Exported earth.

[Evans Electric]

Right here goes , I see yet another reference to exporting the earth from a PME supply ( TNCS) to a remote building etc. My question is , can someone tell me which Regulation covers this . I have to admit that prior to becoming a Forumite I had not come across this, and still cannot see why it is a problem.

PME means Permament Mutiple Earthing , the neutral is spiked to earth along it's run and seperated where it enters a building, so its a reliable connection to earth. TNCS is the new name but nothing has changed, the earth and neutral are combined in the supply and seperated on entering the building.

A sub supply to a remote garage or shed , say, would be classed as TNS, or should I be donning Spec's big pants on my head.

 

[Andy™]

it means protective multiple earthing.

its under ESQCR that it cannot be supplied to certain buildings.

problem is, in a house which can be fairly controlled, any voltage on the neutral shouldnt cause a shock because everything should be at the same voltage, which may well be above ground voltage. exporting it can cause problems, since there may be voltage between something metallic and ground. problem still exists in a house, but can be worse on an outbuilding.

and depending on fault with PME, you can have all metalwork live at 230V to ground, and your RCD is guaranteed NOT to trip

a submain to outbuilding would still be classed as TNCS if you exported it. it cannot change to TNS. although if you dont use the providers earth, you can make it TT

 

[D Brennan]

Right here goes , I see yet another reference to exporting the earth from a PME supply ( TNCS) to a remote building etc. My question is , can someone tell me which Regulation covers this . I have to admit that prior to becoming a Forumite I had not come across this, and still cannot see why it is a problem.PME means Permament Mutiple Earthing , the neutral is spiked to earth along it's run and seperated where it enters a building, so its a reliable connection to earth. TNCS is the new name but nothing has changed, the earth and neutral are combined in the supply and seperated on entering the building.

A sub supply to a remote garage or shed , say, would be classed as TNS, or should I be donning Spec's big pants on my head.

On my assesment i had exported earth and the bloke from elecsa said absolute nothing wrong with exporting earths from pme its just a myth!!

told me i can export pme all day long ???

 

[Andy™]

On my assesment i had exported earth and the bloke from elecsa said absolute nothing wrong with exporting earths from pme its just a myth!!told me i can export pme all day long ???

you can export in under certain circumstances, but id rather not. PME is another example of money over safety from DNO

 

[batty]

Dno say you should not export pme to outbuildings it will be your fault if something goes wrong and remember you signed the cert.

Batty

 

[cheggers]

this debate will go on forever I export and it doesn't worry me I use this as a guild http://www2.theiet.org/Publish/WireRegs/WiringMatters/Documents/Issue16/2005_16_autumn_wiring_matters__complete_no_adverts.pdf the way I think about it is I have come across to many faulty rcd's and never a disconnected neutral . so if you do the maths and actually work out the risk pme all the way

 

[SPECIAL LOCATION]

Right here goes , I see yet another reference to exporting the earth from a PME supply ( TNCS) to a remote building etc. My question is , can someone tell me which Regulation covers this . I have to admit that prior to becoming a Forumite I had not come across this, and still cannot see why it is a problem.PME means Permament Mutiple Earthing , the neutral is spiked to earth along it's run and seperated where it enters a building, so its a reliable connection to earth. TNCS is the new name but nothing has changed, the earth and neutral are combined in the supply and seperated on entering the building.

A sub supply to a remote garage or shed , say, would be classed as TNS, or should I be donning Spec's big pants on my head.

Deke if you go back to post #9 on this thread

http://www.talk.electricianforum.co.uk/showthread.php?t=4409

where I was explain to AndyC that one site can have multiple installations

the Regulations that govern whether you can or cannot export your PME earth are noted.

i.e. Requirements for protective conductors between installations with separate earth arrangements

Regulation 542.1.8 page 126

Where a number of installations have separate earthing arrangements, any protective conductors common to any of these installations shall either be capable of carrying the maximum fault current likely to flow through them or be earthed within one installation only and insulated from the earthing arrangements of any other installation. In the latter circumstances, if the protective conductor forms part of a cable, the protective conductor shall be earthed only in the installation containing the associated protective device.

So if you had a PME & TT joined together i.e. two earth types are present and the connected earth protective conductor MUST be able to carry full fault current.. so if faulty broken neutral at PME end then the earth via the TT rod becomes the neutral path for the whole installation so would need to be 25mmCSA assuming live is 25mmCSA!

The basic requirements for protective earthing from reg 411.3.1.1 page 46.

state that Exposed conductive parts must be connected to a protective conductor and simultaneously accessible parts MUST be connected to the same earth system.

SO referring to the two above points..

Exposed-conductive-parts connected to two earthing arrangements must not be simultaneously accessible.

If such parts could be accessed simultaneously, a physical barrier, physical separation or a non-conducting enclosure must be provided to prevent this.

If the outbuilding has exposed conductive parts AND a single earth system is used e.g. PME exported from house, those exposed parts would be connected back to the PME at the house so the earth conductor MUST have a big enough CSA.

REMEMBER EXPOSED CONDUCTIVE PARTS ARE THOSE THAT INTRODUCE AN EARTH POTENTIAL. e.g. metal pipes, structural steel into foundations etc..

IF the outbuilding has NO exposed conductive parts & thus no introduced low resistance earth path then the PME can be exported.

e.g. total wooden shed on support battens with no pipework entering the shed...

This will meet BOTH of the above regs so OK exported.

Obviously if the house supply was TT or TNS the max fault current down the earth conductor would NEVER be the full load current. e.g. a broken neutral in these situations would just mean NO power! :|

so only a smaller CSA earth needed to safely export earth.

 

[SPECIAL LOCATION]

On my assesment i had exported earth and the bloke from elecsa said absolute nothing wrong with exporting earths from pme its just a myth!!told me i can export pme all day long ???

Not a myth..

there are regs to meet!

as long as you comply then all ok!

542.1.8

411.3.1.1

 

[SPECIAL LOCATION]

this debate will go on forever I export and it doesn't worry me I use this as a guild http://www2.theiet.org/Publish/WireRegs/WiringMatters/Documents/Issue16/2005_16_autumn_wiring_matters__complete_no_adverts.pdf the way I think about it is I have come across to many faulty rcd's and never a disconnected neutral . so if you do the maths and actually work out the risk pme all the way

just make sure you meet these regs Cheggers rather than just assuming things,

then your bottom end is covered in case of flying pooh & fans!

542.1.8

411.3.1.1

and you can sign your certs in confidence.. .....the above work complies with BS7671 blah blah.... etc.

 

[spinlondon]

this debate will go on forever I export and it doesn't worry me I use this as a guild http://www2.theiet.org/Publish/WireRegs/WiringMatters/Documents/Issue16/2005_16_autumn_wiring_matters__complete_no_adverts.pdf the way I think about it is I have come across to many faulty rcd's and never a disconnected neutral . so if you do the maths and actually work out the risk pme all the way

PME are initials that stand for Protective Multiple Earthing.

One of the problems you will encounter with exporting PME, is how far away is the nearest PME.

If the PME is too far away, then you will get a voltage gradient between the PME and real earth at the building you are exporting to.

This could be a problem if there are exposed-conductive-parts.

Best bet if exporting the PME is to ensure that there are no exposed-conductive-parts (use plastic fittings), if in doubt follow the IET guidlines and stake it.

 

[green-hornet]

Just another point worth noting, if you export an earth to say a garage that has its own water supply by means of metal pipe the pipe would require bonding to the main met at the point of installation ie the house.

Most DNO,s will not exept any liability for an exported earth and would often say it was done without their permission.

TT in my view would always be safer.

 

[Theorysparky]

On my assesment i had exported earth and the bloke from elecsa said absolute nothing wrong with exporting earths from pme its just a myth!!told me i can export pme all day long ???

exactly what my assessor said

except in certain conditions

 

[Theorysparky]

Just another point worth noting, if you export an earth to say a garage that has its own water supply by means of metal pipe the pipe would require bonding to the main met at the point of installation ie the house.Most DNO,s will not exept any liability for an exported earth and would often say it was done without their permission.

TT in my view would always be safer.

http://www2.theiet.org/publish/wireregs/wiringmatters/documents/issue16/2005_16_autumn_wiring_matters_electrical_installations_outdoors.pdf

 

[green-hornet]

http://www2.theiet.org/publish/wireregs/wiringmatters/documents/issue16/2005_16_autumn_wiring_matters_electrical_installations_outdoors.pdf

Main equipotential bonding

Another possibility is to include the garage within the

installation in the main dwelling and provide main

equipotential bonding, in accordance with Table 54H

(PME conditions apply), in practice this would mean a

10mm2 main bonding conductor would be

required to connect the water pipe in the

garage with the Main Earthing Terminal in

the dwelling.

Taken from the above guidance note, which is what I said in my original post.

 

[SPECIAL LOCATION]

Main equipotential bondingAnother possibility is to include the garage within the

installation in the main dwelling and provide main

equipotential bonding, in accordance with Table 54H

(PME conditions apply), in practice this would mean a

10mm2 main bonding conductor would be

required to connect the water pipe in the

garage with the Main Earthing Terminal in

the dwelling.

Taken from the above guidance note, which is what I said in my original post.

And what I said in my posts

Basically if your exported earth complies with these regs

411.3.1.1

{covers the need to bond exposed parts etc..}

542.1.8

{That covers having big enough CSA if bonding in outbuilding joined to house.}

its ok...

otherwise TT. :|

 

[SPECIAL LOCATION]

Most DNO,s will not exept any liability for an exported earth and would often say it was done without their permission.TT in my view would always be safer.

AFAIK They don't accept any responsibility/liability for any earthing / bonding etc.. once it has gone past their cut out anyway.

i.e. the DNO have no responsibility or involvement for any additions to supply outbuildings at any later date, that is the contractors responsibility & liability due to the EIC they have (or should have) signed.

According to the NICEIC earthing and bonding snags & solutions book

snag 38 "earthing an electrical installation" basically says..

If DNO have supplied an earth they are obliged to ensure it is a safe earth ref 24(1) of ESQCR.

But the contractor still has responsibility to ensure the earth terminal is suitable for the requirements of the installation,

which basically it will be if it complies with BS7671.