PLC IOs Galvanic Isolation


Thread Starter


Good evening Sirs,

I have worked with SIEMENS S7-300 PLCs for a long time, including solutions design, programming and commissioning. Now I work as a consultant, and I was asked an opinion about a project with a SIEMENS S7-1500 PLC.

In my projects I've always used third-party galvanic isolation modules to the analog inputs and interface relays for the digital inputs and outputs. The PLC IO modules that I've used were isolated, no embedded output relays. The whole idea was to protect the PLC modules. An interface relay or a galvanic isolator are, of course, cheaper than a PLC module. The supplier of the solution tells me that the new PLCs don't need such protections.

I want to have your opinion on this issue, because I still think that the relays and the galvanic isolators should be used.

Does it make sense to expose the PLC modules to the electrical hazards typical of an industrial installation? Or do you think the galvanic isolators and interface relays should be used?

Please share your experiences with PLCs on this matter.

Best regards.
Carlos Melim
I think if you have to rely on isolators you are doing something wrong. 120 VAC inputs can be referenced to ground, so can your input and output modules.

Analog transmitters and control valves are isolated, the modules and 24 Volt DC power supplies should be grounded also.

There was a time when isolators needed to be used, but I haven't bought one in 15 years.
You already have the data to answer the question.

If you've been doing this for a decade or two, and have used isolation, then add up the cost that was saved on point failures vs. the material, labor, and panel space cost of the isolation. If you've saved one module in all that time, you'd have been better off without the isolators. If you've saved 25, then then you're happy you had them (but in that case I agree with Roy Matson, you should look at the operating environment in general).

In my case, I usually do not isolate. My experience has been that triac outputs are fragile so I never use them. I have relay output modules, and of course DC output modules that have been quite reliable in service. The odd case where I do would be something like I need just a couple AC inputs in a cabinet, so it's easier to interpose relays than buy a module, or I'm getting a DI as a voltage rather than a dry contact from someone else's system, and having a relay makes it easy for them to adapt to whatever their control voltage is (plus giving me isolation from a system not of my design).
Slightly off-topic, I put an RS-485 repeater/isolator right at the CPU and at least one more half way down a long-run 485 multidrop network, just in case there's a electrical fault or lightning strike then the CPU and half the network survives. This has paid off at least three times that I'm aware of when half the network devices had blown 485 ports.
In agreement with the two first responders, isolators are not beneficial in typical processes or in manufacturing.

They were common fifty years ago where pneumatics were also common, but not now.