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High Impedance On Domestic Tt System


Guest Jammin

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Guest Jammin

Hi folks,

 

Tested the earth electrode on a  TT system the other day and got a reading of 323 ohms. Normally I would install additional spikes to bring the value down, however the property is completely surrounded by concrete, and the customer will not consent to me digging anything up. so I removed the existing spike and installed a bigger one and got a reading of 282 ohms.

 

The spike I pulled out was in good condition but very dry as it appears not much moisture is penetrating the concrete. I can see no way of getting the reading below 200 with the restrictions the customer has implemented. I thought one possibility would be to get him to ask the DNO to upgrade him to a TNCS  which would probably cost him, so he won't go for that!

 

Would I be right in thinking as the whole installation is protected by a 30ma RCD that because Ra divided by 0.03 allows for 1666 ohms for disconnection to occur it could be left as is. or do I walk away from it?    

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extendable rods...

 

but if the reading is stable then it could be left as is

 

and remind the customer that regulations dont take into account the customer being awkward and not wanting anything done...

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Guest Jammin

Thanks Andy. I will look in to the extendable rods never knew they existed till you just said. I read him the riot act regarding the regulations and ultimate safety I am obligated to enforce, but as you can probably tell the guy is a bit of a di*k. I'm inclined to just walk away from this one, but I suppose my main question is I'm bound to come across this issue in the future, what the heck are you supposed to do if you simply cant bring the value down to comply with regs due to location customer or any other legit reason.  

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First point is this,

Firstly you refer to impedance of the rod,

Then you refer to its resistance,

It makes a massive difference,

How are you measuring it,?

Are you doing a Ze or an Ra of the rod,?


Oh, btw, unless things have changed, there is no reg says it has to be under 200, 

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We had this problem at a tiny tobacconist shop in the middle of an open shopping mall in Stoke , very high Ze .  requested an earth terminal from the DNO , no chance.     The shop was surrounded by block paving  and we were told we could not work in the public area .   Didn't want to drive a rod in there anyway , not in the middle of a pedestrian area shopping mall.

 

I couldn't see anywhere to put rods down within the shop..... not allowed to dig up outside .....no help from DNO .... so we obtained a reasonable Ze from the water connection & put a warning  letter into the landlord ..............never heard anymore .

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given that the current weather is dry and has been for quite a few weeks, the previous recommendation for rods was 500 :ohms and 200 :ohms  is just an update on that, I wouldn't worry too much. For what sounds like a fairly permanently dry piece of ground that reading is pretty good and is likely to be quite stable. The only way of proving that being to return a few times to check it.

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Fit a hanging basket over the rod location and ensure it is regularly watered . . .

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IIRC the <200ohms is just a recommended value for a stable reading,,,,,, you can only do so much

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First point is this,

Firstly you refer to impedance of the rod,

Then you refer to its resistance,

It makes a massive difference,

How are you measuring it,?

Are you doing a Ze or an Ra of the rod,?

Oh, btw, unless things have changed, there is no reg says it has to be under 200, 

 

Yes Steps terminology must be consistent and accurate I believe, looking at the scenario in the OP, I would say it's an impedance measurement, rather than an Ra, due to the difficulty in doing anything about improving it.

 

OP, go for a couple of 5/8" rods coupled.

Else core out the soil etc to a depth of say 2m, then put a rod down and backfill with Bentonite or similar.

 

However, it's only a note to table 41.5 that requires it to be <200 Ohms which refers to 542.2.4.

Difficult one, should be improved and might be unstable are a bit wooly, it could also be OK and very stable @ the values you have.

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Yes Steps terminology must be consistent and accurate I believe, looking at the scenario in the OP, I would say it's an impedance measurement, rather than an Ra, due to the difficulty in doing anything about improving it.

 

OP, go for a couple of 5/8" rods coupled.

Else core out the soil etc to a depth of say 2m, then put a rod down and backfill with Bentonite or similar.

 

However, it's only a note to table 41.5 that requires it to be <200 Ohms which refers to 542.2.4.

Difficult one, should be improved and might be unstable are a bit wooly, it could also be OK and very stable @ the values you have.

 

I shoulda noticed that,  :innocent

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I would suggest that it is avery stable reading, being under a concrete slab. Soil conditions are unlikley to change. I had a job once 3/4s of the way up a hill on bedrock, rod was 450ohms, I popped back a few times, it never changed. I  those delightful Cornish coastal fishing villages like Polperro, now full of holiday homes, it is normal to take 1m drill bits and extended rods to get down to 500 ohms, but the readings are stable, improving in winter when it piddles down for a month.

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i guess in lucky... hit a rod half way in here and its unlikely to be over 100...fully in and its usually 20-60...

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Geordie land is obviously very conductive. Must be those magnetic personalities :slap

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Geordie land is obviously very conductive. Must be those magnetic personalities :slap

It is actually due to,all,the body glitter falling off the lasses as they go home, and it works its way into the water table.usually greater around Bus Shelters as this is what they use for "protection" during the act of .......

No offence

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who cares. either way, it makes my job easier!

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