Earthing Spike

[bernardgreen]

After a bit more thought.

An earth rod would not make a PME system safe if the incoming Neutral went open circuit. In fact it seems almost impossible to ensure there was no hazard when the incoming Neutral goes open circuit. The type and level of hazard depends on where the Neutral goes open circuit.

Having the CPC at Live potential is not a problem for people inside the equipotential zone (*) but presents a hazard to people outside the zone who come into contact with anything that is bonded to the CPC ( via the MET and main bonds ) Gas Meter, external water taps, external lighting with earthed metal work and electric gates made of metal to name but a few.

(*) since walls cannot be bonded to the MET any wall that is damp and touching the ground would provide the second point of contact for anyone holding an earthed item with exposed it's metal parts at 230 volts above Ground. Probably ( but by no means certain ) the impedance of the wall will be high enough to limit the shock current such that the shock is felt as a severe tingle.

Some one said that cases of the incoming Neutral going open circuit are very rare. Hoe rare are they ? Considering the hazards they create and the damage they have caused ther has to be some information / statistics about them. Metal theft of the Neutral links at sub stations and over head routes have not been that un-common

 

[Risteard]

I know TN-C-S is the norm in the R.o.I. Is the electrode also standard there?

It is a requirement of ET101. TT is known as Direct earthing and TN-C-S (by far the most common - almost universal) a Neutralised supply. Both require an electrode. The cable between the MET and the ESB's PEN conductor is known as the Main Protective Conductor or Neutralising Conductor and the cable between the MET and the earth electrode is called the Earthing Conductor.

 

[JohnW2]

After a bit more thought. ... An earth rod would not make a PME system safe if the incoming Neutral went open circuit. ... Having the CPC at Live potential is not a problem for people inside the equipotential zone.

Indeed - I made both those points in my previous post.

In fact it seems almost impossible to ensure there was no hazard when the incoming Neutral goes open circuit. The type and level of hazard depends on where the Neutral goes open circuit.

A common feature of all supply-side neutral faults in a TN-C-S system is that the neutral potential relative to true earth will rise to an abnormal level, to location of the fault determining how high a current may try to flow from the incoming TN-C-S 'earth' (neutral) to bonded low-impedance paths to true earths.

AFAICS, it is only really regulations which make it "almost impossible to ensure there was no hazard when the incoming Neutral goes open circuit". If one had a local earth electrode and had not connected it to the MET/CPCs, then it would be easy enough to detect an abnormal pd between it and the incoming CNE, and use that to trigger a 'disconnector' - but regs would not allow the 'earth'/CPCs (only L and N) to be disconnected by such a device.

Having the CPC at Live potential is not a problem for people inside the equipotential zone (*) but presents a hazard to people outside the zone who come into contact with anything that is bonded to the CPC ( via the MET and main bonds ) ....

Indeed - and if one connects an earth electrode to a TN-C-S system, there would be additional potential hazards for those standing on the ground in the vicinity of the electrode.

Kind Regards, John

 

[bernardgreen]

it would be easy enough to detect an abnormal pd between it and the incoming CNE, and use that to trigger a 'disconnector' - but regs would not allow the 'earth'/CPCs (only L and N) to be disconnected by such a device

Total and automatic disconnectors were used at one time to protect equipment from damage when the potential difference between the local ground mat and the incoming Earth ( the Neutral ) was high causing high currents through Earth / Bond cables. Admittedly these were sites with very low impedance Ground mat systems, in the order of one Ohm or less. Being communication sites they had cable connections to other sites and a failed Neutral had resulted in mains voltages being exported on the screens / armouring of cable to other sites.

Then there was a major re-think and supply arrangements were changed and a version of TT was adopted for the earthing of equipment. PME and similar non Ground eaarthing was banned. Bought these sites in line with petrol stations but for different hazards

 

[PBC_1966]

It is a requirement of ET101. TT is known as Direct earthing and TN-C-S (by far the most common - almost universal) a Neutralised supply. Both require an electrode. The cable between the MET and the ESB's PEN conductor is known as the Main Protective Conductor or Neutralising Conductor and the cable between the MET and the earth electrode is called the Earthing Conductor.

Thanks. I know that whole "neutralized" supply terminology threw me a bit the first time I saw it used over there (to me, neutralizing is something you might do to an RF amplifier stage).

 

[JohnW2]

... to me, neutralizing is something you might do to an RF amplifier stage.

Indeed - or, if you were not a very nice sort of person, what you might do to you 'enemies' of people who disagreed with you (and, of course, if yo were a Chemist ....!!)

Kind Regards, John

 

[ericmark]

To use an earth electrode and have a TN-C-S supply you need something to limit the current to an acceptable level. When I worked in point of Aye gas terminal there was a resistor the size of a car to limit the current on the earth connection between the DNO and the site earth. This was next to an earth pit with 4 rods one in each corner of 8Ω each and copper tape joining the rods not measured after back filled but I would think likely a fraction of an ohm.

The the domestic supply we hope all houses have a similar set-up so if one house has a 6mm² cable connecting an extraneous-conductive-part like a gas pipe or water pipe they all will have a 6mm² cable connecting to the same extraneous-conductive-part so the current in any one cable will be limited.

However in extreme cases like the one I was involved in where a radio ham had built an earth pit around his garden to help with his transmitting and had linked this rather good earth with the house earth with a 4mm² cable when the DNO cable was hit by a road worker disconnecting his neutral/earth his cable started to heat up and even turning off all of his power did not stop the cable heating up and by the time I arrived it was a load of copper balls on the ground.

As to what size cable would be required to stop it melting I don't know. It should have been at least 10 mm² between the DNO and his extraneous-conductive-part but even that may not have taken the load it is an open question. After the even the whole house was put as TT. In theroy with a PME supply as the M indicates there should be multiply earth rods which would have shared the load. As to if a TN-C-S requires multi earth rods I am not sure?

If the rules say every house must have an earth rod that's OK as any fault will be shared between the houses, but if the rules don't require it then safer not to have one.

 

[JohnW2]

To use an earth electrode and have a TN-C-S supply you need something to limit the current to an acceptable level.

In terms of the average (or even 'exceptional') domestic earth electrode, the electrode itself should be more than adequate to 'limit the current'. A 'typical good' domestic electrode of ~50Ω would allow less than 5A to flow, even if fully supply voltage were applied to it.

Kind Regards, John

 

[ericmark]

To use an earth electrode and have a TN-C-S supply you need something to limit the current to an acceptable level.

In terms of the average (or even 'exceptional') domestic earth electrode, the electrode itself should be more than adequate to 'limit the current'. A 'typical good' domestic electrode of ~50Ω would allow less than 5A to flow, even if fully supply voltage were applied to it.

Kind Regards, John

Which is what we rely on. Of cause you can't have an earth electrode with a TN-C-S supply, you may have a rod in the ground, but it's not called an earth electrode it is called an extraneous-conductive-part even if it looks and connects just like an earth electrode. Trick question for the apprentice. It changes name.

The point is there is nothing in the regulations to limit the connection to earth. I have seen where a concrete mat has been laid and the reinforcement has been connected to earth, I think it was really for lighting conductors to connect to but it was connected to earth.

 

[bernardgreen]

If the rules say every house must have an earth rod that's OK as any fault will be shared between the houses, but if the rules don't require it then safer not to have one

Take that one step further into the confusion....... Some houses have an " earth rod " in the form of a metallic service pipe that has to be bonded ( but not earthed ) to the Main Earth Terminal. How does one square this if the rules say no earth rods with a TN-C-S supply. We can comply wih the rules by calling the service pipe "an extraneous conductive part " but naming calling will not prevent a massive current flowing through the bond wire into the metallica service pipe when the MET goes high voltage when teh Neutral goes open circuit.



Today there are ( probably ) not many water supplies that are all metallic but there are still a few and these are very low impedance, as low as 0.1 ohm and normal fluctuations of Neutral potential above Ground of a couple of volts will results in 10's of amps flowing in the bond wire .... Hence the requirment to bond service pipes to the MET with 10mm² cable just in case the service pipe is a metallic pipe with a low impedance. It could be a continuous metallic path to the grounding system at the substation. In days gone by sub-stations would very often use a nearby metal water main as part of a very low impedance Earth. ( or was the water pipe considered "extraneous conductive item" and only bonded ). Houses often used the water pipe as the main earth for the electrical system and it had to be able to carry enough fault current to blow a fuse if there was a Live to Earth fault while keeping the potential on the earth wires ( the CPC ) and any Earthed items below a safe limit ( normally 50 volts ).

There have been several instances where "power surges" have started fires in meter boxes. In many cases the source of the fire has been the bond / earth wire overheating and burning its insulation.

 

[JohnW2]

Of cause you can't have an earth electrode with a TN-C-S supply, you may have a rod in the ground, but it's not called an earth electrode it is called an extraneous-conductive-part even if it looks and connects just like an earth electrode.

Hmmm. Certainly not as far as I am concerned. Just because something constitutes an extraneous-c-p as far as an electrical installation is concerned doesn't stop it "being what it is". Even if they qualify as extraneous-c-ps, water pipes are still water pipes, gas pipes are still gas pipes and structural steel is still structural steel - and that is what I, for one, continue to call them ... and the same for earth electrodes!!

As for whether one considers the cable connected to it to be an 'earthing conductor' or a 'bonding conductor' really depends upon how one chooses to look at it, and maybe on 'which came first'. If the earth electrode and its earthing conductor had been in place for decades and then "along came TN-C-S", one could argue that it was the incoming CNE of the TN-C-S supply that was the 'extraneous-c-p', in which case it would be connected to the MET by a 'bonding conductor'!!

Kind Regards, John

 

[JohnW2]

How does one square this if the rules say no earth rods with a TN-C-S supply.

Despite what a lot of people seem to think, I'm not convinced that there ever has been such a rule, anywhere.

Kind Regards, John

 

[PBC_1966]

We can comply wih the rules by calling the service pipe "an extraneous conductive part " but naming calling will not prevent a massive current flowing through the bond wire into the metallica service pipe when the MET goes high voltage when teh Neutral goes open circuit.

Some years ago on another forum I had a big argument with somebody who was absolutely insistent that connecting a TN-C-S system to a local earth rod was not permitted and highly dangerous. Nothing could get him to see that having a bonded water line connected to a TN-C-S arrangement - which he willingly accepted was necessary and perfectly safe - was effectively no different and, in fact, even if metallically isolated from the mains supply pipe where connected still a far more effective earth electrode than a mere 4 ft. rod driven into the ground.

"But BS7671 doesn't allow water pipes to be used as an earth electrode," seemed to be his only response, as though he thought that was in some way going to change the laws of physics.

 

[PBC_1966]

Just because something constitutes an extraneous-c-p as far as an electrical installation is concerned doesn't stop it "being what it is". Even if they qualify as extraneous-c-ps, water pipes are still water pipes, gas pipes are still gas pipes and structural steel is still structural steel - and that is what I, for one, continue to call them ... and the same for earth electrodes!!

Indeed. If you drive an electrode into the ground with the intention of using it as a connection to earth, then as far as I'm concerned it's an earth electrode.

Despite what a lot of people seem to think, I'm not convinced that there ever has been such a rule, anywhere.

At one time, the IEE Wiring Regs. actually recommended the use of a local earth electrode.

 

[ericmark]

We can comply wih the rules by calling the service pipe "an extraneous conductive part " but naming calling will not prevent a massive current flowing through the bond wire into the metallica service pipe when the MET goes high voltage when teh Neutral goes open circuit.

Some years ago on another forum I had a big argument with somebody who was absolutely insistent that connecting a TN-C-S system to a local earth rod was not permitted and highly dangerous. Nothing could get him to see that having a bonded water line connected to a TN-C-S arrangement - which he willingly accepted was necessary and perfectly safe - was effectively no different and, in fact, even if metallically isolated from the mains supply pipe where connected still a far more effective earth electrode than a mere 4 ft. rod driven into the ground.

"But BS7671 doesn't allow water pipes to be used as an earth electrode," seemed to be his only response, as though he thought that was in some way going to change the laws of physics.

You it seems have also found the apprentice who could not separate English and Electrics yes of course it is an earth rod you could hardly go to an electrical whole sale outlet and ask for an extraneous-conductive-part. And I agree that the whole idea of using two different names earthing and bonding is really not required.

However I did not write the rule book and that book uses two names. That's why on a 13A plug it is clearly marked LNE for line, Neutral or Earth Oh but hang on maybe that should say LNB for Live, Neutral and bonding as what is important is that the appliance is the same voltage as the surroundings what ever that may be if the Line, Neutral and what ever we are going to call it are more than 50 volts away from the voltage of the naturally voltage of the surroundings we have a problem.

We don't even need an electrical supply I have seen huge sparks from a tower crane hook caused simply by the wind.

So a number of points.
1) It does not matter what voltage metal parts are within a house as long as they are the same voltage so in the house we only require bonding.
2) The problem with TN-C-S is three fold when the neutral/earth is lost.
a) The voltage can rise and fall 0 ~ 400 volt. In theroy electrical items should fail safe?
b) The voltage of metal parts within the house is different to the voltage found outside the house.
c) Only if items are also connected to a true earth is there a current capacity problem. In theroy a resistance should limit the current.

The really big problem is moving from the house, be it a whole house or a tram if it goes off the rails you have to jump off so you don't get a shock. In real terms a house is not that easy as one is likely to have a voltage gradient.

However if all houses from the same supply all have approx the same connection to ground then no one house will have earth wires burning. It is only when one house has a greater connection to ground that there is a problem. So if all but one house in an estate have plastic pipes and one odd one say the original farm house that was there before the estate was built has metal pipes then that house will have problems when the neutral/earth is lost.

However in 45 years I am aware of only one time that happened, and no one was injured although a lot of electrical equipment was damaged so if I do a risk assessment the risk has to be very low.

So do you think are the plugs with E marked on the should have it changed to B?