Disconnected wire still lit Tester turns on

Question

I have multiple wires going through an electrical PVC conduit.

Two phase wire carrying 220v AC of 1.5mm gauge. Two neutral wire carrying of 1.5 mm gauge. One wire for earthing (grounding) of 1 mm gauge. Another wire (Faulty One) of 1 mm which is planned to switch on the Bulb is also there. When I use tester on the switch wire it shows as live wire carrying current and the tester lit on. Even when I disconnected the wire from both ends, it still show live on the tester. Above is the tester I am using. I pull out the wire to check if there is any breakage in the wire but all is fine. When I touch this faulty wire with bare hands it does not feel anything but still the Tester turns on. Can someone assist me what is going on?

Answer 1

Two conductors separated by insulator is actually a capacitor.

So when a disconnected wire goes a long distance along a live mains wire, there is capacitive coupling between live wire and disconnected wire, and the capacitance couples AC voltage from the live wire to the disconnected wire.

Your tester is also a high impedance load so it will show that also the disconnected wire is live, even if it is only very weakly live via the capacitance, and not really connected to live.