Strain relief is not there to protect the cable. It is there to keep mechanical load off the terminals. A cable hanging from a gland on a wall, pulled during maintenance, or vibrating on a pump transfers that force down the conductors until something gives — usually the terminal screw, the ferrule, or the conductor strands right where they leave the insulation.
A gland provides strain relief when its clamping element grips the cable jacket hard enough that the load stops there. Whether it does that is a property of the insert, the cable jacket and how the two were matched, not a property of the gland having been installed. Two identical glands on two cables can differ completely in retention.
Almost every gland that provides strain relief does it with one of three arrangements, and they behave differently under load.
| Mechanism | How it grips | Strength | Weak point |
|---|---|---|---|
| Tapered compression insert | Dome nut drives a soft insert down a cone; the insert closes radially on the jacket | Good for round jackets across a wide range | Grip depends on the cable being near the middle of the range |
| Separate clamping ring or saddle | A metal or hard-plastic ring closes on the jacket independently of the seal | Higher retention, and adjustable without disturbing the seal | Needs the exact cable diameter; rings are size-specific |
| Split insert with a clamping cage | Multiple fingers close on the jacket, spreading the load over a longer length | Best for flexible cable and vibration | More parts to lose and to reassemble in the right order |
The first arrangement is what most people mean by “a cable gland”. It is adequate for static installations with a properly supported cable. It is not adequate for a cable that hangs, moves, or carries a connector at the far end.
There is a fourth arrangement worth knowing about, because it is the one buyers most often specify by accident: a gland with an internal clamping cage that has to be assembled in a fixed order around the jacket. It gives the best grip of the four on flexible cable, and it is also the one most often installed with a part missing, because the cage comes in pieces and the pieces look optional. If your specification calls for a clamping cage, ask for an assembly drawing with it.
These two get conflated and the mistake is expensive. Strain relief stops tension reaching the terminals. Cable support carries the weight of the cable so that tension never reaches the gland in the first place. A gland that provides excellent strain relief will still fail if it is the only thing holding 20 metres of vertical cable.
The practical rule: the cable should be supported so that the gland sees no sustained static load. Then the gland only has to resist occasional pull, vibration and thermal movement — which is what it was designed for. If the gland is carrying weight all day, the insert creeps, the jacket necks down at the grip point, and retention falls off over months rather than failing outright.
Retention is a system property of the gland, the cable and the installation. Five factors dominate, and buyers usually control only the first two.
| Factor | Effect | What to do about it |
|---|---|---|
| Cable position in the clamping range | Middle of the range gives the best grip; the extremes give the least | Choose the size from the measured jacket diameter, not the conductor size |
| Jacket material and hardness | Soft PVC and rubber grip well but creep; hard jackets resist creep but are slippery | Ask for the retention figure with the jacket type in mind |
| Insert hardness | Harder inserts resist creep but conform less to an uneven jacket | Match the insert to the application, not to the price |
| Grip length | Longer contact spreads the load and resists necking | Prefer a longer body on large or heavy cables |
| Tightening | Too little leaves the insert unloaded; too much crushes the jacket and reduces the section carrying load | Judge by seal travel, not by force |
The single most common selection error is buying by conductor cross-section. A 4 mm² cable can have a jacket anywhere in a wide band depending on insulation type and manufacturer, and the clamping range is defined on the jacket. The thread size chart shows the ranges; measure the cable before choosing a row from it.
Four failure patterns account for nearly all of the retention complaints we see, and none of them are manufacturing defects.
The material changes how the clamp behaves over time, not just how strong it is on day one.
| Propiedad | Nylon (PA66) | Brass and stainless steel |
|---|---|---|
| Initial retention | Good, with a compliant insert | Good to high, especially with a clamping ring |
| Behaviour over time | The body and insert creep under sustained load; grip relaxes | No creep; the clamp holds the same load for years |
| Under vibration | Better damping, but the nut can walk if not locked | Transmits vibration to the panel; needs a locking feature |
| Effect of over-tightening | Dome nut splits or the thread deforms | Jacket is cut before the body yields |
| Where it fits | Light cable, static installation, cost-sensitive panels | Heavy cable, hanging runs, machinery, outdoor |
That creep behaviour is why a nylon gland on a vertical cable needs support at a shorter interval than a brass one. It is not a defect — it is what the material does — and it is the reason the nylon range and the brass range are specified for different duties rather than ranked.
Cable gland standards include a cable retention test: a defined tensile force is applied to the cable and the movement at the gland is measured. The required force scales with the cable diameter, and the figures live in the standard rather than in a product catalogue — which is why a catalogue that quotes a single “pull-out force” for a size should be read carefully.
In the field, the check that matters is whether the cable moves when pulled by hand after installation. It is crude, but it catches the two failures that actually happen: the insert that never closed, and the cable that was never in the range it was sold for. Do it before the enclosure is closed, on every gland, and do it again at the first maintenance visit on anything vertical or vibrating.
Record the result, even informally. A retention check that is not written down is a check that will not be repeated, and the second inspection is the one that matters because creep and vibration take months to show. A line per gland in the commissioning file — size, cable diameter, pass or fail — costs a few minutes and settles most arguments about whether the gland or the cable caused a failure two years later.
A gland is the wrong tool for four situations, and recognising them early avoids a callback.
| Situation | Why the gland is not enough | What to add |
|---|---|---|
| Long vertical drop | Static weight is continuous; the insert creeps | Cable cleat or support bracket above the gland |
| Moving or vibrating equipment | Cyclic load works the jacket at the grip point | Clamp close to the gland plus a service loop |
| Connector on the far end | Anyone can pull the connector and the load goes straight to the terminals | Internal cable tie-down or a strain relief boot |
| Very large or stiff cable | The gland cannot develop enough grip without crushing the jacket | Dedicated cable entry plate with a clamp bar |
Retention is rarely written down, which is why it is rarely delivered. Four lines in the specification close the gap.
Where the cable is armoured, the mechanical duty changes completely: the armour has to be clamped and earthed, and the jacket grip becomes secondary. That is a separate design covered in the armoured cable gland guide, and the double compression arrangement that separates seal from clamp is compared in single vs double compression.
Only if its clamping element grips the jacket with enough force to stop the load reaching the terminals. A tapered insert at the middle of its range does this well; the same gland at the top of its range seals without gripping.
The requirement scales with cable diameter and is defined in the cable gland standard. The useful field check is simpler: after installation, pull the cable by hand and confirm it does not move.
For sustained loads yes, because it does not creep. For light static cable in a dry panel, a nylon gland with a good insert is entirely adequate and cheaper.
Almost always jacket creep under sustained load, which means the gland is carrying weight it should not be carrying. Add support above the gland and re-check the size selection.
Yes, in most cases. A clamping ring can be fitted on ranges that offer it, or a cleat can be added above the gland without disturbing the seal.
Send us the cable list with jacket diameters and the duty for each entry and we will tell you which sizes need a clamping ring rather than a standard insert. The installation guide covers the fitting sequence, and the torque guide explains how much compression the insert should see.
Single compression uses one elastomer seal to both grip and seal the cable; double compression splits those duties across two seals. Here is where the difference decides an installation.
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