General Q about earth rods

[toasty]

b) completely remove any trace of water from your garden

That would give me a nice low Ze, wouldn't it....

Umm, well you could then flood the garden with , ermm, mercury!
That would work.

(Sorry, pub at lunchtime)

Just out of interest though, I notice you can get earthrods that screw together to form much longer ones, presumably getting down to the wetter denser soil further down would improve Zs?
-Dan

 

[Pensdown]

In the past we have fitted hard drawn uninsulated copper cable in the trench - 70mm. Once back filled we did a test to it prior to terminating and the results where very impressive.

I know quite a few consultants who specify that method along with a copper earth mat under the geny base to extend the PME zone if it's sited away from the main building.

 

[ban-all-sheds]

Just out of interest though, I notice you can get earthrods that screw together to form much longer ones, presumably getting down to the wetter denser soil further down would improve Zs?
-Dan

Presumably.

I wish whoever ends up banging the rod in good luck in driving it through many metres of London clay...

 

[efiste2]

"I notice you can get earthrods that screw together to form much longer ones"

Stackable earth rods. they are threaded at both ends, and have a screw tip and a screw joint piece. so you can hammer as deep as you need to, then add another rod ont top, then keep knocking in until you get the reading below the standard, whatever that is these days,

 

[Pensdown]

How much is the tape Ban and what are it`s dimensions?

Well I've not done much of a search, so maybe the first supplier I looked at after googling is outrageously expensive, but they want £6.42/m inc for 25 x 3mm tape...

But I doubt if it's wildly uncompetitive - that tape is 75mm², and when you think about the cost of cable....

If you're passing with pound notes in your pocket.....

 

[ban-all-sheds]

If you're passing with pound notes in your pocket.....

Hmm - passing where? Sevenoaks or Broadstairs?

You'll find an email address for me in my profile, masquerading as an MSN ID - drop me a line with details of what you've got there, how many pounds I'd need in my pocket etc...

 

[Spark123]

That would give me a nice low Ze, wouldn't it....

The Ze will hopefully be pretty low as it is in effect a TN-S system, the electrode tying it all down to the general mass of earth does need to have a pretty low resistance, iirc it is something like 21 ohms max (2nd opinion req'd!!) but it should be checked with an earth electrode tester.

 

[ban-all-sheds]

With a N/E link it's in effect a TN-C-S system, surely....

 

[Spark123]

I suppose so if you are using a combined neutral and earth for the lead from the genny to the house, though if the link is in the generator end along with the electrode it will be TN-S.

 

[securespark]

If you're talking about TT here, 21Ohms is the reading for the DNO's side of things. As long as Zs on TT is < or equal to 200 Ohms, you're in.

 

[Spark123]

Talking about the connection of a generator earthing point to the general mass of earth, which I wasn't over sure on the max resistance of the rod to ground.

 

[Lectrician]

If you're talking about TT here, 21Ohms is the reading for the DNO's side of things. As long as Zs on TT is < or equal to 200 Ohms, you're in.

Yup, but in effect "you are the DNO" - Your genny is effectivly a substation, and the earth tape/wire/rod or whatever is tying the neutral to earth, and providing a return path to the star point of the generator.

The 21ohm rule does apply It is still only a guide though - BS7671 certainly doesn't cover this scope.

 

[Lectrician]

And how many exhibitions, fairs, temp supplies have you seen with a gen floating away (not litterally)?

It is very rare to see a rod driven near a temporary generator sadly, and if there was one, I doubt it would be near to 21 ohm.

I was chatting to a guy who installed a temp genset at a festival, and questionned the lack of rod........he said "it has an RCD". I tried to tell him how this would be non effective if someone was standing on the ground and touched a live conductor......he couldn't grasp it. (I am aware the RCD would actually likely operate due to the fact there is a sodding great metal generator sat on soil, but thats not the point)

 

[Spark123]

Not really, if the generator is floating then an RCD is pretty much useless.

 

[Lectrician]

Did I not say that?