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jump3mb:
You need to accurately define what you want to know.
If you simply want to know if AC voltage is present at the fan, then I would put a relay at the fan that changes state when when voltage to the fan is present. Whether the output contact should be a NO or NC will depend upon various factors. I would suggest a reed relay or at least silver contacts, not silver-cadmium-oxide. If you use a solid-state relay, then you may need a shunt resistor at the PLC input.
If you want to know if current is flowing, then use a current sensing relay.
If you want to know if current is in an acceptable range, then you need a more complex current sensor.
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