TT system issues???

[MilesSparks]

Hi,
I've had to change a TN-S over to a TT system.
My first problem is that my earth rod resistance is too high. At the moment it's measures at 161ohms (taken it wet conditions). To satisfy NICEIC I need to get it below 100ohms in normally dry conditions. What is the most effective way of doing this? Is it to install a second rod or extend the first rod deeper? At the moment my first rod is 8ft into the ground.
My second problem is the house was wired to 17th edition regs, supplementry bonding in the bathroom was not done as all ccts are rcd protected and all main bonding is in place. Is this now a issue now that the earthing system has changed to a TT system?

 

[Plonker 3]

Just out of curiosity why couldn't you keep the TN-S?

 

[telectrix]

first question is why change from TN-S to TT. a 2nd rod in a different location may well bring the Ra down. 8ft. seems a good depth for your rod. as all circuits are on RCD, i don't think ( although i may be wrong) that supplementary bonding is required in the bathroom.

 

[PEG]

Accepting that the need to TT is a necessity,and assuming there is no reason for your rod not to be driven deeper,why not send it down a bit and check if there is an improvment? This may indicate the direction to go. And i've got a side bet with my missus how long it will take for a certain overseas earth guru to comment on a certain schemes resistance criterea...

 

[telectrix]

with a Ze of 100, you might just as well use a piece of wet string . the great god RCD well provide all the fault protection you require. NOT. ( there. beat him to it ).

 

[Engineer54]

Well i'm assuming that the TN-S earth has failed on the DNO side, i hope so, or the OP is a fool!! lol!!

If you have an 8 foot rod that's only giving you 160+plus ohms in wet sodden conditions, then either the soil type in that area is naff, or your driving the rod through rubble and the like. Your rod position isn't right next to the house outside wall by any chance is it??

You have 2 choices really, 1) Try another rod position in a different area of the property, and at least a metre and a half or so away from any building walls. 2) leave it as it is, and install a S type 100 mA as your main CU switch/isolator.

You could also try as others have suggested add another 1.2m extension to the existing rod and see what that gives you, but as it is, standing at 8 feet, and only giving 160 ohms, i can't see it making much of a difference to be honest!!

The NICEIC don't make the rules thankfully, so ignore any such figures they may be putting forward. A 100 ohm Ra TT system is just as useless as the BS7671 numbty figure of 200 ohm's...

 

[happyhippy68]

I Had to do a similar job in a Cambridge village on a social housing scheme. The previous "Electrical contractor" aka kitchen and bathroom fitters had changed 8 consumer units and converted them all from TT to TNCS without the DNOs Knowledge. Two years later I did a test on them as part of a stock transfer and had some really strange readings. Contacted the DNO and a very nice area manager started accusing me of all kinds of things and threatened a major HSE investigation. Long story short was each property required two rods fitting 2.5m away from the property with concrete paths broken up and reinstated at a considerable cost to the new housing association as the existing rods would not give an earth resistance of less than 400 ohms. The PART P electrician obviously not knowing what he was doing had decided off his own back to convert the lot and fit very nice 16th edition boards, he also ripped out the old 100 ma RCD from the meter to the property. Now I don't bet my life on RCDS to protect me but I would prefer they were present as supplemental protection when required, especially when there is a 9.5 KW shower in a wet room, all nicely bonded but the ZE on the property was 22.2 ohms. OUCH (No gas supplies and plastic water incomers to all properties, incomer overhead cables tapped at post and looped between pairs of properties on a 6mm approx supply DNO fused at 60amp)

 

[PEG]

Wise words and an easily earnt slice of Stollen plus cup of tea from me missus...

 

[bigbaddave]

Hi,
I've had to change a TN-S over to a TT system.
My first problem is that my earth rod resistance is too high. At the moment it's measures at 161ohms (taken it wet conditions). To satisfy NICEIC I need to get it below 100ohms in normally dry conditions. What is the most effective way of doing this? Is it to install a second rod or extend the first rod deeper? At the moment my first rod is 8ft into the ground.
My second problem is the house was wired to 17th edition regs, supplementry bonding in the bathroom was not done as all ccts are rcd protected and all main bonding is in place. Is this now a issue now that the earthing system has changed to a TT system?

Im with the above posts, It would be nice to know why the supply had to be transfered over to a TT supply, I have in the past had poor readings on a TN-S, 99 Times of 100 its a issue with the internal earth being loose disconected. The other one time it was external and a call to the DNO was required to notify them of there fault (Beware of a horrible process if this is the case)

What was the reason to why this action had to be taken?

 

[MilesSparks]

How far from the house should it be I didn't realise this made a difference? It's currently no more than a mtr away.

 

[MilesSparks]

Didn't have much choice, no earth at all!!! I thought putting a rod in was my best option tried contacting DNO but couldn't get a decent response.

 

[happyhippy68]

How far from the house should it be I didn't realise this made a difference? It's currently no more than a mtr away.

It does not need to be a set distance, the problem is builders throw all the rubbish in near the foundations or the floor plate may be a little wider than the wall and it is hard to drive a spike through concrete or rubble/hardcore.

 

[Engineer54]

How far from the house should it be I didn't realise this made a difference? It's currently no more than a mtr away.

It makes a difference because this area is where all the old building rubble is going to be. The further away from building walls the better!!

So WHY have you had to TT this previously TN-S property?? And why didn't the DNO give you a PME connection if it was they're earth provision that failed??

 

[MilesSparks]

It was relatively easy to go in so I don't think it's an issue being that close? Correct me if I'm wrong?

 

[bigbaddave]

Didn't have much choice, no earth at all!!! I thought putting a rod in was my best option tried contacting DNO but couldn't get a decent response.

They tried palming me off the first time I rang to be honast.. Then rang again palmed me off. Third time i just reported it as a dangerous fault.. Eventually they sent a gut out the sort things out.

Be interesting to see if the problem is next doors as well.. If it is then it needs sorting.

Its worrying that once the fault is cleary on there equipment they wont come out. Its very easy to identify the fault and even easier argument that a TNS systmem is safer than a TT system and obviously cheeper for the customer..

My advice dont be fobbed off. They soon get sick of you calling. I mean somtimes they will say it aint financhally feasable but keep calling until you speak to sombody that speaks sence..