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Voltage difference

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  • 14-10-2020 4:54pm
    #1
    TJSparks
    Registered Users Posts: 12 ✭


    Changed a programmer on a domestic 2 zone heating system spurred off kitchen sockets. When I done a voltage test between neutral and earth I'm getting 10 ish volts and when heating is switched on it rises to 18 volts! Circuit is RCD protected but no fault tripping. Scratching the head, any ideas?


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    #2
    MoodeRator
    Registered Users Posts: 1,693 ✭✭✭MoodeRator


    Could it be EMF?


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    #3
    Lenar3556
    Registered Users Posts: 1,356 ✭✭✭Lenar3556


    Would be worth doing a loop impedance test to confirm the status of the earth.


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    #4
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    What is voltage L to É and L tó N?


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    #5
    John.G
    Registered Users Posts: 3,473 ✭✭✭John.G


    Depending on load there is always some voltage drop in the neutral due to its resistance, for example if I have a kettle and a toaster on then I get ~ 2V between neutral and earth but I would be quite alarmed if i got 10v.


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    #6
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    Without testing it, which should be easy to find the cause or condition of the setup, first thought is check for a floating earth conductor. This would be indicated by a lower voltage reading L to E than L to N if that is the case.


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    #7
    alan4cult
    Registered Users Posts: 1,583 ✭✭✭alan4cult


    Check N to E at each socket on that kitchen circuit would be something else you could check. Also, if possible, turn off every other MCB on this RCD and test same, then turn of RCD and test same.


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    #8
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    John.G wrote: »
    Depending on load there is always some voltage drop in the neutral due to its resistance, for example if I have a kettle and a toaster on then I get ~ 2V between neutral and earth but I would be quite alarmed if i got 10v.

    The differences caused by loading may be bigger on non neutralised installations. But wouldnt expect much change with the loading of a boiler heating system.

    It may or may not be an open circuit earth but....

    My first check is usually for indications of a floating (non connected open circuit earth) in the circuit though, for readings similar to the op, by means of quick voltage checks L-E, N-E.

    A voltage test from L to a floating earth wire will often give results near 200v, but not usually as high as L to an intact N. Even a TT earth with relatively high impedance earth rod (example 100+ ohms) will give the full supply voltage L to E, where as the floating earth with impedance in the 100s of k ohms or higher, will give a voltage reading L to E of 100 to 200v, with a digital voltmeter, but usually lower than L to N.

    It is like 2 resistors in series, 1 megaohm multimeter in series with 100 ohm earth rod, meter sees practically the full 230v.

    Put it in series with 100k ohm floating earth, meter sees 210 v or so.

    A 150v reading for example L to E, in my opinion leads to the conclusion of an earth that is not great. I would view it as a completely open circuit earth myself when its a digital meter reading.

    Loop impedance test will also indicate if the earth is open circuit.


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    #9
    TJSparks
    Registered Users Posts: 12 ✭TJSparks


    Bruthal wrote: »
    What is voltage L to É and L tó N?

    L-N 228v and L-E 218v. I was getting 10V between N & E at the wiring centre when heating was switched off and 18v when I called for heat.
    I'm thinking you might be right about the floating earth I'll go back and do a loop test on the circuit as I hadn't the testers with me on the job.
    Thanks for the input I'll give an update.


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    #10
    TJSparks
    Registered Users Posts: 12 ✭TJSparks


    alan4cult wrote: »
    Check N to E at each socket on that kitchen circuit would be something else you could check. Also, if possible, turn off every other MCB on this RCD and test same, then turn of RCD and test same.

    Thanks, I'll take readings from each outlet on the circuit. What's your thinking behind switching off MCBs and RCD?


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    #11
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    TJSparks wrote: »
    L-N 228v and L-E 218v. I was getting 10V between N & E at the wiring centre when heating was switched off and 18v when I called for heat.
    I'm thinking you might be right about the floating earth I'll go back and do a loop test on the circuit as I hadn't the testers with me on the job.
    Thanks for the input I'll give an update.

    Yea the voltage test is just an indicator of a poor or non existant earth. Loop test is the real test to do.


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    #12
    alan4cult
    Registered Users Posts: 1,583 ✭✭✭alan4cult


    TJSparks wrote: »
    Thanks, I'll take readings from each outlet on the circuit. What's your thinking behind switching off MCBs and RCD?
    Just to isolate it down to the lowest possible circuit that still reads 10V e.g. a neutral could be coming from another circuit all under the same RCD.

    I think testing at a few sockets will definitely allow you to see if it's present across the circuit.

    As others have said a loop impedance test is definitely a good idea if you have access to a Megger or other tester.


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    #13
    brightspark
    Registered Users Posts: 2,637 ✭✭✭brightspark


    A bad connection on the neutral?

    As the voltage is increasing with current there must be a bad connection (or insufficient cable csa.) between where you are measuring and the distribution board.


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    #14
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    Bad connection on neutral to a load would reduce the voltage seen by a voltmeter from phase to neutral across a load. Open neutral would show 0v phase to neutral. Load itself wouldn't operate properly or would be off.

    Earth seems to be 10 or 20 volts above N in op setup. Probable floating earth/induced/capacitive voltage. Or poor earth cpc and earth fault/residual current on it.


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    #15
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    alan4cult wrote: »
    .

    I think testing at a few sockets will definitely allow you to see if it's present across the circuit.
    Yes I'd do that alright, across several circuits.


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    #16
    John.G
    Registered Users Posts: 3,473 ✭✭✭John.G


    Not directly related buy could you please explain this.

    My 1972 built house still using fuses and is I think a TCNS system. the main earthing box where all the circuit earths terminate is connected to a earthing rod in the garage, there is also a cable from this earthing box going into the main ESB fuse box, I presume this is connected to the neutral??. Is this where the neutral is earthed and is the neutral itself earthed at various points on the ESB side.
    I have looked at a few TCNS systems but don't recall seeing any house earthing rod shown on them but presume its to give some protection if the main neutral is broken. Thanks.


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    #17
    Bruthal
    Closed Accounts Posts: 13,422 ✭✭✭✭Bruthal


    Neutral is usually earthed at multiple points from transformer to house, including at the house itself.

    Neutral failure outside house can leave the house earth rod as the only return path for house load. But it is usually a high impedance route, so all earthed items in the house often go up toward the supply voltage, magnitude depending on load resistance compared to earth rod.

    In a housing estate, all earth rods are connected together when all is intact, keeping the neutral well grounded.

    The main protection method for the above neutral failure is bonding. It won't bring the voltage down in neutral failure cases, but keeps everything in the installation at the same potential at least.


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    #18
    TJSparks
    Registered Users Posts: 12 ✭TJSparks


    Was back at the job with my MFT and attempted an FL test on the kitchen circuit, with no joy. The kitchen had a refurb obviously by cowboys 6-7 years ago and the "sparks" must have left a JB badly connected in a wall or floor which is anyone's guess if its accessible. Might have to borrow an earth from another circuit. Also found another missing earth on another circuit where the home owner drilled through a cable hanging pictures and reset the MCB after the flash minus the earth.


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    #19
    JimToken
    Closed Accounts Posts: 225 ✭✭JimToken


    TJSparks wrote: »
    Was back at the job with my MFT and attempted an FL test on the kitchen circuit, with no joy. The kitchen had a refurb obviously by cowboys 6-7 years ago and the "sparks" must have left a JB badly connected in a wall or floor which is anyone's guess if its accessible. Might have to borrow an earth from another circuit. Also found another missing earth on another circuit where the home owner drilled through a cable hanging pictures and reset the MCB after the flash minus the earth.

    You can't borrow earth's from other circuits

    Always take a reading first before you extend or spur off a final circuit


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    #20
    JimToken
    Closed Accounts Posts: 225 ✭✭JimToken


    There's at least 2 reasons off the top of my head why you can't borrow earth's ,maybe more

    A final circuit can be disconnected leaving you with no CPC at the appliance where you borrowed.

    Testing and verification


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    #21
    TJSparks
    Registered Users Posts: 12 ✭TJSparks


    I understand you are not supposed to borrow earths, it's a temporary fix as there are fitted floors that the home owner has agreed to replace once the fix is done after Christmas so it's better than leaving them with no earth in the meantime.
    Out of interest does anyone know where it's in the regs that you can't borrow an earth?


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    #22
    [Deleted User]
    Posts: 5,238 ✭✭✭ [Deleted User]


    better to share them Shirley


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    #23
    meercat
    Registered Users Posts: 4,730 ✭✭✭meercat


    You must Issue a notice Of potential hazard.
    Your primary concern is the safety and integrity of the installation and circuits. It’s not your concern what type of flooring is fitted. Run a new circuit back to the db protected by rcd(up through attic with trunking down bedroom wall). Don’t be doing temporary/unregulated work to suit the customer. Set your standards high and stick to them.


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    #24
    TJSparks
    Registered Users Posts: 12 ✭TJSparks


    meercat wrote: »
    You must Issue a notice Of potential hazard.
    Your primary concern is the safety and integrity of the installation and circuits. It’s not your concern what type of flooring is fitted. Run a new circuit back to the db protected by rcd(up through attic with trunking down bedroom wall). Don’t be doing temporary/unregulated work to suit the customer. Set your standards high and stick to them.
    It must be great to live in an ideal world?
    I ran a new cpc from the board and labelled up.


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