Shield Termination Point
The decision whether the shield of a shielded cable will be grounded at one end or at both ends. The common dividing threshold is about 100 kHz: above that frequency of noise, or when the cable length exceeds one twentieth of the wavelength, it is grounded at both ends; for low-frequency noise below it and where there is a risk of an ground loop, at one end.
The decision depends on which problem you are solving. Grounding at both ends provides low impedance at both ends of the shield and lets high-frequency noise current return by the shortest path; in inverter-driven systems the dominant noise is in exactly that band. Grounding at one end, on the other hand, prevents the potential difference between the two ground points from driving current through the shield (an ground loop); this matters with low-frequency mains noise.
- Threshold: in practice the boundary is about 100 kHz; furthermore, if the cable length exceeds one twentieth of the relevant wavelength, a single end already starts behaving like an antenna and both ends are required.
- Typical choice in a crane panel: because the noise the inverter produces is far above this threshold, the shield of the motor power cable is grounded at both ends, at the inverter and at the motor end, with a 360° clamp.
- On signal cables: a single end is common with low-frequency analogue signals and is also what is preferred on low-voltage DC driver modules; on control lines, combining it with a twisted pair is more decisive.
- Rule: make the decision by measuring — changing the connection and watching the number of dropouts is the most practical method.
When shield grounding is done wrongly, the shield can turn into a conductor that carries the noise instead of reducing it.
The context in which this term is used: Wireless Crane Remote Control Selection Guide