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3 Phase quick query

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  • 04-08-2020 1:28pm
    #1
    Effects
    Registered Users Posts: 9,761 ✭✭✭


    I have a quick question for a 3 phase supply that I'm hoping someone who works with it a lot can answer for me.
    The voltages seem a bit off when testing them, and I'm not sure why.

    P1 - N: 237v
    P2 - N: 56v
    P3 - N: 234v

    P1 - P2: 139v
    P1 - P3: 408v
    P2 - P3: 219v

    Any help much appreciated. These were the voltages last week, and now I've had a sparks connect into my electricity supply without permission, so looking to figure things out before I get a REC in to disconnect the work this other guy has carried out.


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    #2
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,536 Mod ✭✭✭✭2011


    Effects wrote: »
    I have a quick question for a 3 phase supply that I'm hoping someone who works with it a lot can answer for me.
    The voltages seem a bit off when testing them, and I'm not sure why.

    P1 - N: 237v
    P2 - N: 56v
    P3 - N: 234v

    P1 - P2: 139v
    P1 - P3: 408v
    P2 - P3: 219v

    Any help much appreciated. These were the voltages last week, and now I've had a sparks connect into my electricity supply without permission, so looking to figure things out before I get a REC in to disconnect the work this other guy has carried out.

    You have lost P2.


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    #3
    Effects
    Registered Users Posts: 9,761 ✭✭✭Effects


    Ah ok, so it's possibly blown a breaker/fuse further back than where I've checked. Can you explain why I'm getting 56v at P2? If it was blown, should I not be getting zero across it and N.

    I've tested it at a fairly old panel with neozed fuses. I'll check what's happening at the main meter panel.
    P3 currently is connected to my 63a fuseboard, which also contains storage heaters with separate 63a fuse that pull a 55 amp load.
    The sparks in question connected another 63a board to P3 without any consultation or checking loads that come off P1 or P3.


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    #4
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,536 Mod ✭✭✭✭2011


    Effects wrote: »
    Ah ok, so it's possibly blown a breaker/fuse further back than where I've checked. Can you explain why I'm getting 56v at P2?

    As there is no current flowing in P2 most meters will show a “floating” voltage such as this.
    Readings such as this are often due to capacitive coupling.
    If it was blown, should I not be getting zero across it and N.

    I suspect that you will get 0 volts if you use a lower impedance test instrument.


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    #5
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,536 Mod ✭✭✭✭2011


    Can you connect a suitable resistive load (maybe a filament type light bulb) between L2 and N ?
    Then measure the voltage with the same test instrument as before between L2 and N and it should read 0V.


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    #6
    Effects
    Registered Users Posts: 9,761 ✭✭✭Effects


    Thanks. I'm using a Fluke T5-1000 as far as I can remember.


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    #7
    Effects
    Registered Users Posts: 9,761 ✭✭✭Effects


    2011 wrote: »
    Can you connect a suitable resistive load (maybe a light bike) between L2 and N ?
    Then measure the voltage with the same test instrument as before between L2 and N and it should read 0V.

    I'll give that a shot. Thanks.


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    #8
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,536 Mod ✭✭✭✭2011


    See link



    “The other day I had a situation where I had 73 volts showing up on a circuit,” says Walstad, a second-generation electrician with more than 30 years experience. The circuit in question: a control circuit on a concrete transfer cart used to move material from the mixer to the molds in a factory making pre-cast concrete light poles. In the wet, dirty plant environment, the potential problems were many. “It should have been either 110 volts, or nothing.”


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    #9
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    If you connect the filament bulb to that phase, you will almost certainly see 0v then. P2 is likely disconnected.


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