TT system.

[gselectrical]

Evening all.

Did a job today involving running a sub main supply to an outbuilding at a tourist centre. Earthing arangements at the main switch room where TNC-S. Therefore earth not exported and a 1200mm earth rod was installed.

The Earth loop reading I got at the DB was 694ohms. What would be the best way to reduce this to below 200ohms?

The ground is mainly slate below for a fair few meters.

Cheers. J

 

[Zeespark]

Try installing a couple of rods GS.

AndyGuinness

 

[batty]

With a high reading like that I think you will struggle to get a reasonable reading what ever you do. Slate must be a poor conductor.

 

[gselectrical]

Try installing a couple of rods GS.AndyGuinness

If I install another, I dont really think it will bring the reading down below 200ohms.

It was pouring it down today, and still got a high reading.

Bleedin slate!

 

[Zeespark]

Just do the best you can J, slate isnt good for a TT install. Is there any way you could record slate as the ground makeup on the EIC at all?

AndyGuinness

 

[sparkytim]

Either try extendable Furze Rods that screw together to get down deeper Or what may be easier add further rods in parrallel but make sure you space them at least their length apart this can often half the resistence

Although the 200 ohm max is RECOMMENDED the regs only require a Max Fault touch voltage of 50 Volts

So for a 30 m A RCD Protected circuit a max earth loop resistence of 1666.66

is still compliant but not indicative of a stable earth connection.

 

[ADS]

Oh my god - not again!

Use the supplied earth - it's allowed, for god sake.

And if the doubters want me to prove it this time, I will!

 

[binky]

Slate is impervious to water, hence used for damp proofing and roof slates, so chances are you will struggle even with a longer rod. Having said that if your on TNC-s which also relies on rods, there must be an area of ground that is useable so options for consideration.

1/ bang in extendable rods (would try 2 and see what diffence it makes)

2/ look around for softer piece of ground further away from the building (perhaps head for the bank of a stream??

3/ try connecting to steel work of building, pipes, or the like and very carefully sign installation/ annotate certs that this has been done.

4/ Invent some new and creative technique unique to old slate quarries (can't drop a long cable down the mine shaft can you?? :coat )

Would also be worth consulting NICEIC, they might have some ideas. Would also discuss the max rod impedence which in theory can be upto nearly 1667ohms. Have a good re-read of section 4 of the regs.

You might also be able to justify high Zra if it is stable, so might mean returning over a few weeks of variable weather to demonstrate that rod resistance doesn't fluctuate with the weather - avoidance of which is the basis of the 200/500ohms recommended max.

 

[ADS]

Would also be worth consulting NICEIC, they might have some ideas.

They'll probably tell you to use the TN-C-S thats suppliedheadbang

 

[gselectrical]

Oh my god - not again!Use the supplied earth - it's allowed, for god sake.

And if the doubters want me to prove it this time, I will!

Outbuilding is a pre-wired metal container. It is a toilet block with various heaters etc.

On the connection outside the unit it clearly says "TNS or TT earthing arrangement only to be used"

 

[ADS]

Outbuilding is a pre-wired metal container. It is a toilet block with various heaters etc.On the connection outside the unit it clearly says "TNS or TT earthing arrangement only to be used"

Well, you should have said that, instead of this......

Evening all.Earthing arangements at the main switch room where TNC-S. Therefore earth not exported and a 1200mm earth rod was installed.

 

[binky]

Has to be said, if ground conductance is poor, then perhaps earth export is less of an issue. I take it the plumbing is plastic and can't be press ganged. How is the building fixed down - big metal stakesray

 

[ADS]

No, I agree, now - gotta be TT'd. He's basically supplying a caravan without wheels. ; \

 

[gselectrical]

Well, you should have said that, instead of this......

Apologies ADS

 

[gselectrical]

Has to be said, if ground conductance is poor, then perhaps earth export is less of an issue. I take it the plumbing is plastic and can't be press ganged. How is the building fixed down - big metal stakesray

Metal feet bolted to the ground! :^O

 

[binky]

Have you considered earth plates/ tape type materials. Sounds like your just going to have to poke around until you get a reasonable number, and then dig trench and bury conductor. Cunning plan - toilets must have had trench dug for poo pipe - try utilising disturbed ground beside it ( not through it :innocent )

 

[paul b b]

install the said deeper rods ans put 2 or 3 in (they need to be 2x the depth apart and earth cable unbroken from 1 to another)

there is also some grid thing you can install but i have never done it, though i bet its a right pain

 

[netblindpaul]

There is also marconite or bentonite.

Try here:

http://www.furse.com/elp/pdf/scagents.pdf

 

[green-hornet]

Installing a deeper rod is more efective than multiple rods. As mentioned some makes do rods with screw connections for adding additional rods.

Different soils will have differing resistance, you can measure the actual soil resistance by drilling with long drill bits and using them as temp rods for testing. This result will give you the actual soil resistance, from this there is a formula that you can use to work out the optimum depth of the rod you would require. Double the length of rod would usually yeald a 40% reduction in resistance, to get the same from additinal rods may take 3 or 4 rods at a spacing where they do not interfere or share the same ground zone, normally a rule of thumb being at least the depth of the rod apart, but I would go for a larger spacing than that.

If you need to drill to get the rod down to the depth you need you would have to infill the hole with a grounding compound, which also helps with reducing the resistance.

The only other alternative on very high resistive soils is to install a ground plate or more commonly a mat, which can be huge dependant on the depth of top soil.

Not easy on some types of soil to get a good reading, I was lucky on my last job I installed two rods and got the reading down to below 50, but the zdb at one board gave me a reading with all bonding of 0.19 now thats what I call lucky and I had to test it with two different testers because I have never had it that low!

 

[gselectrical]

Installing a deeper rod is more efective than multiple rods. As mentioned some makes do rods with screw connections for adding additional rods.Different soils will have differing resistance, you can measure the actual soil resistance by drilling with long drill bits and using them as temp rods for testing. This result will give you the actual soil resistance, from this there is a formula that you can use to work out the optimum depth of the rod you would require. Double the length of rod would usually yeald a 40% reduction in resistance, to get the same from additinal rods may take 3 or 4 rods at a spacing where they do not interfere or share the same ground zone, normally a rule of thumb being at least the depth of the rod apart, but I would go for a larger spacing than that.

If you need to drill to get the rod down to the depth you need you would have to infill the hole with a grounding compound, which also helps with reducing the resistance.

The only other alternative on very high resistive soils is to install a ground plate or more commonly a mat, which can be huge dependant on the depth of top soil.

Not easy on some types of soil to get a good reading, I was lucky on my last job I installed two rods and got the reading down to below 50, but the zdb at one board gave me a reading with all bonding of 0.19 now thats what I call lucky and I had to test it with two different testers because I have never had it that low!

Cheers GH

Grounding compound. Have seen this being used on mats connected to lightening conductors. Never used it myself. Where would I purchase this from? As I did have to drill to get the rod down.