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110VAC mix with 24VDC

  1. Jan 15, 2012 #1
    I work in a factory where an instrument's bus contact we have has two connectors, one for 24VDC (used to power the instrument), and the other is for 110VAC (used for a bulb). Now, for some reason, the two connectors got melted and 110VAC got mixed with 24VDC. Our 24VDC is ungrounded, but for some reason when we measured the Common of the power supply it was reading 24VDC. Fortunately, when the cable that is carrying the digital signal from the instrument was touched (by mistake) by one of my colleagues, he received a slight shock, which I think was the 24VDC. But when measured by a multimeter, it reads 110VAC. Now, I'm confused, what happens exactly when we mix an ungrounded DC supply with an AC supply?
     
  2. jcsd
  3. Jan 15, 2012 #2
    how are they mixed? in series or parallel connection?
     
  4. Jan 15, 2012 #3

    jim hardy

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    if the system is completely ungrounded, it can "float" at any voltage with respect to "ground" (i prefer to call 'ground' EARTH, to distinguish it from system common)

    it can float at 110vac, even.

    what polarity? + or - 24?
    a dmm selected to DC will report 110 ac as zero
    and if your skin is real dry 110 isn't much of a shock to the thick places like callused fingers..
    so reading of -24 makes sense if +24 got shorted to ac line(hot).

    if nothing went up in smoke you're probably okay.
    but you should check around.

    i worked for a manufacturer who insisted on using transorbs from system common to earth, if you had his stuff there'd be some smoked transorbs.
     
    Last edited: Jan 15, 2012
  5. Jan 15, 2012 #4
    They were connected in Series.
     
  6. Jan 15, 2012 #5
    Thanks for your reply jim hardy,

    The weird thing the Common was reading +24, and some other instruments readings that were in the same cabinet as the instrument mentioned in original post were fluctuating from 0 to 1 repeatedly.
     
  7. Jan 16, 2012 #6
    Your common was floating before and somehow the dc supply came between the common and ground.
     
  8. Jan 16, 2012 #7

    jim hardy

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    ""The weird thing the Common was reading +24,""

    hmmm and it isn't a dual supply with plus AND minus 24?
    i'd be scratching my head too.


    ""and some other instruments readings that were in the same cabinet as the instrument mentioned in original post were fluctuating from 0 to 1 repeatedly.""

    i'd guess they were unhappy about the 110V.
    open their manuals see if you don't find capacitors to chassis, or capacitive sensor elements in them.

    I know, i know, real men don't read instruction manuals... =) =)

    have fun !
    old jim
     
  9. Jan 16, 2012 #8
    Thanks a lot Jim and Kholdstare.

    We have a 3.2A fuse which didn't blow, I guess there wasn't enough impedance for it to go over 3.2A.

    And in the factory I work in, believe me you have to read manuals since we have so many control systems and different kinds of instruments.
     
  10. Jan 17, 2012 #9

    jim hardy

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    "And in the factory I work in, believe me you have to read manuals since we have so many control systems and different kinds of instruments"

    :) :) i was kidding on that one.

    ditto where i worked.
    an industrial maintenance man encounters great variety of desigers' clever ideas, and learns from them all.
     
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