At which end is the shield of a screened cable grounded?
For analogue and low-frequency signals the shield is grounded at one end only; connecting it at both ends creates a ground loop. If the signal frequency is above 100 kHz or the cable is longer than one twentieth of the wavelength, the shield is grounded at both ends or at multiple points. In every case the shield goes to chassis with 360-degree contact, not to circuit ground.
The job of the shield is to catch the electric field arriving from outside and discharge it to ground. For it to do this job a low-impedance path to ground is needed; where and how it is connected changes the result completely.
- Single-end rule: on low-frequency analogue measurement cables the shield is connected at one end. Even a difference of a few millivolts between the two ends drives current through the shield when it is connected at both ends, and this current induces noise into the inner conductors.
- High-frequency exception: above roughly 100 kHz a shield grounded at one end starts to behave like an antenna. In that case multi-point grounding is the right choice.
- To chassis, not to circuit ground: shield current is not passed through the sensitive circuit ground.
- No pigtails: twisting the shield and screwing it down with a thin tail leaves the shield useless at high frequency because of the inductance of that tail. Provide 360-degree circumferential contact with an EMC gland or a metal connector body.
- Continuity: the shield must be continuous from end to end, without being broken at intermediate connectors.
Under no circumstances is the shield used as the return conductor of the signal; a separate conductor must be provided for the return.
The full topic: Installation, redundancy and field continuity