Choosing a cable gland — IP rating, strain relief, and EMC shielding

The grounding and EMC in variable-frequency drives guide covers why 360° shielding of the motor cable is essential for EMC performance. This article steps one level further back: to the cable gland itself, the component through which the cable enters an enclosure, which must simultaneously perform three functions that are often underestimated. For the ATEX-specific variant of cable entries, see the ATEX cable entries Ex-d/Ex-e guide; this article covers the general, non-ATEX application.

Function 1 — IP sealing

The first function of a cable gland is preserving the IP protection rating of the enclosure the cable enters. This only works if the gland's clamping ring actually matches the outer diameter of the specific cable — every gland has a specified clamping range (for example M20 with a clamping range of 6-12 mm), and a cable with a diameter outside that range, or a gland simply chosen too large or too small for the cable, compromises the IP seal, even if the gland itself is certified for the correct IP rating.

Function 2 — strain relief

The second function is strain relief: the gland must mechanically clamp the cable such that tensile forces on the cable (for example from dragging a cable bundle, or the weight of a hanging cable) are not transferred to the terminals inside the enclosure. A gland that only provides the IP seal but does not clamp the cable jacket firmly enough lets the tensile force propagate directly to the termination, risking loosening or breakage of the conductor connection over time.

Function 3 — EMC shielding (360°)

For a shielded cable (for example a motor cable on a variable-frequency drive), the gland has a third, optional but essential function in EMC-sensitive applications: bonding the cable shield to the enclosure all the way around (360°), instead of earthing the shield through a single connecting wire ("pigtail" or drain wire). A pigtail connection works fine for low-frequency signals, but at the high-frequency switching edges of a variable-frequency drive, that single wire behaves as an inductance that severely degrades shielding effectiveness at high frequencies. An EMC gland with a 360° contact ring instead makes contact all the way around the outer jacket of the shield, preserving shielding effectiveness over a much wider frequency range.

Note: fitting an ordinary (non-EMC) gland on a shielded cable and then earthing the shield via a pigtail is not, in itself, a safety fault — but it undermines the entire purpose of using shielded cable for EMC suppression. See the grounding and EMC in variable-frequency drives guide for the background.

Practical relevance

When fitting cable entries in a distribution board, switch cabinet, or motor terminal box, it is important to choose the gland based on all three functions at once: the correct IP rating for the environment, the correct clamping range for the chosen cable diameter, and — for shielded cable in an EMC-sensitive application — a gland with a 360° contact ring instead of an ordinary gland with a pigtail connection.

Common mistakes

Choosing a gland based on thread size alone, without checking whether the clamping range actually matches the cable diameter.
Using an ordinary gland on a shielded motor cable and earthing the shield via a pigtail, largely losing the shielding effectiveness at high frequencies.
Neglecting the strain-relief function by not tightening the gland sufficiently, so that cable weight or tensile forces eventually reach the terminals.
Using a gland with a lower IP rating than the enclosure itself, so that the weakest link in the chain determines the IP rating of the whole installation.