Flat Cable Bending Flexibility Testing: Standards, Methods and Acceptance Criteria

Flat cable bending flexibility testing evaluates whether flat and ribbon-style cables can withstand repeated flexing, tight bend radii, and low-temperature installation without conductor breakage or insulation cracking. This article explains the applicable IEC, GB/T, EN and UL test standards, typical sample requirements, core test items, and the methods accredited third-party laboratories use to verify the flexural performance of flat cables used in elevators, cranes, drag chains and automation equipment.

Technical Overview

Flat cables, also known as flat flexible cables or ribbon-style multicore cables, are widely used in applications where space is restricted and the cable must move continuously during service. Because the conductors in a flat cable lie side by side in a common plane, bending concentrates mechanical strain along the cable edges and the conductor-insulation interfaces. A bending flexibility test (also called a softness or flexing test) is therefore essential to confirm that the cable can endure repeated bending, torsion and tight bend radii without conductor strand breakage, insulation cracking or loss of dielectric integrity.

The test family typically combines a mandrel bend test, a repeated flexing (tick-tock) test, a pulley-based flexing endurance test and, for cold-climate service, a low-temperature bend test, followed by electrical verification of the specimen. Evaluation is generally performed against product and test standards published by IEC, GB/T, EN and UL, including IEC 60227-2, IEC 60227-6, IEC 60245-2, IEC 60228, GB/T 5023, GB/T 5013, EN 50396, UL 2556 and UL 758, with low-temperature procedures drawn from the IEC 60811 series. Manufacturers, buyers and accredited third-party laboratories use these tests for type approval, incoming inspection and design validation of flat cables.

Testing Samples

Flat Elevator (Lift) Traveling Cables

These are PVC-insulated flat cables with multicore control and power conductors, frequently incorporating a galvanized steel suspension member, tested in accordance with IEC 60227-6 or GB/T 5023.6. Specimens are cut from finished cable in full cross-section, including the tensile carrier, typically in lengths of 1.5 m to 3 m, and are conditioned at ambient temperature before testing.

Flat Crane and Festoon Cables

Rubber or EPR-insulated flat cables used on port cranes, gantry cranes and festoon systems are heavier-duty samples covered by IEC 60245 or GB/T 5013 series standards. Longer specimens may be required because the flexing travel distance is greater, and the sheath compound and conductor stranding (IEC 60228 Class 5 or 6) directly influence the result.

Flat Drag-Chain and Robotic Cables

High-flexibility flat cables designed for energy chains, linear modules and robot dress packs are tested to verify extremely high cycle counts. Samples are prepared with the conductor orientation and bending plane clearly marked, since flat cables are anisotropic and must be flexed in the direction of intended service.

Flat Ribbon and Appliance Wiring Cables

Light-duty flat ribbon cables and appliance wiring material (AWM) constructions evaluated under UL 758, UL 2556 or EN 50525 are tested with shorter specimens, often 1 m or less, focusing on mandrel bend integrity and insulation cracking resistance at ambient and low temperature.

Testing Items

A complete flat cable bending flexibility evaluation typically covers the following items, selected according to the product standard and the intended service condition:

• Repeated bending (flexing) endurance, expressed as cycles to conductor or insulation failure
• Mandrel bend integrity at ambient temperature, including verification of the minimum bending radius
• Cold bend and cold impact performance at the rated low-service temperature
• Bending stiffness or flexural rigidity measurement for quantitative softness comparison
• Torsional flexibility for cables subjected to twisting motion
• Conductor continuity and strand-breakage inspection after flexing
• Visual examination of insulation and sheath for cracks, crazing or delamination
• Post-test dielectric withstand voltage testing
• Post-test insulation resistance measurement
• Dimensional checks for deformation and reduction of insulation or sheath thickness

Testing Methods

Mandrel Bend Test

The specimen is bent 180 degrees or wrapped around a polished mandrel whose diameter is a specified multiple of the cable thickness, commonly five to ten times depending on the cable type and conductor class. After bending and straightening, the insulation and sheath are examined with normal or corrected vision for visible cracks. This simple test verifies that the cable meets its declared minimum bending radius, which for flat cables is normally expressed as a multiple of cable thickness rather than overall diameter.

Repeated Flexing (Tick-Tock) Test

Following the general principles of EN 50396, the specimen is clamped vertically and its free end, optionally loaded with a defined mass, is deflected back and forth through a specified angle such as plus or minus 60 or 90 degrees at a controlled rate. A monitoring current is passed through each conductor so that strand breakage is detected automatically. The test runs for the specified number of cycles or until electrical failure, providing a direct measure of flexural endurance.

Pulley-Based Flexing Endurance Test

In accordance with the flexing test of IEC 60227-2 and the lift cable requirements of IEC 60227-6, the sample is threaded over a pulley of defined diameter with a counterweight providing constant tension, and the cable is driven back and forth along a fixed travel. Conductors are continuously monitored, and the number of double strokes completed before any conductor breaks or an insulation fault occurs is recorded. For flat lift cables, the specimen is oriented so that bending occurs in the flat plane, reproducing the geometry of the elevator shaft installation.

Low-Temperature Bend Test

To simulate cold-climate installation and operation, specimens and mandrels are conditioned in a climatic chamber at the rated temperature, for example minus 15 degrees Celsius for ordinary PVC compounds or minus 25 to minus 40 degrees Celsius for cold-rated compounds, for at least 16 hours or as specified. The bend is performed while the specimen is still at temperature, after which the sample is allowed to recover and is inspected for cracks. Low-temperature procedures follow the relevant parts of the IEC 60811 series and may be complemented by a cold impact test using a falling mass.

Post-Test Electrical Verification

After mechanical loading, samples undergo a dielectric withstand test, typically 2 kV AC for 5 minutes depending on the rated voltage, and an insulation resistance measurement, often performed on water-immersed specimens with a DC voltage of 80 to 500 volts. Conductor resistance is compared with the values tabulated in IEC 60228 to confirm that no excessive strand breakage or cross-section degradation has occurred.

Applications

Bending flexibility testing of flat cables is applied throughout industries in which cables are installed in continuously moving systems. Typical use cases include:

• Elevator and lift traveling cables subjected to millions of bending cycles over service life
• Festoon and crane power supply systems in ports, shipyards and steel plants
• Drag chains (energy chains) on machine tools and automated production lines
• Robot dress packs, linear gantries and material handling shuttles
• Sliding doors, movable stage equipment and lighting rigs
• Automotive wire harnesses and battery interconnects with tight routing radii
• Household appliances and office equipment with internal moving assemblies
• Cleanroom and medical equipment requiring small, highly flexible interconnects

Beyond compliance verification, the test data support design optimization, guiding the selection of conductor class, insulation and sheath compounds, lay geometry and carrier configuration so that flat cables achieve longer flexural service life.

FAQ

Which standards govern flat cable bending flexibility testing?

The most commonly applied standards are IEC 60227-2 and IEC 60227-6 (PVC cables, including lift cables), IEC 60245-2 (rubber cables), EN 50396 (repeated flexing and torsion methods), UL 2556 and UL 758 (North American practice), together with the corresponding GB/T 5023 and GB/T 5013 series used in China. Low-temperature bending methods are covered by the IEC 60811 series.

What samples are required and how many specimens are tested?

Specimens are cut from finished cable, normally 1 m to 3 m in length depending on the method, in the as-manufactured condition with no removal of insulation or sheath. At least three specimens per test are customarily evaluated, conditioned at 23 plus or minus 5 degrees Celsius before testing, and the bending plane is marked to match the intended installation orientation.

How is a pass or fail decision made?

A specimen passes if no cracks are visible in the insulation or sheath after bending, conductor continuity is maintained throughout the required number of cycles, and the sample subsequently withstands the specified voltage test and meets the minimum insulation resistance. Any conductor strand breakage or dielectric breakdown constitutes failure.

Can the test simulate cold environments?

Yes. Low-temperature bend and cold impact tests condition the specimen at the rated service temperature, such as minus 15, minus 25 or minus 40 degrees Celsius, and perform the bending while cold. This is particularly important for outdoor cranes, northern-climate installations and winter commissioning of elevator systems.

Why do flat cables need a dedicated flexibility test rather than general cable flexing tests?

Because the conductors in a flat cable lie in a single plane, bending stiffness and strain distribution differ significantly from round cables, with stress concentrating at the longitudinal edges. Dedicated flat cable methods therefore specify bending in the flat plane, radii expressed as multiples of cable thickness, and evaluation of the bonding between adjacent cores and the sheath.

How long does a typical test program take?

A mandrel bend test with electrical verification can be completed within one to two working days, while low-temperature tests require additional conditioning time in a climatic chamber. Pulley-based endurance tests may run continuously for several days depending on the specified cycle count and stroke rate.