Max Ze for TT system

[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.

 

[Guest 004]

^ yea!!!!!

 

[Engineer54]

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'?

I'll leave the nightmares to the clever people like of you...

There are a good few real electricians out there that have never touched a TT system in their lives before, and as has been seen from previous threads and posts, that they will also take that 200 ohm figure as meaning the maximum. Like many other areas in BS 7671 things can be a tad on the ambiguous side, especially if you don't take the time to read things through to fully understand what's being stated.

 

[Guest 004]

my work is done, time to move to another thread

 

[D Skelton]

^ yea!!!!!

Before you get too happy Mr, just realise that the chances of a fault occuring that recreates the operation of the test button is impossible! lol

 

[D Skelton]

I'll leave the nightmares to the clever people like of you...

There are a good few real electricians out there that have never touched a TT system in their lives before, and as has been seen from previous threads and posts, that they will also take that 200 ohm figure as meaning the maximum. Like many other areas in BS 7671 things can be a tad on the ambiguous side, especially if you don't take the time to read things through to fully understand what's being stated.

Now that is a statement we can all agree on! lol

 

[i=p/u]

Does rcd not provide fault protection on said circuit with no cpc. Andy leakage and bang trips.

 

[D Skelton]

Does rcd not provide fault protection on said circuit with no cpc. Andy leakage and bang trips.

No, fault protection is provided by the class 2 accessories. The very nature of a class 2 installation is that there is no earth for current to leak to!

 

[spark 68]

I dunno Mr D, outside of the realms of our normal installations I could think of a few exotic cases where an RCD could be used that wouldn't use an earth per se, albeit the chances of most of us ever seeing it would be practically nil.

The nearest thing to this is our good old TNC-S system

 

[Engineer54]

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.

There are always going to be situations such as you have described due to horrendous soil/ground conditions, and you're solution of an upfront S type, backing up any standard 30mA RCD's is the route i would also take, in such circumstances...

I too have a good amount of chalky soil on my property in Cyprus, but thankfully not to the extent of causing me major problems on my own system...

 

[D Skelton]

I dunno Mr D, outside of the realms of our normal installations I could think of a few exotic cases where an RCD could be used that wouldn't use an earth per se

Like what?

 

[spark 68]

Like what?

Certain IT systems use some pretty exotic protection schemas, also some DC systems are another, and I believe someone on here a while back came up with an idea using one (un-conventionally) to protect against a lost Neutral.

But as I said this is way off the beaten track, and is sidetracking this interesting thread, and almost all of us will never ever see it, it was more of an answer to Mr E, and partly the way the test button works (which is why I mentioned TNC-S).
I will not mention this part again, let the thread continue un-abated

 

[D Skelton]

Certain IT systems use some pretty exotic protection schemas

Every IT system that incorporates an RCD will have an earth sensing device to allow the RCD to operate. Although not an earth per se, it still requires a means to detect an inbalance.

also some DC systems are another

Even a DC RCD needs a means of detecting imbalance between pos and neg which isn't going to happen unless a fault loop has been intentionally made at the supply end. Again, functional imbalance detection would need to be employed.

I will not mention this part again, let the thread continue un-abated

Agreed

 

[MBEINST]

An update on the situation with my TT system.

I today called the DNO 'Electricity North West' and explained that I was at a property to install a couple of extra sockets on the downstairs ring and my Zs readings were unacceptable. I requested that the TT system be changed to TNCS since my subsequent Ze readings mean that no matter what I do to the householders installation I cannot get acceptable Zs readings (OSG Table 2d). I also mentioned that the occupants are in fact living with a dangerous installation since they effectively have little or no fault protection.

The nice man at the other end was quite alarmed and told me that an engineer would be with us before the day is out. Unfortunately this turned into tomorrow morning. However the earthing arrangement will be changed free of charge.

This appears to have bolstered my credibility with the householder and he now wants me to install a fully loaded RCBO unit.

I have returned with this update to thank you all for your input and advice. You have succeeded in enlightening a raw but 2391 trained spark (not sure if that qualifies me as a microwave spark but certainly not a DIYer). I like a little banter myself but some of the drivel in this thread boarders on people stuck up their own arses trying to score cheap points. Tone it down guys, its not big and its not clever.

Special thanks to Wirepuller and Engineer 54.

Everyday is a school day!

 

[D Skelton]

I like a little banter myself but some of the drivel in this thread boarders on people stuck up their own arses trying to score cheap points. Tone it down guys, its not big and its not clever.

I can't see any drivel on here, everyone who has commented has a fair amount of knowledge on TT systems. I can assure you that what you might think is point scoring is purely banter.

Special thanks to Wirepuller and Engineer 54.

Interesting how you give special thanks to the two very people that most on here would regard as being at complete opposite ends of the spectrum with regards to their opinions on domestic TT systems yet refer to others opinions as drivel. I find the term drivel quite disparaging if I'm honest, I can't think of anyone here who hasn't contributed in a positive way to the thread?

 

[MBEINST]

What was it you said:

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

Sorry that I'm not interested in winning popularity contests and taking sides in petty arguements.

I thanked everyone on the thread especially those 'wirepuller' & 'Engineer 54' whose advice I followed and which resolved my situation and in fact got me extra work.

However if this leaves you feeling a little under appreciated:

A great big extra special thank you very much to:

D Skelton :broken_heart: :devilish:

 

[baggy61]

There are always going to be situations such as you have described due to horrendous soil/ground conditions, and you're solution of an upfront S type, backing up any standard 30mA RCD's is the route i would also take, in such circumstances...

I too have a good amount of chalky soil on my property in Cyprus, but thankfully not to the extent of causing me major problems on my own system...

Sorry to be thick, but how does an upstream s,type provide more protection, unless you nail the tails ?
Regards

 

[MDJ]

Interesting how you give special thanks to the two very people that most on here would regard as being at complete opposite ends of the spectrum with regards to their opinions on domestic TT systems yet refer to others opinions as drivel?

I was thinking the same, funny world

 

[spark 68]

No petty arguments on the thread ?????

Most of us on here respect E54 highly, he is entitled to his opinion as much as other highly respected posters are, we were pulling his leg somewhat as we already know what his opinion is lol

That doesn't mean we cannot have our own opinions either, and if you notice we did not say he was wrong, just we all follow the regs which are sometimes badly written and open to interpretation, or sloppily written which implies one thing but could equally mean something else entirely.

Discussions are a good thing, no ?

If you actually understood TT systems a) you would not have posted in the first place, and b) you would see both sides of the debate which was going on in the background, but sadly this appears to have been lost on you.

 

[richy3333]

Sorry to be thick, but how does an upstream s,type provide more protection, unless you nail the tails ?
Regards

I'll put my tin hat on and take cover

 

[Guest 004]

Was trying to refrain from posting again on this thread, but that last post by sparky68 was just beautiful!