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 34 replies
  • Subscribers 32 subscribers
  • Views 5466 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

VOLTAGE BETWEEN THE GENERAL MASS OF EARTH AND A PME NEUTRAL/EARTH

John Peckham
Other than under fault conditions or a small difference due to volt drop on a heavily loaded CNE cable can anyone explain why you may get a large potential difference (say 70V) between the general mass of earth and the MET on a an installation with a PME earthing system?


I have not seen this myself. If this does occur how rare or frequent might this circumstance occur?


If this potential difference does occur what sort of duration might this persist for?


Although a DNO may switch occasionally the HV ring for fault or maintenance works transformer neutrals remain bolted to earth and if the HV/LV earths are combined then an earth resistance of sub 1 ohm (in UKPN land that is what they want) so how can the neutral voltage float up more than a couple of volts above the general mass of earth?
Parents
  • 0
    If the idea is for a trip for car charging, not withstanding the unicorn like nature of the device currently described in the regs, I'd be much happier  if it tripped well before 70V RMS , and well before 50V RMS as well, personally I think it really ought to be set to trip at more like the 25V we used to insist on as the maximum exposed voltage for conductive locations before it all got harmonised to 50V everywhere. Cars have large metallic surfaces, charger points may well be in bare earth car parks, or next to grass verges, and folk will at some point touch the car either in bare feet, or while standing in a puddle in shoes that have a leak, one can imagine a situation in a seaside car park  that is almost as good as an outside bathroom.?

    As in posts above, I don't think a healthy network should be anything like 70V off earth for more than a few tens of milliseconds at a time before some ADS sorts it out, and a charge point that puts itself out of order as a precaution during an abnormal event of more than 24V for perhaps half a second, is preferable tp an exposed lethal voltage,  (so long as there is a reset method  at least ).

Reply
  • 0
    If the idea is for a trip for car charging, not withstanding the unicorn like nature of the device currently described in the regs, I'd be much happier  if it tripped well before 70V RMS , and well before 50V RMS as well, personally I think it really ought to be set to trip at more like the 25V we used to insist on as the maximum exposed voltage for conductive locations before it all got harmonised to 50V everywhere. Cars have large metallic surfaces, charger points may well be in bare earth car parks, or next to grass verges, and folk will at some point touch the car either in bare feet, or while standing in a puddle in shoes that have a leak, one can imagine a situation in a seaside car park  that is almost as good as an outside bathroom.?

    As in posts above, I don't think a healthy network should be anything like 70V off earth for more than a few tens of milliseconds at a time before some ADS sorts it out, and a charge point that puts itself out of order as a precaution during an abnormal event of more than 24V for perhaps half a second, is preferable tp an exposed lethal voltage,  (so long as there is a reset method  at least ).

Children
No Data

Community Rules & Guidelines