Inter posing relay 5

[Indy]

Hi,
With regards to a Siemens DO card which has outputs rated 0.5A@24VDC if we are using these output channels to power 24VDC solenoid valves (0.3W &7w)would these require interposing relays as they are within the limits of the channel?

Thanks

 

[RedSnake]

We never do.
One thing to look out for is the total Amp. on the card.
Usually for one 8xOut card it is usually 2 Amps, so they are constructed for valves with 2 coils on one valve so only one side is active at the time.

If you need different power sources like safety or regular control there can be interposing relays if it is a rebuild in a old facilities, but in a new ones you put emergency stopped valves on one card safety or standard and regular controlled on another with a separate feeds.

BR A

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

 

[thebard3]

It sounds like you're well within the rating of each output but like Red says, you need to ensure you're not exceeding the cumulative rating of the module. I don't use Siemens but if their module isn't up to triggering a full inventory of solenoid coils, I would be very surprised.

Brad Waybright

The more you know, the more you know you don't know.

 

[RedSnake]

Siemens have a lot of different cards.
For the 6ES7132-4BD02-0AA0 ET200S 4DO 24VDC 0,5A the total is 2A.
But for 16DO or 8DO it is not always the case, so it is best to check it in the manual for the card to be sure.

Best Regards A

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

 

[analogkid2digitalman]

Don't forget to add external back EMF suppression devices if such is not provided internally on the output module.

-AK2DM

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
"It's the questions that drive us"
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

 

[Indy]

Thanks for the replies.

 

[joezop]

It sounds like you're well within the rating of each output

 

[jraef]

My philosophy, which is NOT popular in some circles, is that it is MUCH MUCH easier to pull out and replace an interposing relay than it is to replace a PLC Output Card...

The unpopularity stems from this: it ends up costing more to add the relays and it takes more space in the control panel. But from my experience any perceived savings in going direct to the card can be wiped out in an instant when the card fails and you don't have one on the shelf, or even if you do, it involves a complete shutdown and more lost production to replace it. I learned that lesson the hard way at a steel mill; welded relay output on a PLC card, replacement card was available, but in another building. By the time we got the card, shut down the PLC (no hot swap capability), replaced it and turned it back on, the crucible was ruined, cost us over $50k, and that was in 1978 dollars! The interposing relays and extra space in the panel would have been far far cheaper.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[IRstuff]

Controller cards are often outrageously expensive to replace; my fridge lost its controller card, and supposedly, its factory was wiped out by the Fukushima disaster, so there were no cards even to be found for replacement, so only solution was to replace the system at a few thousand dollars. I lucked out that the only issue was actually a silly $3 electrolytic cap on the controller board, which I could buy at the now extinct Fry's Electronics.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

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[thebard3]

IMO, installing relays is exactly in opposition to the reason for using a PLC with appropriately rated I/O modules.

Brad Waybright

The more you know, the more you know you don't know.

 

[jraef]

IMO, installing relays is exactly in opposition to the reason for using a PLC with appropriately rated I/O modules.

I disagree. The main purpose of the PLC is for the LOGIC capabilities. If you are old enough to have experienced relay panels, you know that any changes in the control scheme had to be done via physical movement of wires, adding relays, timers, contacts etc. and resulting in a rat's nest of wiring relatively soon after commissioning. PLCs were a godsend in that aspect.

An interposing relay on a PLC output would never need to have the wiring changed or moved, it just mirrors the PLC output, protecting it from the ravages of the real world wiring. As a bonus, I sometimes used the little "ice cube" relays that have a push button or lever to manually force the contacts, those can be really handy for troubleshooting. I had a couple of years working as a System Integrator for a company that did prison systems, all of the door locks were controlled by PLCs. We had interposing relays for every output because the solenoids on the door locks were tough duty for the PLC relay card and if the relay contact welded, it resulted in the door lock being unlatched. In some of those prisons, that would be REALLY bad... When it happened, the guards knew they could reach in and pull the relay, which would drop out the solenoid and re-latch the door.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[itsmoked]

I always include a relay and it has to have a flag AND an indicator. That way on the phone you can have the customer over-ride the PLC for testing.

"The pump isn't running and I think it should be."
"Go to the relay labeled PMP and lift the little flat flipper thing on it. Did the pump run?"
"No"
"There's not a problem with your PLC. The problem is with the pump."

Keith Cress
kcress -

http://www.flaminsystems.com

 

[thebard3]

If you are old enough...

I am.
I will agree to disagree.

Brad Waybright

The more you know, the more you know you don't know.

 

[RedSnake]

Most systems I would build without interposing relays, but then I am as much a PLC and automation programmer as I am a maintenance electrician.
There could be situations as some here describes where they are a necessity.
And in some cases it might be better for the end user.

Where I work we would never put them in if not absolutely necessary and takes them away as soon as we get the chance.
All our electricians make there troubleshooting with the PLC, having interposing relays or other relay systems that coexist with the PLC makes two systems to troubleshoot.
Which makes them very confused .
And since I am the only one that is old enough ;-)
I end up having to take care of the relay problems...

The most common brand of PLC system we use, I can't remember having to change more then maybe 2 or 3 cards in 30 years and we have many.

/A



“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

 

[thebard3]

RedSnake--
I was just gonna say that.

In my (many years) experience, I've seen MOVs used on AC outputs but have never specified them myself. Either way I can't remember having an AC ouput fail unless it is used outside it's specified rating, or when not individually fuse protected and closing into a short. I have cooked a couple of DC outputs when switching into a larger inductive load which can be prevented by using a free wheeling diode.
Keith--
I would expect that if my customer's maintenance had appropriate NFPA70E training, then they should have personnel able to identify whether a particular PLC output was on. If they didn't then they probably shouldn't be opening the control panel anyway.

Again, that's just my opinion.

Brad Waybright

The more you know, the more you know you don't know.

 

[RedSnake]

Well I might add that most of our systems are 24 VDC and we almost never have relay outputs on the PLC cards.
I prefer to use varistors for DC and RC circuits for AC.
Free wheeling diodes that has become "popular" is half good at both but not good enough, in my experience.
On valves a ordinary diod of the right size does the job much better then most products you can buy ready made.

/A


“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

 

[3DDave]

Seems like the takeaway is - if the on-site people are skilled, practiced, trained, and certified in diagnosing PLC issues then the protection of an interposing relay isn't required. I would not have guessed that.

Knowing a number of people I have worked with ... I'd be voting for interposing relays.

Example - I was debugging a mechanical failure and required the use of a system controller and a motor amplifier. It was a constant fight to get this done as various pieces of test equipment kept getting taken to higher priority work**. I did not have an office I could lock it all away - it was about 200 pounds and bulky. You'd think a lab with a cypher-lock was enough.

Imagine my surprise when my red-tagged motor amplifier goes missing - it had been removed from production because the maker had made some change and the DoD didn't want the prototype installed. Where was it? Stolen back by production. They had done work over the weekend and burned up every production amplifier, one after the other. Here's $5k*, here's another $5k, here's another $5k; all of them. So they took my scavanged unit which didn't have the new mod and, apparently, were more than willing to burn that one too.

*Cost to repair. In working condition they were more than that.
**Higher priority meant - cash flow. The mechanical problem was worrying over safety of flight, but hadn't failed completely yet.

 

[RedSnake]

then the protection of an interposing relay isn't required.

What protection.

And where comes the relays in to the picture. ;-)

/A

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

 

[thebard3]

1926.960(b)(1)(ii)
Only qualified employees may work in areas containing unguarded, uninsulated energized lines or parts of equipment operating at 50 volts or more.
This means unqualified personnel SHALL not open a control panel with a 120 volt terminal in it.

Just sayin'...

Brad Waybright

The more you know, the more you know you don't know.

 

[itsmoked]

In my experience savvy customers can change relays but not PLC cards. PLC cards with relay-outs crap-out slightly more often than ice-cubes. I try to avoid relay cards at all costs, instead using much less failure-prone DC or AC (solid-state) output cards driving relays for the aforementioned reasons I gave and the ability of 'savvy' users to replace them via phone guidance. In one case while traveling thru Utah at 80MPH with failed A/C.

I've seen a dozen failed PLC relay outputs and only one failed SS output.

Keith Cress
kcress -

http://www.flaminsystems.com