# When to use interface relays on PLC I/O



## Peewee0413 (Oct 18, 2012)

Mixed votlage.
Load is definitely a good reason. 
PNP/NPN trickery. 


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## gpop (May 14, 2018)

Depends on the cards and a bunch of other things. (especially 120vac verses 24dc)
Generally if a wire goes from the building to the field we use surge suppressors and relays. Some of our cards (24dc) detect broken wire/ load loss so isolation would disable that feature.


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## mburtis (Sep 1, 2018)

I've heard some just say to put an interface relay on all discretes, fuses and surge suppression on all analogue. Seems a little excessive to me. 

I'm slowly developing my opinions on what should be in our cabinets. We pretty much only use AC input cards, relay output cards, and analogue cards. Analogue that leaves the building gets surge suppression now, hopefully someday all analogue will be fused better. Relays make sense for outputs that are more than just a electronic signal. AC inputs I'm not sure about. Most of our inputs are simple feedbacks, ie power goes out from the plc cabinet through the aux contact of a motor starter and back to the input. I suppose something could short and surge the plc, but how likely is that really. Stuff that leaves the building I could see isolating with a relay.


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## gpop (May 14, 2018)

Fuses are good as long as they are the correct size especially on dc because the power supply will go into low voltage lock down before blowing a oversize fuse. I like to see ac output's fused as it saves pulling the card to replace a fuse but im not going to bother with relays for aux contacts.


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## micromind (Aug 11, 2007)

When I build cabinets, digital outputs will always have some sort of an ice-cube relay between the card and whatever it's feeding. These are individually fused. The basic idea is if something goes wrong in the field, it'll either blow a fuse or worst case, it'll blow the relay but it won't blow the PLC card. I never take DOs outside the cabinet. 

Digital inputs, I fuse the hot going out to the field individually regardless of voltage. I do not fuse the signal back, it just goes a terminal block then to the PLC card. 

Analog out, no fusing at all, it goes from the PLC card to the instrument. 

Analog inputs are individually fused on the 24DC+ side.


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## mburtis (Sep 1, 2018)

micromind said:


> When I build cabinets, digital outputs will always have some sort of an ice-cube relay between the card and whatever it's feeding. These are individually fused. The basic idea is if something goes wrong in the field, it'll either blow a fuse or worst case, it'll blow the relay but it won't blow the PLC card. I never take DOs outside the cabinet.
> 
> Digital inputs, I fuse the hot going out to the field individually regardless of voltage. I do not fuse the signal back, it just goes a terminal block then to the PLC card.
> 
> ...


So you hit on something that I am going to start working on for our digital inputs. Right now there is a couple of fused hots in the panel then these get sent out in every direction to the field equipment. I want to go through and fuse every hot that goes out to the field. Right now if you have to work on something there is no way to kill this foreign power at a specific piece of equipment without shutting down half the inputs to the plc. Flip out fuse holders would make it so much easier. 

So do you use isolation relays even when you have a relay output card?


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## micromind (Aug 11, 2007)

mburtis said:


> So you hit on something that I am going to start working on for our digital inputs. Right now there is a couple of fused hots in the panel then these get sent out in every direction to the field equipment. I want to go through and fuse every hot that goes out to the field. Right now if you have to work on something there is no way to kill this foreign power at a specific piece of equipment without shutting down half the inputs to the plc. Flip out fuse holders would make it so much easier.
> 
> So do you use isolation relays even when you have a relay output card?


The flip-out fuse holders are the kind use. They make it easy to kill the power to an instrument. 

The relays in the output cards are easy to burn up and hard, if not impossible, to replace. The ice-cube ones will handle much more power (like a size 5 starter.......lol) and a cinch to replace.


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## mburtis (Sep 1, 2018)

Very valid point. In fact we have one relay output burned out in one of our cards because the contractors who installed it didn't realize the alarm horn load was to much for it. I bought a bunch of slim interface relays, rated at like 6 amps to try out. A lot of our relay outputs are running the coil of an ice cube in another cabinet, basically being used as enable commands for chemical feeders. Most of the rest are open or close commands to valve actuators.


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## glen1971 (Oct 10, 2012)

I've found Weidmueller SAK-R terminals, or equivalent, are handy to field wiring on DI, AI, and AO devices. Usually I've done a dedicated hot to half a DI card, then a jumper to all the other hots, and knife terminals to and from the device. AI's get a fuse, knife terminal(s), and a regular terminal for the shield. AO's get 2 knife terminals and a regular terminal. Makes for the ability to easily isolate whatever device, plus troubleshoot.
DO's get relays depending on voltage and load. A regular solenoid gets a fuse and a knife terminal. Motor coils get a relay, with the same terminals.


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## SWDweller (Dec 9, 2020)

Voltage is obvious, I have a rule that all I/O will be relay rated not Triac.

If the I/O is Triac then you need relays on everything. 

This is a presonal observation just had to much trouble with Triac's especailly after and during the Monsoons we get twice a year.


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## mburtis (Sep 1, 2018)

OK so here is another question that's sort of an off shoot of this. Say you have a running feedback that is an input to a plc. Do you send a hot out from the plc to the aux contact, then bring your signal back to the plc card. Or do you take power from inside the mcc bucket (assume each bucket has its own control transformer) through the aux contact and on to the plc input, via a relay probably. One way you can shut the motor breaker off and everything is dead, the other way requires way less wires because you can use a common hot out to the field panel. Which one is better.


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## SWDweller (Dec 9, 2020)

I always switched the common. Found it worked in the most situations.

This gets very busy inside a large MCC. lots of labeling


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## bill39 (Sep 4, 2009)

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.


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## glen1971 (Oct 10, 2012)

bill39 said:


> 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.


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...


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## bill39 (Sep 4, 2009)

glen1971 said:


> 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...


Oh, and it’s a real joy doing what you described when there’s an irate production manager & foreman breathing down your neck asking how much longer until it’s fixed.
👌👌


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## SWDweller (Dec 9, 2020)

I worked for a brief time at a place that made precast concrete stepping stones for HD.
The boss and I had a disagreement after lunch on the first day. Screaming at me about anything is not the way to find a soft spot in my heart. I got pissed on the 3 day and walked off. I had to go to the break area to get my cooler and as I walked by the PLC while the boss is screaming at god, he turned away and I hit the reset button and every thing started booting back up. He looked at me and said what did you do? I smiled still heading for the door "that's why I deserve more money than you get" I was pretty sure there was a intermittant extra ground that was mucking up the process. I was leaving for a new job where the starting salary was 8 bucks an hour more.

One other thing about PLC panels I believe we all can agree on, excellent grounding, <5 ohms and surge protection with a front panel that has a lock.


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## micromind (Aug 11, 2007)

mburtis said:


> OK so here is another question that's sort of an off shoot of this. Say you have a running feedback that is an input to a plc. Do you send a hot out from the plc to the aux contact, then bring your signal back to the plc card. Or do you take power from inside the mcc bucket (assume each bucket has its own control transformer) through the aux contact and on to the plc input, via a relay probably. One way you can shut the motor breaker off and everything is dead, the other way requires way less wires because you can use a common hot out to the field panel. Which one is better.


It depends on how the bucket is wired, if it has separable terminals then go ahead and run the PLC power to the bucket. When you pull the terminals apart, you disconnect the PLC power from the aux contact. 

If there are no separable terminals, if the digital input is 24DC, obviously, you can't get shocked but grounding the 24 power might do bad things.........depending on how it's fused. 

the DI is 120AC, often it's run to the aux contact and if you're not careful, you'll get bit. The proper way is to run 120 from the aux contact to the PLC cabinet and use a relay for the DI. The reason is you're not expecting more than one source of power in a starter but most PLC cabinets have multiple sources.


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## Jlarson (Jun 28, 2009)

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.


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## Peewee0413 (Oct 18, 2012)

micromind said:


> It depends on how the bucket is wired, if it has separable terminals then go ahead and run the PLC power to the bucket. When you pull the terminals apart, you disconnect the PLC power from the aux contact.
> 
> If there are no separable terminals, if the digital input is 24DC, obviously, you can't get shocked but grounding the 24 power might do bad things.........depending on how it's fused.
> 
> the DI is 120AC, often it's run to the aux contact and if you're not careful, you'll get bit. The proper way is to run 120 from the aux contact to the PLC cabinet and use a relay for the DI. The reason is you're not expecting more than one source of power in a starter but most PLC cabinets have multiple sources.


I hate multiple sources...Not for my safety, I consider myself pretty thorough and safe in a panel, but for others who can't recognize. Safety starts with engineering, and there are usually ways to keep things safe....

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## Peewee0413 (Oct 18, 2012)

@micromind Where did you spend your career at?.. I swear years ago you mentioned waste water....But I've been drunk at least once since then..

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## micromind (Aug 11, 2007)

I've worked on wastewater plants, lift stations, potable water plants, pump stations, water wells and oil refineries for a good portion of my 32 year career. Lots of other stuff too but those are the majority.


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## paulengr (Oct 8, 2017)

mburtis said:


> 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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## paulengr (Oct 8, 2017)

bill39 said:


> 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?






Finder 39 Series | FactoryMation







www.factorymation.com





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.






Finder Interface Relay Modules - 48 Series | FactoryMation







www.factorymation.com


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## paulengr (Oct 8, 2017)

glen1971 said:


> 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.



https://www.automationdirect.com/adc/shopping/catalog/relays_-z-_timers/relay_-a-_timer_accessories/ad-asmd-250



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:



https://www.automationdirect.com/adc/shopping/catalog/relays_-z-_timers/electro-mechanical_relays/octal_relays,_plug-in,_10a_-_16a_(750r_-z-_h750_-z-_755_series)/hazardous_location_(hazloc),_12a_(h750_series)/h750-2c-120a



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:



https://www.automationdirect.com/adc/shopping/catalog/relays_-z-_timers/electro-mechanical_relays/octal_relays,_plug-in,_10a_-_16a_(750r_-z-_h750_-z-_755_series)/general_purpose,_10a_(750r_series)/750r-2c-120a



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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## glen1971 (Oct 10, 2012)

The relays I'm referring to are:


https://www.grainger.ca/en/product/SEALED-RELAY%2C14-PIN%2CSQUARE%2C24VDC/p/WWG1YDG4



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.


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## paulengr (Oct 8, 2017)

glen1971 said:


> The relays I'm referring to are:
> 
> 
> https://www.grainger.ca/en/product/SEALED-RELAY%2C14-PIN%2CSQUARE%2C24VDC/p/WWG1YDG4
> ...


Look at Omrons web site.



https://assets.omron.com/m/778b3ebf6bb72f4c/original/MY4H_Datasheet_en_J131-E1-02-pdf.pdf



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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## glen1971 (Oct 10, 2012)

paulengr said:


> Look at Omrons web site.
> 
> 
> 
> ...


As mentioned, they're controlling a 24 vdc solenoid. They draw 1.37 amps.


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## JRaef (Mar 23, 2009)

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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## Going_Commando (Oct 1, 2011)

Jlarson said:


> 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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## bill39 (Sep 4, 2009)

paulengr said:


> 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?
> 
> 
> 
> ...


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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## JRaef (Mar 23, 2009)

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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## just the cowboy (Sep 4, 2013)

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. 










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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## crwilliams (Oct 26, 2012)

mburtis said:


> 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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## GladMech (Sep 18, 2020)

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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## dvcochran (4 mo ago)

mburtis said:


> 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.


The best way I can say it is "isolation". If you have a downstream device (relay or isolation breaker) you can take that end effector offline and keep everything else running. This is very critical in some applications. It also makes troubleshooting easier. 
In other applications where if one end effector is down everything is down it is not as big of a deal.


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## Wardenclyffe (Jan 11, 2019)

dvcochran said:


> The best way I can say it is "isolation". If you have a downstream device (relay or isolation breaker) you can take that end effector offline and keep everything else running. This is very critical in some applications. It also makes troubleshooting easier.
> In other applications where if one end effector is down everything is down it is not as big of a deal.


That's not the subject here, protecting the I/O card is,...


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## paulengr (Oct 8, 2017)

Since I work for a motor shop quite often I have a lot of contactors. I still use 120 VAC outputs for contactors but inputs are 24 V for everything. I just haven’t found a downside. Truth be told the actual PLC outputs are 24 V because it is much faster to just use a prewired output card with relays.

But I’ve been rethinking this. IEC contactors with with 24 V coils are easily available and this gets rid of the relays.

I use relays sparingly. Dry contacts to be sure such as VFD outputs. I often use 120 V coil relays as interfaces because most everything these days is 24 V. Older equipment is 120 V. But often if I’m doing say a wye delta to soft start conversion as you trace out the circuits often I can just disconnect the control power and reuse. My only concern is putting DC onto formerly AC contacts but it seems to last a long time.


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## dvcochran (4 mo ago)

paulengr said:


> 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.
> ...





Wardenclyffe said:


> That's not the subject here, protecting the I/O card is,...


The OP is "when to use interface relays", but isolation still applies, like the isolation relay link someone posted above. Configured correctly (including suppressors for Triac outputs, not relay), it is going to protect the I/O card.
You would have to be a real noob to but a relay on a panel light or such.


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