Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • Locked Locked
  • Replies 68 replies
  • Subscribers 32 subscribers
  • Views 9350 views
  • Users 0 members are here
Options
You may also be interested in
This discussion has been locked.
You can no longer post new replies to this discussion. If you have a question you can start a new discussion

THE CAMPAIGN FOR REAL EARTHING

John Peckham
I think that we were considering adopting PME earthing systems today on what we know now we would say no thanks?


I strongly believe that the use of PME earthing systems is inherently unsafe. I am keen to hear any technical arguments to defend the use of PME?


Most PME DNO new distribution cable use 3 core Wavecon cables for UG distribution with single phase concentric cables tapped off for single phase users. For overhead open wire supplies of newer installs ABC cable.


There is no reason not to use 4 core Wavecons and distribute a much safer TN-S earthing system other than the cables will be a 1/3rd more expensive.


 I believe that the DNOs having been tentatively asking government  for a £trillion pounds to upgrade their networks for when we stop burning fossil fuels and go all electric. No doubt the DNOs hope that the government, civil servants and politicians will have forgotten that these private companies purchased a public assets for a knock down price with the idea that the public would no longer have to subsidies a public body! 


A good start would be that no new supplies will be PME, no replacement cables will be PME and no repairs to cables will be PME conversions. For instance a new housing estate would have to be an all TN-S installation. I understand that WPD are already installing TN-S earthing systems for new housing estates. If this is the case then well done WPD. Can anyone confirm this?


I am also concerned about the degradation of the of the Global Earthing System with use of all plastic covered cables, no bonding to metallic service pipes and the failure on DNO contractors to install earth rods and joints to save time and money. Will we start to see 442 type over voltages?


Look at my EV charging thread and the measures we are having to deploy due to PME earthing, we are having to do this because the PME system is inherently unsafe!


Is there support for my proposed campaign?
  • Former Community Member
    0 Former Community Member

    mapj1:

     the hard pressed linesmen go into the substation block, or up a pole, do some magic, the power goes off and then is restored, and the lost PEN part misses the official stats.




    Back in a previous life, I worked with ESKOM in South Africa - we had several UK line crew and jointers who had given up the cold and wet life with UK area boards for a life of sun and apartheid. It wasn't that uncommon for linesmen to admit to tightening line taps and then slacking them back a turn to generate call outs that kept their bonus at the highest possible. At one point we were using line taps with shear heads that once tightened and sheared couldn't be re opened (as they had no hex head left). Ditto for the jointers. Similar situation as the "on shift" guys who used to drive around looking for street light outages and submitting them back the next day, until they got an emergency call - one character I worked with kept an air rifle handy in the van to ensure the outages were sufficient to keep his brother in law gainfully employed repairing them.


    To be fair, these were then still nationalized industries  - but old attitudes die hard in some areas.. The nearest factory to me where I grew up used to make ammunition boxes and jerry cans for MoD - almost every house had a red oxide front step and a jerry can green line post and back garden gate. Under the kitchen sink, the shoe cleaning kit was usually in a flip top cannister designed to carry 7.62 link ammunition. Several houses had clear glass tungsten lamps that when lit would project "NCB" onto the front parlour wall.


    Such is life


    Regards


    OMS

  • 0
    I would support a return of real earthing.

    If PME is so wonderful, why is it prohibited or discouraged in petrol filling stations, explosives stores, and on livestock farms.


    One problem might be modern types of cable. Back in the good old days, street mains had wire armouring that was at least partially in contact with the general mass of earth. Even with an outer serving of hessian and bitumen there was a certain amount of fortuitous contact with true earth.

    The armour of the suppliers cables therefore functioned as both a metallic path back to the substation AND as extensive earth electrode.


    Modern cables tend to use a plastic oversheath that provides good insulation between the armouring and the general mass of earth. There might be a case for cable with plastic insulated cores but with the armouring exposed to the general mass of earth.

    Alternatively, perhaps bare copper tape should be buried in the same trench and connected to the armouring at joints.

  • Former Community Member
    0 Former Community Member
    In North Norfolk, where a whole village is upgraded from bare overheads to A.B.C. overheads, U.K.P.N. supplies a white sticky label that is affixed on the main cutout. On it is hand written the Zs value after testing. That is very helpful.


    Wouldn't Ze be more helpful?


    Regards


    BOD
  • 0

    Chris Pearson:




    Kelly Marie:

    In one of the recent EV charging threads there was a link to SSEs LV manual I read some of it and it looked like it said that SSE don't turn existing underground cables into PME by earthing the neutral whenever there is a a repair or extension done if I've read that right maybe DNOs are less than keen on it too. Anyone know for certain if this is the case? I bet UKPN will chirp in soon if he is still about




    I know for certain that SSE "PME'd" my service cable (and urgently too) 'cos I saw them do it. ?


     




    I also know that SSE PME (what was Wessex region at least) service cables, as I said in another thread I used to work for them. The jointer would use whatever they had on the van or what was available at the local sub station, they did not know the difference.

  • 0

    Kelly Marie:

    In one of the recent EV charging threads there was a link to SSEs LV manual I read some of it and it looked like it said that SSE don't turn existing underground cables into PME by earthing the neutral whenever there is a a repair or extension done . . . 




    That is quite correct. Adding additional earths to the neutral does not make it PME. 


    Regards,


    Alan. 

  • 0

    perspicacious:
    In North Norfolk, where a whole village is upgraded from bare overheads to A.B.C. overheads, U.K.P.N. supplies a white sticky label that is affixed on the main cutout. On it is hand written the Zs value after testing. That is very helpful.


    Wouldn't Ze be more helpful?


    Regards


    BOD




    Indeed Ze is a better description. I believe that the U.K.P.N. labels do say Ze as opposed to Zs. But thinking about the TN-C-S system having solid conductors in the supply I used Zs as it included the length of supply cable that may run in a cottage loft for a few metres before being connected to a main cut out, so is not totally literally physically external, but may be regs wise. Some old buildings have overhead incoming positions on the first floor but the consumer unit and main cut out can be several meters away for convenience. I suppose the reading of earth fault loop impedance will be the same if we call it Ze or Zs if the test is carried out at the main cut out in a property. I am not referring to final circuit tests though.


    Z.

  • 0
    So instead of Ze or Zs or Zdb we now have Zcutout!
  • 0

    ebee:

    So instead of Ze or Zs or Zdb we now have Zcutout!




    Why not have Zcutout? Just look up the B.S. 7671 definition of Electrical Installation, Ze and Zs. Is the cutout part of an electrical installation?


    "Electrical installation. An assembly of associated electrical equipment having co-ordinated characteristics to fulfil specific purposes."


    So if the main overhead supply cable enters a house for many meters it could be considered as part of the electrical installation, ignoring ownership, and the term Zs would apply equally as would Ze at the main cutout. A minor matter not worth debating until the cows come home.


    Z.


     

  • 0

    Alan Capon:




    Kelly Marie:

    In one of the recent EV charging threads there was a link to SSEs LV manual I read some of it and it looked like it said that SSE don't turn existing underground cables into PME by earthing the neutral whenever there is a a repair or extension done . . . 




    That is quite correct. Adding additional earths to the neutral does not make it PME. 



    So has "PME" become shorthand for "not TN-S"?

     

  • 0

    Like all these things, we seem to want a Rolls Royce system, but society doesn't want to pay



    I wouldn't call TN-S with an equal sized PE a Rolls Royce solution - if you wanted a properly safe Earthing system you'd go for a PE that had about 5x the conductivity of the line conductors - so that the Voltage on the supplier's earth couldn't exceed 50V - keeping things safe even during long disconnection times (whether 5s BS 7671 ones or quite a bit longer for faults on the distribution network). That would also allow us to solve the 'mixed disconnection times' problem within installations (circuits with long disconnection times have larger c.p.c.s) and have a supplier's earthing terminal that was useful for just about every application.


    As has been said, the larger cost is digging, planning and carrying - a bit of extra copper wouldn't add a huge percentage to the overall cost.


    Alternatively a smaller PE together with a neutral earthing resistor so that during faults the PE remains at close to true earth potential (N is pulled high instead).


    Disconnection of Earth faults at the substation might be problematic if faults on consumer's installations could (false) trigger it. We'd need RCDs at each consumer's intake - but that's exactly what the French do - so not beyond the bounds of possibility.


    While we're about it we might think about going 3-phase for domestic supplies - so we could manage with smaller live conductors and benefit from reduced v.d. 


    And get smart meters to monitor supply voltages and disconnect is they go significantly out of spec to protect consumer's installations from overvoltages (e.g. broken N). They could monitor for loss of Earth while they were at it too - and automatically report back to the DNO (far more efficient than waiting for consumers to notice something dim, tingly or smokey and phone it it manually).


    There's so much we could do far better if we weren't starting from here. I do agree though that there feels to be something fundamentally unsound about the PME approach and alternatives should be considered - even if (like the introduction of 13A plugs) it takes a long time before everyone benefits.


        - Andy.

Community Rules & Guidelines