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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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 30 July 2017 11:29 AM
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sparkingchip

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We need to determine RL the load resistance, so we need to know the loading in watts.

Let's keep this simple and consider one of the flats connected to a single phase incomer that has a E7 supply.

There is a number of loads and varied usage, so let's take one specific daily event that occurs every winter night, that is the radio teleswitch turning on the off peak supply. These are two bedroom flats and have three 3.4 Kw and one 2.5 Kw storage heaters totalling 12.7 Kw. These come on at night so other loads are minimal.

Therefore for the purpose of this project I suggest a total of 13 kilowatts as the load for one flat with a E7 supply.

The load resistance RL is Vs2/ wattage

Therefore:

RL = (230x230) / 13000 = 4.07 ohms

Does that sound sensible?

Andy B.
 30 July 2017 12:28 PM
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AJJewsbury

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

Agreed - I was just looking at the wider implications of the general approach. (I'm still favouring a different strategy - i.e. detect the problem and disconnect the installation.)

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.

Just to be clear, it's the normal load currents we're worrying about at the moment, rather than fault currents. (With the CNE open circuit the MET can go up to something towards line voltage, even with no faults present). I think they're ignoring earth faults during a broken CNE condition (as it would still require two faults to danger - the broken CNE being the 1st fault and the internal L-PE fault the second).

- Andy.
 30 July 2017 12:35 PM
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AJJewsbury

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Therefore:

RL = (230x230) / 13000 = 4.07 ohms

Does that sound sensible?

Adds up for me!

So the next stage, what resistance in series with that would have 70V across it when the whole lot is fed by 230V.

If we're to have 70V across the electrode then there should be 230-70=160V across the installation - so current through the 4.07 Ohms would be about 39.3A. Our electrode would therefore have to have a resistance to the general mass of the earth of 70V/39.3A = 1.78 Ohms.

- Andy.
 30 July 2017 02:00 PM
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sparkingchip

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Here is the diagram from GN5 from the Voltimum website featuring an article on swimming pools originally published in the IET Wiring Matters magazine.

The casual earth connection has to become a correctly installed earth electrode with the introduction of 542.1.201 that will prevent a dangerous touch voltage.

Andy B.

Edited: 30 July 2017 at 02:11 PM by sparkingchip
 30 July 2017 02:55 PM
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Angram

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Is there any guidance on how to achieve 1.78 ohms Ra in Camberley, Surrey where there is only sand, not soil, in which to plant an earth electrode of any kind?

If by some measure, not yet known to science, it becomes possible to achieve 1.78 ohms Ra with a rod, how do we protect the earth conductor from causing a fire when the local distributed load finds this new route back to the 21 ohm TX under a broken neutral ?

One thing the proposed regulation will achieve is harmonisation with Europe just prior to Brexit. Earth rods are the norm there I think. No PME.

Fast disconnection with a rod and RCD yes, but voltage rise prevention appears to be a fantasy as the Guidance Note calculations have demonstrated for some years now.

It is a digression to mention here that the cancellation of railway electrification projects, due to escalating costs, was caused by new harmonised european standards being applied to overhead line clearances after costing the projects. BS7671 rules on projects in hand not being allowed even where bridges and tunnels had been built one hundred or so years ago.

We live in interesting times.

Angram.
 30 July 2017 04:43 PM
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sparkingchip

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Originally posted by: AJJewsbury

Therefore:



RL = (230x230) / 13000 = 4.07 ohms



Does that sound sensible?


Adds up for me!



So the next stage, what resistance in series with that would have 70V across it when the whole lot is fed by 230V.



If we're to have 70V across the electrode then there should be 230-70=160V across the installation - so current through the 4.07 Ohms would be about 39.3A. Our electrode would therefore have to have a resistance to the general mass of the earth of 70V/39.3A = 1.78 Ohms.



- Andy.


Very succinct.

I was going run through the GN5 equation referring to the figure above from GN5 and the IET Wiring Matters article.



Us is the nominal supply (source) voltage, which for this single phase supply is 230 V.

Up is the touch voltage, Andy J and myself agreed on 70 V for this flat in the project, you may disagree.

Re is the external line supply resistance and GN5 says we can ignore it.

RL is the load resistance (Vs2 / wattage), Andy J and myself agreed on 13 Kw for this flat in the project, giving 4.07 ohms at 230 V.

RA is the resistance of the additional earth electrode and the answer we are looking for.

RB is the resistance to earth of the neutral point of the power supply, which GN5 says we can ignore.

Which gives the answer that the RA of the additional earth electrode needs to be 1.78 ohms or less, as Andy J said above.

So now we need to design an electrode system that is likely to have a RA of 1.78 ohms or less.

Remember this is just for one of the seven flats and the landlords supply, so will still have to deal with the other parts of the project.

Andy
 30 July 2017 05:21 PM
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sparkingchip

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Keison.co.uk

So based on table 6.1 and 6.2 in the EV COP we may, if the soil resistivity is exceptionally low, get away with installing three 2.4 metre rods spaced at 3 metres or five 1.2 metre rods spaced at 3 metres, but that is optimistic.

To quote the EV COP:

"Tables 6.1 and 6.2 demonstrate that, in many installations, it will not be reasonably practicable to achieve the required the required electrode system resistances to earth.

However it is recommended that a minimum of two 2.4 m or three 1.2 m electrodes separated by 3 m are installed. If this practice is followed, whilst many installations will not have touch voltages below 70 V, the risk of injury will be reduced."

Given the precise wording of the proposed regulation 542.1.201 is banging in a few rods knowing they may not really to the job acceptable or do we need to go for the full gold plated job?

Andy B

Edited: 30 July 2017 at 05:42 PM by sparkingchip
 30 July 2017 06:37 PM
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Zoomup

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Originally posted by: AJJewsbury

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


Agreed - I was just looking at the wider implications of the general approach. (I'm still favouring a different strategy - i.e. detect the problem and disconnect the installation.)



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.


Just to be clear, it's the normal load currents we're worrying about at the moment, rather than fault currents. (With the CNE open circuit the MET can go up to something towards line voltage, even with no faults present). I think they're ignoring earth faults during a broken CNE condition (as it would still require two faults to danger - the broken CNE being the 1st fault and the internal L-PE fault the second).



- Andy.


Yes indeed Andy I agree with you. I was illustrating the unreliability of the use of earth electrodes in varying soil conditions as a method of preventing dangerous Voltages on earthed metalwork. This preventative method will include the event of a broken P.M.E. C.E.N. conductor carrying a healthy normal load current as you correctly point out, where that normal current tries to route via the earthing electrode and/or bonded underground metalwork. If a sustained current flows for some time the earth electrodes may dry out surrounding soil and increase their resistance to "true" earth.

Some sort of detecting device may need to be installed that cuts off the supply if dangerous Voltages appear on exposed conductive parts.

Just what might that device be?

Bye,

Z.
 30 July 2017 06:58 PM
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Zoomup

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Then we have to add a "corrosion factor" that allows for loss of efficiency of the earth electrodes due to deterioration and decay, or soil drying. Most domestic earth electrodes are never inspected or tested for efficiency or condition. I recently saw one on the coast that was originally galvanized but now was nicely rusting away with no zinc coat at all above the soil. The copper earth conductor was clamped between two rusty washers. Sub two Ohms? Never. Not originally and certainly not now. More like 50 Ohms or above.

A U.S. earth rod corrosion technical report http://www.erico.com/catalog/literature/LT0540.pdf

Z.

Edited: 30 July 2017 at 07:13 PM by Zoomup
 30 July 2017 07:31 PM
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Zoomup

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A technical report referring to copper, earthing, bonding, electrodes etc. Very informative. http://copperalliance.org.uk/d...hing-practice-pdf.pdf

Z.
 30 July 2017 08:06 PM
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sparkingchip

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So, sizing the earth conductor to the rods, I am guessing if there are 25 mm tails it should be a 25 mm earth conductor, but I think most DNO earth terminals in domestic properties are designed for 16 mm. If it goes in as a bare buried conductor it will have to be 25 mm and that will improve the Ra of the earth electrode system.

So where do we locate the electrode earth terminal, can it go in an external meter box?

Will it be linked to the DNO terminal with 16 mm even if the earth conductor is 25 m?

Do we have to use the Adiabatic equation to size the earth electrode conductor rather than the tables?

Andy B.
 30 July 2017 10:06 PM
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AJJewsbury

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Some sort of detecting device may need to be installed that cuts off the supply if dangerous Voltages appear on exposed conductive parts.

Just what might that device be?

I think I mentioned a possibility in another thread - voltage monitoring relays (starting at about £30 for a single phase one) - either monitor the N-true earth voltage (via a rod with an arbitrary resistance to earth) or just monitor L-N voltage (since if the installation's load is increasing the voltage on the broken CNE, the installation will see a corresponding drop in L-N voltage) so no extra electrode needed at all - the statutory supply limits (216.2 to 253V) would be a good starting point - anything outside that disconnects the installation. A suitable delay (say just over 5s on undervoltage) would ensure that normal earth fault clearance times aren't compromised. As a side effect such an arrangement would also provide some protection from equipment damage and fire risks due to overvoltages in all installations (not just PME) from general broken N (not just CNE) problems. Better still, build the functionality into the new smart meters - as we're about to roll out millions of them - you'd get many more installations protected far more quickly than if you wait for existing installations to get an overhaul - plus the responsibility for providing a supply within spec. then remains with the supply industry, which sort of feels appropriate somehow.

Do we have to use the Adiabatic equation to size the earth electrode conductor rather than the tables?

No chance, as the disconnection time will be well over 5s (maybe several days over...). You could use the minimum electrode resistance to deduce the maximum possible current that could flow and look that up in the normal tables (although that might not help a lot). Applying the same logic to main bonding (if you still have metallic services, also bonded in other installations, the resistance could well be a fraction of an Ohm) you could well find that the normal table 54.8 looks significantly undersized too - as that never considered a broken CNE - just diverted N currents in normal service.

What's that Latin phrase about reducing to absurdity?

- Andy.
 30 July 2017 11:15 PM
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sparkingchip

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In a single phase TNCS installation if the PEN fails and the neutral voltage is raised along with that of the earth terminal along with conductive parts etc. the lights will dim and if the voltage rises enough they will go out.

So, if we permanently connect a green lamp between live and neutral if the neutral voltage rises along with that of the earth it will go out indicating there is a problem, just like some of the cheap extension leads on the market.

B&Q extension lead with power and earth indicators

Job done!

Andy B
 30 July 2017 11:28 PM
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sparkingchip

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It looks like the the earth electrode system conductor needs to be the same size as the tails, but 25 mm conductors may not fit into the DNO earth terminal. Though as a bonus, 25 mm bare buried conductors will improve the Ra.

Andy B.
 31 July 2017 11:13 AM
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Legh

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some sort of detecting device may need to be installed that cuts off the supply if dangerous Voltages appear on exposed conductive parts. Just what might that device be?


You could try This method. However, some other regulations would have to also change to accommodate switching an earthing conductor.....

Legh

-------------------------

http://www.leghrichardson.co.uk

de-avatared
 31 July 2017 11:40 AM
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sparkingchip

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Let's not get bogged down with the groundwork at the moment. There are two single phase intakes at the side of the building, each needs electrodes, so let's assume that up the alleyway at the side of the flats we will install a total of six 2.4 metre rods spaced at 3.0 metres in two systems separated by 3.0 metres, so a total system length of 15.0 metres connected with 25 mm buried bare copper cable.

We can detail those installations further when we know what we are doing around the front of the building where the shared Ryfield is.

So, in the main entrance hall cupboard there is the three phase Ryfield with the shared DNO earth terminal that have a maximum demand of 120 A / 120 A / 85 A across the three phases.

Let's take that as 20 Kw / 20 Kw / 14 Kw as those figures seem reasonable for he design purposes.

What earth electrode system Ra do we need to keep the Shared Ryfield DNO earth terminal down to a 70 V touch voltage if there is a failed PEN conductor?

Andy B
 31 July 2017 12:36 PM
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AJJewsbury

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Let's take that as 20 Kw / 20 Kw / 14 Kw as those figures seem reasonable for he design purposes.

What earth electrode system Ra do we need to keep the Shared Ryfield DNO earth terminal down to a 70 V touch voltage if there is a failed PEN conductor?

For 3-phase systems I think you need only worry about the neutral current - i.e. the worst case unbalanced situation - which is presumably 20kW - so treat the same as one 20kW single phase installation (I think that comes out at about 1.16 Ohms).

- Andy.
 31 July 2017 01:34 PM
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sparkingchip

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If the neutral and earth is at 70 volts, what happens to the voltage across connected single phase equipment, is it 160 volts?

I remember sitting in evening classes at the old Kidderminster College that has now been demolished with a geometry set drawing three phase vector diagrams, out of the window we could see quite some way away in the darkness a huge pyre withhe rbodies of cattle shot to prevent the spread of foot and mouth disease being burnt. Needless to say I remember the cattle being burnt better than how to do three phase vector diagrams and calculations.

Andy B.
 31 July 2017 01:45 PM
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sparkingchip

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 31 July 2017 03:01 PM
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sparkingchip

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If one winter night I got a phone call to say someone had a electric shock of one of the storage heater cases and due to a lost PEN, if I started turning the main switches off in the hallway to the flats and got to the point where the landlords supply to the lights in the communal area were connected on one phase (0.180 kw) and one of the other phases still had both flats it supplies connected (20 kw), what would the voltage be at the DNO earth terminal that we need to bring down to 70 volts?

Andy B.
IET » Wiring and the regulations » Application of the proposed regulation 542.1.201

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