Lighting earth

Discussion in 'Electricians' Talk' started by Kev shorter, Nov 14, 2021.

  1. Kev shorter

    Kev shorter New Member

    Hi I am puzzled by a lighting circuit that shows juice on the earth's using a non contact volt alert probe. Can enough power leak to earth from the live in 2&e? Are these volt checking pens so sensitive?
    No issues with breakers etc
    Hope someone can give me a pointer
  2. Teki

    Teki Screwfix Select

    Most competent electricicans don't rely on neon screwdrivers or volt sticks.

    The indication you are seeing is induced voltage from a nearby line conductor.

    Try using a multimeter to take measurements or troubleshooting.
  3. adgjl

    adgjl Active Member

    Volt sticks are for proving conductors live, not proving them dead. Neon screwdrivers belong in the bin.
    Sparkielev likes this.
  4. seneca

    seneca Screwfix Select

    The earth's are obviously not connected to earth!
  5. The Happy Builder

    The Happy Builder Screwfix Select

    I agree it’s a Red Flag that indicates further investigation is required.

    However, are you stood under a fluorescent light? There’s one of the reasons you may get a false positive.
  6. Tony Goddard

    Tony Goddard Screwfix Select

    I would proceed with caution, investigate the continuity of the earth conductor back to the CU.
  7. MGW

    MGW Screwfix Select

    That is clearly wrong, the next two posts show where the use has alerted you there is some thing wrong.
    The volt pen, or neon screwdriver shows there is a voltage between you and the item being tested, you as a large lump are likely at true earth potential, and the wire is at a different voltage, but it could be the reverse, you could be live and earth wire is true earth, but either way need to find out why.

    The pen will activate with static, rubbing on your cloths is often enough to light it. I love my neon screwdriver, we should use a clamp on meter on cables to see if there is and current before disconnection, but if the volt tester shows zero and the tester works in the proving unit we tend to assume dead, but if there is a borrowed neutral on disconnection it can become live, so the neon screwdriver is a good second string to ones bow.

    With LED lights there can be enough capacitance in the switch cables to cause the light to light dim or flash when switched off, especially with two way switching. There is normally a resistor or capacitor in the bulb to try and stop this, but not always big enough.

    Two wires held set distance apart called 300 ohm ribbon cable was good enough to form a slim jim aerial and allow me to transmit 30 watt when selotaped to my flat window in Hong Kong, this 300 ohm ribbon is not so different in spacing to twin and earth, only difference is frequency I was using 145 Mhz and mains is just 50 hz. But we have seen the trick with fluorescent tubes under pylons, clearly even at 50 hz it does transmit a little.
  8. Comlec

    Comlec Screwfix Select

    A volt pen or similar non-contact devices do not require a path to earth.
  9. The Happy Builder

    The Happy Builder Screwfix Select

    But in some circumstances such as if you are standing under a fluorescent light you are not at true earth potential.
  10. adgjl

    adgjl Active Member

    Do you have a multimeter?
  11. MGW

    MGW Screwfix Select

    Correct, it can be a path to any different voltage. Your hand to your jumper for example after you have rubbed the jumper. So if you have built up a static charge the unit can light when you touch anything not charged up.

    Very handy when working on the TBM in Hong Kong to stop getting shocks, only time I have worked on an IT system, hope I never have to work on one again, a really shocking system, pun intended.
  12. The Happy Builder

    The Happy Builder Screwfix Select

    A multimeter can give a false positive as well as can a two pole voltage tester, unless it has a low resistance test.
  13. adgjl

    adgjl Active Member

    True when measuring voltage, but measuring earth continuity to something definitely earthed, it will give the correct result.
  14. adgjl

    adgjl Active Member

    Correct. A “volt pen” measures the alternating electric field strength across the sensing tip of the pen, and does not require a path to earth or any other conductor.

    A “neon screwdriver” needs the user in the path to earth. A large resistance connected between the neon and the terminal the user holds prevents the user receiving a possibly fatal shock. If the resistor gets wet, the user is likely to receive at least a significant electric shock.
    Sparkielev likes this.
  15. The Happy Builder

    The Happy Builder Screwfix Select

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice