Cable screen bonding — single-point or both ends, and why the difference matters

The guide on EMC separation between power and data cables covers why power and signal cables are physically routed apart. This article covers a related but separate choice: how the screen (shielding) of a cable is bonded to earth — at one end, or at both — and why that choice is not arbitrary but depends on the cable type and the frequency of the disturbance being addressed.

What the screen does

A screened cable has a conductive layer (foil, braid, or both) around the cores, intended to limit electromagnetic coupling between the cores and the surroundings — in both directions: the screen keeps external interference out of the cores, and it keeps the cores' own radiation (for example the switching edges of a variable-frequency drive) inside the cable. To perform that function the screen itself must be bonded to earth in a low-impedance way — but "bonded" has two variants with opposite effects.

Single-point bonding

With single-point bonding, the screen is connected to earth at only one end; at the other end the screen remains floating (or only capacitively coupled).

Prevents a ground loop: with two earth points that are not exactly at the same potential (in a large building, or between two building sections, this can already differ by several volts), bonding at both ends creates a loop current through the screen at mains frequency, which induces noise into the cores through magnetic coupling.
Typical application: low-frequency instrumentation and signal cables (sensor loops, measurement circuits), where avoiding a ground loop outweighs shielding against high-frequency radiation.

Bonding at both ends

With bonding at both ends, the screen is connected to earth at both ends — preferably with a full 360° circumferential gland connection, not a single "pigtail" wire.

Lowest impedance for high-frequency interference: at higher frequencies (roughly above ~100 kHz, or when the cable length becomes a significant fraction of the wavelength of the disturbance), a both-ends, low-impedance screen bond is needed for the screen to retain its shielding function; a single-point bond then lets the screen act as an antenna rather than a shield.
Typical application: variable-frequency-drive motor cables, where the fast switching edges (IGBT) generate high-frequency interference that, without a both-ends, low-impedance screen bond, can couple into other cables and equipment via radiation and conduction.

Practical relevance

Gland selection: both-ends EMC bonding requires an EMC gland with 360° clamping of the screen (see the guide on cable glands); a pigtail connection introduces extra self-inductance that works against the high-frequency shielding it is meant to provide.
Current-carrying capacity: for a bonded-at-both-ends armoured power cable, a circulating current can arise in the armour (induced by the load current in the cores). This circulating current contributes to cable heating and can reduce the permitted current-carrying capacity from the installation-method tables (see the guide on reference methods) compared with a cable whose armour is bonded at a single point only.
Variable-frequency-drive installations: see the guide on earthing and EMC for variable-frequency drives for the complete EMC earthing strategy around the drive itself, of which the motor-cable screen bond is one part.

Common mistakes

Bonding a long instrumentation cable at both ends "to be safe" — this actually causes a ground loop that would not have occurred with single-point bonding.
Bonding a variable-frequency-drive motor cable at a single point under the assumption that one earth point suffices — under high-frequency switching interference the EMC shielding then largely fails.
Using a pigtail wire instead of a 360° gland connection for the screen bond of an EMC-sensitive cable — the added self-inductance of the pigtail undermines the low-impedance connection needed at high frequency.
Not consulting the cable manufacturer's current-carrying-capacity table for a both-ends bonded armoured power cable run — the circulating armour current can require a derating that is not captured in the generic installation-method table.
Treating screen bonding as a simple "yes or no" choice without accounting for the type of signal (low-frequency versus high-frequency) — the correct choice depends on the application, not on one fixed rule that applies to every cable.