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Discuss The hoary old topic of PME, TN-C-S and earthing rods (not TT) in the Electrical Forum area at ElectriciansForums.net

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Somewhere along the line I picked up a suggestion from a forum posting or two that the DNO does not like you to connect an earthing rod to his "earth" - even though it is effectively just creating yet another, local to your installation, earthing point for the multiple ones already assumed to exist as a part of his PME installation.

But why?

What possible issue could it cause?

On the contrary, surely it is safer as it ensures that a person adjacent to the local installation will at least be offered a measure of protection against the dreaded, much discussed but in reality extremely rare "broken neutral" problem? Granted, if enough fault current is flowing in from outside of your location as a result of the JCB mediated cable breach in the street it could potentially (sorry!), depending upon the local ground conditions, pull the voltage on your local earth up, but even so it is surely still safer than not having a local earth reference at all?

The topic tends to come up in the context of "exporting" a PME earth along with discussion about the pros and cons of connecting outbuildings using a TT configuration and the point is generally and validly made when it does that the user of the installation must be protected from touching both the earth coming in on the run of SWA down the garden and the local earth provided by the earth spike as there is the potential (sorry again!) for the unconnected "earths" to be at different voltages.

But the scenario I am seeking to explore (purely for the purposes of further enlightenment, no actual or proposed project is involved here) is emphatically not a TT installation because the local earth rod would be connected to the earth coming in from the street.

By now I expect you are mostly wondering what possible reason one might have for wanting to construct such a "bizarre" system. Well, it is admittedly a bit of a minority sport, but one reason is that, if you are a radio amateur (and we are not talking the 2 metre band or handheld VHF transceivers here) it is very common to want to install a good solid local (& low impedance at frequencies > 50Hz) earth for your transmitter. A chap I know is such a person (weird I know, but somebody has to be :) ) and he has just had installed an EV charge point for his Smart car electric runabout. We got to talking about earthing and safety in the context of a broken neutral what with a tin car on a charger being very much the same scenario as a caravan and, as we all know, anything but TT on a caravan is verboten. He then offered up the information that it was alright because he has an earth spike for his transmitter which is also connected to the incoming earth from the DNO in the house.

That's the context, now let's have the opinions, personal abuse for my obvious misunderstandings, seasoned informed advice, and, of course not forgetting the usual off-topic banter which we all know and love so much.:)
 
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davesparks

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As far as I know there is nothing wrong with connecting earth electrodes to the MET of an installation with a Tncs supply, I've done it a couple of times. This is a requirement in many other countries and we are lagging behind them in our standards.

Any installation in a building with lightning protection fitted will have earth rods with less than 10ohms Ra connected to the MET (assuming main bonding is connected)
 

Ian1981

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I suspect that we haven’t heard the last of the requirements for installing earth electrodes to supplement the DNO provided TN earthing arrangement as was in the original 18th edition DPC and I’m sure the notion of having to install one is being tweaked as the 1st amendment looms.
 
N

Nigel

Not sure where you got that from. The DNO would love to have us installing earth rods to support their failing network. In fact it was almost brought in under the 18th to do exactly that. Expect to see this within the next Amendment or two.
 

happysteve

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I can't recall anywhere in the Regs that prohibits this.

I guess the main point is that it won't help with the issues of losing the PEN conductor (or possible touch voltages under fault-free conditions) unless you achieve a really low Ra. Annex A722 of the Regs (page 319 of the BBB) goes into the maths - here related to achieving a maximum voltage of 70V between the MET and Earth in the event of a broken PEN conductor. It depends on what your maximum demand is, but as an example, if your maximum demand is 60A, then the maximum Ra would be 1.68Ω.

Good luck :)

If anyone complains that you've attached an extra earth rod to a PME supply, just say you've bonded this extraneous-conductive part, which happens to look like an earth rod. ;)
 

Strima

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All metallic street cabinets are required to have a supplemental rod on TN systems, if the resistance is above a certain measurement based on max demand then it must be converted to TT.
 
D

Deleted member 26818

All metallic street cabinets are required to have a supplemental rod on TN systems, if the resistance is above a certain measurement based on max demand then it must be converted to TT.
Yep, less than 10 Ohms from what I recall.
Though one REC we dealt with didn’t care about the RA, just required an electrode.
 
Not sure where you got that from. The DNO would love to have us installing earth rods to support their failing network. In fact it was almost brought in under the 18th to do exactly that. Expect to see this within the next Amendment or two.
 
The last thing the DNOs want is consumers banging stakes down with low resistance externally creating problems for their neutral. There are two laws, 1 the DNO network is installed at a certain resistance. 2. The consumers installation requires equipotential bonding so no dangerous voltages exist. All metalwork on the consumers side of meters, plastic inserts must be bonded with 10mmsq leads. The pseudo regulations, correctly called BS7671 requirements, have no bearing on ESQCR regulations.
 

Ian1981

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I can't recall anywhere in the Regs that prohibits this.

I guess the main point is that it won't help with the issues of losing the PEN conductor (or possible touch voltages under fault-free conditions) unless you achieve a really low Ra. Annex A722 of the Regs (page 319 of the BBB) goes into the maths - here related to achieving a maximum voltage of 70V between the MET and Earth in the event of a broken PEN conductor. It depends on what your maximum demand is, but as an example, if your maximum demand is 60A, then the maximum Ra would be 1.68Ω.

Good luck :)

If anyone complains that you've attached an extra earth rod to a PME supply, just say you've bonded this extraneous-conductive part, which happens to look like an earth rod. ;)
That’s for car charges but I can see a similarity with what maybe required
 
D

Deleted member 26818

LEB wanted 10 Ohms or below.
Eastern or Southeastern (can’t remember which) just wanted an electrode.
So you could have a road where one side had to have 10 Ohms, and the other side didn’t matter.

I remember chatting with a director of a company installing cable TV cabinets.
They spent thousands lifting those brick paving things and laying down earth tape trying to get below 10 Ohms in a shopping precinct.
He was not a happy bunny when I asked why he hadn’t just TTd it.
 

davesparks

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The last thing the DNOs want is consumers banging stakes down with low resistance externally creating problems for their neutral. There are two laws, 1 the DNO network is installed at a certain resistance. 2. The consumers installation requires equipotential bonding so no dangerous voltages exist. All metalwork on the consumers side of meters, plastic inserts must be bonded with 10mmsq leads. The pseudo regulations, correctly called BS7671 requirements, have no bearing on ESQCR regulations.
There's a lot wrong with this, the bonding will often have exactly the same effect as the earth electrodes you say are not allowed. Especially in the case of lightning protection systems and structural steelwork.

Where do you get the idea that all bonding must be 10mm?
 
D

Deleted member 26818

10mm2 minimum where PME conditions apply.
That’s pretty much the whole of the public network.
 
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