What is the best way to handle an unused wire inside of a cable.
Let's assume I've got a cable with 4 wires, only 3 of them are used.
I could connect the 4th wire at one end to ground, at both ends or let it float.
In my specific example, the cable connects a Oxford Instruments ILM level meter with a liquid nitrogen level sensor. (Basically two concentric metal cylinders. They act as a capacitor. The nitrogen level modifies it's relative permittivity.) The connections are
Pin Function
1 0V
2 12V
3 Freq.
Best Answer
If you already have a ground connection with one or more other wires, then use the otherwise unused wires to make a more solid ground connection.
If there is otherwise no ground connect, like with ethernet for example, then I agree with Keelan in that you shouldn't create one. That could cause trouble with ground loops. In that case, either leave the extra line floating or connect it to ground at one end only.