Max Ze for TT system

[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!

 

[D Skelton]

Edited last post. Not necessarily!

ADS doesn't rely on the customer being the earth! lol

 

[spark 68]

Just to clarify,

the max recommended values I gave earlier were for run of the mill and domestic type circuits and not for the more specialised installations.

 

[D Skelton]

Just to clarify,

the max recommended values I gave earlier were for run of the mill and domestic type circuits and not for the more specialised installations.

We gathered that Any spark worth his weight will know that it isn't always feasible to provide a TT system that delivers TN values to your average domestic installation. It's when you start getting into systems of 200A and over that unless you want to start spending a few thousand on an adjustable 'B' type RCCB, it's best just to provide TN values to protect your metal switchgear.

 

[Guest 004]

Edited last post. Not necessarily!

ADS doesn't rely on the customer being the earth! lol

My question was only 'will it work!' :devilish:

Do not need to discuss all the diferent scenarios.

 

[D Skelton]

My question was only 'will it work!' :devilish:

Do not need to discuss all the diferent scenarios.

Then the right answer would be - Not always! lol

 

[Guest 004]

^^ agreed post 53
But this forum is 97% house bashing microwave sparks or DIYers :devilish:

 

[Guest 004]

Then the right answer would be - Not always! lol

Final answer?
What about class 2 lighting circuits that have no earth?

 

[Guest 004]

^ Just a bit banter, and I like being devils advocat

 

[D Skelton]

Final answer?
What about class 2 lighting circuits that have no earth?

Yes final answer lol

That is entirely irrelevant. You're relying on a different protective measure entirely, one where RCD's are useless.

Edit: let me rephrase that. The protective measure of using double or reinforced insulation cannot be applied in a domestic setting. Where an existing lighting circuit has no earth and it has not been rewired then you are correct, you are relying on both class 2 fittings and an RCD for addtional protection of the cables buried in walls. This doesn't change my answer though. Bash a nail into that cable and nick the line conductor only, touching that nail doesn't guarantee disconnection

 

[spark 68]

If you really want to be pedantic Mr E then an RCD just provides differential protection, or an imbalance between the current flow between (for simplicities sake) output terminals and relies on whatever leaves one terminal returning on the other.

In the context of BS7671 (EEBADS/ADS) then this usually utilises the earthing system to work, but it doesn't actually need an earth to work, just merely the conditions to create an imbalance will trip it, look at the way the test button works.