Why are normally open, normally closed switches related to high-level and low-level alarms?
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Why are normally open, normally closed switches related to high-level and low-level alarms? Assume that after being fed into the DCS, it is no longer inverted, meaning it is an normally open contact, which triggers a circuit closure action (alarm or interlock). A normally closed contact is an action where the circuit is disconnected. So whether it’s overreporting or underreporting, there has to be a basic principle: if the cable is broken, the DCS should give an alarm so that people are aware of it. In that case, it must be a normally closed circuit. Why are there different options for overreporting or underreporting, and why do many people say the exact opposite?
The last edit to this post was made by zhysen on 2017-6-23 at 10:32. If the cable in a DI circuit is broken, the system will not consider it a fault; instead, it will treat the signal from that circuit as being in a disconnected state. As a result, the DCS will not issue any alarm regarding a circuit fault. In contrast, interlock circuits are usually designed such that even when the circuit is disconnected, an interlock action is triggered
This is because interlocks generally use negative logic; in negative logic, whether it is a high-level or low-level signal, an alarm is triggered on the premise that the node is disconnected. In other words, whether it is overreported or underreported, the node remains closed during normal operation, and only opens in the event of an alarm. Underreporting connects to the normally open state, while overreporting connects to the normally closed state.
Thank you very much for your answer. “I understand everything before “Underreporting connects to normally open”, and I also understand “Overreporting connects to normally closed”. I just don’t understand the concept of a low-level signal that is normally open. If it has already been stated that during normal operation the contacts are closed while they are open in the event of an alarm, then doesn’t a low-level signal that is normally open mean that the contacts are open under normal conditions and closed when an alarm occurs?
Thank you very much for your answer. “I understand everything before “Underreporting connects to normally open”, and I also understand “Overreporting connects to normally closed”. I just don’t understand the concept of a low-level signal that is normally open. If it has already been stated that during normal operation the contacts are closed while they are open in the event of an alarm, then doesn’t a low-level signal that is normally open mean that the contacts are open under normal conditions and closed when an alarm occurs?
Thank you very much for your answer. “I understand everything before “Underreporting connects to normally open”, and I also understand “Overreporting connects to normally closed”. I just don’t understand the concept of a low-level signal that is normally open. If it has already been stated that during normal operation the contacts are closed while they are open in the event of an alarm, then doesn’t a low-level signal that is normally open mean that the contacts are open under normal conditions and closed when an alarm occurs?
Normally open or normally closed means that, in the absence of any external influence, it remains in either the normally open state or the normally closed state. The normally open contact remains in the open state even when the low-level alarm is triggered (for example, an alarm is issued when the liquid level is below 1000 mm; when the level is below 1000 mm, no liquid touches the normally open contact, so it stays in the open state and the alarm continues to sound). When the liquid level rises above 1000 mm, pressure is applied to the normally open contact, causing it to close. When the liquid level drops back to 1000, the contacts will disconnect again, and an alarm will be triggered at this point. We believe that when the liquid level is greater than 1000 mm, it is in a normal state). The same principle applies to overreporting as with normally closed circuits.
Thank you, I understand now. The level switch is a type of instrument that is quite intuitive and easy to analyze. The natural state is when the process medium is not in contact with the level switch, which is the “normal” state. In the case of a low-level alarm, the medium is indeed not in contact with the level switch, and to trigger the alarm it is necessary to disconnect the connection; therefore, such a point is considered a normally open point. However, there are underreports for pressure, temperature, and flow switches. Alarm and disconnection when below a certain value. To determine whether the instrument is in its normal state when the value is below this level, is it necessary to understand its internal structure?