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Engineer54
Ahhhh, the sweet sounds of defeat, i knew i was right all along
Its true that persistence really does pay off
ONLY in your dreams antimatter; ....Only in your dreams!!!!! and you can take that to the bank!!! lol!!!
- Thread starter Robo
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ONLY in your dreams antimatter; ....Only in your dreams!!!!! and you can take that to the bank!!! lol!!!
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antimatter
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antimatter
Broken neutral conductor
The neutral of a supply is often common to a large number of installations. In the (unlikely) event of a broken neutral, all the consumers on the load side of the break could have a combined neutral and earth potential of the same level as the phase system (240 V to earth). This situation could be very dangerous, because all earthed metalwork would be at 240 V above the potential of the general mass of earth.
To prevent such an event, the Electricity Supply Company connects its combined neutral and earth conductor to earth electrodes at frequent intervals along its run. Whilst this does not entirely remove the danger, it is much reduced. For example, the assumed earth resistance values show that the maximum possible potential to earth in this case would be 96 V. In practice, much lower resistance values for the earth connections will reduce this voltage. The Electricity Supply Company goes to very great lengths to ensure the integrity of its neutral conductor.
http://www.tlc-direct.co.uk/Book/5.6.3.htm
The neutral of a supply is often common to a large number of installations. In the (unlikely) event of a broken neutral, all the consumers on the load side of the break could have a combined neutral and earth potential of the same level as the phase system (240 V to earth). This situation could be very dangerous, because all earthed metalwork would be at 240 V above the potential of the general mass of earth.
To prevent such an event, the Electricity Supply Company connects its combined neutral and earth conductor to earth electrodes at frequent intervals along its run. Whilst this does not entirely remove the danger, it is much reduced. For example, the assumed earth resistance values show that the maximum possible potential to earth in this case would be 96 V. In practice, much lower resistance values for the earth connections will reduce this voltage. The Electricity Supply Company goes to very great lengths to ensure the integrity of its neutral conductor.
http://www.tlc-direct.co.uk/Book/5.6.3.htm
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antimatter
Special requirements PME-fed installations
An installation connected to a protective multiple earth supply is subject to special requirements concerning the size of earthing and bonding leads, which are generally larger in cross-section than those for installations fed by supplies with other types of earthing. Full discussions with the Electricity Supply Company are necessary before commencing such an installation to ensure that their needs will be satisfied. The cross-sectional area of the equipotential bonding conductor is related to that of the neutral conductor as shown in {Table 5.9}Table 5.9 - Minimum cross-sectional area of main equipotential
----------------- bonding conductor for PME-fed installations
Neutral conductor c.s.a. (mm²) Main equipotential bonding conductor c.s.a (mm²)
35 or less 10
Over 35 and up to 50 16
Over 50 and up to 95 25
Danger can arise when the non-current carrying metalwork of an installation is connected to the neutral, as is the case with a PME-fed system. The earth system is effectively in parallel with the neutral, and will thus share the normal neutral current. This current will not only be that drawn by the installation itself, but may also be part of the neutral current of neighbouring installations.
It follows that the earth system for an installation may carry significant current (of the order of tens of amperes) even when the main supply to that installation is switched off. This could clearly cause a hazard if a potentially explosive part of an installation, such as a petrol storage tank, were the effective earth electrode for part of the neutral current of a number of installations. For this reason, the Health and Safety Executive has banned the use of PME in supplies for petrol filling stations. Such installations must be fed from TN-S supply systems (RSE booklet RS(C)41
- 'Petrol Filling Stations: Construction and Operation').
An installation connected to a protective multiple earth supply is subject to special requirements concerning the size of earthing and bonding leads, which are generally larger in cross-section than those for installations fed by supplies with other types of earthing. Full discussions with the Electricity Supply Company are necessary before commencing such an installation to ensure that their needs will be satisfied. The cross-sectional area of the equipotential bonding conductor is related to that of the neutral conductor as shown in {Table 5.9}Table 5.9 - Minimum cross-sectional area of main equipotential
----------------- bonding conductor for PME-fed installations
Neutral conductor c.s.a. (mm²) Main equipotential bonding conductor c.s.a (mm²)
35 or less 10
Over 35 and up to 50 16
Over 50 and up to 95 25
Danger can arise when the non-current carrying metalwork of an installation is connected to the neutral, as is the case with a PME-fed system. The earth system is effectively in parallel with the neutral, and will thus share the normal neutral current. This current will not only be that drawn by the installation itself, but may also be part of the neutral current of neighbouring installations.
It follows that the earth system for an installation may carry significant current (of the order of tens of amperes) even when the main supply to that installation is switched off. This could clearly cause a hazard if a potentially explosive part of an installation, such as a petrol storage tank, were the effective earth electrode for part of the neutral current of a number of installations. For this reason, the Health and Safety Executive has banned the use of PME in supplies for petrol filling stations. Such installations must be fed from TN-S supply systems (RSE booklet RS(C)41
- 'Petrol Filling Stations: Construction and Operation').
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antimatter
And remember electricians, don't have nightmares, have nice dreams about a PME earthing system with an outbuilding with extraneous metalwork that is connected to an earth electrode.
zzzzzzzzz zzzzzzzzzzz
zzzzzzzzz zzzzzzzzzzz
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antimatter
This situation could be very dangerous, because all earthed metalwork would be at 240 V above the potential of the general mass of earth.
Common - not, Dangerous - very.
Do you want to take that chance? its your name on the paperwork, not mine.
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Dean182
Which is why equipotential bonding is very important, providing the installation has an RCD and the above happens, and all equipotential bonding is in place and resistance is 1667ohms or lower the RCD will operate.
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antimatter
Do you mean supplementary bonding conductors?
Equi-potential is 16th Edition terminology and if used in an exam, would result in a fail.
Source
Chied Examiner - City and Guilds
Every time I read of the mass deaths from broken supply neutrals on exported TN-C-S system, I will think of you.
You should contact BBC Watchdog with your findings, you are a national hero of electrical engineering and I am turning to landscape gardening to contemplate my ignorance.
You should contact BBC Watchdog with your findings, you are a national hero of electrical engineering and I am turning to landscape gardening to contemplate my ignorance.
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antimatter
as you say Do it once, do it right....
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antimatter
Anyone like my avatar by the way?
In true Darwinian style, it has evolved.
*hopefully like everyone knowledge on this excellent forum*
In true Darwinian style, it has evolved.
*hopefully like everyone knowledge on this excellent forum*
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Phil
Ahhhh, the sweet sounds of defeat, i knew i was right all along
Its true that persistence really does pay off
The above quote is neither helpful nor constructive. I for one have a great deal of respect for members on here like Lenny who have given a great deal of their time to this forum unpaid and all they ask for is the curtiousy of the thanks button being hit every now and then.
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cagedanimal2010
antimatter shhhhhhhhhhhhhhhhhhhhhhhhh!
please respect your elders because the will now a hell of a lot more than you!!!
there is so many blokes on here just trying to help and from what i see all you want to do is cause problems!!!
danny
please respect your elders because the will now a hell of a lot more than you!!!
there is so many blokes on here just trying to help and from what i see all you want to do is cause problems!!!
danny
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Deleted member 26818
I believe the correct term as used in BS7671 (17th edition) is Supplementary Equipotential Bonding. This is a method for providing additional protection.
That aside, there is no regulation in BS7671 or ESQCR that prohibits the exportation of a PME earth.
Individual DNOs can and do put restraints on where and to what they will allow connection to their PME system, but that is their choice, not something contained in any standard or statutory document.
There are prohibitions on providing a connection to a PME system for caravans, petrol stations and marinas, obviously exporting a PME earth in such situations would also be prohibited.
The thread from the IET forum you quoted concerned a mobile shipping container, these are often treated the same as caravans. The OP in that thread erroneously believed that a PME earth cannot be exported, hopefully he went away eductaed to the fact that it can be.
One of the problems with exporting a PME earth to an outbuilding containing extraneous parts, is that those parts may well be at a differnt potential to earth than the exported earth. The use of protective bonding will equalise any potential difference, but an appropriately sized protective bonding conductor back to the point of supply will have to be installed.
Often it is more cost effective not to export the earth, and to TT the out building.
That aside, there is no regulation in BS7671 or ESQCR that prohibits the exportation of a PME earth.
Individual DNOs can and do put restraints on where and to what they will allow connection to their PME system, but that is their choice, not something contained in any standard or statutory document.
There are prohibitions on providing a connection to a PME system for caravans, petrol stations and marinas, obviously exporting a PME earth in such situations would also be prohibited.
The thread from the IET forum you quoted concerned a mobile shipping container, these are often treated the same as caravans. The OP in that thread erroneously believed that a PME earth cannot be exported, hopefully he went away eductaed to the fact that it can be.
One of the problems with exporting a PME earth to an outbuilding containing extraneous parts, is that those parts may well be at a differnt potential to earth than the exported earth. The use of protective bonding will equalise any potential difference, but an appropriately sized protective bonding conductor back to the point of supply will have to be installed.
Often it is more cost effective not to export the earth, and to TT the out building.
I've got a good IET guide for supplies to outbuildings, anyone seen it? 
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antimatter
Because my interpretation of the regs is different from other peoples doesn't mean that im causing problems at all, on the contrary, it is others if you read the earlier posts who are derogatory to me, please read the posts back, this is a forum not a text book, and different people have different opinions danny
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cagedanimal2010
im not saying its a text book, all im saying is iv been on here and needed advice regarding all sorts and someone has help me, the people that have helped me you seem to be having a problem with. imo everyone has there own opinion which is fine but please respect the member on here!!!
danny
danny
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antimatter
Why do you think people have a problem with me?
Hawk81
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The problem was never about your opinion, as you say you are entitled to it. It was the fact that at the start of this thread you told the op that they can not export a tncs earth. You did not state this as your opinion, you stated it as fact. Which it is not.
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antimatter
An opinion is a fact of the person sending the message as a fact, which is his opinion is correct.
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cagedanimal2010
this is what i mean you just seem to argue!
i will say no more!!!!
danny
i will say no more!!!!
danny
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Phil
thread closed.
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Phil
This thread is closed and i will not say it again and that includes private messages being sent to other members regarding it. enough has been said and this is an end to it.
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