PLC Program Question

[jbelectric777]

I Dont know how to start a new thread, I hope someone can see this and give me some advice. I am getting back into PLC Programming and I can't remember if the standard is using N.O momentary PB's to the I/O terminals or using a N.O element in the program but connect it to a N.C limit, stop, etc. Reason is I don't see in the program anywhere to open circuit on a closed loop safety circuit. I guess what I am trying to ask is if they use N.O contacts for inputs to latch and unlatch, or are we still locking the auxilary holding contact in with N.C devices and open on coil drop out. I don't feel it's very safe to use N.O inputs for E Stop conditions. I am kinda old school and from a control issue it's easy so I just want to know which way is accepted most in the industry ? PLC latching & unlatching using N.O PB's or Start with PLC output energize and latch it in the field through a N.C loop... Thanks , Jim P.S , Sorry for the intrusion NM and augies right, It's all by HP

 

[charlie b]

I can?t help with your question, but I can tell you how to start a new thread. You have to begin at the forum home page. You select the topic area into which you wish to post your new thread. There will be a blue field with white letters, saying ?New Topic or Question,? near the top-left edge of the page. That is the button you?ll need to hit.

I have moved this particular question to a thread of its own.

 

[jim dungar]

In a PLC program an N.O. command from an input (also called examine on) will always mimic your field device. I have found very very few reasons to ever use program an input as an NC (examine off) contact.

So, in my opinion, you should purchase your field devices with the contact state you would use if they were hard wired instead of to a PLC.

 

[FNCnca]

I don't feel it's very safe to use N.O inputs for E Stop conditions.

Personally I would never program a TRUE E-Stop in a PLC. If I call something an E-Stop, I hardwire it.

I do know those who will do logical E-stops, I just do not believe in them.

Anytime I program a stop of any kind, I always use NC on the device, that way I get a failsafe condition.

 

[boater bill]

I always hard-wire the e-stop if it is a life safety issue. A machine or process emergency stop can be soft wired (through logic), but I don't like that idea. I would use a contact on the hardwire e-stop circuit to indicate and e-stop condition on the control panel.

 

[dbuckley]

Certainly in Europe anything that interferes with the e-stop button's ability to directly stop whatever it is that has to be stopped is required to be certified for that role, so there are so-called "safety relays", almost always available in fetching red. These relays have special feaures to enhance their reliability. A standard PLC would not be acceptable.

 

[Psychojohn]

I work as an industrial electrician and do a lot of PLC programming.
A start switch is usualy NO and stop switches are usualy NC. Limit switches usualy close when physicaly made. There are no rules here in the US though, so I have seen both.
E-stops have to be hard wires per OSHA I believe, then we monitor the inputs for alarms sake and to shut off non critical equipment. In the EU, double redundant hard wire E-stop switches are required, which can create a troubleshooting nightmear.

 

[Psychojohn]

Just curious, what kind of PLC?

 

[Snorks]

I used to make complicated XP machinery that fractionated solvents under reduced pressure. Although the machinery was constantly monitured via 64 IO ports full of XP and intrinsic sensors including CT meters in the pressurized control panel, we had to hard wire the E-Stop and supply many automated manual NC depressuration and filtering devices to pass UL.

And there were Siemans 200 & 300 PLCs.

 

[petersonra]

In a PLC program an N.O. command from an input (also called examine on) will always mimic your field device. I have found very very few reasons to ever use program an input as an NC (examine off) contact.

So, in my opinion, you should purchase your field devices with the contact state you would use if they were hard wired instead of to a PLC.

Normally one would want the input to be on in the "normal" condition. That way if a wire falls off the switch or the power supply fails you would get an "abnormal" condition that you could alarm.

Consider a level switch. If you are concerned about high level, you would wire to the NC side of a float switch so that it opens on high level or open wire, etc. If you are concerned about low level, you would wire to the NO side of the switch so the input goes off in the alarm condition.

But, often you get what you get and you live with it.

 

[GeorgeB]

Back when I wrote PLC applications and tried to introduce people to the new technology, the NC/NO, examine on/examine off was hard for beginners to grasp. (We're in the mid '70s here). My classic example was "3-wire" motor control with a stop and start pushbutton. For various reasons, young fresh-out-of-school engineers wanted to use all NO contacts and program a NC (examine off)(not for those used to boolean and assembly language) for the stop button.

I was heavily involved with Modicon in the mid 80's and used their capability of hot-swapping IO modules as an example of why this is a bad thing ... start a motor, pull the module with the stop NO contact connected, and try to stop it ... it cannot be done via the control circuit.

I want E-Stop circuits hard-wired with NC contacts, each switch brought back to the panel. I want safety circuits to use both the NC and NO to verify and make it more difficult for bypassing to take place, even by "experts". I want for filter clog indicators to be wired with both NO and NC so that they cannot easily be bypassed or have a loose connection allow incorrect operation. (I'm not a harda** on the filters; clogging should INDICATE, shutting down only after some time in most applications).

As others have said many times, we cannot make things foolproof, but we can make the fools work harder to hurt themselves and others, hopefully getting caught, and corrected, in the process.

 

[petersonra]

I was heavily involved with Modicon in the mid 80's and used their capability of hot-swapping IO modules as an example of why this is a bad thing ... start a motor, pull the module with the stop NO contact connected, and try to stop it ... it cannot be done via the control circuit.

That is one of the reasons why any PLC input that is a stop or alarm contact should be wired so it is normally on. The wire falling off is the classic reason.

But these days, very few pushbuttons are wired to plc inputs. most of the time start/stop signals are generated from some kind of HMI, so it does not make all that much difference if the stop pb is no or nc.

 

[Overkill]

All of the machines i've seen that are built today have 3 sets of contacts for each E-stop button. 1 NC for the "hard wired" E-stop loop which goes to a relay. 1 NC for the "soft wired" E-stop loop which goes to a PLC input. 1 NO which also goes to a PLC input for a location.

The thinking behind the 2 E-stop loops is redundancy. Anyone thats ever worked on older machines knows that the plastic clips that hold the contact on can break. If this should happen, the other loop will stop the machine and through programming, indicate there is a descrepancy with the 2 loops.

 

[petersonra]

All of the machines i've seen that are built today have 3 sets of contacts for each E-stop button. 1 NC for the "hard wired" E-stop loop which goes to a relay. 1 NC for the "soft wired" E-stop loop which goes to a PLC input. 1 NO which also goes to a PLC input for a location.

The thinking behind the 2 E-stop loops is redundancy. Anyone thats ever worked on older machines knows that the plastic clips that hold the contact on can break. If this should happen, the other loop will stop the machine and through programming, indicate there is a descrepancy with the 2 loops.

usually, if there is some need for such a level of redundancy, one would use a self checking safety relay, with two separate estop loops. that way if one contact fails to open when it is pressed, the other one still will. In fact, they now make an estop pb with self monitoring contacts so if the scenario you described happens, the circuit opens and you can't re-close it without fixing the problem.

You would still have a NO contact to the PLC just to tell the operator what PB was tripped. That can be a big deal on a machine or line with multiple estops spread out over a lot of floor space.

 

[Overkill]

usually, if there is some need for such a level of redundancy, one would use a self checking safety relay, with two separate estop loops.

But that is one device monitoring E-stop loops. Personally, I would feel safer having two independant devices monitoring it.

I've replaced a few of thoes self checking relays that have gone bad (always in a fail safe condition). And there not cheap.

To each his own I guess, both systems are much safer than in the past!

 

[petersonra]

But that is one device monitoring E-stop loops. Personally, I would feel safer having two independent devices monitoring it.

I've replaced a few of those self checking relays that have gone bad (always in a fail safe condition). And there not cheap.

To each his own I guess, both systems are much safer than in the past!

The safety relay is two independent devices monitoring two independent estop loops. they just happen to be mounted in the same package with the appropriate self checking to make sure that if either loop fails, the relay will not come on.

They are not cheap, but they can be had for under $100.

 

[comjos]

PLC Instruction

PLC Instruction

If a NC stop button is used in a ladder diagram for the purpose of stopping an output device, it would typically be listed in the Ladder Logic as an XIC (Examine if closed) instruction. When the NC stop button is in its resting position, it will be closed and the instruction will be true. If you use an XIO (Examine if Open) instruction, the NC stop button would have to be held open in order to make this instruction true and change the binary bit from 1 to 0. This is the only way that current could flow to the output device in the PLC rung.
I do agree with a previous reply that E-stops should be hardwired according to OSHA and plc manufacturers.

 

[S'mise]

I agree that e-stops should be hardwired rather than logic tied. Your e-stops should go to your CRM relay and an input on that can be programed into your logic circuits. I have seen plc's lock up or get hung uo on some sub-routine. Yes, the N.O (XIC) contacts are the norm, and monentary pb's in the field. I have seen double contacts used on buttons too, that way you have hardwire and I/O for your plc. If safety is really an issue consider using watch-dog circuits.