Contact switch (on/off signal) Proximity switch (NAMUR signal)
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Hello, sea friends! A few days ago, I was repairing the feedback switch (contact switch) of a control valve; it was of the normally open/normally closed type and had been damaged due to corrosion by alkaline substances. I then replaced it with a proximity switch that uses NAMUR signaling. The feedback signal from this valve goes directly to the DI card, without passing through any safety barriers or relays. After the replacement, the feedback matched the actual position of the valve, and the system worked properly. I’m not sure what the principle behind this is; I hope experienced colleagues can help me understand it. Thank you!
Theoretically, it is not possible, because digital signals are voltage-based detection signals, while NAMUR signals are current-based signals; conversion through a safety barrier is required. In your case, I can only analyze two possible reasons: one is that your probe does not emit NAMUR signals – not all proximity switches generate NAMUR signals; the second reason is that the control system hardware itself is capable of accepting NAMUR signals. I can’t think of anything else.
Since there were no digital feedback switches available at that time, a NAMUR feedback switch was temporarily used as a replacement. When the valve was opened or closed, the voltage values measured were 20V and 8V. I suspect that this DI card has a threshold for detecting low voltages – that is, it considers a connection to be disconnected when the voltage falls below a certain level
This thing of yours is pretty amazing; I’ve never encountered anything like it. Generally, NAMUR feedback switches come with conversion safety barriers, and the voltages are 8V and 4V. Why isn’t your voltage matching? You can give it a try yourself if you get the chance in the future
Are you using a TOPWORKS reply device and proximity switch?
Are you using a repeater from TOPWORKS? It features contactless magnetic induction passive dry-contact position switches; although they are also proximity switches, they actually consist of passive dry contacts.
Yes, according to the regulations, a safety barrier should be installed; we will add one during the next maintenance opportunity
The voltage is abnormal because the DI card was connected directly, without going through a safety barrier
NAMUR is 24V to 8V, and it is itself a safety barrier. I also have Phoenix NAMUR safety barriers; feel free to contact me if you need them