Max Ze for TT system

[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!!