Armoured Cable Query

[bernardgreen]

If the garage has a TT installation then the armour is an ecp, and maybe it should most definitely be connected to the garage earth terminal.

NO because if it was then the house "earth" would be connected to earth of the TT system.

The armour has the be connected to the MET at the end where the power is coming from. In this case the house.

IF the garage has a TT set up with earth derived from a ground rod then the armour of the supply cable has to be insulated and not be able to come into contact with anything in the garage.

 

[ban-all-sheds]

NO because if it was then the house "earth" would be connected to earth of the TT system.

No, the extraneous conductive part introducing a potential to the garage would be bonded to the garage earth.

IF the garage has a TT set up with earth derived from a ground rod then the armour of the supply cable has to be insulated and not be able to come into contact with anything in the garage.

What if there was a metal water pipe T'd off the house water supply serving the garage? Would that be insulated and not be able to come into contact with anything in the garage?

 

[bernardgreen]

What if there was a metal water pipe T'd off the house water supply serving the garage? Would that be insulated and not be able to come into contact with anything in the garage?

My cottage is semi-detached to a retail unit. Previously they were both owned by the shopkeeper who lived in the cottage and one electricity meter in the retail unit fed both shop and cottage. It was and still is a TT supply. The water service was also common. I had a new electricy supply installed. TN-C-S. It was made very clear to me that nothing in my cottage that was connected to my MET ( earthed or bonded ) should be able to ( even accidently ) make contact with the CPC and CPC earthed items in the retail unit. This required some care as one of my bedrooms is above the retail unit. The water supply to the retail unit is from my meter and there has to be a metre of plastic pipe to pevent my MET bonded pipes making electrical contact with the TT earthed pipes in the retail unit.

The bottom line from the DNO engineers and project manager who arranged my new supply was ""TT earths and PME earths must not be joined.""

 

[ban-all-sheds]

So what, electrically, is the difference between an earth rod and a length of pipe emerging from the ground?

 

[ericmark]

So what, electrically, is the difference between an earth rod and a length of pipe emerging from the ground?

One is an earth electrode the other is an extraneous-conductive-part, there was some apprentices who said you can't have an earth electrode with a TN-C-S supply which is correct, it's not called an earth electrode it's called an extraneous-conductive-part but it is still a copper coated steel rod in the ground only the name changes.

The garage would become TN-C-S if the earth is connected through the SWA of the cable even if the earth rod is also still connected. Earth is always connected on supply side so with TT one would not connect the SWA at garage side and something requiring a tool to remove it should be fitted to the cable so the SWA can't be touched. Simple method is a plastic stuffing gland.

 

[bernardgreen]

So what, electrically, is the difference between an earth rod and a length of pipe emerging from the ground?

A ground rod connects only to the ground. A length of pipe could be metallically connected to other electrical systems.

A ground rod can only provide a reference voltage that is true ground potential. A length of pipe could be at any potential depending on the condition of the other electrical systems that it may have connections to.

If you look at the history, methods and development of earth leakage detection you will get a better idea of the functional difference between the two.

 

[JohnW2]

So what, electrically, is the difference between an earth rod and a length of pipe emerging from the ground?

A ground rod connects only to the ground. A length of pipe could be metallically connected to other electrical systems. A ground rod can only provide a reference voltage that is true ground potential. A length of pipe could be at any potential depending on the condition of the other electrical systems that it may have connections to.

All technically true, but it does alter the fact that we are obliged to connect (bond) the water pipe to the installation's earth system - whether that be TN-C-S, TN-S or TT.

As for the adage that "one must not connect an earth electrode to a TN-C-S earth", for the reasons you explain that presumably would represent a lesser potential problem than the (obligatory) bonding of a water pipe (at 'unknown potential') to a TN-C-S earth, wouldn't it?

Kind Regards, John

 

[bernardgreen]

Connecting a water supply pipe to the MET where the "earth" is derived from the Neutral removed the potential problem of extraneous voltages getting into the equipotential zone of the building and the subsequent risk of electric shock. Where the water supply pipe is not low impedance ( ie plastic ) then the MET and the bond cable can ensure the water pipe is at the same potential as all earthed items inside the building not matter what potential the incoming neutral may be..

The difficulty arises when the water pipe ( or other supply ) is metallic and low impedance to ground. The bond cable then has to cope with signifcant currents which can be in the order of amps between MET ( incoming Neutral ) and the water pipe. There have been instances where the bond cable has become warm or even hot when it is a metallic water supply system and the supply Neutral has gone to a potential a few volts above ground. In at least one extreme case when a Neutral was disconnected somewhere in the network ( metal theft ) the bond wires in four adjacent properties became hot enough to ignite fires.

Hence where there is a low impedance route to ground via a service pipe or structural metal work bonding these to the MET of a TN-C-S or a TN-S supply can present a hazard when there is a fault in the supply network. It seems sensible in these circumstances to install a TT supply using the low impedance routes to ground paralleled with a ground rod to provide the system Earth. While the ground rod may seem surplus one has to consider that at some time in the future the pipe work / structure may be altered and the Earth via them removed.

 

[ban-all-sheds]

So basically you don't like anything other than TT supplies.

 

[JohnW2]

The difficulty arises when ....

Yes, we know what the potential (exttremely rare) problem is. However, as I said,that does not help to explain why a lot of people seem to believe that it is hazardous/disallowed to connect a (relatively high impedance to earth) earth electrode to the 'earth' of a TN-C-S installation.

Kind Regards, John

 

[bernardgreen]

However, as I said,that does not help to explain why a lot of people seem to believe that it is hazardous/disallowed to connect a (relatively high impedance to earth) earth electrode to the 'earth' of a TN-C-S installation.

People believe it because it is implied as being hazardous in the regulations. Why are bond wires to "high impedance" ( >200 ohm ) ground rods and pipes specified as needing to be at least 4 mm²

The regulations are written as a best fit to cover all situations without the need for the installers to have to think. This removal of the need for the installers to think is sensible since some ( maybe too many ) installers who also "design" the installation cannot think and can only blindly follow the instructions.

Earth from a TN-C-S or TN-S can connect to a ground rod if the impedance of the ground rod is such that the current into the ground due to any Neutral ( CPC ) potential can never be a current high enough to be a hazard. What is the current that would melt a 4 mm² copper wire ? , Why is the bond wire to a pipe coming out of the ground specified as 4 or even 10 mm² ? What current is the regulation considering as possible ? If the impedance of the ground rod or pipe is 230 ohms then the maximum current would be 1 amp. So why not use 2.5 mm² ? If the impedance of the pipe or ground rod is 0.1 ohm ( metallic water supply for example ) then the current due to the Neutral rising to 10 volts above ground potential would be 100 amps. Not good on 2.5 mm²

The regulation have to try and ensure all installations designed by non thinking cable installers will be safe. Hence they have to consider the possibility of the 0.1 ohm metallic connection to ground and reduce the risk should a cable installer connect a Neutral derived "Earth" to that 0.1 ohm metallic connection to ground.

( Neutral----MET----bond wire -----metal in ground or Neutral----MET----CPC -----boiler---pipe work----metal in ground )

It would confuse the non thinking cable installers if the regulations related the required size of bond wire to the known range of impedance that could be found in metal items in the ground ( pipes or ground rods ) and allowed the cable installer to calculate the maximum possible current and thus select an apprporiate size of bond wire. The range of impedance is wide. From less than 0.1 ohm ( metallic water mains, structural steel etc ) to several million ohms ( short ground rod in dry sandy soil ). The calculation would also have to consider a MET potential that ( extreme worst domestic case ) could be as high as 230 volts above ground.

Too many cable installers call themselves electricians when they have little if any ablity or knowledge to be able to think and calculate sensible cable sizes and protection methods to be used in the design of an installation.

 

[JohnW2]

However, as I said,that does not help to explain why a lot of people seem to believe that it is hazardous/disallowed to connect a (relatively high impedance to earth) earth electrode to the 'earth' of a TN-C-S installation.

People believe it because it is implied as being hazardous in the regulations.

Where do the regs imply that? If the authors believed it to be hazardous, they would surely say so, and forbid it?

Why are bond wires to "high impedance" ( >200 ohm ) ground rods and pipes specified as needing to be at least 4 mm² ... What is the current that would melt a 4 mm² copper wire ? , Why is the bond wire to a pipe coming out of the ground specified as 4 or even 10 mm² ? What current is the regulation considering as possible ? If the impedance of the ground rod or pipe is 230 ohms then the maximum current would be 1 amp. So why not use 2.5 mm² ?

Is that perhaps the 'implying" that you're talking about? If so, I think it's a red herring, probably there because of considerations of the mechanical integrity ('damagability') of the conductor, not its CCC - any single protective conductor which does not have 'mechanical protection' is required to have a CSA of at least 4mm², even if it's just providing a CPC to, say, an old, 'cpc-less', lighting circuit.

No real-world (non-industrial!) earth electrode is ever going to have a low enough impedance to represent a threat to a 1mm², let alone 4mm² or larger conductor - so I don't understand why anyone (who is able to think) can believe that any harm would/could be done by connecting it to a TN-C-S earth!

Kind Regards, John

 

[ban-all-sheds]

I had a new electricy supply installed. TN-C-S.

You were not obliged to use it. Just because they make their earth terminal available to you does not mean that you are not free to make your own arrangements if you so choose.

 

[PBC_1966]

However, as I said,that does not help to explain why a lot of people seem to believe that it is hazardous/disallowed to connect a (relatively high impedance to earth) earth electrode to the 'earth' of a TN-C-S installation.

I had an argument with a (supposedly) "fully qualified electrician" or another forum some years ago about this. He was absolutely insistent that it was dangerous to add a local earth rod to a TN-C-S system, but seemed incapable of explaining why he thought so, or how he thought it was going to pose any more of a danger (current in conductor with neutral fault etc.) than bonding water pipes and other underground services which would also form very effective earth electrodes.

He seemed to be completely stuck on the regulation stating that water pipes may not be employed as the required earth electrode for an installation (TT), but nothing I said could elicit an explanation as to how he thought that regulation was going to change the laws of physics and prevent multiple tens of feet of buried metallic piping from behaving as an effective electrode nonetheless (and far more effective ones than the typical 4 ft. earth rod).

He also seemed to think that adding the electrode changed the installation to TT (even though still firmly bonded to the supply neutral), and eventually, as I recall, came up with some final excuse about danger arising because a DNO worker might see the electrode, assume the house had been converted to TT and remove the neutral bond. (Well, any installation can become dangerous if somebody goes around disconnecting earthing/bonding conductors without checking what's going on first.)

I think he must have been a "qualified BS7671 cable installer" rather than a real electrician. And let's not forget that at one time the old IEE Wiring Regs. did actually suggest a local earth rod as an extra safeguard with PME.

 

[JohnW2]

I had an argument with a (supposedly) "fully qualified electrician" or another forum some years ago about this. He was absolutely insistent that it was dangerous to add a local earth rod to a TN-C-S system, but seemed incapable of explaining why he thought so, or how he thought it was going to pose any more of a danger (current in conductor with neutral fault etc.) than bonding water pipes and other underground services which would also form very effective earth electrodes.

Indeed - the trade seems to have more than its fair share of these 'urban myths', but they can only exist and persist because of people who are incapable of thinking and/or have no understanding of basic electrical principles.

Kind Regards, John