Control Panel With 70v AC residual voltage .

Discussion in 'General Electronics Chat' started by t00t, Sep 14, 2015.

  1. t00t

    Thread Starter Member

    Jan 22, 2015
    Dear all ,
    I am currently studying a really old control panel. There is 60 - 90 v on a turned off temperature controller (which controls a few heaters). Why does the live wire still carry 60 - 90 v. It should be either 230v when ON or 0v when OFF just like the other temperature controllers. There also isn't any thing that is giving off this range of voltages on the control panel. And I think it is this residual voltage that causes my temperature controller lcd screen to spoil .

    The connections are as follows ---> Temperature controller - PLC - Heaters

    So my question is .

    1) where does this voltage comes from ?

    2) Could this voltage be messing around with the input and output on the PLC ?

    3) Could this be due to the fact that in this panel all the control wires are stacked very closely with power wires ?

    Thank You.
    Last edited: Sep 14, 2015
  2. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    Do you think you have provided enough information for anyone to answer your question?

    According to you the voltage comes from nothing so are you just inviting people who have never seen the panel to guess what this nothing is?

    You then refer to a PLC......
    Last edited: Sep 14, 2015
    Johann likes this.
  3. strantor

    AAC Fanatic!

    Oct 3, 2010
    You leave no details from which I could provide an informed guess, but i can still say this: it could be bogus induced voltage from parallel conductors in conduit, or IT COULD BE REAL, DEADLY VOLTAGE. BE CAREFUL.

    I have seen both cases plenty of times. One time i was servicing a cabling machine and found an oddball voltage where none should be, about 137v IIRC. I checked all disconnects associated with that machine and there was no power coming to the panel from any source. I even turned off the breakers for 120v lighting in the area to be safe, but that voltage was still there. I made the bad assumption it must be phantom induced voltage. After it shocked the piss out of me, I traced it back to a wirenut in a conduit full of rainwater, that conduit shared conductors for another machine that had a DC motor with 180V power applied (also via submerged wirenutted conductors ).

    When working in a plant environment, especially one that has old machines that have been in operation for decades, assume that every new task is a test; a trap set by some former employee who never got to see the intended victim suffer.

    EDIT: a good investment is a DMM with a low impedance measuement function. For example, see Fluke 289. This will provide a clue about phantom induced voltage or real power.
    Last edited: Sep 14, 2015
  4. t00t

    Thread Starter Member

    Jan 22, 2015
    could it be due to wires being packed to closely together causing capacitive coupling ?

    If so I can just ground the shielded part of the wire and solve the problem ?

    Thank you
  5. MaxHeadRoom


    Jul 18, 2013
    Using a high impedance type meter leakage or inducted voltages can often be detected, one way of verifying if the source is because of this is to connect a 120v/240v lamp between the measured source and ground, if the source is high impedance in nature, the voltage should collapse.
    Johann likes this.
  6. Johann

    AAC Fanatic!

    Nov 27, 2006
    My thoughts also, Max!
    I am from the old school and when in doubt, still firmly believe in using a so called test lamp, just to make sure if the apparent voltage is real or not!
  7. #12


    Nov 30, 2010
    Electronics designers don't guess very well. Perhaps you should ask a Fortune Teller or a Psychic.
  8. BillB3857

    AAC Fanatic!

    Feb 28, 2009
    We were supposed to use these
    to check for the difference between stray and current carrying voltages. It has a solenoid that pulls a plunger down various depths to indicate the voltage present. Phantom voltage will not provide enough current to energize the solenoid function. The little clear plastic cap contains a magnet on a shaft. It will indicate polarity for DC voltages.
    strantor likes this.