The following discussion is part of an occasional series, "Ask the Automation Pros," authored by Greg McMillan, industry consultant, author of numerous process control books, and 2010 ISA Life Achievement Award recipient. Program administrators will collect submitted questions and solicits responses from automation professionals. Past Q&A videos are available on the ISA YouTube channel. View the playlist here. You can read all posts from this series here.
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Recently I had to explain to vendors difference between “Open to Alarm”, “Close on Status” and NO / NC. I was surprised to know that lot of switches vendors are not familiar with the term. As you know, alarms are very important for situational awareness. The more engineers make their contacts open to alarm, the safer the system gets.
Let me know your thoughts.
Starting with “nomenclature” the NC means “Normally Closed”, meaning the condition of the contact as it sits in its shipping container with no connections to anything is “closed”. It stems from the way electromechanical relays were built. A “closed” contact presents to a circuit just like any conductor. NO means “Normally Open” and presents exactly like there is no connection at that point.
In the “old days” control logic was implemented using relays. They could be interconnected with control equipment and alarm annunciators, often in complex ways, to implement symbolic logic functionality. If a contact was open it was usually presumed to represent a “logic 0”. If it was closed, it would represent a “logic 1”. From that, it was possible to implement logical functions such as “AND”, and “OR”. For example, if the state of three circuits were represented by three relay contacts in series they all (one AND two AND three” – all of them) would have to be closed for AND to be true. On the other hand, if the same sort of contacts were wired in parallel either one OR two could compete the circuit. That is but the beginning of an adventure into symbolic logic, most of which is done in software these days.
If there was a need to know if some equipment was working, or is in in its expected condition an electromechanical relay with a NO contact could be easily used to signal if something was “on” or “working in the expected way” (closed circuit through NO contact on a relay monitoring the equipment or its condition), presenting a “logic 1” to interconnected equipment. The contact opening would indicate something wasn’t working anymore and could be used to signal so.
When PLCs appeared the programming usually involved “relay ladder diagrams” in which all this logic could be implemented in software in the PLC just like it used to be implemented in “relay panels” often containing very many relays, hardwired to implement the functionality of the logic.
The “control magic” is, of course, the logic design itself. The goal of any implementation design is to reliably implement it. Starting with simplicity, perhaps an annunciator needs to display if the control system is active. A NO contact closes when the control system powers up, pronouncing “Yes! We are on the job and working.” If someone trips over the power cord, the system goes down, the relay loses its power and goes to its normal state (NO), opening the circuit, triggering the annunciator.
We are dealing with several different technological approaches that are all trying to accomplish the same thing. Operationally, what one does with relays, or programmable controllers, or process control computers, or logic components is focused on implementing a logical function of some sort. Sometimes a designer may have lots of education in logic design but struggle with how to optimally get it implemented with the components his company normally applies to such problems / situations. It’s good and useful to discuss this, as has been done here, with a focus on the desired outcome and then think through the use of the available technology to implement it.
Obviously, there is a lot of room in design for often-complex considerations but all that is translated for the control equipment and operators in often-complex symbolic logic designs. Hopefully these thoughts help!
To add to Ed’s answer, “shelf state” is commonly used to describe the contact condition (open or closed) when the device is unenergized or unactuated. This has the benefit of avoiding the confusion between the “normal” state and the contact condition during normal operation. For example, for a PSL, the shelf state is the alarm state but for a PSH the shelf state is the normal state (for alarms).
For projects, there is benefit in defining (tabulating) the device shelf state and the state for the alarm condition, equipment operating mode or operator commands. This is particularly important when there is a physical interface between the programmable system and the controlled device (for example MCC), to aid in the design, reduce problems during commissioning and aid trouble shooting.
The shelf state contact status of the measuring device, primarily switches (e.g., level, pressure, temperature, flow), must be considered in combination with the desired detection point. "Low" process variable concerns would use a NO contact for the alarm or interlock while "high" variable concerns would use the NC contact.
Achieving the desired detection trigger point, or failure of the contact wetted voltage, would provide a logic 0 open circuit for alarm or interlock purposes. In addition, some field switches are externally powered and their contact outputs are inverted on power up to provide a failsafe operation on power loss (these contacts can be considered NO held closed or NC held open when the device is powered).
For valve position detection NO contacts are used for each position but only the actual position contact will be held closed and indicated as a logic 1 closed circuit.
In Programmable Logic Controller (PLC) ladder logic, to complete a rung to energize a coil for example, what appears as a NO and NC contact in series would really represent a closed contact logic 1 followed by a NOT closed contact logic 0. The terms Examine ON and Examine OFF are used to indicate the true concern of the logic input state which is not to be confused with whether a field contact is wired as NO or NC.
Alarm switches should always be wired with “open to alarm” contact.
In normal process state, alarm contact should be closed. In abnormal process state, contact should open to alarm to annunciate alarm event. This is important from wiring perspective. If a broken wire occurs, there will be a false alarm that will be discovered and fixed by maintenance tech. If Open contact is wired in normal process state, then broken wire will cause alarm to never annunciate causing potential hazard. A false alarm is a lesser evil then putting alarm out of service unintentionally.
For switch contact, NO & NC are popular terms and need little explanation but important detail to note is that these are off the shelf condition. Meaning at atmospheric conditions. It is preferred to use Form C contact so that appropriate contact can be used during commissioning. Form C contact comes with one common wire, one terminal for close and other terminal for Open. Only two wires are used for input.
Example 1.
PSL-100 set at 40 PSIG with normal operating pressure of 100 PSIG. Following above, for normal process at 100 PSIG, switch contact should be closed, PLC or DCS program should be configured using NC contact. As we know that NO or NC are off-the shelf conditions, at atmospheric pressure, which is below 40 PSIG set-point for the PSL, switch contact is already in alarm, as soon as it is put in process normal conditions, it will change state. Hence to accomplish “Open to Alarm” we should use NO contact on the switch.
Example 2.
AC POWER FAIL RELAY CONTACT with Normally energized relay. As we know that NO & NC are off the shelf conditions, for an energized relay, contacts switch state. Since Relay is energized with AC Power, when power is interrupted, relay contacts will return to their normal state. Hence for this alarm, with relay normally energized, use NO contact (C and NO) that will close during normal process condition upon relay getting energized. DCS or PLC program contact used should be NC. Similarly, think through flow and temperature switches.
Always speak with factory providing switch contact to confirm their nomenclature.
I propose to make this term “Open to Alarm” popular with Switch Vendor, Applications Engineer, Inside Sales Engineer, Sales Engineer, and Consulting Engineers. It may save life or prevent an incident. I suggest users help prospective switch vendors with awareness of the term.