Can earth Rod be retained after TT to TN-C-S conversion?

[batty]

I am sorry but I really cannot see what is going on hear. You either have a TT system so you need a modern 17th edition board with total RCD protection or you are going to leave system relying on DNO earth. Why would you want two earthing systems. If DNO neutral fails you are not going to be able to relie on your earth rod to solve this problem. Why would DNO earth/neutral fail anyway unless it real old cabling with poor joints and I don't think they would give pme earth if it was unless this is DIY pme earth you really are worrying about nothing.

 

[Sidewinder]

Ahh, Batty, good point, however overhead PME is very susceptable to joint issues.

Also lack of maintenance on DNO part, which is actually none of our concern in law, however, morally is another issue.

Also there are requirements in the brb to add an earth rod to TN-C-S/PME supplies...

 

[Apache]

Ahh, Batty, good point, however overhead PME is very susceptable to joint issues.Also lack of maintenance on DNO part, which is actually none of our concern in law, however, morally is another issue.

Also there are requirements in the brb to add an earth rod to TN-C-S/PME supplies...

Go on, remind us.

---------- AUTO MERGE Post added at 00:31 ---------- Previous post was at 00:17 ----------

Was it the dialysis huts?

 

[Sidewinder]

Nope Patch, not the Dialysis huts.

However, as these are verging on Group 1 medical locations it is something to consider.

I'll look the reg no. up later & put it up.

 

[NeilMcC]

Neil,As long as the 132 ohm is stable and the install is RCD'd as per TT requirements then the potential PEN failure should be no concern.

Else you really need the Re below 20 ohms, unless you are going to rely on the distributors earth.

HTH

Hi,

I still think Plumber was right when he said the RCD would not operate to protect someone from high touch voltage in a PEN failure scenario. In this instance the MET has a high touch voltage which is on the other side of the RCD - the RCD still sees a balance between it's Live and Neutral conductors.

Even if the Earth Rod was say 10 ohms, if there was a high load (7KW would mean 30 amps flowing and be equivalent to a load resistance of 7.5ohms, right?) then over half of the 230V would still appear on the earth rod?

I possibly am worrying over nothing; but when I dont understand properly there is room for doubt and I'm a cautious type of person.

 

[Chr!s]

Hi Neil

Hi,I still think Plumber was right when he said the RCD would not operate to protect someone from high touch voltage in a PEN failure scenario. In this instance the MET has a high touch voltage which is on the other side of the RCD - the RCD still sees a balance between it's Live and Neutral conductors.

Even if the Earth Rod was say 10 , if there was a high load (7KW would mean 30 amps flowing and be equivalent to a load resistance of 7.5, right?) then over half of the 230V would still appear on the earth rod?

I possibly am worrying over nothing; but when I dont understand properly there is room for doubt and I'm a cautious type of person.

Well Neil you are right, and yes you should be concerned, you need to remember sometimes it is difficult to maintain an equipotential zone, often electrical systems exist outside where's there's an increased risk of shock and also in places like workshops for example where the floor may well be at earth potential. So if the pen was to break then a electrode at the correct Ra for the assumed load would give the protection in those areas. Think areas of increased electric shock if your looking for a Reg/recommendation

---------- AUTO MERGE Post added at 13:18 ---------- Previous post was at 12:15 ----------

Oh and just one other point.

In many instances recommendations for electrodes on pme supply's isn't necessarily just for the broken pen fault, its also to do with perceived electric shock.

Take for example a wet floor shower room, the floor may be close to earth potential yet bonded metal work will be at system potential, the difference between the to potentials may rise to the sensation of electric shock. Placing an electrode in the system ties the bonded metal work down close to a similar potential removing the sensation of electric shock.

 

[NeilMcC]

I found some statistics:

• "Chance of any one consumer experiencing a PEN failure 1 in 236,000 per year.
  
• Risk of PEN failure causing a reportable injury to a person 1 in 5,260,000 per year
  
• It should be noted that there have been no fatalities from PEN failures over the 13 years to May 2003 that records have been kept."
  

And for comparison:

• Risk of death by traffic accident across the population 1 in 10,204 per year
  

Source: http://www.bbc.co.uk/safety/pdf/safety-ratuesl-03.pdf

Page 20.

So I think I can stop worrying..

 

[Adrian]

I wonder what the statistics are for a failed rcd on a tt system 1 in a 100, I suppose. A good reason to pme and not tt

 

[Robin Spark]

I wonder what the statistics are for a failed rcd on a tt system 1 in a 100, I suppose. A good reason to pme and not tt

And probably less than that if its a Wylex RCD!

 

[Sidewinder]

Found the reg. he pulled me on the following item in the regs!

Yes only a NOTE! The actual reg. was complied with!

702.410.3.4.3(ii)

...

Note: When the supply to the swimming pool is part of a TN-C-S system it is recommended that an earth mat or earth electrode of a suitably low resistance, e.g. 20 ohms or less, be insalled and connected to the protective equipotential bonding.

The reasoning for this is as per Plumber suggests above, also, it will ensure no complete loss of earth in the event of a PEN failure.

This install is an underground 3ph PME supply, it is jointed and rodded outside the property grounds and at the repair within the property grounds where a JCB went through it!!!

You can simulate PEN faults in a controlled manner to experiment with the failure modes and the responses of the various protective devices.

 

[Larnacaman]

I would ALWAYS ground any Swimming Pool re-bar system, not matter what the incoming Earthing arrangement was. All pool accessories always include an earthing point anyway, such as in pool ladders and diving boards etc.

I bonded and grounded the re-bar on my swimming pool here and as i'm on a TT System here, all that steelwork reduced my overall rod resistance considerably

 

[Sidewinder]

Larnacaman,

This was not a new install, it was a CU change & partial rewire, also, no rebar or steps etc!

 

[Larnacaman]

New install or not, Means should always be provided to earth and bond S/Pools and in ground Spa's/Whirlpools etc.

The S/Pool you have mentioned, should have been earthed and bonded to the main earthing system when it was built/constructed. It would in all probability, have fulfilled the requirements in that note in the Reg's you quoted too!! ...lol!!

All TT Systems, or come to that any roded installation (except Lightning Protection etc) ''Should'' be a minimum of 20ohms or less. But we know that is almost never the case in modern domestic installations, because of the now common total reliance on an RCD, for the installations earth fault protection.

 

[Sidewinder]

Larnacaman,

It almost certainly should have had a much more robust earthing system when built, however, I had no control over that, and as it was built & wired by scaffolders then the fact it is as good as it is, is almost certainly a bonus!

One cannot have control over that which was done in the past!

 

[Larnacaman]

Agreed.... lol!!

It would not surprise me in the least, if my S/Pool is the only one that has been Earthed and bonded in Cyprus. Then again, i would hope that the many hotel S/Pools have been so earthed/bonded, but i certainly wouldn't like to put money on it !!!! lol!!!

 

[Chr!s]

Theres no requirement to earth a swimming pool, it just requires supplementary equipotential bonding.

 

[Larnacaman]

haha!!, So what do you think all that steelwork in the S/pool base and walls are then??

And what do you think you are doing by bonding that steelwork??

 

[Chr!s]

The requirments are to connect all extraneous conductive parts in zones 0,1,2 by supplementary protective bonding to the protective conductors of exposed conductive parts of the equipment in these zones, bonding and earthing are not the same thing.

 

[Sidewinder]

Plumber, Larnacaman,

This is where things get complicated!

You are both correct, however, I suspect you will never agree!

You cannot "bond" as such reinforcing materials in structural concrete as they would not be accessible.

However, if a connection point were available then you could be connectin g your bonding which would be required to a very "massive" earth electrode which would be providing an earth, however, via a bond, however, you are allowed to use structural steelwork as your earth electrode...

Thus this connection could be your main earthing conductor...

 

[Chr!s]

The rebar system of a swimming pool should be bonded, clamps are purposely made for either the ties or the rebar, though the requirement is to supplementary bond this to the exposed conductive parts of equipment, not for it to be earthed. The idea is to create a equipotential plane.