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Discuss Backup generator (earthing) in the Commercial Electrical Advice area at ElectriciansForums.net

W

wattsup

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Hi guys
I’ve been asked to provide a standby generator for a residential/small home business property.
I will be needing something like a 6 kVA diesel genset, manual electric start and transfer switch….anyhow

I’ve seen conflicting advice regards earthing arrangements when power is supplied via the genset.
Existing earthing is pme.

The generator housing will be roughly 30 metres from the main building, the feeder cable buried 3-core swa. Two earth rods adjacent the housing and bonded to the genset framework, and carried through the swa windings and one core to the transfer switch, and thus to the distribution board.

Now, does the main utility supply earth need breaking via the transfer switch, as well as L&N when the genset is running?

I’ve got some notion it does, because possible earth fault from genset backfeeding to utility supply, via E&N fault.

I’ve read tech documents from the generator supplier but they are pretty vague, probably because their sets are used worldwide and different regs apply in different countries, not to mention alternative earthing methods.
Hope someone can advise
 
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S

Shakey

  • Thread Starter Thread Starter
  • #2
Hi guys
I’ve been asked to provide a standby generator for a residential/small home business property.
I will be needing something like a 6 kVA diesel genset, manual electric start and transfer switch….anyhow

I’ve seen conflicting advice regards earthing arrangements when power is supplied via the genset.
Existing earthing is pme.

The generator housing will be roughly 30 metres from the main building, the feeder cable buried 3-core swa. Two earth rods adjacent the housing and bonded to the genset framework, and carried through the swa windings and one core to the transfer switch, and thus to the distribution board.

Now, does the main utility supply earth need breaking via the transfer switch, as well as L&N when the genset is running?

I’ve got some notion it does, because possible earth fault from genset backfeeding to utility supply, via E&N fault.

I’ve read tech documents from the generator supplier but they are pretty vague, probably because their sets are used worldwide and different regs apply in different countries, not to mention alternative earthing methods.
Hope someone can advise

Well think of when your genny is running, presuming it is not a floating power supply (i.e. neutra isolated from earth in the genny?), the star point / neutral is connected to chassis which is earthed through the spike.

Think of a line to earth fault of a circuit supplied by the genny - the fault current will flow from the genny line, down the circuit, through the fault and will return through the SWA to the genny star point/N/Earth. Genny outputs are generally wired as TNS, because you are running a physical earth cable out.

The point being, the fault current has no reason to go anywhere else other than back to its origin - it is not referenced to the mains supply earth, other than they are both connected to the same lump of rock that we all stand on, but their supplies are entirely independant of each other

With gennys, because you using TNS, the spike is effectively 'bonding' (yes i know its the wrong term) the genny frame to the general mass of earth, and barring the main earthing cable going open circuit, is not part of the fault current path

so yes, leave all your 'earths' effectively connected together, and just isolate your supplies
 
W

wattsup

  • Thread Starter Thread Starter
  • #3
Thanks for that shakey, I guess I got confused somewhere regards centre tap stuff / floating supply and rcd's
Cheers!
 
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