Shield Pigtail
This is the cable shield being gathered together and taken to the grounding point as a thin tail. It is easy to apply, but the inductance of the tail makes the shield ineffective at high frequency. As the tail length increases, the effect of the shielding at high frequency falls rapidly.
The shield works as a sleeve surrounding the signal conductor along the cable. If this symmetry is broken at the termination point and the shield is reduced to a single wire, that wire behaves like an inductance and forms a high-impedance obstacle to high-frequency noise current. The current cannot flow comfortably to the chassis and finds its way through the signal conductor.
- The correct way: bonding the shield to the body with 360 degree circumferential contact, through a shield termination clamp or a conductive gland.
- If unavoidable: the tail is kept as short as possible; every extra centimetre reduces the suppression at high frequency.
- Symptom: noise that continues while the motor runs despite the use of shielded cable, and noise on a radio or sensor line that will not go away.
Terminating shielded cables with a conductive gland or a shield clamp at panel entries is markedly more effective than connecting them to a terminal with a long tail. Deciding the shield termination method up front in the installation plan is easier than hunting noise afterwards.
Context in which this term is used: Installation, redundancy and field continuity