# Proper way to wire a PLC



## mutabi

Ok guys, I have been going some reading on here about PLCs. My company currently uses AB PLC 5's and 500's(and of course one oddball Siemens Step 7). We also have a lot of VERY antiquated electrical equipment in use e.g. MG sets to generate DC voltage, cabinets with hundreds of relays surrounded by a rats nest of wiring. 

So, someone (MDSKUNK) commented on here, that when he wires up a PLC, he only uses the outputs to turn on relays, and not the true output. I am trying to figure out why. Sure, PLC cards are not cheap, but relays are not exactly dirt cheap either and buy a few hundred of them, and it sure aint cheap. 

Then you get to the troubleshooting factor. I have spent hours tracing through hard wired relay logic trying to find why a machine is breaking cycle, only to find a set of contacts on a relay is bad. And the worst thing is, its usually an intermittent failure. 

So whenever we build a new cabinet for a machine, we dont use relays, unless its for a special purpose. We drive our outputs directly. Saves a lot of space, time, and money (?). We do fuse all our o/ps or run them through a CB. 

Where is the flaw in my thinking? And I am sure there is a flaw as I know using relays with a PLC is a common enough practice. 

Thanks guys.


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## IMM_Doctor

mutabi said:


> Ok guys, I have been going some reading on here about PLCs. My company currently uses AB PLC 5's and 500's(and of course one oddball Siemens Step 7). We also have a lot of VERY antiquated electrical equipment in use e.g. MG sets to generate DC voltage, cabinets with hundreds of relays surrounded by a rats nest of wiring.
> 
> So, someone (MDSKUNK) commented on here, that when he wires up a PLC, he only uses the outputs to turn on relays, and not the true output. I am trying to figure out why. Sure, PLC cards are not cheap, but relays are not exactly dirt cheap either and buy a few hundred of them, and it sure aint cheap.
> 
> Then you get to the troubleshooting factor. I have spent hours tracing through hard wired relay logic trying to find why a machine is breaking cycle, only to find a set of contacts on a relay is bad. And the worst thing is, its usually an intermittent failure.
> 
> So whenever we build a new cabinet for a machine, we dont use relays, unless its for a special purpose. We drive our outputs directly. Saves a lot of space, time, and money (?). We do fuse all our o/ps or run them through a CB.
> 
> Where is the flaw in my thinking? And I am sure there is a flaw as I know using relays with a PLC is a common enough practice.
> 
> Thanks guys.


Please don't call Marc a Skunk. If fact he is a very knowledgable and kind electrician.

As far as PLC outputs, there is NO "One-Size-Fits-All" answer. It is generally recognized to individually fuse the outputs with "BFI" (Blown fuse indicators). You can also get output cards that sniff for opens and shorts, and will report back to the supervisory PLC, so that you can annunciate on the operator interface.

If the output is an AC load, then a triac or SS output card is typically used. NEVER use a "dry-contact" card to switch a DC solenoid, as you will have infant mortality. You can drive a large current DC solenoid via a downstream SS relay such as 700-HLSZ24. This gives you reliable, fequent switching of DC loads. Also don't forget to get DIN plugs with a "surge-suppression" diode right at the DIN connector to snuff the DC spike.

Use Triacs Output Cards for AC loads
Use Relay outputs for infreequently switched isolated loads
Use DC FET outputs for signaling and LV gates for SS power devices
Use DC High Current outputs for DC solenoids, (and use snubber diodes at the load).

One advantage to using relays for each and every plc output is that you can usually swap out the relay (pluggable), economical fix. Up fornt cost, but later reduced cost for maint.


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## McClary’s Electrical

I can tell you exactly why you never wire an output to anything other than a relay. I was called to crane at a local lumber yard. The crane operator could not open his grapple. In the PLC room I could see the input from his trigger to the micro logix 1000. The output light would light up from the 120v signal,,,,,that went STRAIGHT to a solenoid on the hydraulic valve to open his grapple. This is a huge crane so it goes through 400 of conduit, through the flexible cable going to the grapple and then the solenoid. The conduit on the valve had become damaged and shorted to ground. This, in turn, burnt up the output of the PLC. The light would light up, but no voltage. There are no cards in a micrologix 1000 so, instead of burning up a $20.00 relay, you smoked an $800.00 PLC. and hopefully you know a contractor (like me)who works with RS linx and RS logix to get it back going for you. All because they did not use a $20.00 relay. Never wire straight to an output


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## drsparky

The output relay cards also tend to "leak", a voltmeter will read voltage even when they are off, and they will not have enough current to throw a solenoid but sure can throw off your troubleshooting.


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## TheRick

I have always used relays for PLC outputs, not only does it protect your PLC by isolating it from the load.....but it also adds functionality as most have multiple sets of NO and NC contacts.

Yeah the initial install is quicker and cheaper without the relays, but extended downtime, and damage to equipment aint cheap either!


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## 5volts

For PLC5s we wire the outputs directly to the solenoid valves. Depending on the card you may have a fuse blown indicator that lights up when the fuse is blown or possibly fused at the swing arm.

Pretty easy to troubleshoot of you have a wiggy handy. I never use a digital meter 120 Volt AC output cards.

For control logix most output cards we use are electronically fused, and the outputs wire directly to hundreds of solenoid valves.

If you have access to your PLC for monitoring and troubleshooting this design works fine. Controlling devices directly with an Output card if designed properly is much easier to troubleshoot and much cheaper in my opinion. 

Remember you must be familliar with the PLC you are working with to ensure you circuit is fused properly so you don't burn something up.


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## mutabi

All the reasons given so far have been valid, yet I am still struggling justifying using relays in addition to a PLC. To me it just adds another potential failure point in the circuit. And one of the big advantages of using a PLC, is that it allows us to get rid of cabinets full of relays. 

Now dont get me wrong, I am not trying to say my way is the right way. We have had outside contractors come in, and wire up a machine, using a PLC and relays on the outputs. I am just trying to figure out why. I have a feeling there is one major reason for it, one of those 'AHA' reasons, that will make it all clear. I am just trying to find out what it is, so next time I go build a cabinet, I can make my case to my boss, that we need to use relays as well. 

Thanks for everyones input so far guys.


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## TheRick

I have driven solenoids, fused directly off an output card.....but the vast majority of the outputs I have done are motor starters, and ALL of these I have done with relays.


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## McClary’s Electrical

You're right, the PLC can eliminate LOGIC relays, like used in the old days to make a machine do what you wanted it to do, you're right they CAN eliminate those by using programmed logic to decipher and determine what's next,,,,,,,but I consider this an isolation relay,,,,,,it's not meant to be eliminated by PLC programming,,,,it's meant to protect the PLC


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## mutabi

Wouldnt a properly sized circuit breaker for each output also protect the PLC?


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## MDShunk

mutabi said:


> Wouldnt a properly sized circuit breaker for each output also protect the PLC?


Or a fuse, or an optoisolator.


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## PLCMentor.com

Fuse your outputs and make sure you have diodes or MOV's (depending on DC or AC). A properly designed PLC circuit does not need interposing relays. The problem listed above where the conduit was damaged woud have been prevented with properly fused outputs. Now if it makes you more comfortable to put interposing relays and you can afford it - that is your choice. Most of our clients are trying to cut costs at every corner. I would rather them cut costs in an area where it will have no negative affect. I have never seen an AB output fail that was properly fused and had proper surge protection at the device (well except for the guy that came in and pulled a PLC5 card out while hot - but technically that was blown cards). Mutabi, I have been using AB for over 20 years and this works.


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## Richard Rowe

When you get ready to build your next box up do a quick cost run on fuses vs relays and give your owner an option. I am sure if they are like most owners they will go with the cheapest way. Here I had some that ran to motors direct(we bought it used and that's how it was set up) and we blew some outputs so we went to relays. That was my call but I don't think I even thought of using fuses at the time..... Single minded I guess.


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## PLCMentor.com

Richard Rowe said:


> When you get ready to build your next box up do a quick cost run on fuses vs relays and give your owner an option. I am sure if they are like most owners they will go with the cheapest way. Here I had some that ran to motors direct(we bought it used and that's how it was set up) and we blew some outputs so we went to relays. That was my call but I don't think I even thought of using fuses at the time..... Single minded I guess.


Yeah if we give them the option, they eliminate it all... Then we are usually left fusing the ouput card as a whole. Nobody is happy with that in the long run. Unless its a code issue, we can only suggest the proper way to go.


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## petek57

*When you say to fuse your outputs...*



PLCMentor.com said:


> Fuse your outputs and make sure you have diodes or MOV's (depending on DC or AC). A properly designed PLC circuit does not need interposing relays. The problem listed above where the conduit was damaged woud have been prevented with properly fused outputs. Now if it makes you more comfortable to put interposing relays and you can afford it - that is your choice. Most of our clients are trying to cut costs at every corner. I would rather them cut costs in an area where it will have no negative affect. I have never seen an AB output fail that was properly fused and had proper surge protection at the device (well except for the guy that came in and pulled a PLC5 card out while hot - but technically that was blown cards). Mutabi, I have been using AB for over 20 years and this works.



Do you mean you install like a 1/2 amp fuse in series with the output?


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## mutabi

> Do you mean you install like a 1/2 amp fuse in series with the output?


In our case, we run out o/ps down to the terminal strip that have DIN mounted CB's installed on them. That makes it real easy to see at a glance if there is a problem with an output.


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## mutabi

> Fuse your outputs and make sure you have diodes or MOV's (depending on DC or AC).


Ok can anyone explain the use of diodes or MOV's. MOV's i believe are used for AC, and diodes for DC loads, correct? 

Lets say I am turning on a DC relay. What good would a diode do? Does it precent spikes going back to the output card? or does it perform some other function.


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## lectricboy

TheRick said:


> I have driven solenoids, fused directly off an output card.....but the vast majority of the outputs I have done are motor starters, and ALL of these I have done with relays.


I agree, most output cards (AB & Sq D anyway) will handle a solonoid, but not a pilot duty coil in a motor starter.


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## mutabi

Pardon my ignorance but what is a pilot duty coil? and what makes it so different to a regular say relay coil?


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## Mach

I used cube relays because they take more abuse from voltage spikes plus they are usually rated for 10amp. Wish I could give you a more technical answer. oh and the engineer usually tells me what to put in


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## MDShunk

mutabi said:


> Pardon my ignorance but what is a pilot duty coil? and what makes it so different to a regular say relay coil?


I think he made a mistatement. "Pilot duty" is used when we're referring to controls that handle very light loads. Smaller pushbuttons are typically referred to as "pilot duty" since they only control the light load of a relay coil and not the connected load (like a motor) directly. That's just one example. Pressure switches and float switches are often pilot duty also, but not necessarily. They sometimes directly control loads. 

Sometimes people will puzzle on what the difference between a relay and a contactor is. I really don't think there's a real good answer to that, since they are often the same part. You'll typically find a part called a "relay" when it is used in a pilot duty application. You'll typically find a part called a "contactor" when it controls some load directly.


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## plcguy

what plc and card type are you using? are you on slc's or siemens etc..?are you triac or relay output? do you jump things when you troubleshoot?
over the years i have kinda got set in my ways i always use cube relays and now i use cube relays that have manual pullins just because i plan on everything breaking down at some point and i plan whatever way is the simplest and will lead to the fastest troubleshooting can you get around cubes? yes defintly is it the best way 99 times out of 100 no imho


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## lectricboy

MDShunk said:


> I think he made a mistatement. "Pilot duty" is used when we're referring to controls that handle very light loads. Smaller pushbuttons are typically referred to as "pilot duty" since they only control the light load of a relay coil and not the connected load (like a motor) directly. That's just one example. Pressure switches and float switches are often pilot duty also, but not necessarily. They sometimes directly control loads.
> 
> Sometimes people will puzzle on what the difference between a relay and a contactor is. I really don't think there's a real good answer to that, since they are often the same part. You'll typically find a part called a "relay" when it is used in a pilot duty application. You'll typically find a part called a "contactor" when it controls some load directly.


Wrong use of terms, sorry.


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## DavisIMI

> mutabi
> Quote:
> Fuse your outputs and make sure you have diodes or MOV's (depending on DC or AC).
> Ok can anyone explain the use of diodes or MOV's. MOV's i believe are used for AC, and diodes for DC loads, correct?
> 
> Lets say I am turning on a DC relay. What good would a diode do? Does it precent spikes going back to the output card? or does it perform some other function./QUOTE]
> 
> You're close, when a relay makes and breaks there is a voltage spike for a nano second, the diode removes this spike and protects the PLC card from damage.


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## smeric28

mcclary's electrical said:


> I can tell you exactly why you never wire an output to anything other than a relay. I was called to crane at a local lumber yard. The crane operator could not open his grapple. In the PLC room I could see the input from his trigger to the micro logix 1000. The output light would light up from the 120v signal,,,,,that went STRAIGHT to a solenoid on the hydraulic valve to open his grapple. This is a huge crane so it goes through 400 of conduit, through the flexible cable going to the grapple and then the solenoid. The conduit on the valve had become damaged and shorted to ground. This, in turn, burnt up the output of the PLC. The light would light up, but no voltage. There are no cards in a micrologix 1000 so, instead of burning up a $20.00 relay, you smoked an $800.00 PLC. and hopefully you know a contractor (like me)who works with RS linx and RS logix to get it back going for you. All because they did not use a $20.00 relay. Never wire straight to an output


using a relay is silly get a midget fuse tb with blown fuse indicator that would eliminate the problem. or you could use a better plc manufacturer like say B&R which has a fuse for the whole output card that would also prevent this damage. Adding relay's can introduce jitter and they can take 200ms to switch. i only use a relay if i'm doing all the logic with them or if i need to control an odd ball like say i want to switch on and off an florescent light with an all 24vdc setup. oh yea relays cost like 25 bucks apiece. try 2 bucks for miget fuse block and 50 cents for the fuse

( i like the saying about AB you can always buy better but you'll never pay more)


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## John Valdes

mutabi,
I see total logic in your question. What the heck is a PLC for, if I have to use relays. You could just build a relay logic panel and forget the PLC.
But, I also see the advantages of using relays for larger loads. And that is how I have always approached the PLC output. If the PLC will drive a relay, it certainly will drive many loads of suitable current.

I have always used PLC outputs to drive small loads that are withing the current requirement of the PLC manufacturer. If the manufacturer specs an output for 5 amps thats my limit, and fuse or breaker accordingly.
Since you have breakers for all the outputs, why not just make a list of all the current requirements for the outputs and see if they are suitable for the output. If not use a relay. Yes, you can drive loads with PLC outputs. Knowing when outputs can be used and when they cannot be used is very easy to determine.

I am certain you will find outputs suitable for direct switching and some that will require some help.

Documentation is the most important thing you can do. Build from prints and edit prints when things are changed. This lessens the impact of troubleshooting the systems.


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## bill39

I believe this answer depends on the area of industry you are in.

My experience in the automotive industry has been to run outputs directly off of the PLC output points unless there you are interfacing to a device w/a different voltage level or a different system or if the load is greater than the output can handle (ex: large motor starter coils). 

My experience in the water treatment area is that the customer/consulting engineer wants to run PLC output points to a relay about 99% of the time. They are usually concerned w/power surges from lightening strike (doesn't always help). The other reason is because for them, MANUAL control is all hardwired, bypassing the PLC completely. Running the PLC output to a relay provides a simple transition for this.

In automotive, most of the time *in my experience* MANUAL control was still wired through the PLC. Not always, but usually.

Fused or Non-fused outputs is an entirely different subject.


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## smeric28

bill39 said:


> I believe this answer depends on the area of industry you are in.
> 
> My experience in the automotive industry has been to run outputs directly off of the PLC output points unless there you are interfacing to a device w/a different voltage level or a different system or if the load is greater than the output can handle (ex: large motor starter coils).
> 
> My experience in the water treatment area is that the customer/consulting engineer wants to run PLC output points to a relay about 99% of the time. They are usually concerned w/power surges from lightening strike (doesn't always help). The other reason is because for them, MANUAL control is all hardwired, bypassing the PLC completely. Running the PLC output to a relay provides a simple transition for this.
> 
> In automotive, most of the time *in my experience* MANUAL control was still wired through the PLC. Not always, but usually.
> 
> Fused or Non-fused outputs is an entirely different subject.


I work extensively for the automotive and all manual controls are handled through the plc via a software button on the HMI. That's interesting about water treatment plants, that's one place i've yet to work...

Question though in the water treatment situation, how is a relay output card different than a relay. Most industrial quality relays cost 30 bucks when you add up the bits an pieces. And i'm assuming a lighting strike is going to take out more than one. even an allen bradley io card isn't more than 200 bucks and they can be replaced in like 30 secs without wiring. relays would all have to be rewired. I guess that's job security  i mean if the engineers could come out and swap it what would they need me for haha. 

I'm just trying to follow the engineering / cost justification in case i ever do any water plant work.


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## bill39

smeric28 said:


> I work extensively for the automotive and all manual controls are handled through the plc via a software button on the HMI. That's interesting about water treatment plants, that's one place i've yet to work...
> 
> Question though in the water treatment situation, how is a relay output card different than a relay. Most industrial quality relays cost 30 bucks when you add up the bits an pieces. And i'm assuming a lighting strike is going to take out more than one. even an allen bradley io card isn't more than 200 bucks and they can be replaced in like 30 secs without wiring. relays would all have to be rewired. I guess that's job security  i mean if the engineers could come out and swap it what would they need me for haha.
> 
> I'm just trying to follow the engineering / cost justification in case i ever do any water plant work.


Typically, 24VDC sourcing output modules are used and wired to small ice-cube relays. I believe the reason for not using relay output modules is because of the problems with lightening. They're trying to keep things the field wiring away from the PLC rack or other modules. Plus w/the relay there is usually a 2nd contact available to use if needed.

In more than one case we have had to take all field mounted discrete input devices (PB, etc.) and wire them to a relay in the PLC panel, then take contacts from those relays to the PLC input card.

On analog circuits they require a surge/isolator device both in the panel and at the field end of the instrument wiring.

Money's no object. This is all gov't money used on municipal projects. It's your taxes hard at work. But seriously, if it prevents a major blow up in a panel or enviromental spill then it is probably worth the extra expense.


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## TheRick

bill39 said:


> MANUAL control was still wired through the PLC.


How is that "manual" control if it is still wired through the PLC? 



smeric28 said:


> relays would all have to be rewired.


:blink: Why? Just pull the relay out of it's socket, and plug the new one in, not even a swing arm to move out of the way.


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## smeric28

bill39 said:


> Typically, 24VDC sourcing output modules are used and wired to small ice-cube relays. I believe the reason for not using relay output modules is because of the problems with lightening. They're trying to keep things the field wiring away from the PLC rack or other modules. Plus w/the relay there is usually a 2nd contact available to use if needed.
> 
> In more than one case we have had to take all field mounted discrete input devices (PB, etc.) and wire them to a relay in the PLC panel, then take contacts from those relays to the PLC input card.
> 
> On analog circuits they require a surge/isolator device both in the panel and at the field end of the instrument wiring.
> 
> Money's no object. This is all gov't money used on municipal projects. It's your taxes hard at work. But seriously, if it prevents a major blow up in a panel or enviromental spill then it is probably worth the extra expense.


I gotcha hahaha. Actually Pheonix (and others) makes some very skinny single contact form c relay terminal blocks. I've used those a few times when i needed to adapt to some weird IO voltage. and i take it back about the rewiring relays i forgot that you can just pull the cube when you weld a contact. ( i guess i've ben spoiled working on wiring new stuff all the time)

What would scare me is if any single failure could cause an environmental spill. See i actually design control systems too, and by definition "fail safe" means that the system will fail in a safe manner. Which is why light curtains use dual NC contacts for outputs ect. So even if lighting did cause a million dollar plc to explode with sparks i'm crossin my fingers that they design these things to not open the valve to dump the toxic wate :jester:


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## smeric28

TheRick said:


> How is that "manual" control if it is still wired through the PLC?
> 
> 
> 
> :blink: Why? Just pull the relay out of it's socket, and plug the new one in, not even a swing arm to move out of the way.


yea i was just posting my retraction lol...


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## bill39

smeric28 said:


> I gotcha hahaha. Actually Pheonix (and others) makes some very skinny single contact form c relay terminal blocks. I've used those a few times when i needed to adapt to some weird IO voltage. and i take it back about the rewiring relays i forgot that you can just pull the cube when you weld a contact. ( i guess i've ben spoiled working on wiring new stuff all the time)
> 
> What would scare me is if any single failure could cause an environmental spill. See i actually design control systems too, and by definition "fail safe" means that the system will fail in a safe manner. Which is why light curtains use dual NC contacts for outputs ect. So even if lighting did cause a million dollar plc to explode with sparks i'm crossin my fingers that they design these things to not open the valve to dump the toxic wate :jester:


You're right about the failsafe definition. But consider the difference between a production assembly line and a wastewater treatment plant.

On an assembly line a backup may cause the production mgr. to have a heart attack.

In simple terms, on a wastewater treatment plant, people don't know the plant is down and keep on flushing. If the poo water can't be processed then a manual valve gets opened and it goes straight into the river.
They want to be able to flip the switch to manual, bypassing all the computer hardware & software to continue processing the goods.


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## Chryse

Depending on the appplication that you are using, I would say that under certain circumstances it would be okay to wire outputs to things other than relays.

Basic fundamental PLC logic has to be taken into account when the program is written. You can use "internal" relays to power those devices, and then the relay would not go bad until the PLC went bad, based on the programming of the PLCs, all the outputs would be directly wired to the output side of the PLC, then by using internal relays when there was no power to the internal relay you would find the piece of equipment that stopped the operation. Just my humble thought I may be wrong.


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## Mr.C

Don't use a relay for every output this only makes troubleshooting from the plc a nightmare. If your using 5's and 500's each output is rated for 5 amps so your good. Yea anything is possible and could come in contact with a wire that goes to a plc and smoke a card but that's business. But to design a system with a plc and then throw relays on each one is crazy. We got a huge mobile stacking system that someone at Jarvis Webb decided to do this on 20 years ago. Try troubleshooting something with 500 outputs going to 500 relays the going to 500 devices IT REALLY SUCKS!!! You just screwed up the reason why we use programmable logic controllers which is to have the flexibility to program or reprogram your control process. If everything's going to relays why not just use relays and timers then cause you just lost the flexibility of the plc. Sorry if I sound passionate about this but I have spent to many winter nights troubleshooting on the plc and then having to go out and find which stupid relay needs a shot of canned air.


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## Chryse

*PLC loads*

I know that you have small and large motors. When using a large motor on three phase all you would simply need to use from the output side of the PLC is both the contactor either NO or NC whichever the application calles for, and then use the overload attached to the contactor, and still the PLC would not get smoked. But in terms of there being a possible direct short to ground the design should have thought about how the equipment was being used in the first place. You can program the PLC to not let current go through to the grapple IF ANY device shorted out.


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## TheRick

Mr.C said:


> Try troubleshooting something with 500 outputs going to 500 relays the going to 500 devices IT REALLY SUCKS!!!


Why do the relays make the troubleshooting any harder? I don't follow you there.



Mr.C said:


> You just screwed up the reason why we use programmable logic controllers which is to have the flexibility to program or reprogram your control process. If everything's going to relays why not just use relays and timers then cause you just lost the flexibility of the plc.


The relays actually add MUCH more flexibility, by adding more sets of contacts per PLC output. You can still change your programming at will, just be aware of what each output does and DOCUMENT EVERYTHING!



Mr.C said:


> Sorry if I sound passionate about this but I have spent to many winter nights troubleshooting on the plc and then having to go out and find which stupid relay needs a shot of canned air.


No you don't sound passionate, *(please don't take this the wrong way)*it just sounds like you like to do all your troubleshooting from the office at a computer and not out in the field with a meter. 

I agree not every output on every PLC needs a relay, but they can add some flexibility and functionality if your outputs are limited.


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## gesparky221

*Proper way to wire a plc*

We always fuse all outputs. We also use a relay for high current loads. Other than that, the output drives the device. Do you guys bring all the I/O wiring out to a terminal strip or wire directly to the card? We require all I/O to be terminated. Makes for easier add ons later. I use only GE plc's and some Siemen's. Some of our old plc's, Series Six, had the fuse for the output mounted right on the card. That is a big pain when a fuse blows.


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## RIVETER

*plc*



mcclary's electrical said:


> I can tell you exactly why you never wire an output to anything other than a relay. I was called to crane at a local lumber yard. The crane operator could not open his grapple. In the PLC room I could see the input from his trigger to the micro logix 1000. The output light would light up from the 120v signal,,,,,that went STRAIGHT to a solenoid on the hydraulic valve to open his grapple. This is a huge crane so it goes through 400 of conduit, through the flexible cable going to the grapple and then the solenoid. The conduit on the valve had become damaged and shorted to ground. This, in turn, burnt up the output of the PLC. The light would light up, but no voltage. There are no cards in a micrologix 1000 so, instead of burning up a $20.00 relay, you smoked an $800.00 PLC. and hopefully you know a contractor (like me)who works with RS linx and RS logix to get it back going for you. All because they did not use a $20.00 relay. Never wire straight to an output


Ok, what did you do to fix the problem? Not even a call to Rockwell?


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## Mr.C

> Why do the relays make the troubleshooting any harder? I don't follow you there.





> The relays actually add MUCH more flexibility, by adding more sets of contacts per PLC output. You can still change your programming at will, just be aware of what each output does and DOCUMENT EVERYTHING!


You just answered half your question The Rick. And the second part is not everyone is going to have this special documentation especialy when your VNC'd into a system and everything you have is on the screen in front of you.
And yes I would love to be able to troubleshoot everything from the computer that would be great I admit, but I am really talking practibility the system is down and the company is losing money , more money per minute than a disposable card card is and the stacker system is 4000' long. It makes no sense to muddy up the field with additional hardware that will fail and is not necessary. And as far as saying the enviroment is to harsh for a processor to be in you would be surprised what these processors will endure, how about 5's 500's and 5000's all in an electric shovel thats about 120 degrees getting all beat to crap 24/7 with dust coating your processors and sometimes throughout the day you clip that 130 degree sweet spot where the processor freaks out and decides it just dosnt feel like turning on that output. The only time I ever had to have an output go to a relay is when I had a 2 mile loop on an estop and the other was when an output had to be used to drive a different plc's input (seperate systems intergrating together yes the hmi could have done this but no the two intergrators would not budge on exchanging move bits).


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## Mr.C

gesparky221 are you using GE funac with the genius i/o's?


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## TheRick

Mr.C said:


> You just answered half your question The Rick.


Sorry still not seeing it being "harder" you just have to know what you are looking at, and what it does.



Mr.C said:


> And the second part is not everyone is going to have this special documentation especialy when your VNC'd into a system and everything you have is on the screen in front of you.


This in my opinion is the single biggest problem in the industry! Nothing special, you just need to DOCUMENT your system, and if you make a change DOCUMENT that change. If your DOCUMENTATION is up to date and saved to the system what is the problem? 



Mr.C said:


> And as far as saying the enviroment is to harsh for a processor to be in you would be surprised what these processors will endure.


I don't believe I ever said that.......having worked in the cement industry I would not be "surprised" I have seen racks buried in cement dust keep chugging along without a problem.

Us continuing to debate this would be like the resi guys arguing the ground up ground down thing. I see some added benefit to using relays in certain applications, and you do not, we will have to just agree to disagree.


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## gesparky221

We do use some Genius I/O. Mostly on carriages that move in and out. We use about every series of GE plc, Series One, Series Six, Versamax, 90/30, 90/70, Rx3i, Rx7i.


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## scaledwithparameters

mcclary's electrical said:


> I can tell you exactly why you never wire an output to anything other than a relay. I was called to crane at a local lumber yard. The crane operator could not open his grapple. In the PLC room I could see the input from his trigger to the micro logix 1000. The output light would light up from the 120v signal,,,,,that went STRAIGHT to a solenoid on the hydraulic valve to open his grapple. This is a huge crane so it goes through 400 of conduit, through the flexible cable going to the grapple and then the solenoid. The conduit on the valve had become damaged and shorted to ground. This, in turn, burnt up the output of the PLC. The light would light up, but no voltage. There are no cards in a micrologix 1000 so, instead of burning up a $20.00 relay, you smoked an $800.00 PLC. and hopefully you know a contractor (like me)who works with RS linx and RS logix to get it back going for you. All because they did not use a $20.00 relay. Never wire straight to an output


*Or if your lucky you have a spare output - minor logic change - add interposing relay..yes / no?*


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## gnuuser

mutabi said:


> All the reasons given so far have been valid, yet I am still struggling justifying using relays in addition to a PLC. To me it just adds another potential failure point in the circuit. And one of the big advantages of using a PLC, is that it allows us to get rid of cabinets full of relays.
> 
> Now dont get me wrong, I am not trying to say my way is the right way. We have had outside contractors come in, and wire up a machine, using a PLC and relays on the outputs. I am just trying to figure out why. I have a feeling there is one major reason for it, one of those 'AHA' reasons, that will make it all clear. I am just trying to find out what it is, so next time I go build a cabinet, I can make my case to my boss, that we need to use relays as well.
> 
> Thanks for everyones input so far guys.


there are many ways to wire plc/slc!
I use a ssd relay like these
http://www.alliedelec.com/crydom-dr-odc24/70131327/
the led lamp on the relay helps a lot when troubleshooting and they protect the plc/slc from feedback
also fuse the output of the relay

but you can hard wire leds to the output of the plc as well( we do this because weve had some cards burn an output but its led was still working)
(for triacs we wire in neon pilots)
a little bit of a pain to do but pays off in the long run by the amount of time saved in troubleshooting.:thumbsup:
and in a production environment less downtime is makes for much happier bosses and less butt chewing while your trying to figure out whats going on:laughing:


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## gnuuser

John Valdes said:


> mutabi,
> I see total logic in your question. What the heck is a PLC for, if I have to use relays. You could just build a relay logic panel and forget the PLC.
> But, I also see the advantages of using relays for larger loads. And that is how I have always approached the PLC output. If the PLC will drive a relay, it certainly will drive many loads of suitable current.


a lot of people forget the reasoning for plc in the first place
sure you can build control panels with mechanical timers, relays, buttons and all but if you want to radically change the behavior of the equipment (changing sequence of operation etc). you had to hard wire the changes
this adds up to extended downtime and loss of production!
plc/slc only needed to be hardwired once and changing the machines behavior was as simple as loading the altered program

i am no stranger to building panels either plc or relay logic, each has its benifits and weakness! 
its the mark of a good panel man who designs and builds his panels for all contingencies always plan your panels with this in mind
while you know your work someone else may have to work on it later.
if your design makes troubleshooting very simple and your programs and panels are well documented you significantly reduce the downtime of the equipment


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## triden

Zombie thread...


...With that said, I like to protect all my digital outputs with a thermomagnetic breaker.


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## DesignerMan

TheRick said:


> I have always used relays for PLC outputs, not only does it protect your PLC by isolating it from the load.....but it also adds functionality as most have multiple sets of NO and NC contacts.
> 
> Yeah the initial install is quicker and cheaper without the relays, but extended downtime, and damage to equipment aint cheap either!


Agreed...Using an interposing relay is a little more expensive upfront, but will pay-off in troubleshooting and up-time down the road. :thumbsup:


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## triden

DesignerMan said:


> Agreed...Using an interposing relay is a little more expensive upfront, but will pay-off in troubleshooting and up-time down the road. :thumbsup:


Just put a breaker on it. Your devices should be fed from the control panel through a breaker anyhow so you don't have multiple power sources in the control panel.


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## DesignerMan

triden said:


> Just put a breaker on it. Your devices should be fed from the control panel through a breaker anyhow so you don't have multiple power sources in the control panel.


Breaker? Not sure I follow...
Most relay output cards are good for 1-2A for each point. By using a relay to control the load you protect the output point from excessive current.


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## triden

DesignerMan said:


> Breaker? Not sure I follow...
> Most relay output cards are good for 1-2A for each point. By using a relay to control the load you protect the output point from excessive current.


Yes, a breaker, much like this. Make sure you use a breaker that's designed for this application. I wouldn't do it any other way.


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## DesignerMan

triden said:


> Yes, a breaker, much like this. Make sure you use a breaker that's designed for this application. I wouldn't do it any other way.


That would be breakers (as in plural) which would certainly work.
The problem I'd have with that is the cost would be more expensive than relays and you're still limited by the small amperage that the output point/ CB can handle.


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## triden

DesignerMan said:


> That would be breakers (as in plural) which would certainly work.
> The problem I'd have with that is the cost would be more expensive than relays and you're still limited by the small amperage that the output point/ CB can handle.


Well sure, but the original question wasn't about the limitation of the output card - it was about how to protect the outputs. If your customers cannot afford a $30 breaker on a digital output, I'm not sure I'd want to be working for them. A breaker solution would also take up much less panel space.


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## KennyW

I try to use high current, DC line driver style outputs that have individual open circuit and short circuit detection/protection. This solves many woes. 

In reality if you use interposing relays you should still be using breakers anyways for loads that go out to the field. Otherwise a short in the field trips some up stream breaker that feeds a whole group of outputs, at which point troubleshooting becomes an Easter egg hunt. If one wants to remain in the dark ages in terms of the type of plc io they use, an individual breaker or fuse per field Device is good practice.


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## triden

KennyW said:


> I try to use high current, DC line driver style outputs that have individual open circuit and short circuit detection/protection. This solves many woes.


I try to use those a lot more now as I am able to convert most things to 100% 24vdc on new projects. Relay outputs are nice for their versatility with multiple logic levels.


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