When is it necessary to perform the alternate flexions test?

This test is performed on cables subjected to frequent flexions, such as lift and service lift cables. The objective of this test is to demonstrate that both the construction of the cable as well as the materials utilized in its manufacture are the most suitable for so demanding application, guaranteeing its durability.

In certain applications, cables are subjected to frequent flexions while they are in service. Examples of these installations are bridge-cranes, lifts and service lifts, robots, home appliance cables, extensions rolled up in drums, and similar. In these cases, cables must be utilized that are suitable for mobile service. The difference between a fixed installation cable and a mobile service cable is that the latter must pass the “alternate flexions test”.

In this manner, manufacturers demonstrate that both the construction as well as the materials utilized are suitable for this so demanding application.
Only if the cable resists this test satisfactorily can a suitable duration be guaranteed once in service.

What the test consists:

The test consists in subjecting a sample of cable to 30,000 complete cycles of alternate flexions. During the test, an electrical current circulates through the conductors of the cable and tension is applied between conductors.

That is, during the test, the cable is subjected to mechanical, thermal and electrical efforts (due to the flexions, the current and voltage applied). For this a test bed is utilized as in the figure. On this bed, the central carriage is mobile and moves in a back-and-forth movement of 1 metre of width, at a speed of 0.33 m/s. This means that it makes a complete cycle every 6 seconds

The cable is placed in an S between the pulleys of the carriage, in such a way that the first pulley flexes in one direction and on passing the second, la flexion is produced in the opposite direction. In each cycle the cable passes twice through each pulley. The ends of the cable are connected to the corresponding terminals and weights are hung to maintain the correct mechanical tension.

The diameter of the pulleys, the weights, the intensity and the voltage applied depend on the section and number of conductors. These test parameters are defined in norms UNE21027 and UNE 21031. For example, a type H07RN-F 3x1.5 cable is tested with 120 mm Ø pulleys, 1.5 kg weights and 16 Amperes per conductor, with a voltage applied of 400 V between conductors and 230 V to ground.

The test bed is equipped with a control console that detects any short-circuit between conductors, fugas to ground or lack of continuity of the conductor from breakage of the copper. In any of these cases, the test stops automatically.

30,000 cycles without failure

All cables suitable for mobile service must exceed 30,000 cycles of alternate flexions without failure.

To know the type of service foreseen for a certain cable, one only has to look at the mark that appears on the cover of the cable. All those that end with “–F “ are cables suitable for mobile service (such as, for example, the Topflex VV-F H05VV-F cable, the Flextel ES05VV-F cable and the Xtrem H07RN-F) cable.

In all cases these are cables suitable for mobile service

The Topflat H07VVH6-F cable, recommended for use in lifts and service lifts, successfully passes the alternate flexions test.

Diagram that shows how the work bed performs the alternate flexions test.