Energy collected by the shielding on 2 core cable

I recently used a shielded two core cable as temporary emergency patch for some instrumentation as I was out of three core cable, this in in a remote region of Africa . i.e. I twisted the shielding on the two core to use as the third wire. The sensor being connected to was a passive sensor sending a continuous low voltage DC signal of max 3v. The length of the shielded two core cable was about 120cm.

The patch with the two core shielded cable patch did not work correctly as the voltage from the sensor was now fluctuating wildly. I am assuming that the shielding was receiving a signal from some external source.

Does this seem reasonable ? 120cm shielded cable receiving RF interference to have an effect on a DV signal of max 3v ? Anyone else had this problem ?

Problem fixed by using single wire strands as patch.

It all depends.

You have a two core cable plus a shield. The shield is being used as the third cable which is not unusual. This shield is being used as the signal for the sensor.

The noise depends on the sensor impedance - what is connected to the other end? What are the impedances? Low impedance sources will have lower noise because noise is dependent on the source resistance.

Use the shield as the ground for the 3V signal. Perhaps there will be considerable improvement.

Hi,

The shield should never carry some current or voltage related to the sensor signal.
If this is possible with your sensor, then it is OK.

Additionally best is to connect the shield to the system GND with low imedance (especially at high frequencies).

Klaus