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Discuss High Ze on PME question in the Electrical Forum area at ElectriciansForums.net
You can't have the generator earthing arrangement connected to the DNO supply because an internal fault when running on the generator may direct voltage back to the DNO supply on which someone may be working.
That was my original understanding, however I do understand Richard's scenario, which is essentially the reverse of the often discussed issue of exporting a PME earth outside its equipotential zone and the difference in potential between the exported earth and mother earth, under a fault condition on the DNO side. In this case, the issue is a fault on the client side at the generator, with the DNO operative subject to the rise in potential at the DNO end of the PME.The earth fault loop path for whichever supply is being used, must be complete.
When the DNO supply is in use, the generator supply becomes an extraneous conductive-part, and as such requires bonding to the DNO supply earth.
Most changeover switches, certainly all those that I've seen, only switch live conductors. I understand you could use switch the supply earths, however this doesn't overcome the extraneous aspect.The only acceptable method to divorce the two supply earths is to use a changeover switch which will disconnect the DNO earth at the same time as the live conductors.
I am thinking that you could for example have something like the below diagram.Understood, @Richard Burns So is my understanding correct as below, on the basis the changeover switch only switches the two or four live conductors (depending on whether single or three phase) and not the CPC:
- Generator enclosure & SWA = TT
- The installation always utilises the DNO's PME at all times, irrespective of position of the c/o switch
- The changeover switch cabinet should be bonded to the PME, as this earth will be of lesser resistance
- The SWA armouring from the generator to the c/o switch should only be terminated at the supply end and specifically NOT terminated at the c/o cabinet inlet, to ensure separation of the local and DNO earthing paths
In such a scenario, I would expect the EFLI to be lower via the generator than via the DNO supply.I am thinking that you could for example have something like the below diagram.
When the TNCS supply stops the changeover switch is operated, the earth link removed and the generator started.
When the TNCS supply is restored the generator is stopped, the earth link replaced and the changeover switch operated.
When running from the DNO supply the generator earth is bonded as an extraneous conductive part to the supply earth by the generator earthing conductor.
When running from the generator the link is removed so the Line, Neutral and Earth from the DNO supply are isolated from the installation.
View attachment 46636
As I was drawing this I have realised that the alternative solution to the question in the OP is that the earth link has been removed from the installation even though the DNO supply is still connected leaving the installation as a TT system. Assuming the protection for the installation is correctly set up to be suitable for either supply this should not cause danger to the installation but would be a less effective means of protection than to use the DNO supply earthing.
Reply to High Ze on PME question in the Electrical Forum area at ElectriciansForums.net
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