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  1. #16
    Join Date
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    Quote Originally Posted by .RC. View Post
    I thought in your house neutral is connected to ground simply to complete the circuit back to the power station.

    A DC to AC inverter has no grounded neutral.
    As I understand it, your house wiring is isolated from the power station by the transformers between them, so the neutral goes between your home and the transformer supplying it. At both the transformer and at your home, the neutral leg will be grounded, so if a fault occurs where an active wire contacts the body of an appliance, the current path will be to ground via the earth wire, in this case it would be a virtual short circuit and would hopefully blow the fuse. This would protect the appliance from further damage, and anyone from the danger of electrocution if contacting the live appliance body. If the device was supplied via an RCD, then any current to earth either via the earth wire or an unfortunate operator of the device, would unbalance the forward and reverse currents in the RCD, so causing it to trip and disconnect the supply. I have no idea what the arrangements for the High voltage wiring is, perhaps the power station alternator has its star point grounded, or perhaps not, I simply have no idea, nor do I know what the setup is with all the intermediate transformers is, but the final one always has its neutral grounded as far as I know.
    Just on the situation of an otherwise isolated from earth alternator becoming earthed when sitting on damp ground or if a machine connected to it was similarly situated on the ground, while I agree that it is possible for it to become partly earthed for want of a better term, I would point out though that most modern portable 240 volt alternators such as the Honda EU series machines are encased in plastic covers and have rubber feet, as would a considerable number of tools that get plugged into them, so I think on balance that mostly any earthing effect would be pretty marginal, and it would generally be a pretty high impedance connection to earth. I will check our EU20i tomorrow if I get a chance to see if it has a N-E link, but it seems that BobL has a 2MΩ impedance between neutral and earth points on his Honda genny, so it would seem that a good few do not have that link. If that is the case then there would not be much point using an earth stake without first making a change and grounding the neutral, if you want to get protected by using an RCD device.

  2. #17
    Join Date
    Aug 2010
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    Near Bendigo, Victoria, AUS
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    Every RCD has a test button. If I understand correctly how it works, that button connects active to ground/earth conductor via a resistor to divert 30mA or so out of the circuit. THat is intended to trip the RCD as there is now a fault current not returning via neutral.
    If the genset and attached equipment is not earthed. then this test button should not be able to divert any current and the RCD should not trip.
    Simon, could you test tis next time you set the genset up? I would be interested in the context of learning. Next time I have my little genny connected to the campervan, I'll also test the RCD.

    Here is another related curly one: In my shed, if I make a connection between NEUTRAL and earth, the RCD trips.... I cannot measure a voltage between them, suggesting that it should not be possible to have a current flow.... There is no connection between neutral and earth at the shed, but there is at the house - 30m away. Any guesses? Sending 30mA from active to earth WILL of course trip the RCD as well....
    Cheers, Joe
    retired - less energy, more time to contemplate projects and more shed time....

  3. #18
    BobL is offline Member: Blue and white apron brigade
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    Quote Originally Posted by jhovel View Post
    Here is another related curly one: In my shed, if I make a connection between NEUTRAL and earth, the RCD trips.... I cannot measure a voltage between them, suggesting that it should not be possible to have a current flow.... There is no connection between neutral and earth at the shed, but there is at the house - 30m away. Any guesses? Sending 30mA from active to earth WILL of course trip the RCD as well....
    This is normal is the circuit is is carrying even a small load.

    When you say zero V between N and E do you mean 0.0000000V and were you doing the measurement at the switch board or a GPO?

    With just the lights on in my shed (~2A in the shed A-N circuit) with a GPO switch on I measure 0.17V between N and E at the GPO, and interestingly a fluctuating 0-2V with the switch off.

    If 30 mA is needed to trip the RCD, using Ohm's law the highest R needed between N and E to prevent RCD tripping at 0.17V is ~6Ω and I any N-E resistance should not be this high even if the N-E connections is 30 m away.

    A good E-N connection should be <1Ω so for a 30mA current to flow you only need 0.03V between N & E (with the GPO switch on) to trigger the RCD.

    At the GPO, with the switch off, the N-E resistance at the GPO is >2MΩ because the GPO switch disconnects the N from the GPO outlet pins, and I can't measure the resistance with the switch on because the small 0.17V AC signal on the N effects the meter.

    The resistance of a human body between a fingertip and a foot (ie goes past your heart) is nominally a couple of thousand ohms so at 240V that pathway will 120mA, currents as low as 80mA can kill a healthy human and even 50mA can cause problems. I were to touch the N-E with the fingers of opposite hands with 0.2V on the N live circuit above the current through me would be 0.2mA and should not trigger the RCD.

  4. #19
    Join Date
    Jun 2010
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    Canberra
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    Quote Originally Posted by jhovel View Post
    Every RCD has a test button. If I understand correctly how it works, that button connects active to ground/earth conductor via a resistor to divert 30mA or so out of the circuit.
    An RCD unit does not have an earth connection, just A & N connections. The test button places a resistor between the active and neutral on opposite sides of the unit (i.e. load active to supply neutral).

    Back to generators though. If there is no N-E link on the generator, I'm not sure how an RCD is ever going to trip?

  5. #20
    Join Date
    Aug 2008
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    near Rockhampton
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    Quote Originally Posted by RustyArc View Post
    An RCD unit does not have an earth connection, just A & N connections. The test button places a resistor between the active and neutral on opposite sides of the unit (i.e. load active to supply neutral).

    Back to generators though. If there is no N-E link on the generator, I'm not sure how an RCD is ever going to trip?
    ecause it measures power going in and power going out. If there is a discrepancy it shuts the circuit down.
    B
    Gold, the colour of choice for the discerning person.

  6. #21
    Join Date
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    Quote Originally Posted by .RC. View Post
    ecause it measures power going in and power going out. If there is a discrepancy it shuts the circuit down.
    Sure, but if there is no link between neutral and earth, then there is no other return path from active except for neutral, hence the RCD will never see a discrepancy and will never trip.

  7. #22
    Join Date
    Oct 2015
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    melbourne
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    But unless your appliances are all double insulated, they will have a 'chassis/ground' which is returned to the 'chassis/ground' of your generator truck, via the 'earth' wire of the supply cord.
    So there is an alternative return path, although to be fair there is maybe a slightly lower probability of a fault. And of course your appliance could easily be connected to both the truck earth and the 'real' earth eg by sitting on the ground.


    Russ

  8. #23
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    Quote Originally Posted by russ57 View Post
    But unless your appliances are all double insulated, they will have a 'chassis/ground' which is returned to the 'chassis/ground' of your generator truck, via the 'earth' wire of the supply cord.
    So there is an alternative return path, although to be fair there is maybe a slightly lower probability of a fault. And of course your appliance could easily be connected to both the truck earth and the 'real' earth eg by sitting on the ground.
    If there is no N-E link, then while there's a possible path along the earth wire, chassis and all that, there simply can't be a circuit as the earth isn't connected to neutral, hence active and earth have no potential between the two.

    I'm going to have to do a bit of checking on the earth and neutral pins of the inverter and generator I've got access to...

  9. #24
    Join Date
    Oct 2015
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    melbourne
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    That makes sense

    It would behave like an isolation transformer in that case.

    Interesting...


    Russ

  10. #25
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    Jul 2006
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    Athelstone, SA 5076
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    Quote Originally Posted by .RC. View Post
    ecause it measures power going in and power going out. If there is a discrepancy it shuts the circuit down.
    B
    ine does this from time to time...why????does it drop the M below the top line or in RC's case the B..I'm glad its just not me...apologies for the hi jack on such an important topic.
    M

  11. #26
    Join Date
    Sep 2012
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    York, North Yorkshire UK
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    Hi Guys,

    It doesn't seem to be a browser issue because it only seems to happen on these forums.
    My browsers are Firefox and Chrome ! I do have another browser, Konqueror, but I've never used it to browse the internet.
    Best Regards:
    Baron J.

  12. #27
    Join Date
    Jun 2010
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    Canberra
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    There's a lot of kookiness on this forum - the text editor in the Reply section often doesn't allow standard Windows behaviours, but it seems to be random.

    I also received two email notifications yesterday for replies to threads that didn't actually appear in the thread itself.

    Edit - for those last two, the posts have appeared - I'm guessing moderation has been turned on for those threads (or posters) delaying their post appearing until approved.

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