Earth to Earth - when not to connect earths together.

[MikeSharp01]

The electric company (UKPN) are due to come and move our meter and install a new supply on Friday. I am installing a local earth rod - not quite sure why though, see image below. As things stand we have an overhead supply from a pole on our land. It is a TT supply to the old house, we have an earth rod providing the earth local to the old meter position. The other houses on the pole seem to have an earth provided by rather poorly connected earth rod at the base of the pole. When they come on Friday I think I am expecting them to provide a PME (TN-C-S) supply, just like our neighbours, which uses the earth rod at the base of our pole. Are they allowed to give new properties (this is a knock down and start again project) TT supplies? Given the regs I have decided to provide a commando socket to power up stuff outside (it is not designated as a site supply), although might be used on the site if it power cannot be derived from the garden room external sockets (which itself will be supplied by the distribution panel). The earth rods, ours and the UKPN one will be within 2m of one another.

 

My question / puzzlement / what am I missing is / are:

 

1. Given the proximity and cable run between the the two earth rods why should the two earths NOT be connected together in the distribution panel.

 

I did doodle a resistor network to try and get my head around this and am not sure I can see an issue - but it maybe just circumstances.

[Jeremy Harris]

DNOs won't provide a TN-C-S (PME) supply as a temporary site supply, they will only allow a TT supply.  I've no idea why, in our case it was barking mad, as before we flicked the isolator switch to connect power to the house and test everything, we moved the PE connection to the combined PE and neutral on the head, and pulled out the earth rod.  The 16A Commando socket that we used as the temporary site supply is still there as an outside power point, but the earth for it is now via the TN-C-S connection, rather then the TT arrangement that was in place until the house was wired.

 

In your case, there will be several intermediate earth to neutral connections along the supply line to your house.  By chance it sounds as if you have one at your nearest pole.  The network uses several intermediate earths to ensure that the neutral is close to earth potential and that the impedance to earth is kept low (usually lower than some TT earth systems, because often the intermediate earth may be buried earth mats, rather than slim rods).

 

 

[ProDave]

There is nothing wrong with you connecting your own earth rod to the main earth terminal to supliment a PME earth. After all, the ethos of a protective MULTIPLE earth system is there will be an earth rod, or pigtail at every connection point.  Indeed the draft of the 18th edition of the wiring regs looks like it will become a requirement to add a local earth rod to a PME install.

 

In my own case I have the supply head and meter remote from the house, and a local earth rod by the house to supliment it.

 

A static (indeed any) caravan should not be connected to a PME earth.

[Onoff]

If you have an existing TT supply and don't want to pay out for UKPN to upgrade the supply cabling coming in then they're OK with you sticking with TT. They can't force you to. 

 

https://www.ukpowernetworks.co.uk/internet/en/our-services/earthing/

 

I've apparently been "upgraded" to PME. This happened without my knowledge, undertaken by UKPN. We were on holiday and had a friend periodically popping round to check things. One day he heard a "cracking" like an air pistol. A tree branch had come down and part cut through the overhead. UKPN came in and replaced PART OF the cable from pole, where there was PME, to the house (always TT) leaving an unsightly, taped up "joint", if you can call it that under the soffit. Originally the cable was one length. As a leaving gesture they whacked a PME sticker on.

 

The one constant is my two 5/8" earth rods and they're staying!

 

I have wondered about this thing in the 18th. Locally we had a spate of thieving scroates cutting the intermediate earth straps off the poles along the road just as high as they could reach. A rod on your property at least leaves you with some reference to earth!

 

 

 

 

 

[Onoff]

If you have an existing TT supply and don't want to pay out for UKPN to upgrade the supply cabling coming in then they're OK with you sticking with TT. They can't force you to. 

 

https://www.ukpowernetworks.co.uk/internet/en/our-services/earthing/

 

I've apparently been "upgraded" to PME. This happned without my knowledge, undertaken by UKPN. We were on holiday and had a friend periodically popping round to check things. One day A tree branch had come down and part cut through the overhead. UKPN came in and replaced PART OF the cable from pole to the house leaving an unsightly joint, if you can call it that under the soffit. As a leaving gesture that whacked a PME sticker on.

 

 

 

 

 

 

[MikeSharp01]

Thanks chaps - Looks like all will be well. I have asked for a PME supply and the surveyor for UKPN was OK with that so I assume they will give me that. Roll on Friday!

[MikeSharp01]

8 hours ago, ProDave said:

static (indeed any) caravan should not be connected to a PME earth.

I guess this is the reason the earth is separated in the regs but is it the confusion over 'site' as in caravan 'site' or building 'site'. I have no caravan, it is all fixed buildings but it is a building site although one building - the garden room, is in place. For caravans, neatly insulated from ground by rubber tyres, I also guess the DNO don't want to be responsible for how far their earth can get above ground potential depending upon distance to a good earth in their system. Does that make sense?

[Jeremy Harris]

The daft thing is that it really makes very little difference which arrangement is used, as long as the PE has a low enough impedance to accept potential fault currents without raising the potential at the connected equipment earth to an unacceptably high level.

 

Some of the arguments for moving away from TT were that local earth rods and their connections were more liable to damage (have a look at some older installations where there will often be no protective box over the cable clamp and decades of old paint over the thing), plus some soil types can give a poor, or very variable, impedance.  Without measuring the local soil resistivity (which can be a bit of a palaver, I did it as an experiment before deciding to TT my workshop) there's no way of knowing how consistently good, or bad, a TT earth really is.

 

My personal view is that a good TT system, with a properly protected cable clamp at the top of the rod, and a properly protected earth cable run to the board, may well give a better level of protection than a PE that has been imported along a long length of cable.  Add in that intermediate earths on poles may well get damaged (or nicked) and having a decent earth connection locally makes a fair bit of sense.

 

Quite why the DNOs insist that a temporary supply, even one connected directly to an adjacent permanent incomer that has a combined neutral and PE, should not use that PE but have a local rod and be connected TT I don't know, but I do know that they were adamant about it - there is still a felt pen written warning inside our meter box about it, together with their measured Ze on the combined neutral and PE they fitted..................

Edited October 24, 2017 by JSHarris

[Dee J]

PME earth has the neutral and earth connected together... and earthed to rods at multiple points on the distribution network. Generally ok until there is a neutral cable fault and local load and fault currents pull local customer earths more than 50v from true earth. If you are in an equipotential zone (services and steelwork all bonded) then the danger is reduced. But outdoors, 'pme earthed' metalwork may present a shock hazard. Caravans are TT to protect people outside them. Site supplies are also TT to protect workers holding earthed tools whilst standing on mud.

Hope that helps.

[Jeremy Harris]

4 minutes ago, Dee J said:

PME earth has the neutral and earth connected together... and earthed to rods at multiple points on the distribution network. Generally ok until there is a neutral cable fault and local load and fault currents pull local customer earths more than 50v from true earth. If you are in an equipotential zone (services and steelwork all bonded) then the danger is reduced. But outdoors, 'pme earthed' metalwork may present a shock hazard. Caravans are TT to protect people outside them. Site supplies are also TT to protect workers holding earthed tools whilst standing on mud.

Hope that helps.

 

Fully understood, but when the impedance of a TT earth is allowed to be a higher than the impedance of a TN-C-S earth it does tend to throw that official argument for using TT for a site supply into question, doesn't it?

Edited October 24, 2017 by JSHarris

[Dee J]

1 minute ago, JSHarris said:

 

Fully understood, but when the impedance of a TT earth is allowed to be a higher than the impedance of a TN-C-S earth it does tend to throw that official argument for using TT for a site supply into question, doesn't it?

Not at all. The TN-C-S figure is misleading and is a function of the neutral connection. Lose the N further up the line and the results can be lethal... and no protective trip will operate. With a local TT earth, no influences external to the site can cause it to rise to dangerous potential. Local site faults should be quickly isolated by the RCD.

[Jeremy Harris]

As before, I full accept this, but fail to see how a remote PE/neutral fault is worse for a site connection than for a domestic installation.  All site equipment will be running on  transformers, anyway, lowering the local risk.  If there is a network fault that pulls the neutral up at the incomer then that will affect domestic installations every bit as badly.

 

My 16 A Commando is now connected to the combined PE and neutral, yet I am still using that as a supply in the garden, and it ran my car charger for a while before I installed the proper charge point.  Is it more dangerous now than it was when it was feeding site transformers?

Edited October 24, 2017 by JSHarris

[Dee J]

17 minutes ago, JSHarris said:

As before, I full accept this, but fail to see how a remote PE/neutral fault is worse for a site connection than for a domestic installation.  All site equipment will be running on  transformers, anyway, lowering the local risk.  If there is a network fault that pulls the neutral up at the incomer then that will affect domestic installations every bit as badly.

Inside a house, with all metalwork and services bonded, there is an equipotential zone. The cpc of your kettle and the bonded water tap have a guaranteed low impedance connection between them, holding fault potential less than 50v.

Outdoors no bonding applies. So the metal case of your welder could, under network fault conditions, be at lethal potential wrt the gatepost you're about to work on.

Ok its all belt and braces. Sites should use 110v balanced supplies, but the dno are providing a 230v supply and must make that as safe as possible.

Also note your 'safe' 110v transformer isolates you from incoming L &  N, but centre references the 110v  to the incoming earth. If the PME earth wanders off to dangerous levels, so does your 'safe' 110V.

HTH

Dee

[dpmiller]

"all site equipment will be running on transformers"

 

Umm, no, can't guarantee that.

[Jeremy Harris]

Simple illustration.

 

Fitted immediately under our meter box is our old site supply, a 16 A Commando. 

 

When it was used as a site supply the DNO insisted that it have a TT earth.  Now that the house power is on it has a TN-C-S earthing arrangement.  There was far more outside work done after the TN-C-S earth was connected and the  house switch was thrown for the first time than there ever was before the house went up.  At a guess, 90%+ of the use of mains connected outside electrical appliances were used on site after it was switched over to TN-C-S.

 

The 16 A Commando is still there, and I still use it.  Is it now far less safe to use than it was when it was connected with a local TT earth?

 

I can add a clue.  Like all these small site supply boxes it has a 16A 30 mA, DP RCBO.  An RCBO doesn't have anything to do with the earth arrangement, but trips within 30ms when it senses an imbalance of greater than 30 mA between the current in the line and neutral.  I contend that, no matter what the earth arrangement, the practical safety of that 16 A site supply was pretty much the same, and that the fact that the vast majority of the time it has been used has been after it was connected  as TN-C-S supports the view that insisting on TT for the short time before the house was wired and the house isolator switch thrown for the first time did not, and does not, make practical sense.

Edited October 24, 2017 by JSHarris

[ProDave]

The PME earth and caravan thing is to do with failure of the cable.

 

With a PME, it comes into the house, usually in a concentric cable with the outer being a combined neutral and earth.  It is not unknown for that CNE conductor to fail.  If it does and a caravan is connected, then the metal caravan skin (which should be bonded to earth) can raise to L potential.  This can be VERY bad news for someone entering or leaving the 'van with one foot on the ground and reaching up to grab the door handle.  A TT earth avoids that issue, and while Neutral might rise to L potential, the caravan skin will remain at local earth potential.  Same issue for site tools (if not 110V via a transformer)

 

This I believe is also the reason for the change to the regs to fit an additional rod, as usual we the electricians are having to take on the responsibilty of sorting someone elses shortcomings.

 

[Jeremy Harris]

I was peripherally involved in some of the consultations for the LV Directive and harmonisation, and what was interesting was how focussed the UK was on earthing, and how almost disinterested some other member states were about it.

 

From talking to some who were involved in the decision to adopt PME, which sort of morphed into TN-C-S (although they are not always the same) I gather the major concern at the time (it was before I'd even qualified, let alone was teaching) was that earth rods were often found to be damaged, the cables or clamps broken and that a lot of the time the real earth protection was being provided via earth bonded iron pipes, rather than the earth rod. 

 

There seems to have been a lot of resistance to the idea of PME, which I think stemmed from the shift of responsibility from providing the PE from the householder to the supply corporation (as it was at the time).   My view is that they would have been better off tackling the cause of earth rod/mat failures, as they have in the current regs for a TT connection, and specified protection for both the cable clamp to the rod and the PE cable coming into the house.

 

The other relatively recent change that questions the PME/TN-C-S model is that the incomer rarely comes into the house now, anyway.  External meter boxes have been the norm for decades and some of us here have gone one step further and fitted the box away from the house altogether.  That then creates the "caravan problem" for the house, in effect, although with far less conductive material around in a house the risk is  great deal lower.

 

I quite like the idea of having a local intermediate earth, for the same reason as I wasn't too keen on exporting the earth out to my workshop, so wired that as a local TT installation, with a very well protected earth rod.  It's reassuring to know that you have a fair degree of local protection should the local distribution network develop a CNE fault.  Having said that, so many appliances are double insulated now, and as RCD or RCBO protection is mandatory, the risk for users has reduced a great deal.

[MikeSharp01]

9 hours ago, JSHarris said:

I quite like the idea of having a local intermediate earth, for the same reason as I wasn't too keen on exporting the earth out to my workshop, so wired that as a local TT installation, with a very well protected earth rod

Makes perfect sense but I am now a little more confused than I was when I started this topic - drawing the threads together:

 

@ProDave said:

 

"There is nothing wrong with you connecting your own earth rod to the main earth terminal to supliment a PME earth"

 

Also the consensus appears to be that the local earth rod required for Caravans, metal cased plant etc is just to protect the 'local' users from a Neutral fault on the DNO network as it ensures that there is an earth connection beyond the DNO network in which the fault can occur.

 

@JSHarris said - in terms: that the the RCD that protects his Commando socket will trip if there is an in-balance in the L / N  beyond the device and I think he implied that this would happen even if the earth were not connected as it is one of the 'any arrangement's  and makes sense because the earth is not part of the in-balance.

 

So it seems to me that:

1. In drawing up the 'regs' diagram for a site supply the powers that be have erred on the side of safety by drawing a break between the incoming DNO earth and the site supply earth, connection via an earth rod, so as to ensure that an earth rod is provided and not ignored because 'the (PME) TN-C-S earth did the job'. If it looks like double bubble someone will cut back and use the TN-C-S earth thus rendering the thing dangerous.
2. Any earth rods you have can be connected together across the 'property'.

 

This means that: In our case we have the garden room, about 45m from the electric incomer and fed with a 3 core cable (16mm²). The Garden room also has a local earth rod which I will now be connecting to the earth cable exported down the garden - it was going to be isolated TT as Jeremy's workshop. For the GR the exported erath becomes the backup earth, protected from a rising voltage by the other local earth rod at the utilities centre end. Our earth rod local to the utilities centre, where the incomer is, will be connected to the earth connection provided by the DNO, assuming they supply one. That way I will have three earth connections any one of which should keep the place safe if the RCDs are out of the loop (pardon the pun).

 

 

[Jeremy Harris]

My workshop arrangement was done primarily because I have machine tools in there, a lathe, two milling machines, a bandsaw, mitre saw and pillar drill, plus a half-built CNC router.  This kit all has lots of exposed metal, so I was keen that it be as close to the potential of the floor as possible, in the event of a fault.

 

This is really belt and braces stuff, as the incomer to the workshop is a 40 A DP, 30 mA trip,  RCBO, so any imbalance in current between line and neutral, as might be created by earth leakage from a fault, will turn the power off, anyway.

 

It can be surprisingly difficult to get a low earth impedance with an earth rod, and it's a PITA to assess the true earth resistivity (there's a method here, which is the one I used:  http://www.wiley.com/legacy/wileychi/eca_wiringregulations/supp/Appendix_12.pdf ).  Our soil is pretty wet clay, which is about as good as it gets, but to get a really low impedance I ended up driving two 4ft rods into the clay under the workshop slab.  To make the installation neat and well-protected, the workshop earth rod goes down through the inside corner of the floor slab and is terminated inside a boxed in area, so is well protected from damage.

 

For some soil types a rod may not be OK, and you may need to bury a grid or mat.

Edited October 24, 2017 by JSHarris

[MikeSharp01]

8 minutes ago, JSHarris said:

For some soil types a rod may not be OK, and you may need to bury a grid or mat.

Yes - luckily we are on clay,  I will drive the rods tomorrow in readiness for Friday.

[Onoff]

DON'T DO THIS AT HOME, IT'S REALLY DANGEROUS...

 

Take a small wattage 240V lamp and stick it in a batten holder. Connect one side to your earth rod banged in just a little and the other side to live. The lamp should glow brighter the better the earth as in the deeper you bang the rod in...

 

Best to switch the power off before banging deeper...

 

[dpmiller]

nah, keep it connected and keep banging it in until the trip goes

 

..but best not hold onto it to start with...

[SteamyTea]

3 hours ago, Onoff said:

DON'T DO THIS AT HOME, IT'S REALLY DANGEROUS...

My Father was an electrical engineer and used that method to check the earth.

Glad to see some thing have not changed in 70+ years

 

I have a feeling that I still have the bulb, holder and cable from when we wired my place up in Weymouth back in 91 (only seems like yesterday).

Edited October 24, 2017 by SteamyTea

[Onoff]

6 hours ago, SteamyTea said:

My Father was an electrical engineer and used that method to check the earth.

Glad to see some thing have not changed in 70+ years

 

I have a feeling that I still have the bulb, holder and cable from when we wired my place up in Weymouth back in 91 (only seems like yesterday).

 

To be honest the idea is "grounded" () in sound, basic science. Nothing more visual than to see more current flowing to earth as the resistance to that current lessens as more surface area of the rod comes into contact with "earth'.

[Jeremy Harris]

1 hour ago, Onoff said:

 

To be honest the idea is "grounded" () in sound, basic science. Nothing more visual than to see more current flowing to earth as the resistance to that current lessens as more surface area of the rod comes into contact with "earth'.

 

And it's easier and more practical to do, too, especially with a nice high wattage bulb so you get a decent earth current.  The snag is that you don't have an exact number to enter into some form to show that the impedance is low enough, not that knowing the exact impedance makes it any safer, it just keeps the paperwork police happy.............

 

Edited October 25, 2017 by JSHarris