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TNS and PME connected]

lyledunn

I would appreciate a wee bit of advice on this one. Quarry with everything supplied by one transformer. There is an overhead bundle to the office block which DNO notice says PME. The quarry supply is TNS and the workshop is also supplied via the main panel in the quarry intake. There is a gen set that feeds into a changeover switch in the quarry intake that offers a back up to the quarry and to the workshop. A new SWA cable was installed from the panel in the workshop to a metal clad changeover switch in the office block to allow the office to benefit from the back up generator. The PME earth is now exported to the quarry. Issue?

one.

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    Is the office block the only customer on the PME side of the supply?

    If so, I think I can see a point of view that with the change-over switch in the generator/TN-S position there would be no loads connected to the TN-C-S system and therefore not potential differences along the PEN conductor (whether due to normal voltage drop along the N in normal conditions, or broken PEN situation as there'd be no connected loads to pull up the voltage on the severed end). Thus the customer's end of the PEN conductor should be at the same potential as the transformer's electrode and, hopefully, that throughout the TN-S system. So even with the bonding to the PME situation in place, the hazards of PME shouldn't be present, in this particular situation. (Unless I've overlooked something obvious again).

       - Andy.

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  • 0

    Is the office block the only customer on the PME side of the supply?

    If so, I think I can see a point of view that with the change-over switch in the generator/TN-S position there would be no loads connected to the TN-C-S system and therefore not potential differences along the PEN conductor (whether due to normal voltage drop along the N in normal conditions, or broken PEN situation as there'd be no connected loads to pull up the voltage on the severed end). Thus the customer's end of the PEN conductor should be at the same potential as the transformer's electrode and, hopefully, that throughout the TN-S system. So even with the bonding to the PME situation in place, the hazards of PME shouldn't be present, in this particular situation. (Unless I've overlooked something obvious again).

       - Andy.

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