Can DPDT hand switch pass 120Vac & 12Vdc without interference?

[NoHoCa]

I'm tasked with adapting an old hard-wired control system to be a manual backup to a new PLC-based system. An operator must be able to designate which analog signal (4-20 mA) controls a pump by using a hand switch. Easy enough to route either one analog or the other through the contacts of a hand switch.

But I also want a discrete I/O indication that the switch is in one position. Since the discretes are 120Vac, it seems that a DPDT switch handling 12Vdc on one set and 120Vac on the other set of contacts would not be a good thing.

Contrary to my forum ID, I'm an ME who regularly works with control systems. Would you mind clueing me in on this subtle problem? Thanks...

 

[itsmoked]

The switches are voltage rated. That also means between the different sections of the switch. If it's rated for it you can mix the the voltages. No Problem.

Many might argue with that philosophy as being treacherous or a bad idea but, I would not. I do it all the time. I believe the customer who is generally interested in the lowest price to be quite happy with that methodology. I also believe you will have a much more trouble free and less complex system for someone to troubleshoot in the future. Not doing it will often create much more hardware in a system. Always a bad thing, (unless you're a salesman).

You would not generally want low voltage logic(5V or 3V) sharing a switch with 24VAC or above for two reasons. 1) Noise from the higher voltage/energy circuits coupling to the low voltage. 2) Contacts rated for low voltage are generally made of different material than for voltages over 12V so the switch contacts would not be compatible for both voltages in one switch.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[NoHoCa]

I think what you're saying here is yes, but no. Are dual-rated switches easy to find? Your comments about noise and coupling are exactly what I'm concerned about. Unsure if there are other solutions that I'm just now aware of since I'm not that familiar with electrical switches and supply catalogs.

Unfortunately our client's insistence on keeping the old system based on single loop controllers, alongside the new PLC system, is resulting in much more hardware than one would design from scratch...

 

[JLSeagull]

...keeping the old system based on single loop controllers, alongside the new PLC system...
Many old timers have seen this sort of thing before. It is surprising to see it today.

I would bring multiple analog signals into the PLC and use a software selector from the operator workstation HMI.

 

[ScottyUK]

JL,

It's not just old timers who see it, unless I am one now! I have this problem with a couple of items of plant, the Flat Earth Society won't let us replace the old single loop controllers even though there is an SLC500 in the cubicle (replacement for an elderly TI-505) that has more than enough capacity to replace the three controllers. Mind, if the FLS members were riding in a square-wheeled cart and I offered to fit round wheels they would refuse that too...


----------------------------------

Sometimes I only open my mouth to swap feet...

 

[NoHoCa]

...I would bring multiple analog signals into the PLC and use a software selector from the operator workstation HMI...

Thanks JL. But what I neglected to mention is that in the case of a PLC failure (very unlikely), the old system must operate on its own. I guess that's the root of my difficulty. Perhaps this client is also a member of the Flat Earth Society.

 

[dpc]

There are no code or safety issue mixing 120 V with low level signals on the same contact block, as long as the contacts are isolated, but there's no need to do that.

You can purchase selector switches with multiple contact blocks. Most suppliers have hermetically sealed contact block options for signal level circuits.

The 120 V discrete can be on one contact block and the 4-20 ma on the hermetically sealed block. I would not expect much electrical interference or noise issues with a 4-20 mA signal, unless you are switching high amperage on the 120 V ac.

Check out Square D 9001 series selector switches, or equivalent by about anyone else.