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Topic Title: Application of the proposed regulation 542.1.201
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Created On: 29 July 2017 12:35 PM
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 29 July 2017 12:35 PM
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sparkingchip

Posts: 9953
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Here is the proposed new regulation:

542.1.201 The main earthing terminal shall be connected with Earth by one of the methods described in Regulations 542.1.2.1 to 3 , as appropriate to the type of system of which the installation is to form a part and in compliance with Regulations 542.1.3.1 and 542.1.3.2 . Additionally, there shall be an earth electrode, supplementing any earthing facility provided by the distributor, in accordance with one of the requirements of Regulation 542.2.3, to prevent the appearance of a dangerous touch voltage in the event of the loss of the main connection to Earth.

I would like to see some worked examples of the application of this proposed regulation, let's start with an installation I completed around nine years ago that is currently considered safe being earted to the DNO TNCS-PME earth terminals, but will no longer be considered so if the installation work were to be undertaken after the implementation of the 18th edition of BS7671.

The project was a conversion and extension of a large Victorian house into seven flats with a landlord supply to the common areas.

Central Networks ran in a new supply from the road, they installed a new PME electrode at the joint in the road, then around 60 metres of new cable to a joint with another PME electrode. From this joint there is a 10 metre branch cable to two single phase intakes at the side of the building for single flats and the main cable runs on to a Ryfield three phase head that belongs to the DNO this supplies five flats and the landlords supply, this three phase head has a single shared DNO earth terminal.

Three of the flats have gas supplies maximum demand 40 amps and have a single rate meter as does the landlords supply with a minimal demand, the other four flats are all electric with storage heaters and a maximum of 80 amps on dual rate E7 meters.

To comply with this new regulation does it have to have three installation electrodes or can the installer interlink the three DNO earth terminals to one electrode system? What is the maximum permissible Ra of the electrode system to be installed and what specification would you anticipate for this site with sandy soil?

The three phase Ryfield with the shared DNO earth terminal has a maximum demand of 120 A / 120 A / 85 A across the three phases, the two separate single phase intakes have a maximum demand of 80 A each.

Andy B

Edited: 29 July 2017 at 12:45 PM by sparkingchip
 29 July 2017 12:54 PM
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dustydazzler

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one rod per application one would presume , correctly labelled at the stake and the corresponding terminal
 29 July 2017 01:00 PM
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sparkingchip

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Originally posted by: dustydazzler

one rod per application one would presume , correctly labelled at the stake and the corresponding terminal


Maximum resistance Ra and expected length for a site with sandy soil, also the maximum permissible touch voltage?

Andy B
 29 July 2017 01:06 PM
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dustydazzler

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I'm going to have to tag in one of the masterminds on this one
Too many variables and not enough experience installing earth electrodes in sandy conditions
 29 July 2017 01:42 PM
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sparkingchip

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Originally posted by: dustydazzler

one rod per application one would presume , correctly labelled at the stake and the corresponding terminal


If it were a new building then they could share the foundation earthing system, couldn't they?

Andy B
 29 July 2017 03:10 PM
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Parsley

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My guess is one supplementary electrode is required per DNO cut-out or DNO owned Ryefield DB if that's where the PME N-E link is.
If this was a new install and building was steel framed the DNO would only supply one cut-out that was PME all the other services would be SNE supplied ideally from one intake position, to prevent diverted neutral currents joining all PME supplies to one common additional electrode might not be the best idea in your example.

The reg doesn't seem to have been probably thought out, it refers to the 542.2.3 which only mentions foundation electrodes.
I added my comments to BSI site yesterday, there were only 20 other comments on this reg proposal.

I personally think it can only benefit new installs and the reg needs to reflect that. I don't believe a resistance value will be given.

Regards
 29 July 2017 03:16 PM
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Parsley

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The IET are advertising for a senior engineer at the moment, the role involves supporting JPEL 64.

Regards
 29 July 2017 03:26 PM
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dustydazzler

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Originally posted by: Parsley

My guess is one supplementary electrode is required per DNO cut-out or DNO owned Ryefield DB if that's where the PME N-E link is.

If this was a new install and building was steel framed the DNO would only supply one cut-out that was PME all the other services would be SNE supplied ideally from one intake position, to prevent diverted neutral currents joining all PME supplies to one common additional electrode might not be the best idea in your example.



The reg doesn't seem to have been probably thought out, it refers to the 542.2.3 which only mentions foundation electrodes.

I added my comments to BSI site yesterday, there were only 20 other comments on this reg proposal.



I personally think it can only benefit new installs and the reg needs to reflect that. I don't believe a resistance value will be given.



Regards


THIS ^^^
Reserve this new fandango earth spike stipulation for brand new installs
Leave us poor saps to continue merrily using the suppliers provided earth provision
 29 July 2017 04:44 PM
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psychicwarrior

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..and I thought there was only a very rare chance that the supplier N would be lost (thinking about all the discussion relating to TN-C-S (im leaving out the is it PME but :-) ). Is it now the case that this (loss of connection to earth) is not as rare as it first came across to me !
 29 July 2017 06:49 PM
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Alcomax

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The recent and proposed amendments appear to contain a subtle shift of risk from DNO to electrician. First the model forms were adapted to include a pass/ fail on suppliers equipment and , indeed, "distributors earthing arrangement". Now the existing risk latent defects of the network and the efficacy of "supplier earth facility " are potentially being further shifted.

Someone asked a question in the last few weeks regarding metal service heads, not sure if it was answered. Anyhow, these were routinely changed and pretty damn quick when highlighted by an electrician and/or inspector. The last couple of years this has changed and appear not bothered anymore. On PILC service cables below the head, in the past would sometimes observe significant bulges in the cabling. These were actioned PDQ, not anymore...will send a jointer who will say "that's okay" , but will not put anything in writing. For clients it is a problem, particularly commercial. As there is this element of doubt. There is "no sign off" of risk.
 29 July 2017 08:07 PM
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sparkingchip

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The reference documents I am using for this design exercise are:

IET Guidance Note 5. Protection against electric shock. IET 17th edition wiring regulations BS7671:2008 incorporating amendment number 1:2008. 14.5 Additional earth electrode for PME supplies. Page 100.

IET Code of practice for electric vehicle charging equipment installation 2nd edition fully updated to BS 7671:2008+A3:2015. Section 6.8.3 Installing an earth electrode system. Page 40.

Both have the identical figure illustrating the open PEN situation, however GN5 considers the Ra of casual earth connections and includes water and gas pipes with touch voltages of 50 V and 100 V. The EV code of practice considers the Ra of an additional electrode connected to the main earth terminal with a touch voltage of 70 V.

In the project I have detailed all the gas and water supply pipes are plastic, so do not help to lower Ra.

Before the design can really get underway we have to agree what constitutes a dangerous voltage, despite years of considering that maximum touch voltage should be 50 volts I think we are going to have to agree that we are going to consider a maximum safe touch voltage can be 70 volts inline with the guidance within the EV code of practice.

So to get the ball rolling can we agree that a dangerous touch voltage is a voltage in excess of 70 volts?

Andy B.

Edited: 29 July 2017 at 08:20 PM by sparkingchip
 29 July 2017 08:14 PM
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AJJewsbury

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So to get the ball rolling can we agree that a dangerous touch voltage is a voltage in excess of 70 volts?

For the sake of debate, I'd say yes. (I'd prefer 50V, but the result will be almost as ridiculous anyway it makes no difference).

I'd suggest the next step is to establish what sort of current the electrode will be required to sink - which depends on where the break in the CNE is assumed to be and the connected loads downstream of the break. For the sake of an initial debate, I'd suggest assuming that the break will affect only your individual installation (or equivalently that all other installations have equally effective electrodes).

- Andy.

Edited: 29 July 2017 at 08:22 PM by AJJewsbury
 29 July 2017 08:16 PM
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sparkingchip

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Originally posted by: Alcomax

The recent and proposed amendments appear to contain a subtle shift of risk from DNO to electrician. First the model forms were adapted to include a pass/ fail on suppliers equipment and , indeed, "distributors earthing arrangement". Now the existing risk latent defects of the network and the efficacy of "supplier earth facility " are potentially being further shifted.



Someone asked a question in the last few weeks regarding metal service heads, not sure if it was answered. Anyhow, these were routinely changed and pretty damn quick when highlighted by an electrician and/or inspector. The last couple of years this has changed and appear not bothered anymore. On PILC service cables below the head, in the past would sometimes observe significant bulges in the cabling. These were actioned PDQ, not anymore...will send a jointer who will say "that's okay" , but will not put anything in writing. For clients it is a problem, particularly commercial. As there is this element of doubt. There is "no sign off" of risk.


I agree completely, for some years now in the Central Networks, now Western Power area where I am, new DNO TNCS customer earth terminals have come with a disclaimer saying they have not been tested by the DNO and it is the customers responsibility to engage a qualified electrician to check that the earth terminal is suitable for use by testing it; and the electrician should make the final connection of the installation main earth conductor to the DNO earth terminal with metering services only connecting the live and neutral tails into a isolation switch, leaving the electrician to make the final connections of the live and neutral tails along with the main earth conductor, with the electrician absolving both the DNO and metering services of responsibility for ensuring there is correct polarity and a sound earth connection.

Andy B.
 29 July 2017 09:22 PM
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AJJewsbury

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Some interesting notes from a DNO about earthing here - http://library.ukpowernetworks...on+Earthing+Design.pdf especially tables B-2 and B-3 on page 50 which give likely resistances for differing number of driven electrodes and lengths of buried conductor/mats in various soil resistances.
- Andy.
 29 July 2017 09:59 PM
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sparkingchip

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Originally posted by: AJJewsbury

Some interesting notes from a DNO about earthing here - http://library.ukpowernetworks...on+Earthing+Design.pdf especially tables B-2 and B-3 on page 50 which give likely resistances for differing number of driven electrodes and lengths of buried conductor/mats in various soil resistances.

- Andy.


The IET GN5 gives that substation earth as RB the resistance to earth of the neutral point of the power supply, it advises we can neglect it in our calculation as this errs on the safe side, we can also neglect Re, the external line supply resistance as it is small compared with RL and RA.

I will have a go at typing the formula tomorrow, trying to type them out on this forum is too much of a challenge for this time of night.

Andy B.
 30 July 2017 10:25 AM
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AJJewsbury

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That's a point. The supplier's substation electrode will also have some resistance - typically next to nothing for a substantial urban distribution system perhaps, but for two or three rural customers fed from a pole pig transformer fed from overhead HV lines, it could well be in the region of 20 Ohms. So even if we somehow got the customer's electrode down to some sub-Ohm values to keep touch/step voltages around the home within safe values, we'd be creating a potentially dangerous situation around the transformer's electrode - and not for the few seconds that rise of earth potential calculations are normally based on, but for many hours perhaps even days; and that potential would be transferred to exposed-conductive-parts upstream of the break too.
- Andy.
 30 July 2017 11:00 AM
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Zoomup

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The idea of a reliably large earth fault current being carried through the ground at low Voltage is bonkers. It can never be assured to be reliable with large loads. It can never be assured to be large enough to operate a fuse or circuit breaker. The whole idea is flawed. Bring back some common sense. Ground soil is varying in its conductivity and is unreliable as a conductor at low Voltage. This whole idea of additional earth electrodes replacing the safety of a P.E.N. conductor if it fails is completely nuts. It can never work in most cases.

GGGGRRRRRrrrrrrr. Mumble ,mumble

Z.
 30 July 2017 11:08 AM
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sparkingchip

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Originally posted by: AJJewsbury

That's a point. The supplier's substation electrode will also have some resistance - typically next to nothing for a substantial urban distribution system perhaps, but for two or three rural customers fed from a pole pig transformer fed from overhead HV lines, it could well be in the region of 20 Ohms. So even if we somehow got the customer's electrode down to some sub-Ohm values to keep touch/step voltages around the home within safe values, we'd be creating a potentially dangerous situation around the transformer's electrode - and not for the few seconds that rise of earth potential calculations are normally based on, but for many hours perhaps even days; and that potential would be transferred to exposed-conductive-parts upstream of the break too.

- Andy.


If we refer to the Introduction of the IET On-Site Guide BS 7671:2008+A3:2015. 1.1(d) page 11 it says:

"For a TT arrangement, 21 ohms is the usual stated maximum resistance of the distributor's earth electrode at the supply transformer."

So that pole transformer supply installed as TNCS-PNB with a single DNO electrode or indeed TNCS-PME with only one DNO electrode left connected to the Tx may presumably have relatively high earth resistance that may cause issues when trying to comply with 542.1.201.

However for the purposes of this calculation and design we can ignore the resistance of the DNO electrode.

Andy B.
 30 July 2017 11:09 AM
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Zoomup

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If the soil was such a good electrical conductor we would not need metal cables to distribute electricity at low Voltage. The ground is unreliable as a conductor at low Voltage. Why can't the electrical engineers understand that simple premise? Metal reliably conducts. Soil does not reliably consistently conduct. A primary school child could tell us that. Blinkin Ek!

Z.
 30 July 2017 11:20 AM
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Zoomup

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The old system of "earthing" in towns and cities years ago when we used underground water pipes as an earth return was not "earthing", it was "water piping". That allowed us a solid metal "earth" return to the supply transformers. The only part of the term "earthing" that was true was that the metal water pipes were buried in the ground, or earth.

Most people do not understand just what "earthing" means. It is very much misunderstood.

"In contact with the general mass of earth" should read "in contact with an unreliable inconsistent generally electrically insulating dry or wet soil-like substance of unknown or unreliable electrical conductivity".

Z.
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