Is TNCS worse than TT earthing?

[Chr!s]

The thing is, you are not 'rodding' a PME supply if you connect the 'rod' to the MET - you are 'rodding' the consumer's installation, and indeed, changing the characteristics of the installation earthing system.To 'rod' the PME supply, you need to connect your 'rod' to the supply neutral (PEN) - which you are not allowed to do.

Im referring to rodding the at the met not the pen, as to which sidewinder is also referring.

 

[ADS]

Plumber, thanks for 702.410.3.4.3 (ii) See note - I hadn't actually noticed that before.

 

[ianmacd]

Certainly 411.4.2 Note PE, 702.410.3.4.3 (ii) See note ,

Now can you quote which reg this practice contravenes?

First one, are you seriously suggesting interfering with the DNO's equipment (only place to access PEN)?

Second one, When did you ever see an intentional swimming pool at the cut out? It actually suggests adding it to the Protective Equipotential Bonding too and not MET or anywhere else.

As above, contravenes 542.1.1

 

[Andy™]

he never said anything about interfering with DNO's equipment

and 542.1.1 does not say you cannto install an earth rod to TNCS.

and then you also have 542.1.5 which says earthing arrangements can used jointly....

 

[ADS]

[quote name='Andy

 

[Chr!s]

First one, are you seriously suggesting interfering with the DNO's equipment (only place to access PEN)?

You seem to be getting confused between met and pen ian

second one, When did you ever see an intentional swimming pool at the cut out? It actually suggests adding it to the Protective Equipotential Bonding too and not MET or anywhere else.

Think you best re read.

As above, contravenes 542.1.1

Best re read again.

Well they best re write the regs, GN5 , energy networks association recommendation G12/3

 

[steptoe]

first off,

plumber, you should feel priviledged as this is the first time I have went to my van and pulled the 7671 to look up a reg,

411 relates to rodding the PEN, which in a normal installation we are unable to do, that is the DNO's part in ensuring we have a proper PME supply, as opposed to the normal TNCS(we can debate this point forever that all TNCS 'should be PME)

702 , relates to swimming pools, which are NOT IMHO part of a normal domestic, and are special locations,

this point was raised to me by sidewinder previously and I feel I gave a coherent answer to it,

again, and I re-iterate, this is not a normal installation, and if someone is asking this question then perhaps they should not be working in installations containing swimming pools,#

it is still not simply a matter of driving in a rod and getting satisfactory Ra readings on it, there is much more to it,

the end game still results in the same scenario, in my humble opinion anyway,

you should not be mixing earthing systems within/out a normal domestic(or any other) installation unless you fully understand the full and final consequences of what your actions may cause, especially in the event of a fault, or someone elses actions.

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542.1.8

 

[ianmacd]

You are confusing Met and PEN as TN-C-S is rodded at PEN not MET AFTER its split.

Title to that section is Swimming pools and other basins so I'm sure I'm not the only one to wonder what this has to do with anything.

Key in 542.1.1 is the word ONE not some.

Not arguing about this again. We have had this too many times and some people only seem to post to argue.

 

[steptoe]

What are the risks on pme, broken pen, electric shock?If we place an electrode of a suitably low impedance to match the diverted current, what will happen to the touch voltage?

where will the current flow through you at say 1000 ohms or through the electrode at say 2 ohms?

plumber,

first off you need to understand the difference between TNCS and PME,

yes, according to law all TNCS should be PME,

real life is different,

search the forum and you will find all the answers you need,

BTW, read 542.1.8

if you cant understand the part about SEPARATE let me know and I'll try and explain.

or you could read 542.1.1 , and you will notice the word ONE is used.

says it all.

 

[ADS]

Ha Ha - You know he's gonna throw 542.1.5 at you.....which is about as ambiguous as the 'regs' get.

 

[Sidewinder]

You are confusing Met and PEN as TN-C-S is rodded at PEN not MET AFTER its split.Title to that section is Swimming pools and other basins so I'm sure I'm not the only one to wonder what this has to do with anything.

Key in 542.1.1 is the word ONE not some.

Not arguing about this again. We have had this too many times and some people only seem to post to argue.

Ian,

I think you will find that Plumbers reference to section 710 is in relation to my comment where by it may be necessary to add a rod to a TN-C-S / PME install.

This came up on my annual assessment, and yes it was a swimming pool and, yes it was a domestic install.

 

[steptoe]

Ha Ha - You know he's gonna throw 542.1.5 at you.....which is about as ambiguos as the 'regs' get.

I dont see that it is if you can understand the reasoning behind the way it is worded,

this is one of the main reasons I would not advocate using different earthing systems in a standard installation,

people reading the regs to suit themselves and not being competent enough to understand the implications of them.

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Ian,I think you will find that Plumbers reference to section 710 is in relation to my comment where by it may be necessary to add a rod to a TN-C-S / PME install.

This came up on my annual assessment, and yes it was a swimming pool and, yes it was a domestic install.

and I think I answered that in what would be a reason behind its installation

 

[davetheglitz]

Can we come back to first principles please!

What is the hazard of adding a rod at the MET on a PME system

Is this allowed?

If not - why not in electrical terms?

How is this different to bonding to a grounded pipe?

Does this bonding/earthing make a TNCS system safer than if no bonding/rodding?

Is this safer than a TT system given the potential for a N-E fault?

We should be examining the principles - not the regs - and interpret the regs after the principles are understood!

 

[Sidewinder]

I think reference to GN5 sections 14.5 where it advocates connection of an earth electrode to the building MET in a PME supply, which in the UK according to ESQCR all TN-C-S supplies are would be a good start.

Also section 14.6.3 again advice to add a rod to a PME / TN-C-S supply.

Also reference should be made to BS7430 which is officially a Code of Practice which will hold more relevance in a court of law then will BS7671.

I will go and get my copy in a minute and post a few other points.

However, in the mean time there is also industry guidance from the CDA with regard to earthing practice in their document 119, which IIRC is now available for download as a pdf.

---------- Post Auto-Merged at 23:03 ---------- Previous post was made at 23:00 ----------

BTW, I would assume sidewinder is referring to a structure that is within touch distance(as defined in BS7671) and therefore 'classed' as the same location. (at least I hope he is...)

Yes I was Steps!

---------- Post Auto-Merged at 23:04 ---------- Previous post was made at 23:03 ----------

and I think I answered that in what would be a reason behind its installation

You did, but before the post I replied to IIRC, however, yes as you can see that was my other point.

---------- Post Auto-Merged at 23:06 ---------- Previous post was made at 23:04 ----------

Is this allowed?

Yes it is and as I have said even required in certain circumstances.

Off to get 7430 brb.

 

[steptoe]

my main concern with the addition of a rod in a normal domestic is that the TNCS/PME supply earth may get withdrawn or disconnected, thereby leaving the installation relying on the rod, making it a TT system,

which may sound ok, but it has been designed as a TNCS system.

this is one of the reasons why I prefer a TT system from the outset,

the installer is in control of the earthing system, not a 3rd party.

 

[Sidewinder]

Both BS7375 & 7909 may require the addition of rods to a TN-C-S supply.

Can't find 7430 so I'm printing another copy.

Will check and come back.

 

[Manator]

Looks to me that some people are confusing earthing arrangements. I have recently done some remedials, awaiting a rewire. There are three substations on site and over 300 remote DB's, I have attached rods to some of the main boards, keeping the PME arrangement in place. I find no problem with this, and I would not consider any installation to be TT. PME is just that.

 

[Sidewinder]

Thinking about it IF you rod a PME/TN-C-S at the MET, then IF the supply N/E link fails then you have a PNB installation, IF the supply N fails then your N conductor is still tied to E at your MET, thus the potential rise is limited by the EFLI of your "rod".

This is IMHO where Steps comes from with regard to banging a single rod in and having Ra/Ze of just less then 200 ohms, this is not really on is it.

In the reg 710... referred to above the earth rod impedance must be <20 ohms.

If this cannot be achieved with a single rod, then you will have to put a better system in!

Had enough now guys, time for bed, I'm just too much of a lightweight!

Will look at 7430 later if I get a chance, I've put it with my 7671 now.

 

[davetheglitz]

Agree with you on this Steps

my main concern with the addition of a rod in a normal domestic is that the TNCS/PME supply earth may get withdrawn or disconnected, thereby leaving the installation relying on the rod, making it a TT system,which may sound ok, but it has been designed as a TNCS system.

I'm not advocating that the rod makes a TNCS safer as - it's so it reverts to a TT system if the neutral is lost - hence the statement on post 1

Overall - I think that the TNCS is probably a better earthing system than TT provided it is all RCD protected and any bonding provides a route to ground.

With TT from the outset you still have the problem of N-E faults upsetting the trip times - though there is an argument that this makes it a DIY TNCS at the fault point - so in most cases will cause the MCB's to trip in the case of a fault.

 

[steptoe]

So the basic answer to OP is yes, you are allowed to rod a TNCS,

But, only if certain conditions are met and under particular circumstances.

I would not advocate just adding a rod to just any TNCS system,

This is why I always say it is a no no, as I said, asking a question such as can I just add a rod brings up 2 things,

Does the poster understand the implications enough to be doing this,

And do the circumstances permit this?