Different Voltages Same Pushbutton

[farmaped]

Hi, I wanted to get the opinion on using multiple voltages on a stackable contact pushbutton. For example, the same pushbutton in an industrial control panel switching 110VAC and 24VDC. These are signal, PLC input (power limited) ckts. To me, it is poor practice, however, it's out there. What's your thoughts? Thanks.

 

[jim dungar]

Hi, I wanted to get the opinion on using multiple voltages on a stackable contact pushbutton. For example, the same pushbutton in an industrial control panel switching 110VAC and 24VDC. These are signal, PLC input (power limited) ckts. To me, it is poor practice, however, it's out there. What's your thoughts? Thanks.

As long as you use independent contact blocks for each voltage it is not a problem. this is a common practice with H-O-A switches that have 24VDC into the PLC and 120VAC starter coils. Because they are control and signaling wires internal to a control panel conductor to conductor isolation is not required.

 

[tkb]

I don't see a problem with it as long as the different voltages use separate blocks on the operator.

I think it it done all the time.

 

[Rick Christopherson]

I have the very same situation. I am using a momentary pushbutton as a mechanical interlock for operator safety to energize a 120 volt circuit, but at the same time, I need to tell the PLC that the operator is pushing the button to activate a subroutine in the PLC. The contact blocks in an industrial pushbutton are not only electrically isolated, but they are physically isolated and can be removed and swapped out independently.

 

[Jraef]

It's done all the time. But the issue of separate contact blocks is important. If one side of a movable contact welds, it can allow a bridge between the separate voltages.

 

[Ranch]

energize a 120 volt circuit, but at the same time, I need to tell the PLC

And using a shielded pair from a switch contact block to signal a PLC input may be a good idea (as I am sure you know) if that circuit runs alongside a 120VAC interlock circuit for any distance.

I believe we are still a long ways off from weaning ourselves away from the extent we use 120VAC control in the US yet quite a ways into DC. I prefer DC for signals (PLC) and 120VAC for items such as panel cooling fans and large contactor coils. I think good systems are those that combine the two with good practice.