Should the pressure switch be connected to NO or NC?
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1. A low-pressure interlock is required; the DCS specifies no interlock. Should the field pressure switch be connected in NO or NC configuration? 2. A high-pressure interlock is required; the DCS specifies no interlock. Should the field pressure switch be connected in NO or NC configuration? I would like to ask: do the terms NO and NC for pressure switches refer to the condition when the pressure is 0, or when it is at its normal level?
The NO and NC settings of the pressure switch refer to the condition when the pressure is 0 (that is, when it is at atmospheric pressure). Therefore, a low-pressure interlock or a high-pressure interlock is required; the DCS specifies a 0-interlock setting. When the field pressure switch is connected in NC mode, the interlock is disconnected. Of course, it’s also possible to accept NO, but the logic for input needs to be inverted.
The DCS requirement is 0 interlock: the field pressure switch is connected to NC, and the interlock is thus disconnected.
This 0 refers to the trigger signal; it is used to disable the alarm. Under normal conditions, the circuit is connected, but when an alarm occurs, the circuit is disconnected!
Just remember one thing: Normal Open/Normal Close refers to the state when there is no stimulus.
Any state can be chosen, as long as it is determined which state will be executed upon power-up. It is determined during configuration in DCS.
Generally speaking, choosing to disable the alarm means that the normal state is one in which the switch is closed; therefore, a low-level alarm is connected to NO while a high-level alarm is connected to NC. The reason for this is that in the case of a low-level alarm, there is no pressure or the pressure is below the set point, but under normal conditions the pressure is higher than the low-level set point. Thus, under normal circumstances the low-level switch is also closed. As for the high-level switch, no further explanation is needed.
The fail-safe principle, right!
It should work either with NC or NO; it can be modified on the cabinet side
It should work either with NC or NO; it can be modified on the cabinet side
I used to think that when a pressure switch is connected to NO, as long as the pressure does not reach the interlock value, it remains in the NO state – whether the pressure is normal or not. Isn’t that the case? @xh8618 @HEJIYUER @1111111