TT, multiple earthing.

[Chris.J]

My detached garage is fed with SWA cable via fuse/breaker from directly after the meter.
The garage has it's own Cons. unit with RCCD & earth rod.
The rods are 30 mtr apart.
The house & garage earths are connected together via the SWA, is this correct, or should they be isolated?

 

[Lectrician]

If they are both TT, best to leave them connected.

If your garage CU contains the RCD to protect that end, is there an RCD Protecting the armour to it (you say switch fuse??). If not, what earthing arrangements are used for the swa and metal (i assume) switch fuse??

 

[ban-all-sheds]

So 2 earth rods, with conductor(s) connecting them.

Let's hope you don't get one of these nearby, which creates a voltage gradient between the two rods and therefore current flowing between them:

Mind you - if it's a risk, surely it must also be so for PME supplies, and I can't work out why not. A lightning strike will create a voltage gradient across the ground, so if you have copper rods and cables linking two points with a PD between them, current will flow...

 

[Adam_151]

I'd be more wooried about holding class 1 equipment and being a bit of a distance away from the earth rod, while making cotact with the ground (you could be in an attached garage, and just happen to have no footwear on, you could just be getting a screwdriver and rest your hand on a metal cased socket as you lean across a pile of junk, for example)

 

[Lectrician]

So how come there are crazy rules on advertising on this site - but none about huge, monsterous pictures

Sort it out bas

 

[ban-all-sheds]

ooh-err.

Is it a problem?

 

[Steve]

ooh-err.

Is it a problem?

its bloody massive! a thumbnail link would have been better.

 

[ban-all-sheds]

OK - sorry 'bout that. I've got automatic image resizing turned on, so when I view the image directly, it doesn't appear full size.

But put it inline, and it grows a bit.

I've resized and re-hosted it.

If anybody wants the full-size original, it is here: http://teslamania.delete.org/photos/Lightning/Lightling_Kane_Quinnell.JPG

 

[plugwash]

Mind you - if it's a risk, surely it must also be so for PME supplies, and I can't work out why not. A lightning strike will create a voltage gradient across the ground, so if you have copper rods and cables linking two points with a PD between them, current will flow...

lightning stikes are considered sufficiantly rare in this country that domestic installations are not designed with them in mind.

 

[Chris.J]

Thanks Guys.
Lectrician, you've raised a good point, the SWA has no leakage protection.
Two solutions come to mind : --
1) I could put a 100mA type S RCCD to feed it, if I did this, then should the rod be left at the Garage end, or be at the House end, or both?
I understand Ban's point about the danger from Lightening strikes, a single earthing point seems best.
However, my incomer is PME capable,(the Board put me an earth terminal from their Neutral some years ago, but it is'nt being used as I preferred to stay with seperate TT systems).
So, 2) instead of the slow RCCD, I suppose I could use the PME earth to protect just the SWA, disconnecting the armour before it enters the Garage CU? The 30A fuse should then blow if the cable fails.