DCS and SIS
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NO: NORMALLY OPEN; NC: NORMALLY CLOSED should indicate the normally open or normally closed state of the relay in the interlock circuit; CC: CONTACT CLOSED, CO: CONTACT OPEN should indicate the normally open or normally closed state of the contacts in the interlock circuit； Generally, in a DCS, internal interlock alarms are assigned 0 for NO and 1 for NC; it’s not clear why NO must correspond to 0 while NC corresponds to 1. And how is it set in SIS?
Be sure to check upstairs whether inverse logic was used during configuration. Some of the interlock circuits nowadays tend to be counterlogical
It’s uncertain; some people do both, depending on the designer’s habits – there are no strict rules!
It shouldn’t be the case; companies like Yokogawa and Honeywell have NO corresponding to 0, and NC corresponding to 1； Since I haven’t worked with SIS interlocks, I’m not sure if it’s like this Another way of understanding it is that as long as the alarm function operates in an open-state configuration, it’s fine; a closed-state configuration represents a non-alarm condition
There are no strict rules for this; it mainly depends on design habits
Both NO and NC refer to the behavior in the event of a fault with the field instruments; they are used in DCS systems as conditions for internal interlocks. Whether it is triggered by 1 or by 0 depends on the designer’s preference
This normally open and normally closed function differs a bit in electrical systems and in instruments