Introduction
Cable cold flow, also called creep, is the slow plastic deformation that occurs when a polymer cable jacket is held under pressure for months or years. Inside a cable gland, the seal ring is deliberately squeezed against the cable jacket to block water and dust, so the same force that creates the seal also loads the cable continuously. This article explains how that load reshapes the jacket, why the deformation opens a leak path in an otherwise correct IP68 cable gland seal, and which installation practices prevent the strain relapse that follows.
What Cable Cold Flow Actually Is
Cable jackets are made from viscoelastic materials such as PVC, TPE, rubber, or cross-linked polyethylene. These materials respond to load partly like a spring and partly like a slow-moving fluid, so they recover from a brief squeeze but continue to deform when the pressure stays in place. Engineers call this time-dependent deformation creep, and in cable glands it is often called cold flow because it can occur at ordinary service temperatures.
The rate depends on polymer type, hardness, filler and plasticizer content, temperature, and stress. Soft PVC and some rubber compounds flow faster than harder thermoplastic elastomers, and every material flows faster as temperature rises. Creep is a normal material response to continuous compression, and it becomes a problem only when the installation does not allow for it.
How Cold Flow Attacks a Cable Gland Seal
A typical nylon cable gland uses an NBR seal ring compressed between the gland body and the cable jacket. When the locknut is tightened, the seal is squeezed radially against the cable and axially against the enclosure wall, which creates the IP68 seal by filling every microscopic gap between the cable surface and the gland bore.
At first the jacket is round and pressure is evenly distributed. Over time the jacket flows away from the high-pressure zones, the cross-section becomes oval or flattened, and the seal itself takes a compression set. Once the jacket relaxes even a fraction of a millimetre, the radial sealing force drops and a micro-channel can form between the seal and the cable surface. Water vapour and fine dust can travel along that channel, which is why cabinets sometimes show internal condensation or dust without any visible dripping. This is different from the pressure-pumping caused by thermal expansion; cold flow changes the geometry of the seal interface itself.
Why Strain Relapse Happens Once the Grip Loosens
Strain relief supports the cable so that pulling, bending, or vibration loads are not carried by the gland body and seal. When a gland is new and the seal is tight, the cable is held firmly in place and external forces are absorbed by the enclosure wall. As cold flow reduces seal pressure, the cable can slide or rock inside the gland, and every movement rubs the jacket against the seal and opens the leak path a little more.
This progressive loss of strain relief is what we mean by strain relapse. Vertical cable runs are especially vulnerable because gravity constantly pulls the cable downward against the seal, and outdoor cabinets exposed to wind or traffic vibration add a dynamic component that accelerates wear.
Installation Best Practices to Prevent Cold-Flow Seal Failure
The compression that seals the gland also loads the cable jacket continuously, so installation discipline matters more than the IP rating alone. The following practices keep both the seal and the cable in the right condition over the service life.
- Size the gland to the actual cable diameter. A cable at the bottom of the clamping range leaves the seal under-compressed, while a cable at the top over-compresses the jacket and drives faster creep. Measure the actual outer diameter at several points along the run with calipers, then choose the gland size that places the measurement near the middle of its range.
- Torque to the supplier’s documented value. More torque does not always mean a better seal. Once the seal is seated, extra tightening mainly distorts the cable jacket and the seal itself. Tighten the locknut with a calibrated torque wrench, seat the panel seal fully against a clean, flat enclosure surface, and recheck torque after the first few weeks of thermal cycling.
- Support the cable close to the gland. A strain relief saddle or bracket should be fixed close enough that the cable cannot bend, pull, or vibrate at the entry point. If the relief is too far away, the unsupported cable acts like a lever and loads the seal directly. On vertical runs, support the weight of the cable independently of the gland.
- Maintain a smooth bend radius at the entry. Bending the cable sharply against the gland mouth creates a point load on one side of the seal and accelerates local cold flow. Install the gland so that the cable leaves in a smooth curve that meets the cable manufacturer’s minimum bend radius.
- Allow slack for thermal movement. Repeated hot and cold cycles make both the jacket and the NBR seal work harder. Creep at high temperature plus differential contraction at low temperature can widen a micro-leak path faster than a steady temperature would. Leave enough cable slack near the gland so the seal is not pulled or twisted during temperature swings.
- Select the right seal material for the environment. NBR is the standard seal in most IP68 nylon cable glands and performs well across -40°C to 100°C with good resistance to oil and water. For extreme cold, high ozone, or extended UV exposure, EPDM or silicone seals may be considered, although the final choice should be checked against the cable jacket compound and the expected temperature range, as discussed in our article on selecting elastomeric seals for marine cable glands.
- Use spiral or flex-rated cable glands where the cable flexes. Spiral cable glands protect cables that bend or move near the entry point by absorbing the bending radius and reducing the dynamic load that would otherwise accelerate cold flow and wear at the seal interface.
- Inspect after the first thermal season. Look for cable movement, oval deformation of the jacket at the seal, dust trails, or moisture inside the enclosure. Early detection allows the gland to be retorqued or replaced before a major leak develops.
Standards and Test Methods That Validate Long-Term Seal Integrity
Buyers often treat the IP68 rating as proof that a cable gland will stay sealed for years, yet the test behind that rating is a short-term static immersion performed on a new sample at a controlled temperature and depth, which means it confirms the design can resist water under laboratory conditions but does not predict how the seal interface will behave after months or years of sustained compression, temperature cycling, and mechanical load. Understanding which standards apply, and more importantly where each one stops, helps engineers specify glands that are less likely to fail from cold flow rather than glands that simply passed an entry-level water test.
IEC 60529 and the limits of the IP68 test.
IEC 60529 defines the degrees of protection provided by enclosures, including the IP68 rating that cable gland suppliers routinely quote. The standard specifies that IP68 means protection against dust ingress and continuous immersion in water under conditions agreed between manufacturer and user, but it leaves the exact depth and duration to negotiation, and the test is normally carried out on a static sample in clean water at room temperature. Because the test does not include long-term creep loading, thermal cycling, or vibration, a gland that earns IP68 on day one can still develop a leak path once the cable jacket cold-flows and the NBR seal takes a compression set.
EN/IEC 62444 for cable gland construction.
EN 62444, now aligned with IEC 62444, sets out the construction and performance requirements for cable glands used in electrical installations, covering thread forms, clamping ranges, seal materials, and mechanical strength. The standard provides a useful baseline for dimensional compatibility and short-term sealing, but it does not mandate a creep-life test or require the supplier to demonstrate how the seal behaves after years of continuous compression, so specifiers should treat compliance as a minimum gate rather than a lifetime guarantee.
UL 94V-2 and UL 50E for fire and enclosure safety.
UL 94V-2 is a flammability classification that confirms the gland material will self-extinguish within a specified time after flame exposure, while UL 50E covers environmental enclosures and includes supplemental tests for gaskets and seals. Neither standard is designed to measure creep resistance or long-term seal compression set, so a UL-listed gland may still be vulnerable to cold-flow leakage if the installation practice ignores clamping range, torque, and strain relief.
Seal material tests that matter for creep.
The seal material itself can be characterised with tests such as ASTM D395 compression set, which measures how much an elastomer remains permanently deformed after a fixed period under compression at elevated temperature, and ASTM D573 accelerated aging, which evaluates property retention after heat exposure. A low compression-set NBR or EPDM seal will recover more of its original shape after unloading, which directly translates to better long-term sealing force against a creeping cable jacket, although the ideal value depends on the cable compound, the gland geometry, and the service temperature.
| Standard or test | What it covers | Where it stops short |
| IEC 60529 (IP68) | Dust and water ingress under static conditions | No long-term creep, thermal cycle, or vibration |
| EN/IEC 62444 | Cable gland construction, threads, clamping ranges | No mandated creep-life or compression-set test |
| UL 94V-2 | Material flammability and self-extinguishing | No seal longevity or cold-flow assessment |
| UL 50E | Enclosure environmental performance | Does not predict seal relaxation over years |
| ASTM D395 | Elastomer compression set | Lab value must be mapped to actual gland geometry |
| ASTM D573 | Accelerated aging of rubber | High-temperature aging may not match field cycles |
Conclusion
Cable cold flow is a slow material process, not a sudden failure, which is why it is often overlooked until water or dust appears inside an enclosure. The compression that creates a cable gland seal also reshapes the cable jacket over time, and once seal pressure drops, small cable movements can open a leak path. Controlling the failure means choosing the right clamping range, tightening to the correct torque, and providing strain relief close to the gland, while checking EN/IEC 62444 and IEC 60529 for the gland and seal rating. Spiral Cable Glands and careful installation discipline do more to keep an IP68 seal intact than trusting the IP rating alone.
Niuli Electric manufactures Metric Cable Glands, GER Standard Cable Glands, UK Standard Cable Glands, and USA Standard Cable Glands from virgin UL-approved Nylon PA66 with NBR seals, rated IP68 and UL 94V-2 from -40°C to 100°C.
FAQs
Does cold flow mean the cable gland was poor quality?
Cold flow is a normal material response of the cable jacket to sustained compression, not a sign of poor quality. Even a correctly installed gland with a good NBR seal can develop a leak path if the cable jacket creeps and the installation does not provide strain relief or the right clamping range.
Can cold flow cause an IP68 gland to leak?
IP68 testing is carried out under static immersion conditions at the time of testing, so it does not predict how the seal interface will change after years of compression, temperature cycling, and movement. Cold flow can open a micro-leak path without visible damage to the gland body.
Which cable materials creep the most?
Soft PVC and some rubber compounds tend to creep more than harder thermoplastic elastomers or cross-linked polyethylene. The exact rate also depends on temperature, filler content, and plasticizer type, so check the cable datasheet.
Is a higher IP rating the answer?
A higher IP rating is not usually the answer, because it does not change the fact that the cable jacket will creep under compression. Correct sizing, controlled torque, and proper strain relief keep the seal interface stable.
How can I tell if cold flow has started?
Inspect the cable at the gland entry. If the originally round jacket has become oval or flattened, if the cable can be moved by hand, or if there is dust or moisture inside the enclosure, cold flow is likely contributing. Retorquing may help temporarily, but replacing the gland with a better-sized unit and adding strain relief is usually the durable fix.
