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Generator earthing

Discussion in 'Electrics UK' started by PaulAH, 22 Oct 2005.

  1. PaulAH

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    Evening all

    Plagued by power cuts in our rural office, we recently bought a cheapo 2.2Kw generator from Aldi to keep the small computer network running. It works fine (although we're probably taking a risk with what must be a very lumpy output).

    It's got me thinking. As I understand it, the mains supply works by swinging 240V AC around a neutral that is nominally at earth potential. As a body standing on earth, you are also earthed; if you touch Live you'll be forcefully reminded of the fact.

    But what happens with an unearthed generator? Presumably it's delivering 240V across what it calls Live and Neutral, but these are relative to thin air. If you touch either (but not both) there is no complete circuit and you wouldn't feel a thing. Or would you?

    Any comments other than "try it"?

    (Perhaps this should be on another forum - I'll leave that to Mod)

    Paul
     
  2. andy

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    there was a thread about this a few weeks ago. go look for it. lots of info there
     
  3. oilman

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    It's here

    If you had a 2-stroke gen, you might be able to de-tune it and run it on kerosene so you could take a few litres to work from your oil tank.

    Have you done any changes to the flue to send it up the chimney? :D
     
  4. Agile

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    A friend of mine plagued by local power cuts in his rural village separated the boiler and pump so he could supply them from his small gene and keep the heating on.

    Unfortunately a bad cut lasted four days! He was eating prawn and lobster and other delights as the freezer warmed up!

    Most small gene have one side connected to the earth connector at its socket so it will not be floating.

    Tony
     
  5. andy

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    should have got a bigger genny to take the fridge and freezer aswell!

    also, i wasnt aware genny's had to be plumbed in.
     
  6. PaulAH

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    Agile writes
    Now that IS interesting. Dosen't that mean the chassis of any connected appliance will reach generator potential unless it is physically earthed to ground? IE you'd be in for a shock (or not)

    Thanks for the link, Oilman. It doesn't actually answer the question. Plenty of acronyms lifted from tech college textbooks but no evidence of a grasp on the underlying physics.

    As for your kerosene idea, the generator powered five computers for a whole day on perhaps a couple of pints of petrol. Not worth the effort. Therefore your suggestion goes up the chimney - but I'm not sure if the liner is strong enough to take it. Is suggestion-proof liner a matter for Building Control?
     
  7. oilman

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    It would be the same as a transformer secondary, and as you say there's no complete circuit. It would be a bit different though if someone (out of sight) decided to try the same experiment and touched the other wire :eek:
     
  8. slippyr4

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    The PD between phase and neutral from the generator would be a Uo of 230v. Theoretically speaking this floats anywhere it likes.

    You're concerned about the safety aspect of this. Picture what happens when an (earthed) human touches a conductor which is linked to either phase or neutral at the Genny. What is the Pd between them? 0v. That person completed the circuit to "tie" that conductor to earth, albeit through a rather higher impedance.

    Which is where the danger happens. If you're standing there "tieing" the neutral, and someone else comes and grabs the phase, then it'll get exciting for both of you.

    slip
     
  9. Agile

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    As far as I am aware ALL small generators connect the neutral and earth at their output connection.

    To do anything else would be potentially dangerous.

    Are you aware of any unit which does not?

    Tony Glazier
     
  10. PaulAH

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    Since we have established that an un-earthed generator won't give you a shock (unless you play ring-a-roses with someone gripping the opposite pole) why should separating Neutral and Earth be "dangerous"?

    Come to that, who decides which pole is Neutral when each is at the opposite potential to the other?
     
  11. ChrisR

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    Just an instance of TN-C-S innit? Don't see an issue here :confused:
     
  12. Agile

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    Paul, you are not looking objectively at the potential hazards.

    Any floating supply can potentially be at ANY voltage above earth. A floating generator winding could rise to say 3000 vdc and punch through its insulation if it did not give you a shock first.

    Any 115 v transformer uses a CT winding and the CT is earthed because 55 v to earth is less dangerous than 115 v which would arise if either side were earthed or due to asymetric leakage to earth.

    Surprisingly the manufacturers decide which side of the winding to connect to earth, usually based on which end of the winding is better insulated.

    Tony
     
  13. Spark123

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    My generator doesn't!!
    A small generator will normally supply a "separated electricity supply" which comes under the scope in the iee regs of "electrical separation". If you require a generator source for an EEBADS supply, the generator needs to be earthed by a suitably tested earth electrode at source and N-E link fitted in such a way that it cannot be run without the electrode connected. I always incorporate an RCD when doing this.
    --------------------
    Thinking about what you are saying, if the N-E link is fitted without the electrode and a fault occur in the system between L and true earth, the chassis of the generator will become live with respect to true earth.
     
  14. ChrisR

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    EEBADS?
     
  15. Spark123

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    Earthed Equipotential Bonding and Automatic Disconnection of Supply.
    Generally TT, TN, IT systems.
     

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