Max Ze for TT system

[MBEINST]

Hi Gents and ladies,

I have been asked to quote on installing a couple of extra socket on a TT system.

When checking the existing installation my Zs readings on the ring are coming out at 39.8 Ohms (most remote). This is unacceptable (table 2d OSG). I then checked Ze readings which are coming out at 37 Ohms.

I'm relatively new to the game and never dealt with a TT system before, so went back to my books and have an hand written note in my 2391 notes stating 21 Ohms for TT system? Can't remember why I wrote this but assume it referred to the max Ze allowable for TT system. However OSG under Earth Electrode Resistance states 'if being used with an RCD should not exceed 200 Ohms'. I'm dealing with an old installation and no sign of RCD protection. Can't convince the customer to upgrade Consumer Unit.

It looks to me like this is going to turn into a major job and obviously I don't want to quote for a couple of extra sockets to find out that I need to put in another rod.

Any help to nurse this TT virgin through would be greatly appreciated.





'The best friendships are built on a solid foundation of alcohol, sarcasm, laughter and a dislike of the same people.'

 

[Octopus]

If you can't convince the client to change the CU, how are you going to provide RCD protection? Is the bonding in place?

Back to the TT Ze, 200 ohms is good enough - but the lower the better so 37 ohms is OK IMHO.

 

[Engineer54]

If you haven't any existing RCD protection in place, then any new socket outlets will either need to be RCD outlets or a separate circuit supplied at origin via a supplementary RCD 1 way CU or the like!!

200 ohms or anything like it, is NOT good enough, it never has and never will, no matter what it say's in BS7671. 21 ohms is the maximum for a DNO substation, but you'll be hard pressed to see/find any substation with an Ra approaching anywhere near that high a value, it'll be more like 1 ohm or less!!

If you can't convince your customer to upgrade the existing unprotected CU to a modern protected CU, then i fear you'll not convince them on providing RCD protection on these additional sockets either, in which case you should simply walk away....

 

[slocm3105]

If you haven't any existing RCD protection in place, then any new socket outlets will either need to be RCD outlets or a separate circuit supplied at origin via a supplementary RCD 1 way CU or the like!!

200 ohms or anything like it, is NOT good enough, it never has and never will, no matter what it say's in BS7671. 21 ohms is the maximum for a DNO substation, but you'll be hard pressed to see/find any substation with an Ra approaching anywhere near that high a value, it'll be more like 1 ohm or less!!

If you can't convince your customer to upgrade the existing unprotected CU to a modern protected CU, then i fear you'll not convince them on providing RCD protection on these additional sockets either, in which case you should simply walk away....

Well said that man!!!

 

[Deleted member 9648]

Hi Gents and ladies,

I have been asked to quote on installing a couple of extra socket on a TT system.

When checking the existing installation my Zs readings on the ring are coming out at 39.8 Ohms (most remote). This is unacceptable (table 2d OSG). I then checked Ze readings which are coming out at 37 Ohms.

I'm relatively new to the game and never dealt with a TT system before, so went back to my books and have an hand written note in my 2391 notes stating 21 Ohms for TT system? Can't remember why I wrote this but assume it referred to the max Ze allowable for TT system. However OSG under Earth Electrode Resistance states 'if being used with an RCD should not exceed 200 Ohms'. I'm dealing with an old installation and no sign of RCD protection. Can't convince the customer to upgrade Consumer Unit.

It looks to me like this is going to turn into a major job and obviously I don't want to quote for a couple of extra sockets to find out that I need to put in another rod.

Any help to nurse this TT virgin through would be greatly appreciated.





'The best friendships are built on a solid foundation of alcohol, sarcasm, laughter and a dislike of the same people.'

With the Zs values you state and no RCD as it stands the existing install has no earth fault protection whatsoever, and you should advise the client that a dangerous situation exists.Do not carry out any work without upgrading.
If the install has to remain TT then the ideal solution is a DB with a 100ma S type main switch and RCBO's....less ideal is a standard dual RCD board,but it would still comply. The maximum Zs where final circuits are 30ma RCD protected is 1667 ohms...a maximum Ra of 200 ohms is advised.
Even better convert it to TNCS if available

 

[Engineer54]

The maximum Zs where final circuits are 30ma RCD protected is 1667 ohms...a maximum Ra of 200 ohms is advised. Only by plonkers!!
Even better convert it to TNCS if available

Now that's GOOD if not the best advice, where a PME supply is available....

 

[D Skelton]

200 ohms or anything like it, is NOT good enough, it never has and never will, no matter what it say's in BS7671. 21 ohms is the maximum for a DNO substation, but you'll be hard pressed to see/find any substation with an Ra approaching anywhere near that high a value, it'll be more like 1 ohm or less!

Yes, as this guy said, 200 ohms is rubbish and 20 ohms is better, although as professional electricians we have no idea why as a fault occuring with both these values of Ra would trip both a 30mA and a 100mA RCD???

Dogma at best

 

[i=p/u]

No sense in wasting earth spikes if you not going to get what you want out of it .

 

[LeeH]

This is what I don't understand ^^^^^

Logic dictates that 20 is better then 200 but the RCD will still operate within 300ms?

 

[i=p/u]

Weather you get it to 2 ohms you still relying on rcd for circuits within board

 

[MBEINST]

Thanks for the feedback fellas.

Although work is tight and I need to keep the wolf from the door like everyone else. I was going to walk away unless I can convince the customer to upgrade his consumer unit.

I have been fitting the Denmans fully loaded RCBO units of late and think that this would be the solution. Obviously it would be great to change the earthing arrangement to TN-C-S but surely this would be done by the supply company and would cost an arm and a leg.

:uhoh2:

'The best friendships are built on a solid foundation of alcohol, sarcasm, laughter and a dislike of the same people.'

 

[telectrix]

This is what I don't understand ^^^^^

Logic dictates that 20 is better then 200 but the RCD will still operate within 300ms?

RCD on a TT must operate within 200mSec. this is achieved by a Zs value of <1667 (30mA RCD).

 

[Engineer54]

Yes, as this guy said, 200 ohms is rubbish and 20 ohms is better, although as professional electricians we have no idea why as a fault occuring with both these values of Ra would trip both a 30mA and a 100mA RCD???

Dogma at best

When i first came into this industry the recognised max for a Roded earth electrode was 10 ohms, FACT!! Never saw one anything like at that value mind, all were generally in the lower single figures.

So now were being sold a complete crock crap that 200 ohms is the max figure to aim for!! If for no other reason, a 200 ohm earth electrode, or anything remotely like it will ''NOT'' provide a stable TT system.

 

[Engineer54]

Thanks for the feedback fellas.

Although work is tight and I need to keep the wolf from the door like everyone else. I was going to walk away unless I can convince the customer to upgrade his consumer unit.

I have been fitting the Denmans fully loaded RCBO units of late and think that this would be the solution. Obviously it would be great to change the earthing arrangement to TN-C-S but surely this would be done by the supply company and would cost an arm and a leg.

:uhoh2:

'The best friendships are built on a solid foundation of alcohol, sarcasm, laughter and a dislike of the same people.'

It shouldn't cost a penny, the old regional boards used to convert TT systems for nothing, in fact that was one of the main reasons the electricity boards invested so much cash into PME'ing overhead and later failing underground cables, to provide homes and businesses with a stable means of earthing!!

Seems now the private DNO's think it's fine to charge for this once totally free service. But seeing as it takes about 2 to 3 min's to convert (depending on the service head) it shouldn't cost an arm and a leg!!

 

[Engineer54]

Weather you get it to 2 ohms you still relying on rcd for circuits within board

It may not trip a protective device out in the required time but it WILL take out protective devices!! On the other hand the 200 ohm crap you think is fine will be whistlin dixie for ever and a day....

 

[LeeH]

RCD on a TT must operate within 200mSec. this is achieved by a Zs value of <1667 (30mA RCD).

Of course, my bad.

 

[MBEINST]

Hope this isn't a stupid question,

Given what has already been discussed regarding TT systems, how is it possible to get acceptable Zs readings which will comply with table 2D OSG.

 

[Guest 004]

Hope this isn't a stupid question,

Given what has already been discussed regarding TT systems, how is it possible to get acceptable Zs readings which will comply with table 2D OSG.

Simple you can't

 

[D Skelton]

When i first came into this industry the recognised max for a Roded earth electrode was 10 ohms, FACT!! Never saw one anything like at that value mind, all were generally in the lower single figures.

So now were being sold a complete crock crap that 200 ohms is the max figure to aim for!! If for no other reason, a 200 ohm earth electrode, or anything remotely like it will ''NOT'' provide a stable TT system.

I got a bite! A big one! C'mon lads, get the landing net out ready, this one's gonna be a wriggler!

 

[Guest 004]

Was going to reply, then realised how bigoted some posters are, so will just watch and read with delight.

 

[telectrix]

you seem to be confusing bigoted with banter. we all know eng54's ideas on rodding ( not that sort of rodding, geordie).

 

[Guest 004]

Oh yes you are correct, I feel really stupid now

 

[telectrix]

that gives me an idea. let's ask dan to create a "really stupid" forum. any poster thinking his question might be stupid could post in there and be assured of stupid answers. the mostr stupid response of the week could get a prize of a broken light bulb. :6:

 

[Deleted member 9648]

I got a bite! A big one! C'mon lads, get the landing net out ready, this one's gonna be a wriggler!

 

[SPARTYKUS]

It shouldn't cost a penny, the old regional boards used to convert TT systems for nothing, in fact that was one of the main reasons the electricity boards invested so much cash into PME'ing overhead and later failing underground cables, to provide homes and businesses with a stable means of earthing!!

Seems now the private DNO's think it's fine to charge for this once totally free service. But seeing as it takes about 2 to 3 min's to convert (depending on the service head) it shouldn't cost an arm and

recently quoted 140-160 by the DNO - in the end after tea and biscuits, no charge. And a free isolator. Yippee.

 

[baggy61]

When i first came into this industry the recognised max for a Roded earth electrode was 10 ohms, FACT!! Never saw one anything like at that value mind, all were generally in the lower single figures.

So now were being sold a complete crock crap that 200 ohms is the max figure to aim for!! If for no other reason, a 200 ohm earth electrode, or anything remotely like it will ''NOT'' provide a stable TT system.

With respect, how big were the rods & how close to the ss would that have to be.
I never seem to see much below 30 ohms.
Regards

 

[baggy61]

Sorry, for ss read PMT>

 

[i=p/u]

My first and last one was 14 .8 ohms and 150m from transformer and I got rods in at 3+4 formation

 

[Engineer54]

With respect, how big were the rods & how close to the ss would that have to be.
I never seem to see much below 30 ohms.
Regards

Well they certainly wasn't the short thin twigs you see today!! 10' / 5/8''-3/4'' foot rods were the standard in those days. In fact i have only recently (last year in Cyprus) seen for the first time a 1.2m 5/8'' rod, and was told they only use them as an extension. Myself, i've never used anything other than 3m 3/4'' rods on any of the projects i've been involved on.

As for you only seeing 30 ohms, i can assure you sub 1 ohm values can be gained in the UK in many areas, in some without too much trouble at all...


You can listen to these silly buggers scoff, but you'll not find another country, (certainly none that i'm aware of) that has a ridiculous max Ra of 200 ohms on a roded or any other earth electrode system!! Even the Yanks have a max of 20 ohms for a single rod (oh and they like most other countries still use 10' copper clad rods)!!!

 

[Engineer54]

Originally Posted by D Skelton
I got a bite! A big one! C'mon lads, get the landing net out ready, this one's gonna be a wriggler!

I have no need whatsoever to wriggle, especially on this subject!!

 

[D Skelton]

Wriggling is exactly what you do on this subject! lol

Even the most trivial of questions pop up on TT systems and every time you've gotta stick your two cents in with regards to 200 ohms being rubbish. Every time without fail! You bite, get reeled in and then get thrown back into the pond for the next chump to throw you a 'TT' line

The most straight forward response to the OP's question would be something like; "200 ohms is the max recommended Ra by definition of the regulations, your 38 ohms is fine on a domestic install, stick an up front type-s in on the tails and RCD protect the existing board with one or two 30mA jobbies. If the customer doesn't like the idea, walk away."

That is all the OP needed to know, your thoughts on whether or not you think 200 ohms is a good value to achieve are not really relevant as he already has one of your magical Ra values at 40 ohms. It also doesn't answer his question.

I'm not gonna get into another '20 ohms or 200 ohms' debate with you, we all know and I hope respect our differing views on the matter. I'm purely pointing out how I giggle when I read a TT related thread knowing any minute you'll pop your head round the door with "Argh! 200 ohms this! 200 ohms that!" haha!

 

[spark 68]

I will fetch the net, lol

 

[Engineer54]

Sorry but that's not wriggling, Facts are facts and 200 ohms is just a ridiculous and laughable farce at best. But you're right i will keep stating that fact, in the hope that those that know no better will realise that fact. The chumps here, are those that just accept and or try to defend the ridiculous!!

 

[D Skelton]

The chumps here, are those that just accept and or defend the ridiculous!!

What? Like 20 ohms being better than 200 ohms?

*Nope! Zip it Mr Skelton, you can't be seen biting the bait you're casting out!* haha!

 

[Engineer54]

What? Like 20 ohms being better than 200 ohms?

*Nope! Zip it Mr Skelton, you can't be seen biting the bait you're casting out!* haha!

Well i damn well know what i would prefer to see between the two, even if you dont!!

Cast away, makes no difference to me one way or the other!! lol!!

 

[spark 68]

Come on E54 lighten up a little

Looking at the 15th edition the max RA for VOELCBs was 500 ohms (including the rod and earthing conductor), the 16th just gives the usual Zs Idn=< 50V for RCDs and this will comply, it doesn't state a max although the 16th OSG recommends that it should not exceed 220 ohms , the 17th edition actually lowered the RA to 200 ohms max.

 

[Guest 004]

How about this:
Will a RCD work even without a main earth!

I also like the new range of Eaton RCBO's which come without an earth flylead, much neater in the board. Took them long enough to realise that.

 

[richy3333]

Don't really want to add to/become embroiled in the usual argument that kicks off about TT systems, but the Regs do not say 200 ohms is a max. As D Skelton says the Regs just say anything over 200 ohms can be 'unstable'. I doubt anyone would disagree with this fact?

 

[spark 68]

No Richy not disagreeing at all,

in my post above yours these are the max recommended, just I omitted that for clarity, we all know this already.

The important point is the formulae given in all editions, where the rod should not be able rise above 50V, and in some cases 25V, and it should be matched to the protective device.

The 16th regs are a little unclear, as it gives the Zs formula also in the above post, and this implies that so long as the Zs complies at the end of the final circuit/s then the rod also complies, it is only in the 16th OSG that mentions 220 ohms, but unlike the 17th does not give any reason why (ie. unstable) nor does it give a reg number, because there isn't one.

 

[Engineer54]

We know what it say's, but i'll guarantee you, that virtually every poster that comes on this and other forums, that isn't knowledgeable on TT systems, take that figure in BS 7671 as being the stated maximum and that's what they inevitably will work too!! ...And why i always Poo it off!! lol!!

The whole section on TT earthing needs a thorough sorting out and a bit more guidance wouldn't go amiss either!!

I'm going home (Cyprus) for Xmas this year, and while home, i'll be checking my own TT system. Earlier this year (early summer) it was around 0.93 ohms which has been steadily dropping year on year from around 4'ish ohms.

Here on site, our earth fields were tested middle of last month and recorded 0.17 ohms and 0.21 on the other. The highest value recorded this year was 0.28 and 0.30 respectively, which corresponds to similar test results conducted last year.

It's surprising what can be achieved when you know what you're doing!! lol!!

 

[richy3333]

Spark 68 - comment wasn't directed specifically at you matey.

Incidentally my DNO manager recently mentioned '200 ohms' when we were taking about TT systems so I don't know if the figure comes from ESQCR?

 

[D Skelton]

How about this:
Will a RCD work even without a main earth!

In a word, no. Fault current needs a path to earth for an RCD to operate. If that path is through you then yeah, it will operate if your bodily resistance combined with the resistance of whatever you happen to be standing on is low enough to allow more than the rated current of the RCD to flow. That said, the requirements of fault protection will not be met if a fault can occur leaving an exposed conductive part live and ready for you to become the earth path!

Im assuming that I'm providing an answer to a genuine and not a rhetorical question

 

[richy3333]

We know what it say's, but i'll guarantee you, that virtually every poster that comes on this and other forums, that isn't knowledgeable on TT systems, take that figure in BS 7671 as being the stated maximum and that's what they inevitably will work too!! ...And why i always Poo it off!! lol!!

The whole section on TT earthing needs a thorough sorting out and a bit more guidance wouldn't go amiss either!!

I'm going home (Cyprus) for Xmas this year, and while home, i'll be checking my own TT system. Earlier this year (early summer) it was around 0.93 ohms which has been steadily dropping year on year from around 4'ish ohms.

Here on site, our earth fields were tested middle of last month and recorded 0.17 ohms and 0.21 on the other. The highest value recorded this year was 0.28 and 0.30 respectively, which corresponds to similar test results conducted last year.

It's surprising what can be achieved when you know what you're doing!! lol!!

So, is your nightmare an illiterate Electrical Trainee, Part P 'registered' trying to 'do' a TT install

Please give people some credit - the wording is not that technical. There's a big difference between 'over 200 ohms can be unstable' and '200 ohms is the maximum permissible value'?

 

[Guest 004]

In a word, no. Fault current needs a path to earth for an RCD to operate. If that path is through you then yeah, it will operate if your bodily resistance combined with the resistance of whatever you happen to be standing on is low enough to allow more than the rated current of the RCD to flow. That said, the requirements of fault protection will not be met if a fault can occur leaving an exposed conductive part live and ready for you to become the earth path!

Im assuming that I'm providing an answer to a genuine and not a rhetorical question

I said will it work, and you have said yes! In saving your life!

 

[i=p/u]

In my head if it doesn't show balance between live conductors and leaks to North Pole or earth it will still trip. Il be put right if I'm taking bull dog

 

[Guest 004]

If you touched a live cable and you was not earthed, you would not get a shock as there is no path to earth. If you make a path to earth then rcd opens simple.

 

[D Skelton]

Here on site, our earth fields were tested middle of last month and recorded 0.17 ohms and 0.21 on the other. The highest value recorded this year was 0.28 and 0.30 respectively, which corresponds to similar test results conducted last year!

Bearing in mind that we are talking about commercial/industrial applications here and not your average domestic house. I have never left a commercial/agricultural TT system with more than half an ohm for an Ra value. The lowest Ra value I ever reached was a steady 0.3 ohms.

Domestic wise however, I'd be happy with a couple of 4 footers, an Ra value of 200 ohms and a type-s up front. Last year I happily left a domestic with an Ra value of around 250 ohms even after punching in 8 foot of copper into solid chalk. I went back every month for six months in a row to check its stability and the value hadn't budged.

 

[i=p/u]

There's brands out there that don't have earth fly lead.

 

[D Skelton]

If you touched a live cable and you was not earthed, you would not get a shock as there is no path to earth. If you make a path to earth then rcd opens simple.

But the whole point is that the system should disconnect before you can touch what is live! If there is no earth you are left merely hoping that the path to earth that you provide to the fault current is higher than the rating of the RCD!

What if you clamped hold of a live part protected by a 30mA RCD but your resistance meant that a fault current of only 25mA flowed? Give it a few seconds and then you're dead!

 

[Guest 004]

But the whole point is that the system should disconnect before you can touch what is live!

Yes indeed, but it will still work!