Mixing 110VAC & 24VDC: Causes & Effects

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Discussion Overview

The discussion revolves around the mixing of 110VAC and 24VDC in a factory setting, specifically concerning the effects and causes of this mixing on equipment and safety. Participants explore the implications of an ungrounded DC supply interacting with an AC supply, as well as the resulting electrical measurements and potential hazards.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant describes an incident where the connectors for 24VDC and 110VAC melted, leading to confusion about the resulting voltage readings and safety implications.
  • Another participant questions how the two voltages were mixed, asking whether it was in series or parallel connection.
  • Some participants suggest that an ungrounded system can "float" at any voltage, potentially leading to unexpected readings and safety issues.
  • Concerns are raised about the polarity of the 24VDC supply and how it may have interacted with the AC line, with one participant noting that a multimeter set to DC could read AC as zero.
  • Participants discuss the possibility of other instruments in the same cabinet being affected by the 110VAC, with fluctuating readings observed.
  • One participant mentions the importance of checking manuals for equipment that may have capacitors to chassis or capacitive sensor elements that could be impacted by the mixed voltages.
  • There is a mention of a fuse that did not blow, suggesting that the current did not exceed its rating, which raises questions about the impedance in the circuit.

Areas of Agreement / Disagreement

Participants express uncertainty about the exact nature of the mixing and its implications. There is no consensus on the cause of the readings or the safety of the situation, with multiple viewpoints and hypotheses presented.

Contextual Notes

Limitations include the lack of clarity on the grounding status of the system, the specific configuration of the connections, and the potential for fluctuating readings due to the interaction of AC and DC supplies.

Who May Find This Useful

Individuals working in industrial maintenance, electrical engineering, or those involved in troubleshooting mixed voltage systems may find this discussion relevant.

s666
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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?
 
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how are they mixed? in series or parallel connection?
 
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.

Common of the power supply it was reading 24VDC
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:
Kholdstare said:
how are they mixed? in series or parallel connection?

They were connected in Series.
 
jim hardy said:
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.

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.
 
Your common was floating before and somehow the dc supply came between the common and ground.
 
""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
 
jim hardy said:
""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

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.
 
"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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