Discussion in 'Electricians' Talk' started by Ozwald Cobblepot, Jun 14, 2019.

  1. Came off my ring main recently to install an appliance and noticed the voltage was approximately 340v. Any idea why this is and/or should I be concerned?
  2. Hans_25

    Hans_25 Screwfix Select

    What were you measuring exactly? Mains voltage is 230V RMS which is a peak of 320volts.
  3. Mains voltage. I was putting a volt meter across the + and - in the junction box just to see if I had removed the correct fuse (old fuse box, not labeled), and noticed the high voltage.
  4. unphased

    unphased Screwfix Select

    I think you must have misread the reading. 340 V is too high for your supply. You would damage equipment with that level. The official voltage for single phase supplies to domestic premises is 230V +10% / -6% or 216.2 V to 253.0 V. This is information provided in Appendix 2 of the wiring regulations. If you are absolutely sure you measured it correctly report it to your electricity supplier.
  5. Bazza

    Bazza Screwfix Select

    Do you have your meter on the correct range? You may get something like that if you have it set to DC instead of AC.
    What make/model of meter do you have?
  6. Bob Rathbone

    Bob Rathbone Screwfix Select

    I would check the meter on another supply in a friends house. I reckon the meter is inaccurate.
  7. nigel willson

    nigel willson Screwfix Select

    + & - on an ac supply , interesting
  8. This is the reading just now. Am I being a pleb, or is something wrong?.....OR is something wrong AND I'm a pleb anyway :))

    Attached Files:

  9. Bazza

    Bazza Screwfix Select

    Interesting, but tells us nothing. Have you done this yet:

  10. I haven't, just because I live in the sticks
  11. Pollowick

    Pollowick Screwfix Select

    What are you seeing from Live to Earth and Earth to Neutral?
  12. ajohn

    ajohn Screwfix Select

    I'd suspect a new decent meter is needed. Possible probe / wires on them problem. Maybe poor contact - a brillo pad on the probe tips might help with that. If it has a diode check facility / continuity check you'll probably find it difficult to obtain zero if it's the probes / wire.

  13. JustPhil

    JustPhil Active Member

    Could be the meter is displaying peak to peak voltage... 230v RMS = 325 V pk-pk.
    Not a meter I recognise, guessing it’s a cheapie?
    Also, is that a low batt warning on the display... could be that.
  14. Woloumbo

    Woloumbo Active Member

    batteries need changing for sure
  15. Pollowick

    Pollowick Screwfix Select

    Your maths is way out ... 230v rms = 650v pk-pk
  16. seneca

    seneca Screwfix Select

    I think you would be replacing your light bulbs daily if the voltage was that high!
  17. JustPhil

    JustPhil Active Member

    Ah yes, I’m out by a factor of two.
  18. peter palmer

    peter palmer Super Member

    Its 325v from 0v in both the positive and negative half cycles which is how most people understand it, you will never get a potential difference of 650v no matter how you measure it.
  19. ajohn

    ajohn Screwfix Select

    I'm not going to work it out but it's mains voltage times square root of 2 peak each side of neutral.

    :) Well I have, it's 325v at 230rms. Ours is always higher. A junk meter might read 300v plus but if a dodgy lead or probe any old number can crop up.

  20. ajohn

    ajohn Screwfix Select

    Personally if I wanted to test for mains there or not and makes sure live is where it should be I wouldn't be without something like this. I always check mains is there with it first but to be honest I have never had one break.


    Ideally longer than 150mm and capable of 440+ volts

    Cheap meters often aren't much good especially in the area of the probe tips and security of the wire at each end of the test probe.


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