Pme again with workshop help please

[steptoe]

Hi Steps, I agree with what you say 100% TT is obviously more reliable, unless the world falls off the end of your rod, AND, TT would be FAR preferable to relying on the DNO and their not too clever wiring... BUT for one thing... Look at the failure rates for RCD's.. MUCH MUCH more likely to have one of them fail, than a problem with the DNO supply.....

john...

Hi Steps, Yes i know i did, but in the context that the OP was referring to!!!!

john..

You've not seen reverse polarity on a TNCS or PME system then,?

 

[apprentice87]

You've not seen reverse polarity on a TNCS or PME system then,?

No, but I can see that it would make all exposed conductive parts [and extraneous ones too] somewhat live.... But then, on TT, when you have an earth fault AND the RCD does not operate, you will be in VERY much the same position, as i doubt very much, that, unless you have an extremely low Zs, that anything but the smallest MCB would operate, and even then it would take absolutely AGES...

john...

 

[Sidewinder]

With regard to number 5 above.................How would you test for Zs ?

I would be inclined to terminate using the correct gland into an insulated box, taking the tails on through into the cu & set up cu as TT.

 This way you have you submain circuit cpc(armour) at the insulated box for a Zs reading.

The only thing with that is then you have two different earthing systems accessible.

The TN-C-S at the SWA gland into the insulating box, and the TT for the rest of the building, so you could have a potential difference between the two, and a potential for a current flow, with no protective device to prevent shock and ensure ADS...

 

[leesparkykent]

I don't see what all the fuss is about. Joint the SWA in an accessible dappy box or bury a resin joint and extend it to the new position. Use the DNO supplied earth for the shed. MCB at the supply end with a small garage unit with 30ma RCD within the shed.

 

[Sidewinder]

I don't see what all the fuss is about. Joint the SWA in an accessible dappy box or bury a resin joint and extend it to the new position. Use the DNO supplied earth for the shed. MCB at the supply end with a small garage unit with 30ma RCD within the shed.

Ok, fair point, but, if the supply is PME as well as TN-C-S, then will the DNO allow the transfer of their PME earth outside the equipotential zone established around the point of supply, as their permission to do this is required by statute law in ESQCR, if you are going to export the PME earth outside the equipotential zone surrounding the presentation of supply.

So would you be exporting a PME earth?...

 

[steptoe]

I exported an earth only the other day and added my own rods for a localised earthing, I am still waiting for the DNO/Scam police to come and arrest me....

ah,

now, this is a fair point,

I dont think the DNO have any jurisdiction over localised PME,

that is what I have at my house, AFAIAC its nothing to do with the DNO how I earth my own property, or any other property I work on and certify as meeting the requirements

 

[Davethsparky]

I'm currently wiring a glorified shed for a friend which I will be using for a scheme assessment. But I'm making the installation a little bit different from the norm because I like to see just how much the scheme assessors actually know, and I am confident in my knowledge and ability.

so this shed is being connected to the PME earth via a 3 core SWA, of which the third core will be there purely as a bond which will connect directly to the water pipework adjacent to the CU without terminating in the CU, the CPC will be he armour which will terminate via the gland onto a brass gland plate. That same brass gland plate will have all of the final circuits glanded onto it. All final circuits are wired in 2L1.5 (including the ring). 

The last time I had a scheme assessment I passed it despite having a blazing row with the assessor and making a complaint about him. He completely missed the extraneous part about 2' away from the CU which I had 'forgotten' to bond and he tried to pull me up on an earth rod which complied with the letter of the regs but did not meet what he would prefer to see done. 

 

[leesparkykent]

Ok, fair point, but, if the supply is PME as well as TN-C-S, then will the DNO allow the transfer of their PME earth outside the equipotential zone established around the point of supply, as their permission to do this is required by statute law in ESQCR, if you are going to export the PME earth outside the equipotential zone surrounding the presentation of supply.

So would you be exporting a PME earth?...

I would extend the equipotential zone to the shed. The OP has a 10mm 3core so as long as the supply neutral is under 35mm the 10mm will be adequate to bond any extraneous conductive parts which I doubt there will be anyway.

 

[Davethsparky]

Flookie wookie, lol fluke 1654b tested meter.

No,  no jointing of cables,  can't extend pme to outbuildings I'll never pass my assessment for part P,  surely,   I've got an existing pme that's been extended but not exported to a house extension with a 5 way fuse board, from this I have an armoured going into the garden "exported" outside, this is what want to TT not extend it, 

Didn't think my write up was that complicated, 

What do you mean you won't pass your assessment by carrying out an installation which complies with 7671??

if you lack the knowledge and skills to carry out a compliant installation in a garden shed then you shouldn't be doing the job unsupervised in the first place 

Thank you Mr Essex , so  what is with all the TT ,ing  I respectfully ask?

All the TT'ing is fundamentally a product of people who lack knowledge believing rumours and old wives tales rather than actually reading and understanding the regulations and having confidence in their own abilities. 

 

[steptoe]

No,

TTing an installation is actually understanding not only the regs, but why TT is used,

By your own admission you are unsure of TT installations, so maybe you are not the best person to comment. 

No,

TTing an installation is actually understanding not only the regs, but why TT is used,

By your own admission you are unsure of TT installations, so maybe you are not the best person to comment. 

 

[steptoe]

@Ampsyaman

For the advice you are trying to get here a solicitor would cost you a small fortune,

Have you thought about upgrading your membership,,,,,,? 

 

[Davethsparky]

No,

TTing an installation is actually understanding not only the regs, but why TT is used,

By your own admission you are unsure of TT installations, so maybe you are not the best person to comment. 

No,

TTing an installation is actually understanding not only the regs, but why TT is used,

By your own admission you are unsure of TT installations, so maybe you are not the best person to comment. 

'TTing'???

where exactly did I admit to being unsure of TT installations? I can't imagine I would ever have said such a thing.

TT is used when and where a DNO cannot provide a suitable earth connection or where the DNOs earth connection is not suitable to be used or where it would be uneconomical to run a suitable main bond to an outbuilding in a larger installation. The example for the last item given in guidance note 8 is an installation where a supply to an outbuilding is something like 16mm but a main bond would need to be much much larger and so it is more sensible to create a new earthing system via a TT for that outbuilding. 

In a situation where extending the main bonding to an outbuilding is a viable option and the installation is not subject to any special considerations then for a small installation such as domestic there is no reason not to use the DNOs Earth connection. 

If if it is economically viable to create a TT earth which has a suitably and reliably low Ra such that reliance on an RCD can be avoided then I can see why you might consider the TT option. 

PME is not the big bad scary wolf that some people seem to think it is, although I am in favour of the idea of connecting a suitably low Ra earth electrode to the MET of a PME fed installation as is practiced in many other countries. 

 

[steptoe]

What do you mean you won't pass your assessment by carrying out an installation which complies with 7671??

if you lack the knowledge and skills to carry out a compliant installation in a garden shed then you shouldn't be doing the job unsupervised in the first place 

All the TT'ing is fundamentally a product of people who lack knowledge believing rumours and old wives tales rather than actually reading and understanding the regulations and having confidence in their own abilities. 

'TTing'???

where exactly did I admit to being unsure of TT installations? I can't imagine I would ever have said such a thing.

TT is used when and where a DNO cannot provide a suitable earth connection or where the DNOs earth connection is not suitable to be used or where it would be uneconomical to run a suitable main bond to an outbuilding in a larger installation. The example for the last item given in guidance note 8 is an installation where a supply to an outbuilding is something like 16mm but a main bond would need to be much much larger and so it is more sensible to create a new earthing system via a TT for that outbuilding. 

In a situation where extending the main bonding to an outbuilding is a viable option and the installation is not subject to any special considerations then for a small installation such as domestic there is no reason not to use the DNOs Earth connection. 

If if it is economically viable to create a TT earth which has a suitably and reliably low Ra such that reliance on an RCD can be avoided then I can see why you might consider the TT option. 

PME is not the big bad scary wolf that some people seem to think it is, although I am in favour of the idea of connecting a suitably low Ra earth electrode to the MET of a PME fed installation as is practiced in many other countries. 

ermmm,

in the first post I have quoted,

stating that it is for people that lack knowledge,?

TT is for people that understand how earthing systems work and know when to use it, and , sometimes, when not to, something you fail to understand.

 

[Duane B]

@Ampsyaman

For the advice you are trying to get here a solicitor would cost you a small fortune,

Have you thought about upgrading your membership,,,,,,? 

Very good Steps Id have a little chuckle lol, 

City bloke many jobs but not on TT, looks like a can of worms has been opened oh dear, just wanted to get my head around it and safely I'd got an idea as many have, lol

Sorry guys if I've up set the applecart, 

 

[Davethsparky]

ermmm,

in the first post I have quoted,

stating that it is for people that lack knowledge,?

TT is for people that understand how earthing systems work and know when to use it, and , sometimes, when not to, something you fail to understand.

I did not state that it is for people who lack knowledge, I stated that 'all the TTing which Essex referred to (that is apparently obsessive desire which domestic installer types have for starting threads about wanting to TT outbuildings unnecessarily) is a product of a lack of knowledge (Ie, lack of knowledge on the part of those DI's)

please do read it again in taco text of the quote I was replying to. 

TT is not 'for' any particular type of person, TT is one of the possible earthing systems which has its advantages and disadvantages, if it is 'for' anything then it is for installation where it is th most suitable option. 

What at are you suggesting that I fail to understand? 

 

[steptoe]

The fact you stated very clearly that, 'fundamentally TT is a product for people that do not understand earthing' 

That grinds with me, as IMHO, as a 5ww, TT is actually a far better system than any pretend PME your DNO is going to give you. 

 

[Davethsparky]

The fact you stated very clearly that, 'fundamentally TT is a product for people that do not understand earthing' 

That grinds with me, as IMHO, as a 5ww, TT is actually a far better system than any pretend PME your DNO is going to give you. 

Go back and read it again! I quite clearly said a product OF, not FOR! And clearly was replying to Evans' question about 'all the TTing' which is a reference to the huge number of topics from people who lack knowledge and common ere with questions that include phrases such as 'My mate told me' or 'I've heard that'

How are you defining a pretend PME as opposed to a proper PME? They aren't terms I've found in any formal document on the subject.

The massive flaw with a 5ww or any other idiot attempting to install a TT system is that they almost always seem to expect a single 4' skinny rod to be an adequate earth electrode which as long as a low current loop test goes a value under 200 ohms they consider to be acceptable. 

If the earth electrode is made up of multiple deep rods and/or earth plates, tapes, other suitable buried metal, ufer earths etc and has been tested with a recognised earth resistance testing method giving an Ra which is low enough to actually do some good and not result in reliance on bloody RCDs then yes I will agree that it is a better system.