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How can my a/c be reading 6.7V?

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  • 21-02-2020 4:12pm
    #1
    case_sensitive
    Registered Users Posts: 852 ✭✭✭


    TL;DR the supply coming out of the stud wall in the bathroom downstairs meaures 6.7V on a known good multimeter, and unsurprisingly, won't power an LED mirror.

    Longer version
    Bought a house built in 1991 in Dublin 18 this summer, been doing bits to it, proper sparky and his team worked on rewiring kitchen. House wiring is ... eccentric.

    I took down a manky mirror and up-until-recently working 230V a/c bathroom-friendly fluorescent light fitting with shaver socket with 110v step-down transformer built into it.

    We'd bought a fancy demister + backlit LED mirror for upstairs bathroom, it arrived with a 3cm crack, and while the shop debated whether they'd take it back for nearly 2 months, we used it upstairs without a hitch.
    Shop replaced expensive mirror, and let me throw them a few quid for the cracked one, now I have two.
    Point is, I'm certain the mirror works.
    Install it downstairs and nothing doing. Took mirror down and checked the permanent live supply at the back of it, and it measures 6.7V.
    Measure it a dozen times, different ways clean the leads, unplug them, test other known good 230V sockets and lights, get ~236v on all of them. Back to the bathroom wall, 6.7V. Next day, still 6.7.
    I flip a few light switches in case it's somehow on a double-pole setup, and no combination of 6 switches makes it read higher than 7V, within measurement error.

    The bthroom light stopped working a few weeks or months ago, I assumed it was a dead bulb, didn't bother noting what if anything was going on at the time. 95% certain leccy works in kitchen were finished before it happened, and nothing more than paint has gone anywhere near anything on the ground floor since.

    Any idea what could be causing this?


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    #2
    [Deleted User]
    Posts: 7,499 ✭✭✭ [Deleted User]


    You need to find the other end of that cable.
    Is it wired in singles or one cable with 3 cores?
    It probably goes to the light switch.


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    #3
    Maewyn Succat
    Registered Users Posts: 506 ✭✭✭Maewyn Succat


    case_sensitive wrote: »
    TL;DR the supply coming out of the stud wall in the bathroom downstairs meaures 6.7V on a known good multimeter, and unsurprisingly, won't power an LED mirror.

    Longer version
    Bought a house built in 1991 in Dublin 18 this summer, been doing bits to it, proper sparky and his team worked on rewiring kitchen. House wiring is ... eccentric.

    I took down a manky mirror and up-until-recently working 230V a/c bathroom-friendly fluorescent light fitting with shaver socket with 110v step-down transformer built into it.

    We'd bought a fancy demister + backlit LED mirror for upstairs bathroom, it arrived with a 3cm crack, and while the shop debated whether they'd take it back for nearly 2 months, we used it upstairs without a hitch.
    Shop replaced expensive mirror, and let me throw them a few quid for the cracked one, now I have two.
    Point is, I'm certain the mirror works.
    Install it downstairs and nothing doing. Took mirror down and checked the permanent live supply at the back of it, and it measures 6.7V.
    Measure it a dozen times, different ways clean the leads, unplug them, test other known good 230V sockets and lights, get ~236v on all of them. Back to the bathroom wall, 6.7V. Next day, still 6.7.
    I flip a few light switches in case it's somehow on a double-pole setup, and no combination of 6 switches makes it read higher than 7V, within measurement error.

    The bthroom light stopped working a few weeks or months ago, I assumed it was a dead bulb, didn't bother noting what if anything was going on at the time. 95% certain leccy works in kitchen were finished before it happened, and nothing more than paint has gone anywhere near anything on the ground floor since.

    Any idea what could be causing this?

    Are you measuring between live and earth or live and neutral?


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    #4
    hesker
    Registered Users Posts: 1,490 ✭✭✭hesker


    Suspect broken or loose neutral


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    #5
    case_sensitive
    Registered Users Posts: 852 ✭✭✭case_sensitive


    BrownFinger wrote: »
    You need to find the other end of that cable.
    Is it wired in singles or one cable with 3 cores?
    It probably goes to the light switch.

    It's meant to be a permanent live, and isn't dependent on the position the light switch. There's a double-pole switch for the hall light and the main bathroom light, and none of 4 possible combinations of the the 2 makes the voltage at the back of the mirror deviate from 6.7-7V.
    The wiring going into the mirror is 3-core, the mirror only has live and neutral.
    I've measured across all combinations, but the 6.V measurement is, of course, between Live and Neutral / brown and blue. Earth is stripped, so definitely not got the wrong one in.
    I wouldn't discount the wiring itself being done wrong, but the light I removed appeared to have all 3 in the correct terminals.
    I'll run a long wire from the stud to the bathroom door and confirm continuity when the wife is gone.


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    #6
    case_sensitive
    Registered Users Posts: 852 ✭✭✭case_sensitive


    Maewyn Succat wrote: »
    Are you measuring between live and earth or live and neutral?

    Live and neutral


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    #7
    Maewyn Succat
    Registered Users Posts: 506 ✭✭✭Maewyn Succat


    case_sensitive wrote: »
    Live and neutral

    What's the voltage between live and earth?


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    #8
    blackbox
    Registered Users Posts: 4,151 ✭✭✭blackbox


    Maewyn Succat wrote: »
    What's the voltage between live and earth?

    ...and between neutral and earth.


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    #9
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,536 Mod ✭✭✭✭2011


    If I were doing this:

    First off check the settings on the meter. Is it set to AC? Is the range setting correct? Are the leads plugged into the correct points?

    Test the meter on a supply you know is healthy. See if you get a reading of around 230VAC. If so it would be a confirmation that the meter, leads and settings are ok.

    Assuming the above has been done, a reading of 7V most likely means either no phase (live) or no neutral, or both. So first work out if any of the wires are live as follows: Test between a known earth and phase. What do you get? Also consider the wire that you may expect to be the phase (due to cable colour or other reason) may in fact be the neutral of earth. On that basis measure the voltage between all wires and a known earth. Once you have determined if any cable is live you can check if any are connected to neutral.


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    #10
    iHungry
    Registered Users Posts: 409 ✭✭iHungry


    6.7v sounds like just pick up.


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    #11
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,536 Mod ✭✭✭✭2011


    This is pretty basic stuff.

    As this seems to be causing so much confusion it would be best to get a qualified electrician to look at it.


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    #12
    adrian92
    Registered Users Posts: 563 ✭✭✭adrian92


    hesker wrote: »
    Suspect broken or loose neutral

    I suspect hesker may be correct.
    I think you are measuring an induced voltage only, connect a light bulb accross your measuring points and see if this voltage goes to zero to confirm


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    #13
    [Deleted User]
    Posts: 7,499 ✭✭✭ [Deleted User]


    case_sensitive wrote: »
    It's meant to be a permanent live, and isn't dependent on the position the light switch.

    Yes,
    Where do you think its getting its feed from?
    I've seen houses where the cable for the shaver is wired but not connected because no shaver light has been fitted..


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    #14
    case_sensitive
    Registered Users Posts: 852 ✭✭✭case_sensitive


    Update:
    The light switch in the adjacent room was acting up, only working when the faceplate was pressed. Eek. Seemed like a wire had popped loose, and the faceplates were all stainless steel, so not a good look.

    After knocking off the breaker, popped off faceplate and not only was the live wire for the light switch loose, but there were a pair of lost neutrals floating about in there too. Seems some bright spark thought to power the feed for the bathroom from a light switch 1.5m away and didn't snug up the terminals. The mind boggles. Even if it did work, it would only have worked when the adjacent room's light was on.
    I still don't know where 6.7V reading came from, but once disconnected from the switch, and a feed taken from the supply side the mirror light is back in commission.


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    #15
    adrian92
    Registered Users Posts: 563 ✭✭✭adrian92


    iHungry wrote: »
    6.7v sounds like just pick up.

    Agree with iHungry.

    Sounds like you are picking up an induced voltage.

    there is no supply where you are measuring. Some opencircuit.


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