Voltage issue with valve travel switch

There is electricity in the circuit; otherwise, how can the system determine that the switch is in place?

This circuit definitely has 24V. By \"passive,\" you are referring to the limit switch on-site; it doesn’t have its own power supply, which is why it’s called passive. The DCS cards, terminal boards, or safety barriers provide the voltage for this circuit – that’s what I mean. If there’s anything wrong with my answer, please point it out. Let’s all learn and improve together. Thank you

Travel switches are generally passive. The DSC output is referred to as powered distribution or unpowered distribution. Four-wire instruments do not require power distribution, but two-wire instruments usually do. Active and passive types are in one-to-one correspondence with the presence or absence of power distribution; generally, passive switch-type instruments require power distribution, and vice versa. The above is a conceptual explanation; the following are examples to illustrate this: For instance, a gas alarm device has alarm terminals with hard-wired connections. The active version can be used to connect directly to an alarm light – when the output is 1, the voltage between the two terminals is 24V, while when the output is 0, the voltage between the two terminals is 0V ； Passive means dry contacts; to connect them to an alarm light, an external power supply is required to be connected in series with these contacts. If the two terminals of these contacts are measured separately, it is also possible to determine whether 24V is present or not. Got it~! ?

If there is a 24V voltage, does the DCS have an isolation relay?

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A travel switch is merely a switching point in the control process; depending on the requirements of that process, it can have either a closed or an open position. You are making it too complicated. It is a contact in the control circuit that serves to supply power to a certain circuit. Depending on your control system requirements, there are those that control the connection of N or M, and those that control the connection of L. It may be 220V or 24V relative to the ground or the enclosure, depending on the voltage of the control circuit.