garage.  Supply to the garage will be via a 3 core SWA 16mm.

So the obvious question... export the TT earth from the existing Rod or install a separate rod for the garage?  As an aside, the ground around the new garage has been made up.  

 

[batty]

You could do it either way. What sort of RCD is house end?

 

[MeFil]

Main distribution board has 100mA time delayed RCD Main switch. This board will feed the garage via the SWA

 

[Sidewinder]

Why 3 core SWA?

 

[MeFil]

The client has already run the cable in. He has some large single phase woodworking machines he is putting in to the garage.

 

[Sidewinder]

So?

OK, I can see that you are now stuck if the client has run the cable in, but, if you are cert'ing it, then you must be OK with the calcs.

Remember that the SWA & the core MUST both be able to meet the fault current requirements independently.

 

[Manator]

Properly installed TT earthing is the best you can get, a two core armour would get you to the garage, a new rod near the garage would enhance the earthing arrangement. If Steptoe was here he would bang a rod in.

 

[MeFil]

I'm happy with the cable size.  I'm suggesting 40A supply for the garage. 

 

[batty]

I personally would put supply through a 63 amp switch fuse with its own TD RCD beforehand less chance of tripping out if he is using machinery. 

 

[MeFil]

Properly installed TT earthing is the best you can get, a two core armour would get you to the garage, a new rod near the garage would enhance the earthing arrangement. If Steptoe was here he would bang a rod in.

I can see the arguments both way, hence the question.  

As the ground is made up around the new garage I can foresee having to put the new rod next to the existing rod.  The extra expense to the client of rods, cable, RCD main switch, testing, etc.. seems a bit pointless in that situation.

 

[Manator]

Have you allowed step voltage calculations within your installation?

 

[MeFil]

I personally would put supply through a 63 amp switch fuse with its own TD RCD beforehand less chance of tripping out if he is using machinery. 

That's an interesting idea....

Have you allowed step voltage calculations within your installation?

I shall express my ignorance at this point and ask for more info from you... :/

 

[Manator]

Your earthing arrangement will be confined to the area of installation. Outside of this (equipotential zone) the effects upon a person could be exaggerated if within one or two meters from supply the ground resistance is more or less.

 

[steptoe]

Im lost as to your use of 'made up'

second,

why the second rod beside the first?

just drive a deeper rod in,

have you done a proper survey to see if a rod will work, or just needs to go deeper,?

 

[MeFil]

Im lost as to your use of 'made up'

second,

why the second rod beside the first?

just drive a deeper rod in,

have you done a proper survey to see if a rod will work, or just needs to go deeper,?

I mean the ground all around the garage has been dug up, removed, and new soil brought in. Might not be the most stable of soil. The nearest stable ground, which hasn't been disturbed by building work, is ironically quite close to the the existing earth rod.

No surveys been done. Just at a planning stage right now. Which is why ive asked for some advice. Given this situation:-

1. Would you treat the garage as a separate equipotential zone and install a rod for this part of the installation?

2. Would the fact that the ground around the garage has been disturbed and made good affect this choice?

your advice is greatly appreciated