Cant get Ra <200ohms

[dave2]

Went to a job to replace a water heater last week.

Its a TT system and the and a loop test on the circuit measured 269 ohms!

R1 + R2 was 0.67 ohms.

30mA RCD protecting cct.

Earth spike was existing but I decided to get longer/bigger rods to improve the reading.

2 rods screwed together and sunk to depth of about 1.8m only to hit solid rock. Could not better 269ohms.

Tried several more times up to 30m away and still no better.

Tried rods in parallel but no difference.

Customer says the site is built on rock.

I know 1667 ohms allowed on 30mA RCD but IEE say 200ohms max for Ra.

Where do I go from here?

Dave2

 

[Handy]

Ring the local supply company , I've come across unrodded tranformers , or nicked rods and cables at the suppliers transformers . Especially on transformers mounted on poles . But sometimes it's just down to the location. As long as it complies with bs 7671 and the touch voltage rule then there is not much you can do . Just make sure you flag it up on the report .

I have heard of earth plates instead of rods for shallow soil areas but have never installed one .

Edit .

I can't remember offhand , but I think that bs7671 states "anything over 200ohms should be considered unstable " and doesn't say it can't be used .

It's a bit of a contradiction when you find the one about touch voltage to be less than 50v .

Grey area ? can anyone give a difinitive answer ?

 

[binky]

If its the best you can get, then it will have to do. There is one last trick which is buy some soil wetting agent (copper sulphate solution). As the regs say you are allowed upto 1667ohms, which still maintains the 50V touch voltage on a 30mA RCD. If my calculations are correct then 269ohms gives a touch voltage of only 8.07V, so don't fret too much

 

[Apache]

a bucket of this?

Gallagher United Kingdom - Bentonite Super Earthing Kit

 

[Apache]

what leaving a hose running arround the rod for half an hour?

 

[Badger]

what leaving a hose running arround the rod for half an hour?

this seems the cheaper option

 

[SPECIAL LOCATION]

IMHO

taken into context with IEE guidance note 3, Inspection &amp; testing

Page 48, table 2.3

gives the maximum Ra for various RCD ratings:-

30ma 1667ohm

100ma 500ohm

300ma 167ohm

500ma 100ohm

Then gives the comment that values above 200ohm may not be stable as soil conditions change due to drying and freezing..

If your tests were during DRY or FROZEN conditions.. these could be the max it reaches?

if warm &amp; wet... perhaps a bit more concern..

but again I take the 200ohm bit more in context with a 100ma RCD where 200+ starts getting nearer to max permissible 500ohm

whereas 30ma (&amp; 17th circuits need 30ma now), less of an issue IMHO? :|

 

[davetheglitz]

As far as I am concerned you should be working to the regs - not heresay or any other guidelines. In all the properties I've dealt with (principally insurance maintenance) in an area where TT is the norm - I'd be well chuffed with the readings that you are getting!!! Don't beat yourself up!!!

 

[SPECIAL LOCATION]

As far as I am concerned you should be working to the regs - not heresay or any other guidelines. In all the properties I've dealt with (principally insurance maintenance) in an area where TT is the norm - I'd be well chuffed with the readings that you are getting!!! Don't beat yourself up!!!

Applaud Smiley

 

[Barx]

Sometimes a good wee is all it takes

 

[dave2]

Thanks for the replies.

Just to say that the soil up here is absolutely saturated see i doubt a pee will make any difference.

The first 3 foot of rod could be pushed in by hand.

The client says he will get someone to dig a hole then we can drill through the rock and hopefully reach clay but i am happy now to leave at 269 ohms.

dave2