Discuss 3 phase generator earthing in the Commercial Electrical Advice area at ElectriciansForums.net



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Hello Fellas,

Just some thoughts on a problem I encountered with a 3 phase 200KVA diesel generator. It was supplying a 3 phase + E 75Kw motor. The problem I encountered was that the earth was floating between the phase. Expected to see 415V between earth and phase but encountered about 120v under no load conditions.

Generator was being used in a marine enviroment so not too sure about earthing arrangements on marine vessels. Any thoughts ? I was told later that it would be wise to tie the earth and neutral together on the generator by a link ?? Not really sure about that would like to hear anyone's thoughts, cheers
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  • #2
It is not wise to tie up genset with e/n link; rather tie up supply point of incommer from the set.
That way the set is left unmodified when disconnected, earth spikes are also normally used.
But it depends on normal supply if n/e linked then do the same (so normal rcd's will still operate on the consumer side. Not sure about dockside though, you will need to check normal earthing arrangments

If it's for a single operation, ie, 1-circuit I would leave alone and allow a floating earth. Connect as is...grey area I know, I would however ground the genset framework

btw you would not see 415v between phase and earth whatever, you would normally see 240v. With a floating earth you would see 120v. If you tested between phases conventional system then you should see 415v

Whatever the load (1000amps), you would not get more than 120v ish phase earth -;)
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If it is a ex marine generator then it may not have a neutral, as ships I sailed on was 3 phase throughout with the lights getting 240v across 2 phases via transformer from the 415 generated by the generator. Earth was obtained via ships hull.


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  • #4
Cheers Lads,

It was just out of interest question really but I appreciate your replies. Obviously had a blond moment with the P-E 415V. Appreicate it's a grey area. Genset frame was earthed to deck and all equipment feeding off it was earthed through a main earth which was the ships hull. Earth loop impedance reading was correct for devices. Protection was by MCB type 2-3. It's just that was no main earth terminal on the genset to connect to and was interested in how to adequately earth equipment if using genny in different conditions. Thanks for the replies did'nt expect to hear anything so cheers for the advice. :D


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  • #5
hey all fella, for any question on the diesel generator, i can be supported and connected by:*********************
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I was under the impression that all gennys must be spiked. obviously you cant spike a ship without causing certian other problems but most ships have sacrificial anodes that are in contact with 'earth' (water). this would be the place to get your ground.


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  • #7
Hi markythesparky… from my knowledge the sacrificial anodes are for corrosive protection of the ship hull. Ian spoke right …the ship don’t have neutral only phases. You can ground the generator body without problems, but you don’t have a neutral connection. Hope it is helpfull

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