MDShunk

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

lectricboy

Wrong use of terms, sorry.

DavisIMI

[quote]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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DavisIMI

smeric28

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)

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.

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.

smeric28

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.

bill39

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.

TheRick

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

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.

__________________
Rick Boudreau
TASK FORCE SAFE INSPECTION TEAM LEADER
Tal Afar, Iraq
"fool-proof systems do not take into account the ingenuity of fools"

smeric28

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

smeric28

yea i was just posting my retraction lol...

bill39

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.

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.

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.

Chryse

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.

TheRick

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!

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.

__________________
Rick Boudreau
TASK FORCE SAFE INSPECTION TEAM LEADER
Tal Afar, Iraq
"fool-proof systems do not take into account the ingenuity of fools"

gesparky221

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.

RIVETER

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

Mr.C

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).