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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?
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    Actually the choice of not more than 5 seconds not less than 4 thing is as arbitrary (read silly) as the choice of a permitted touch voltage of 70V.  It has the feel of being a spec being devised by a committee that do not normally look at such matters, and in effect re-inventing the wheel, and in the process coming up with a rather unusual square shape. ? It is not a safety of life spec like the ADS times for fuses and circuit breakers.


    If the voltage is there for 4 seconds it might as well be there for 40, as far as electrocution is concerned, the question is how much current will flow in a victim for the time it takes to trigger  a heart fibrillation, which is a function of current flow and duration. That current has only a loose connexion to the applied voltage and a more lot to do with size of contact areas on the live metal and the ground, and the dampness of the exposed skin, or if there is clothing in the way, if it is wet, and if the wetness is fresh water or salty.

    (more info  on shock times http://www.industrial-electronics.com/elec-safety_6.html)

    In terms of building one, the electronics to detect any voltage you like and introduce any delay you like is a matter of pennies in bulk, and a few tens of pounds for a one off design. The more expensive part would be the contacts,  though a 3 pole contactor, or a 3 pole MCB or switch strapped to  a shunt trip coil to act as the actuator from the electronics would be standard parts.  The harder  step would be deciding what to test it and declare compliance  to, as the standard is not properly  fixed.
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    Actually the choice of not more than 5 seconds not less than 4 thing is as arbitrary (read silly) as the choice of a permitted touch voltage of 70V.  It has the feel of being a spec being devised by a committee that do not normally look at such matters, and in effect re-inventing the wheel, and in the process coming up with a rather unusual square shape. ? It is not a safety of life spec like the ADS times for fuses and circuit breakers.


    If the voltage is there for 4 seconds it might as well be there for 40, as far as electrocution is concerned, the question is how much current will flow in a victim for the time it takes to trigger  a heart fibrillation, which is a function of current flow and duration. That current has only a loose connexion to the applied voltage and a more lot to do with size of contact areas on the live metal and the ground, and the dampness of the exposed skin, or if there is clothing in the way, if it is wet, and if the wetness is fresh water or salty.

    (more info  on shock times http://www.industrial-electronics.com/elec-safety_6.html)

    In terms of building one, the electronics to detect any voltage you like and introduce any delay you like is a matter of pennies in bulk, and a few tens of pounds for a one off design. The more expensive part would be the contacts,  though a 3 pole contactor, or a 3 pole MCB or switch strapped to  a shunt trip coil to act as the actuator from the electronics would be standard parts.  The harder  step would be deciding what to test it and declare compliance  to, as the standard is not properly  fixed.
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