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Chief Flunky
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As the title says, what's everyone's opinion on when it is worth using interface/isolation relays on PLC I/O. Non of our stuff has anything isolated unless it was actually required to drive the load.
There are 7 common uses for relays:
1. The first one is if you are trying to drive a bigger load. Many PLC output cards can only handle 1 A and/or 250 VA. Even miniature relays handle 10 A and 1000+ VA so they can easily drive even the largest size 9 contactors with economizes.
2. Different voltage systems or different grounds. The most common issue is a 24 VDC output driving an AC load. But you can run into problems where you want/need to isolate different grounds/neutrals from a remote signal where an isolated inout card is fairly expensive so you need a relay on one end or the other, turning the IO into a dry contact.
3. Safety. Safety circuits are designed to operate independently of the control system so they even if a PLC programmer does something stupid, the safety system prevents disaster. For example a flame safety relay only allows a burner to light if everything is done in the proper order and it detects a flame.
4. Relay logic. Kind of defeats the purpose of having a PLC though.
5. Isolation. PLC transistor outputs are semiconductors. The “off” state leaks a little voltage/current. It causes two problems. The big one is if you try to drive say a small 24 VDC pilot light, the light won’t actually turn off if the output is off. Adding a relay solves this. Another common one is say someone puts the start and stop buttons in the same conduit with the motor leads in a long conduit run that is now buried in concrete or dirt. The motor starts fine but won’t stop because the induced voltage on the control wiring holds the starter in. Ideally the EC pulls a second conduit and/or the greenhorn EE gets ripped a new one but what is done is done so insert a big heavy machine tool relay with a hefty coil to switch the inout and go on with life.
6. Fusing. Ideally you fuse every output. Some people just add relays and use them as “fuses” if a load fails. It’s not a good idea but it’s common.
7. Along the same lines often they get added as a troubleshooting tool. A lot if old school troubleshooters like to use relays as disconnect switches. Just pull the relay out if you want to disconnect an output instead of lifting wires.

However every relay you add does three things:
1. It eats PLC IO unless you add surge suppressors from inductive kickback. This one is solvable but surge suppressors are also consumables.
2. It adds another layer of parts so another point of failure.
3. PLC transistor outputs typically last 10-20 years. Relays last between 100,000 and 10 million cycles. If you have a panel light and blink it once a second in a weekend with a line down the light blinks 86,400 times. Within a year even the best relay has to be replaced. This applies to any IO that is constantly switching.

So I’m a relay minimalist. They should be added when needed but not just because it can eliminate problems. As an example I’ve seen upgrades where the installers put relays in EVERY IO for isolation. This eliminates any need to trace out/figure how what the original IO was wired but within a couple weeks/months the failures showed up.
 

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Chief Flunky
Field Service Engineer
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In my experience, using relays on discrete outputs (DO) is very common in water treatment plants for isolation. Sometimes also on discrete inputs (DI) but not as often. The consultants always specified 10 amp relays which are relatively huge and take up ALOT of panel space. I get the need for isolation due to lightening or few of mixing up voltage sources but extra panel space, increased material costs & labor costs to wire them are the downside.

Relays were rarely used in the industrial/machine control jobs I worked on due to the need for fast response time and the large number of cycles per day. Relays would quickly wear out. Neither of these is much of a problem in the water industry.
An octal relay does take up quite a bit of space. The square style just isn’t much better. But have you looked at the slim interface relays made for the task?


1/4” wide each, just slightly larger than a 5 mm terminal block and it is laid out like a triple terminal block. Lots of vendors for these. The relays themselves are just PCB relays. The downside is only 6 A contacts. But that runs most everything.

If you must have 10 A then the best you can do is miniature relays. They are similar to the slime lines but usually use 2 terminals side by side. The one below is 15.8 mm (3/8”). Not picking on Finder, just that lately Automation Direct is out of stock on almost everything.

 

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Chief Flunky
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We have a few Zone 2 panels that have relays in them and they are constantly burning out. I got tired of tossing them, so I tried moving the wiring down a set of contacts. On a 14 pin relay with only one set of contacts being used, it was easy to track what was going on. The odd time it has been a coil failure, but mostly just contacts. Then the odd person gets in the mix and changes the relay and doesn't return the contacts to the first set. So it is far from a perfect system, that's for sure...
Most of the time contact failures are one of three things.

First one is inductive kick. When you switch a big inductive load off the magnetic field in the coil has to go SOMEWHERE. This is also a bigger problem for TRIAC and transistor outputs. The solution is add RC snubbers or diodes. You can buy them prepackaged like these.


Second is violating either the VA rating or current rating or both, or using the relay as a fuse. They don’t give a short circuit rating but either VA or just assuming 30 times the contact rating is generally pretty close to the correct number. This should be obvious when you check ratings.

Third is not paying attention to the contacts. General purpose contacts come in 3 types: DC, AC, and “dual rated”. AC contacts are often silver and tarnish quickly. They rely on the AC arcing a little to vaporize the oxide coating and the contact has a slight cam to it so that it mechanically wipes the oxides off to get to a fresh surface. None of this works with DC. Instead DC contacts have to remain oxide free and most DC signals are low current, no arcing. If you mix them up the AC current burns up the DC contacts and the DC quickly fails because the AC contacts corrode. There are “universal” contacts. These are AC contacts with a thin coating of gold and minimal, if any wiping. On DC they work great. On AC they vaporize the gold coating leaving the regular AC contact. Only thing you can’t do is swap…do not put DC on a previously AC contact. This is a lot more subtle problem to check.

All of these point to the same thing. Relays and contacts are not universal. Make sure you check ALL the specs. For instance here is a common one:


Look down in the specs and you will see a minimum required switching current of 100 mA @ 5 VDC. Many 24 VDC loads are under 100 mA, so this relay will have failure issues due to excessively low current.

This one is also kind of strange because the top part of the specs says silver cadmium alloy gold flashed which sounds like a universal relay but the contact specs are quite different.

From the same manufacturer:


This one says silver alloy, gold flashed. It has a minimum 10 mA @ 17 VDC current, a 10 A sustained output current, and it doesn’t quite give VA ratings but does list motor HP ratings which are usually close enough. These are clearly universal contacts unlike the hazardous location relay that is clearly AC only.
 

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Chief Flunky
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The relays I'm referring to are:

They are controlling a 24 vdc solenoid on a slide valve on a screw compressor. They are programmed to load slowly and unload slightly faster than loading. I'll see if I can get a pic of the solenoids.
Look at Omrons web site.


330 VA max, silver alloy, “3 A” but it’s 8 A @ 110 VAC, only 1.5 A @ 24 VDC. Everything else is pretty vague. 330 VA gets you up to the smaller contactors and similar loads. 3 A is not much and most DC power supplies are larger than 1.5 A. It’s easy to see why someone might mistakenly think a “3 A” relay is “3 A” but unable to drive a bigger contactor or switch a decent load without burning up,

They don’t give minimums on current but the endurance chart stops at 250 mA for 24 VDC and 400 mA @ 110 VAC, and as per previous post gold is what you want with DC.

What is the load and voltage? This relay can easily be trouble with DC or AC, both high or low current.
 

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Look at Omrons web site.


330 VA max, silver alloy, “3 A” but it’s 8 A @ 110 VAC, only 1.5 A @ 24 VDC. Everything else is pretty vague. 330 VA gets you up to the smaller contactors and similar loads. 3 A is not much and most DC power supplies are larger than 1.5 A. It’s easy to see why someone might mistakenly think a “3 A” relay is “3 A” but unable to drive a bigger contactor or switch a decent load without burning up,

They don’t give minimums on current but the endurance chart stops at 250 mA for 24 VDC and 400 mA @ 110 VAC, and as per previous post gold is what you want with DC.

What is the load and voltage? This relay can easily be trouble with DC or AC, both high or low current.
As mentioned, they're controlling a 24 vdc solenoid. They draw 1.37 amps.
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I recommend interposing relays on any PLC outputs driving a large solenoid (contactor, valve, actuator etc.), because the inrush and inductive kickback is hard on contacts, but it's easier to replace a burned out relay than it is an entire PLC card when one output goes bad.
 

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All our process control stuff/SCADA is 100% interface relay. We've been using a ton of these lately https://www.automationdirect.com/ad...mechanical_relays/slim_interface_relays/52003

All the inputs get optocouplers too.

The machine control side is a little more varied.
The sawmills I work at use the IDEC version of those. They burn out every couple years, but they definitely save on the PLC relay outputs, and they are easy to pop the little relay card thing out and and pop a new one in. They had some controls that ran directly through the output cards, and more than once the output cards have had to be replaced. They seem like cheap insurance, if annoying.
 

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An octal relay does take up quite a bit of space. The square style just isn’t much better. But have you looked at the slim interface relays made for the task?


1/4” wide each, just slightly larger than a 5 mm terminal block and it is laid out like a triple terminal block. Lots of vendors for these. The relays themselves are just PCB relays. The downside is only 6 A contacts. But that runs most everything.

If you must have 10 A then the best you can do is miniature relays. They are similar to the slime lines but usually use 2 terminals side by side. The one below is 15.8 mm (3/8”). Not picking on Finder, just that lately Automation Direct is out of stock on almost everything.

Paul, thanks for the reply. Yes, I’m familiar with those slim terminal strip style relays. We were able to convince the consultants to use them when the load was appropriate. These guys wanted 4-pole 10-amp relays on every point, PLC inputs and outputs. The specs also said all field wiring had to go to terminals, not to he relay itself. There simply wasn’t enough room to install a control panel that big.

As far as the octal relays I really detest the ones where the relay covers up the terminals on its base, making it almost impossible to get test leads on the screws.

Did I mention how much I hate consultants & their boilerplate specifications?
 

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I did a PLC project at a state penitentiary once in Washington. Almost every PLC output was driving a locking solenoid, so they insisted on Potter Brumfield (at the time the most well known) octal base relays as the interposing relays. What we did was to have the PLC in its own nice sleek cabinet in the guard station, but the interposing relays were in a separate "Marshaling Cabinet" in each cell block (not in the prisoner area though). Those cabinets were huge and even with the little octal socket relays, they made a lot of noise. Because the marshaling cabinets were in the cell blocks they were built like safes, just in case there was a breakout and one of the prisoners was savvy enough to figure out that if they jumped out the coils, they could open the doors from there. The cabinets were within eyesight of the common room window, so every time we worked on them, the prisoners would do stuff in the windows to get our attention. It was "memorable" to say the least...
 

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Here is what we do. Things that leave the building are AC due to voltage drop.
Things that originate in the building are DC.
Every input and output is relay isolated for three reasons:
1). They are sacrificial for shorts and lightning so PLC card is saved.
2). They can be replaced with out shutting whole system down.
3). They can be a good troubleshooting tool for maintenance. Mark what is does on label is easier that reading IO numbers. Also the AD ones we use have the override feature on them, if something need to be state changed till repairs can be made.

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As for analogs fuse is all we do. we have had more downtime due to surge suppressor failing than lightning hitting and making it to cards.
 

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As the title says, what's everyone's opinion on when it is worth using interface/isolation relays on PLC I/O. Non of our stuff has anything isolated unless it was actually required to drive the load.
Anytime on-off type wiring comes from or goes to locations outside the plc enclosure I use a relay. There are DIN rail mount relays that are the same width as passive DIN terminal blocks so really don’t take up additional space. You never know what the next guy’s going to do. If he’s using 120 Vac for signaling (not unusual in my experience) you can have that voltage inside the enclosure when your power is off and isolated. Using the DIN connector relays makes it finger safe.
 

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We don't use a whole lot of relays. Interconnection to foreign panels is getting rarer. Even though most outputs are rated 3A, some of our paper mill customers specify interposing relays for any solenoids over 1A.

I have an unusual testimony. I have a PLC in my house. Due to space constraints, I could only fit 10A 2 pole Idec relays (qty 48). So I paralleled the contacts. Paralleling the contacts almost doubles the rating. I have been using 10A Idec relays to switch all of my 20A lighting circuits for 25 years. I haven't replaced a relay yet!
 
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