×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Are you an
Engineering professional?
Join Eng-Tips Forums!
• Talk With Other Members
• Be Notified Of Responses
• Keyword Search
Favorite Forums
• Automated Signatures
• Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

#### Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

# Inter posing relay5

## Inter posing relay

(OP)
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

### RE: Inter posing relay

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

### RE: Inter posing relay

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.

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

### RE: Inter posing relay

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

### RE: Inter posing relay

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"
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

### RE: Inter posing relay

(OP)
Thanks for the replies.

### RE: Inter posing relay

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

### RE: Inter posing relay

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 ### RE: Inter posing relay 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: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

### RE: Inter posing relay

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

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

### RE: Inter posing relay

#### Quote (thebard3)

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

### RE: Inter posing relay

2
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

### RE: Inter posing relay

#### Quote (jraef)

If you are old enough...
I am.
I will agree to disagree.

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

### RE: Inter posing relay

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

### RE: Inter posing relay

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.

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

### RE: Inter posing relay

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

### RE: Inter posing relay

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.

### RE: Inter posing relay

#### Quote (3DDave)

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

### RE: Inter posing relay

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'...

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

### RE: Inter posing relay

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

### RE: Inter posing relay

Apparently Keith and I run into more 'bottom feeders' than others. I also always opt for interposing relays for the same reasons Keith and Jeff cite.

### RE: Inter posing relay

Danw2--
Don't go there. A couple of weeks ago I received a call about a problem at the plant. I was pretty sure I knew what it was and called the electrician to check it out. He reported back that everything I had him check was fine so I had to go in. I discovered that he was looking in the wrong control panel.

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

### RE: Inter posing relay

#### Quote (ItSmoked)

In my experience savvy customers can change relays but not PLC cards.

danw2 then you are using the wrong PLC cards, the ones we have used for the last twenty years, you exchange as easily as a relay it's just plug and play.

But we never have to.

/A

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

### RE: Inter posing relay

I've seen the same argument for using additional terminals beyond PLC cards as the maintenance gibbons onsite somehow manage to break the PLC cards, so providing terminals helps prevent that, but I suspect that the overall consideration is a little more nuanced than that.
There have also been improvements in automation equipment over the years that make replacement easier (pluggable terminals for each PLC card as an example), and there's also the cost, both in equipment and assembly for the panels to include the additional relays. My preference is generally additional relays, as it's then possible to run with a single type of output card, and everything else gets interposing relays or equipment suitably rated to drive whatever's at the other end.

### RE: Inter posing relay

LOL... "maintenance gibbons "...

I use the term "Bubba". In my experience the worst maintenance tech in any given facility is invariably named (or nicknamed) Bubba. He has no neck, hands like meat hooks, fingers like sausages and a brain like Emmantaler cheese.

This guy is a Bubba...

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

### RE: Inter posing relay

I have only come across one like that during my years, but his nickname was like "weld guru" and he was actually good at his job and he made great sponge cake.

/A

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

#### Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

#### Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

#### Resources

Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a partâ€™s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

# Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

• Talk To Other Members
• Notification Of Responses To Questions
• Favorite Forums One Click Access
• Keyword Search Of All Posts, And More...

Register now while it's still free!