PLCs galvanic isolators and interface relays 2

[Carlos Melim]

Good morning everybody,

I have worked with SIEMENS 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 new project.

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 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.

But maybe I´m being too conservative.

I appreciate your expert advices on this issue.

Best regards.

Carlos Melim

 

[itsmoked]

Hi Carlos,
I've never used any kind of isolators and have PLCs in all sorts of weird environments. All I've seen have extensive I/O protections.

If you are running really long lines far out into the field then extra protection could be worth the hassle.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[danw2]

I don't think it's new vs old, it's what the AI was designed as, with or without isolation.

Most PLC manufacturers have a selection of Analog input cards: the good, better, best options. You get what you pay for.

The low end AI cards are usually single ended inputs with a common for all the AI (-).

Those boards can tolerate maybe 10V of common mode before the inputs saturate and drive offscale. The classic symptom is "I added an input to the x number that were already there and when I did that all the previous inputs went crazy and drove offscale. What happened?"

The solution for that is external isolators. Is external isolation always needed at the outset? Not on the higher end AI cards which have isolated inputs.

There are even single ended DCS inputs (at 3x the cost per input as a PLC) on the assumption that the field instruments are isolated 2 wire 4-20s (eliminating a field side ground) and there's common DC loop power supply.

After all these years I still can't tell people how to predict whether they'll encounter a common mode ground loop problem. No, one exception the home brew plant methane burning electrical generator made from salvaged truck engine and a PTO coupled generator. I insisted on external isolators for the AI's and quoted such but they didn't buy the isolators or use any. They commissioned it themselves and the AI's fried immediately during commissioning. When they called me, I pointed to line 3 on my quote and asked why they didn't buy what I recommended and hung up the phone.

I just figure that any project worth doing is worth the premium cost of the better/best isolated AI inputs.