TT system - Why less than 200 π ?

[HappyHippyDad]

I have had a read through a few threads on a TT system needing to be below 200Ω and I wondered why the regulations state this figure?

I dont mean why cant we just let it be higher (as it will be protected by an RCD - so in theory (not good practice) just needs to be below 1667Ω)), I mean why 200? Where does this figure come from? Who decided on this figure and more importantly why? What happens at 200Ω that makes it safer than 250 or 300 for example?

Why not have a much lower figure that might actually aid in disconnection times. Is it to do with the logistics of having to add maybe 3 or 4 rods to achieve such figures or other reasons?

Thanks all...

 

[Engineer54]

Your guess is as good as mine, ...mine being that the figure of 200 has been picked from thin air!! Strange that no other country has a minimum TT earthing figure even approaching that arbitrary figure.... lol!!

 

[HappyHippyDad]

Your guess is as good as mine, ...mine being that the figure of 200 has been picked from thin air!! Strange that no other country has a minimum TT earthing figure even approaching that arbitrary figure.... lol!!

I was hoping you'd be the one person that may know the answer to this question E54!!

I shall investigate... :detective:

 

[derek]

I'm just guessing but is it anything to do with having an upfront 100mA RCD. Just wondering if changing ground conditions could change an Ra of 200 enough to risk going over max Zs?

 

[NDG Elecs]

Is it possibly related to the stability of the Ra value.... I have no idea, just throwing it in there! I have measured Ra's of 400 ohms that have fluctuated massively by a couple of hundred ohms at a different time of year. My own TT hovers around 20 ohms and seems fairly stable.

E54 do other countries normally have max/recommended Ra's, and if so what are they in China for instance.

 

[HappyHippyDad]

Stability due to fluctuation does seem to be mentioned on other sites as a possible reason, so I think you may be right GayOwl!

 

[Dizzy_Maskell]

Simply

Why not

 

[Knobhead]

If as it would seem they picked a number out of the air, pity they didn’t knock a 0 off the end.

 

[Marvo]

If as it would seem they picked a number out of the air, pity they didn’t knock a 0 off the end.

Lol, pity they didn't knock both zeros off it and halve it as well, all of a sudden you'd have a fully functional earth as well as a protective one.

 

[D Skelton]

It is a nonsense value, I personally believe it has no relevance to stability. I hear chatter about values potentially fluctuating up to 8 times their original value, but I can't see it really (that is unless you are using a 6 inch earth rod?!). The fact is, whatever the value, whether it be 1 ohm or 1000 ohms, you will make it stable by going in deep.

 

[Knobhead]

Lol, pity they didn't knock both zeros off it and halve it as well, all of a sudden you'd have a fully functional earth as well as a protective one.

Only ever done one TT install, that was way below 1Ώ.
Had PME been available back then I wouldn’t have allowed the NWEB (DNO) to connect neutral to my earth nest. It was better than theirs.

This may surprise you, it’s from a UKPN document. The document is actually about interconnecting MV and LV earthing. <1Ώ they can be connected.

Just look at the maximum LV value.

 

[NDG Elecs]

I knock in around 8 rods a year at a guess and am occasionally troubled about the advice I give, which impacts on a customers cost. If I am getting say 30 for the Ra off one rod do I insist on bashing another one in... Customer, if they catch wind of the max values, may say I am doing unnecessary work and be a bit miffed. Obviously the decision is mine to make but it could bite me on the bum one day.

Another example I had was I knocked in two four footers coupled together and was only getting Ra of 65. Do I insist on bashing in more rods at other end of garden to see what gives.... That occasion I didn't as customer refused to accept further upheaval in their garden. I would have tried going deeper still but the last two ft of the rod ie 6-8 ft deep was taking 50 whacks of a lump hammer to move 1 cm further in. I always try and get below 20 if I can which too is a figure I have plucked outta nowhere!

Tony, getting <1 on your only TT is bloody good going!

 

[D Skelton]

Buy an earth rod driver for £35ish and 8 feet of rod will go in in about 30 seconds. Whatever value you get then will most likely be stable. When I have rodded a system, I will always go back for one year and check it quarterly to confirm it's stability. In two cases so far I have driven in multiple 12ft rods in crows foot formation to achieve sub 1 ohm values therefore negating the need to RCD protect distribution equipment, I went back monthly for one year, in both cases, to confirm stability, and in both cases, the values hadn't budged.

 

[Engineer54]

E54 do other countries normally have max/recommended Ra's, and if so what are they in China for instance.

It's 25 ohms over here, the same as the States.... The nearest to that of the UK as far as i'm aware is France 100 ohms, but they have an upfront 500mA RCD on every TT installation....

 

[Engineer54]

Tony, getting <1 on your only TT is bloody good going!

It's not as difficult as one may think, especially if like Tony you are installing a rodded earth nest or earth field.... But you aint gonna get anywhere near sub 1 ohm values with those short thin twigs they have the cheek to call earth rods these days. Those things are not much better than the test stakes i use with an earth tester....

I have never used 1.2m 5/8'' earth rods ever!! Only ever used the 3.2m (or is it 3.4m?) 3/4'' copper clad rods on all my projects, and on quite a few occasions multiples of, depending on ground conditions.... Whenever you use a screwed coupler to join two earth rods together, the Ra value will improve over time (12 to 18 months) as the soil consolidates around the upper length of rod.

 

[nickblake]

if you look at regulation 411.5.2 , note 2 states that the resistance of installation of the earth electrode should be as low as practicable a value exceeding 200 ohms may not be stable

 

[Engineer54]

if you look at regulation 411.5.2 , note 2 states that the resistance of installation of the earth electrode should be as low as practicable a value exceeding 200 ohms may not be stable

True, but because a value has been given as to be not exceeded, that then becomes the minimum value. For many these days, the industry has become the same as painting by numbers, so no number and they are up S*** creek without a paddle!! lol!!

 

[nickblake]

True, but because a value has been given as to be not exceeded, that then becomes the minimum value. For many these days, the industry has become the same as painting by numbers, so no number and they are up S*** creek without a paddle!! lol!!

Totally agree

 

[Marvo]

I definitely wouldn't want an Ra reading above say 50 ohms regardless of upfront RCD's. The tendency seems to be to look at it only from the protective angle and ignore the functional side of the earth. For correct operation of surge arrestors and communications systems even 50ohms can be too high.

 

[Engineer54]

I definitely wouldn't want an Ra reading above say 50 ohms regardless of upfront RCD's. The tendency seems to be to look at it only from the protective angle and ignore the functional side of the earth. For correct operation of surge arrestors and communications systems even 50ohms can be too high.

Try convincing some here of that, ....ooops i forgot you have, and sadly like myself, failed miserably!! ...lol!!

 

[NDG Elecs]

I am liking this post as I have a strange fascination with earthing! As an aside I have always fancied getting into lightning protection design and install, but not too crazy about the heights involved!

I have been meaning for ages to get a metal workshop to weld a rod driver up for my SDS plus. Take it the SDS plus would do the job most of the time....rather than shelling out for a SDS max drill and driver.

I am liking the 25 max figure also. That kind of value I have found not easy to attain in my neck of the woods without going eight ft down. Where I do most of my work has a historical (from collated data from MET office over last 50 yrs or so) micro climate which is much drier than just a few miles inland. Maybe this area has a resultant lower water-table.

A Ra max of 25 would hopefully push all leccys to try n get to <10....

Noticed that HHD has been moonlighting and has a pseudonym elsewhere! Someone on there reckons the 200 figure originated with the NIC...

 

[Knobhead]

I have had a read through a few threads on a TT system needing to be below 200Ω and I wondered why the regulations state this figure?

I dont mean why cant we just let it be higher (as it will be protected by an RCD - so in theory (not good practice) just needs to be below 1667Ω)), I mean why 200? Where does this figure come from? Who decided on this figure and more importantly why? What happens at 200Ω that makes it safer than 250 or 300 for example?

Why not have a much lower figure that might actually aid in disconnection times. Is it to do with the logistics of having to add maybe 3 or 4 rods to achieve such figures or other reasons?

Thanks all...

This worries me, asking why it can’t be higher because RCD’s are used.

Sorry HHD but while everyone else is aiming as low as possible, you show what I think are your true colours.
It makes me wonder if you’ve just done a TT install and looking for an excuse to leave it in a dangerous condition.

But thanks non the less for starting the topic.

 

[D Skelton]

I have been meaning for ages to get a metal workshop to weld a rod driver up for my SDS plus. Take it the SDS plus would do the job most of the time....rather than shelling out for a SDS max drill and driver.

Your SDS plus would struggle. You really would need an SDS max breaker to give the rod the oomph it needs.

 

[Engineer54]

Your SDS plus would struggle. You really would need an SDS max breaker to give the rod the oomph it needs.

Depends really, on the ground conditions your driving the rod into. Yes ideally a SDS max, but a good SDS plus will probably suffice most of the time.

 

[NDG Elecs]

Ta. Will give the plus a go; decent Bosch one, and see how it goes. Gotta be better than a lump hammer haha!

 

[D Skelton]

Depends really, on the ground conditions your driving the rod into. Yes ideally a SDS max, but a good SDS plus will probably suffice most of the time.

Very true, if you're using straight rods in clay then it'll work, but as soon as you hit a pebble or as soon as you introduce a coupler it'll start to cough lol.

 

[NDG Elecs]

Ta. Will give the plus a go; decent Bosch one, and see how it goes. Gotta be better than a lump hammer haha!

 

[D Skelton]

Ta. Will give the plus a go; decent Bosch one, and see how it goes. Gotta be better than a lump hammer haha!

Regardless of which you use, you'll need a grinder ready if you're using coupled rods so that you can neaten up the thread before attaching the next rod.

 

[davesparks]

Use a driving stud and an sds to 1/2"square drive with an impact socket.

 

[HappyHippyDad]

This worries me, asking why it can’t be higher because RCD’s are used.

Sorry HHD but while everyone else is aiming as low as possible, you show what I think are your true colours.
It makes me wonder if you’ve just done a TT install and looking for an excuse to leave it in a dangerous condition.

But thanks non the less for starting the topic.

Sorry to discredit you Tony, but you're wrong again! Try the glass half full way of thinking

I had an EICR where the Ra was 256Ω, Obviously wasn't happy with this as I see it as a high figure and assumed it would be a c2, although after reading many threads it seems its a c3. The customer was completely happy to have another rod sunk, It was difficult to find a suitable spot to locate it but I got it down to around 75Ω (approx- cant remember exactly), still not great but better, but it made me think what have I actually achieved??

 

[davesparks]

What you've achieved is getting it closer to a useful value.

 

[Knobhead]

Sorry to discredit you Tony, but you're wrong again!

Then why did you not say it was part of an EICR first off?

You’re happy with 75Ω are you?

 

[Deleted member 9648]

Sorry to discredit you Tony, but you're wrong again! Try the glass half full way of thinking

I had an EICR where the Ra was 256Ω, Obviously wasn't happy with this as I see it as a high figure and assumed it would be a c2, although after reading many threads it seems its a c3. The customer was completely happy to have another rod sunk, It was difficult to find a suitable spot to locate it but I got it down to around 75Ω (approx- cant remember exactly), still not great but better, but it made me think what have I actually achieved??

Then why did you not say it was part of an EICR first off?

You’re happy with 75Ω are you?

Aside from Marvo's point about certain equipment requiring lower earth resistance values than are generally achieved on UK TT systems,which I accept but which generally dont apply to most of the TT installs discussed on here,nobody has ever come up with a satisfactory explanation of what HHP's 75 ohms will do for fault protection that the original 256 ohms wont.
IMO you either get a TN value,or it doesnt really matter if it's 10 ohms or 200 for a small TT install. I would agree with E54 that 7671 needs an overhall regarding TT systems,I believe the recommendation should be for a sub 1 ohm value,or if that is not practical the installation must have an S type back up RCD.

HHD you have met the obligation regarding an EICR with an Ra value of 75 ohms,it meets the requirements of 7671 which is the only purpose of an EICR. Dont take any nonsense *removed*.

 

[HappyHippyDad]

Then why did you not say it was part of an EICR first off?

You’re happy with 75Ω are you?

Pharrell Williams - Happy (Official Music Video) - YouTube :daisy:

 

[SPARTYKUS]

Sorry to discredit you Tony, but you're wrong again! Try the glass half full way of thinking

I had an EICR where the Ra was 256Ω, Obviously wasn't happy with this as I see it as a high figure and assumed it would be a c2, although after reading many threads it seems its a c3. The customer was completely happy to have another rod sunk, It was difficult to find a suitable spot to locate it but I got it down to around 75Ω (approx- cant remember exactly), still not great but better, but it made me think what have I actually achieved??

I know what the ground's like round your way....

 

[ipf]

Never seen a hippy without a few decent spots, usually covered in 'em

 

[Marvo]

I have a question that's slightly off track but not completely off-topic maybe one or two of the senior guys might remember the answer.

What was the maximum allowable Ra impedance value on a TT earthing arrangement in the 16th or 15th or maybe 14th edition before RCD's were generally available or before there was an option to sell you soul and put all your eggs in one basket by using a 100mA upfront RCD to achieve your disconnection times?

 

[HappyHippyDad]

I have a question that's slightly off track but not completely off-topic maybe one or two of the senior guys might remember the answer.

What was the maximum allowable Ra impedance value on a TT earthing arrangement in the 16th or 15th or maybe 14th edition before RCD's were generally available or before there was an option to sell you soul and put all your eggs in one basket by using a 100mA upfront RCD to achieve your disconnection times?

I really hope this gets an answer.. Would be very interesting!

 

[davesparks]

I've got one for you Marvo:

According to Stubbs the IEE wiring regulations (10th edition I think at that time) require that earth leakage protection is required for a supply up to 100A if the earth continuity conductor is not connected to a town's metallic water main.

It goes on to say the protection is afforded by an earth leakage circuit breaker.

A figure of 40Volts potential rise is mentioned in various entries as a limit to the maximum permissible rise of the earth conductors potential above that of the general mass of earth.

Also the same regulations appear to require that the resistance between any part of the earth continuity conductor and the point of connection to earth must not exceed 1 ohm.
The earth continuity conductor appears to be what we now call CPCs

 

[Engineer54]

When i first came into the industry, the accepted minimum TT value was 10 ohms which more often than not was always bettered. But in those days the rods were all extendable 5/8'' - 3/4'' 8 to 10 footers, not the now typical short 1.2m affairs and certainly not those useless 3/8'' twigs they now jokingly sell as earth rods. When i used to help my uncle after school and at weekends, he's method was 2 single rods at suitably spaced positions...