Earth for an emergency genorator set.

[Gritishbasspark]

Hi I have recently been asked to wire in a emergency 3 phase and N genorator in a work shop. I have a break before make change over switch, serge protection with a contactor incase of a spike on the genorator. The mains supply is a 3 phase and N TN-C-S system. What is trubbling me is what will I do with the earth. Due to the potentual currents on the neutral cable which in turn could come down the earth cable I don't want to connect everything to the supplys earthing system, but in that case my only opption is to put everything on to a TT system which isn't so great any ideas would be greatly welcome

David

 

[Guest123]

Hi.


Well when the genny is running, assuming it is not a floating power supply (i.e. neutral isolated from earth in the genny?), the star point / neutral is connected to gen frame which is in turn earthed through the rod(s) you will have installed.

Think of a line to earth fault of a circuit supplied by the genny - the fault current will flow from the genny line, down through the circuit, through the fault, and will return through the SWA (or 3rd core) to the genny star point thus operating any protective devices in the geney. Generator outputs are by and large TN-S systems, because you are running an actual CPC in the cable, the fault current has no reason to go anywhere else other than back to its origin.

Any fault should not flow through the main supply earthing as they are not referenced together other than at their orgins via earth rods into the rock we all live on.

Likewise the rods on the gen frame ordinarily would not form part of the earth fault path of the geney system unless your installed CPC was damaged.

My opinion is to leave all earthing connected together as it is, with just the live conductors made or broken upon changeover.


My grey cells are pretty much dead for today now.....I hope this helps a bit.

 

[baldsparkies]

And double check the neutral feed connection, especially so from 3 phase genny packs. I was once tasked with replacing lots of single phase appliances thet had fried because of this.

 

[Bernieg]

I agree with both of the above. Earthing is important and if you do not make a good neutral connection, Youy single phase circuits will have some unacceptable high voltages.