Relays for isolation of PLC input signals.

[AutomationTechBrian]

I'd like to talk this through a little:

Relays are regularly used for PLC outputs, regardless of a need for signal conversion. But what about 24vdc input signals? Most of my experience has no signal isolation for 24v discrete input signals into PLC cards.

I'm in a conversation with a application engineer. My customer is replacing a panel. The integrators replacing the panel thought the discrete input signals were using 120vac, so they sent a panel with 120vac relays to convert the input signals for the 24vdc PLC input card. The input signals are not 120vac, they're 24v. And this is a replacement of a legacy panel, with the 24v input signals previously coming straight into the PLC (no relay).

In switching out the 120vac relays, I didn't think twice about not using a relay for 24v input signal isolation. In my eyes, the input card doesn't "pull" amps, and the relays become another point of failure. But the engineer is casting doubt on my decision with admonishments that they would not be held responsible for burnt-out inputs. I don't come across many burnt-out inputs... none that I can remember. What is your take on this issue? Do you use relays on input cards?

 

[Cheeseface]

If they don't share the same common, use a relay.

 

[padees]

I'd like to talk this through a little:

Relays are regularly used for PLC outputs, regardless of a need for signal conversion. But what about 24vdc input signals? Most of my experience has no signal isolation for 24v discrete input signals into PLC cards.

I'm in a conversation with a application engineer. My customer is replacing a panel. The integrators replacing the panel thought the discrete input signals were using 120vac, so they sent a panel with 120vac relays to convert the input signals for the 24vdc PLC input card. The input signals are not 120vac, they're 24v. And this is a replacement of a legacy panel, with the 24v input signals previously coming straight into the PLC (no relay).



Relays can most certainly be another point of failure, and if you have dedicated IO, you may have to drop in a few interposing relays to address this. As long as the circuitry and devices are sized properly, should not be an issue.

 

[ojz0r]

Its faster and cheaper to replace a burnt-out relay than a whole I/O card.

 

[padees]

Its faster and cheaper to replace a burnt-out relay than a whole I/O card.

Yep! As long as you know what fried.

 

[jaden]

Adding relays to PLC inputs could reduce reliability if the input draws less current (a few ma.) than the minimum current rating for the relay.

 

[robertmee]

Also need to consider operate and release times of relays adding inherent delays to the input signal. Depending on relay design could be as little as 3ms or as much as 50ms. I've rarely seen relays used on common 24vdc inputs. Most input modules are well protected against shorts. Now putting 120vac to a 24dc input, all bets are off.

 

[strantor]

I'd like to talk this through a little:

Relays are regularly used for PLC outputs, regardless of a need for signal conversion. But what about 24vdc input signals? Most of my experience has no signal isolation for 24v discrete input signals into PLC cards.

I'm in a conversation with a application engineer. My customer is replacing a panel. The integrators replacing the panel thought the discrete input signals were using 120vac, so they sent a panel with 120vac relays to convert the input signals for the 24vdc PLC input card. The input signals are not 120vac, they're 24v. And this is a replacement of a legacy panel, with the 24v input signals previously coming straight into the PLC (no relay).

In switching out the 120vac relays, I didn't think twice about not using a relay for 24v input signal isolation. In my eyes, the input card doesn't "pull" amps, and the relays become another point of failure. But the engineer is casting doubt on my decision with admonishments that they would not be held responsible for burnt-out inputs. I don't come across many burnt-out inputs... none that I can remember. What is your take on this issue? Do you use relays on input cards?

If the Engineer says it's ok that you remove the relays then he is formally acknowledging that they shouldn't be there (or didn't need to be there). If they weren't needed then either they upsold you (or your client, unclear), or they did a **** poor recon job prior to building the panel. Either way, you remove those relays and they'll have questions to answer. Nobidy likes answering those kinds of questions.

 

[JeremyM]

I'd like to talk this through a little:

Relays are regularly used for PLC outputs, regardless of a need for signal conversion. But what about 24vdc input signals? Most of my experience has no signal isolation for 24v discrete input signals into PLC cards.

I'm in a conversation with a application engineer. My customer is replacing a panel. The integrators replacing the panel thought the discrete input signals were using 120vac, so they sent a panel with 120vac relays to convert the input signals for the 24vdc PLC input card. The input signals are not 120vac, they're 24v. And this is a replacement of a legacy panel, with the 24v input signals previously coming straight into the PLC (no relay).

In switching out the 120vac relays, I didn't think twice about not using a relay for 24v input signal isolation. In my eyes, the input card doesn't "pull" amps, and the relays become another point of failure. But the engineer is casting doubt on my decision with admonishments that they would not be held responsible for burnt-out inputs. I don't come across many burnt-out inputs... none that I can remember. What is your take on this issue? Do you use relays on input cards?

1) Resistive stuff (input cards) draw a load. Probably no more than 4 mA per point normally.
2) Inputs usually aren't isolated, so the relay layer could eat a lighting strike / surge without knocking out the card. However, the cards use opto-isolators anyway to protect the rest of the rack, so there's that.
3) Replacing the cubes on relay bases already installed and wired isn't the worst thing that could happen.

 

[mikeexplorer]

There could be valid reason to do this. In my case I am using this solid state relay block as isolation for the inputs because these inputs are connected to my model railroad layout track to sense where the train is on the layout.



In the event of a derailment, AC power from the transformer could feed back into the inputs potentially blowing one out, so this isolation relay block will prevent this. Worst case, it pops the relay and I just replace it.


Mike

 

[chud]

Is the 24v going out to the devices thats giving the inputs fused? That should be sufficient

 

[AutomationTechBrian]

Thanks for all the comments. I've always relied on email notifications to alert me to the replies. I didn't get any notifications, and the week was so busy, I didn't check back until now.

This panel is a bit of an oddity, starting with the 24v not coming from inside this panel... Killing the 120v does not kill the 24v. The panel has been in service over 20 years, and things have been changed piece-meal over that time. I'm not sure that all the 24v input signals originate from the +24v distribution terminals in this panel. In fact, I know some originate from the panel next to this one. So I can't be certain fusing the +24v distribution terminals will affect all of the input signals. But I suppose throwing a few fuses to the distribution blocks would not be a bad idea.

I talked through the input relay question with the engineer, and he's now good with just using terminal block for the inputs. We'll just make sure all the electronics are fused, as you should.

Here's a visual of the panel we are replacing. The wires were not marked well, and tracing them to where they go would be a nightmare. So this pic was taken while I was in the process of labeling all the wires with shrink-wrap labels, even the ones not used.

Btw, those are Modicon PLCs.

 

[padees]

Yikes!

 

[AutomationTechBrian]

Lol, I didn't think I'd be the one installing the new panel. They told me the manufacturer would install it. I have a huge extruder project with another customer that is in the testing stage. Fortunately, there are mechanical glitches that they can work on while I'm doing this.

Here's an updated pic. This was taken while I was converting the I/O relays. The input relays have been removed, and replaced by terminal block. And the bottom two output relay blocks have been changed to 24v wiring (blue).

 

[JLand]

Coming from the integrator side of things, isolation relays are really important. We can never be 100% certain what is happening on the other side of the fence. We could waste a bunch of time ensuring all documentation is 100% accurate and then integrate our common (and even then there are a lot of things that could be problematic due to age), or we could just add some cheap ice cubes and call it a day. Ultimately it's cheaper and much more foolproof just to add the isolation relays. Isolation relays ensure we know exactly what is going into our hardware and ensure any unpleasantness in crusty old equipment goes no further than a cheap and easily replaced relay. It's usually in everybody's best interest to use them.