Programming stops, OL's, and other safety inputs

When given the choice, do you program safety inputs such as stop buttons and overloads as normally open or closed? For example the basic motor starter circuit get wired with the stop and OL normally closed so when either one opens, the circuit deenergizes. In a PLC program, they can be programmed either way and the field wiring would need to be wired accordingly.

I have seen these inputs program both ways. Strictly out of my curiosity, how would you program a safety input, open or close, and why presuming the field device can be wired either way too?

I would when given the opportunity to, I would program them as open and wired the device normally closed. This way if the circuit is compromised or open for an emergency, the process would stop. If wired "backwards"

Safety logic would be going via safety relays however hardwiring stop buttons to the plc would be normally closed in most of the cases. For my MAP_IO subroutine I would take XIO I:0/0 and map that to OTE Stop_PB so all my logic looks for the “button pressed” state. Auxiliary contacts on the gate switches we use are the same way, normally closed on the monitoring and open when activated.

Also the normally closed contacts for my overloads are wired in series with the output and the normally open contact is a plc input (in case plc output welds close and the motor overload is not reset)

Many (all?) safety inputs and motor OL’s should be hardwired with a second contact going to the PLC for alarming. Whether you program it as NO or NC probably depends on your alarm strategy.

sparkyinak said:

Programming stops, OL's, and other safety inputs

When given the choice, do you program safety inputs such as stop buttons and overloads as normally open or closed?...

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I think you may be a little mixed up with the terminology you are using? Try replacing the word "safety" with "control interlock"?

Standard control interlocks wired to standard PLC inputs would not be classed or termed as "safety" inputs. Interlocks such as a standard machine Stop push button, an Overload relay contact, a High or Low Level switch, or an Overtravel micro switch, for instance, would all be classed as control interlocks.

Safety, by today's standards, is an entirely separate Engineering discipline. So we really are best advised not using this term loosely when referencing standard control interlocks. Think Emergency Stop (Red) actuators, light curtains, safety gate interlocks, and so on. These are "Safety" related devices. However, how and what they are wired to is a quite involved discussion, so we'll keep it simple as you appear more interested in standard control interlock wiring to inputs.

Wiring control interlocks as failsafe, however that may be achieved, would indeed be good practice.

Regards,
George

Safety or not, Stop buttons are best N/C so if the power to the button fails i.e. a wire comes away or breaker/fuse trips. Often PLC card input sensors/buttons are fed from a separate fuse/breaker per card, although not considered safety (and should not be) a stop button (soft stop) should also be considered important so if the operator presses the stop button it stops either immediate. or under control as expected. A failure of supply to a N/C button will be considered a command. This can also be applied to position sensors so if a conveyor is energised to run until a sensor is triggered, N/C makes sense as a loss of supply to that sensor will be seen as a stop.

parky said:

Safety or not, Stop buttons are best N/C so if the power to the button fails i.e. a wire comes away or breaker/fuse trips. Often PLC card input sensors/buttons are fed from a separate fuse/breaker per card, although not considered safety (and should not be) a stop button (soft stop) should also be considered important so if the operator presses the stop button it stops either immediate. or under control as expected. A failure of supply to a N/C button will be considered a command. This can also be applied to position sensors so if a conveyor is energised to run until a sensor is triggered, N/C makes sense as a loss of supply to that sensor will be seen as a stop.

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While I agree with everything you are saying, readers of this should keep in mind the difference between wiring a NC contact and programming a NC contact.

This has been beat to death here but needs to be mentioned so newbies will be warned.

I find that a bit weird, programming it is obvious even for a newbie as if you set it as a N/C in the program then the process would stop or not even start as it is "Energised". To be honest, if a programmer cannot tell the difference then perhaps they should not be programming.
Just saying....

parky said:

I find that a bit weird, programming it is obvious even for a newbie as if you set it as a N/C in the program then the process would stop or not even start as it is "Energised". To be honest, if a programmer cannot tell the difference then perhaps they should not be programming.
Just saying....

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Here in the US, the average PLC "programmer" at most plants is a 60 yr old electrician/maintenance guy who resents the fact that he has to use a computer at all. Employers don't pay enough to hire people who can do the job well, so they cross train other teams as the primary support path, and hire a local integrator when the in house team can't cut it.



We are forced to program so it can be supported by the lowest common denominator, which is pretty low.

dogleg43 said:

Many (all?) safety inputs and motor OL’s should be hardwired with a second contact going to the PLC for alarming. Whether you program it as NO or NC probably depends on your alarm strategy.

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This is how I handle everything. Most customers require hard-wired safety unless an approved safety controller is used.

Then a NO contact is used for each switch/button/etc to send a signal to the PLC that the switch or whatever is closed. It isn't used for any safety, but more as an indication of why something isn't working.

mk42 said:

Here in the US, the average PLC "programmer" at most plants is a 60 yr old electrician/maintenance guy who resents the fact that he has to use a computer at all. Employers don't pay enough to hire people who can do the job well, so they cross train other teams as the primary support path, and hire a local integrator when the in house team can't cut it.



We are forced to program so it can be supported by the lowest common denominator, which is pretty low.

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I was a 50 year old electrician and got into programming full time, and worked for quite a while for several of those "integrators" that you called. In my last job, hardware design wired everything "failsafe"; NC contacts so that if a wire breaks, as pointed out above, or a contact fails to close, then logic sees that and reacts accordingly.

Rson said:

This is how I handle everything. Most customers require hard-wired safety unless an approved safety controller is used...

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Rson & dogleg43 in post #3,

Just another little reminder that we need to be careful in referring to Safety Function wiring as the topic of the thread is "apparently" not in relation to actual Safety Function wiring and/or Safety Controllers, but is more in relation to standard control interlock wiring to standard PLC inputs. Of course, some clarification on the matter from sparkyinak would help us be double sure here. But otherwise, I think discussing Safety Function wiring here just creates confusion, and possibly not just for the OP?

sparkyinak said:

...I would program them as open and wired the device normally closed. This way if the circuit is compromised or open for an emergency, the process would stop...

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As I mentioned, wiring control interlocks as failsafe is good practice. If you stick to the above thinking you are indeed following good practices.

But do bare in mind that circuits can be "compromised" in more than one way. Consider the opposite scenario where there is a short on the wiring after the Stop actuator. With normally closed wiring, and examine-if-closed logic, pressing the Stop actuator will do nothing. Whereas, with normally open wiring, and examine-if-open logic, the short would assert a Stop.

While those two particular types of faults, open circuit and short circuit, are often debated as to which is more likely, especially on a Stop actuator circuit, the likelihood of an open circuit is usually deemed more prevalent, based on years of experience.

To break my own "rule" here now...when all else fails though, that is what your Safety Emergency Stop actuator is for - an abnormal condition.

Regards,
George

I suppose this could go on for many days, but one thing for sure a N/C contact wired to a PLC is more likely to loose the supply than short, and of course we are talking about a stop that is not a safety, I have been in this game for over 35 years and on only a couple of occasions have seen a short but seen many with a loss of supply , however, even if double contacts are used some types (and I mean even E-stops the contacts can depart from the actuator this is a dangerous thing to happen on a safety circuit for example, Telemechanique (now taken over) used to be screw mounted, these are now click fit and I have been to panels where the contacts are hanging in the air either broken off the body or detached somehow. Even if the likelihood of short or open is the same it makes sense to me to wire them as you would an E-stop. I also feel that any non-experienced "60 year old Maintenance engineer" would only change a program not design one, and as for them changing a contact (plc code wise) would stop the process unless he totally re-programmed it.

I was taught years back a rule that applies here;

Current flow is required for motion to happen.

E-stops & Stop PB's are N.C., limit switches are closed when travel/movement is allowed, level controls conduct when operation is OK & open to stop or alarm, etc.

If current flow stops then the machine stops whether it is because someone pressed a button or a wire broke.

#1 Aabeck. If people still followed these rules there would be less confusion.

Aabeck said:

I was taught years back a rule that applies here;

Current flow is required for motion to happen.

E-stops & Stop PB's are N.C., limit switches are closed when travel/movement is allowed, level controls conduct when operation is OK & open to stop or alarm, etc.

If current flow stops then the machine stops whether it is because someone pressed a button or a wire broke.

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Good advice. Any field device I set up if all possible, it is set up in fail safe mode or a secondary device or function to act as it's fail safe.