Cable and insulated wire safety requirements—Part 3: Fixed vs. flexible applications

When you are determining what type of cables to use in an application, you must consider whether that application is a fixed installation or a flexible application.

Requirements for fixed installation
Normally, cables for fixed installation have solid or stranded conductors. In certain circumstances, e.g. for greater ease of installation, the conductor may be Class 5 according to HD 383 or DIN VDE 0295.

Cables should not be in contact with, or close to, hot surfaces if the cables are not intended for such conditions.

Cables should not be buried directly in the ground, unless approved, and should be properly fastened or connected without over tightening.

The cable should not be damaged by any mechanical restraint used for attachment. Cables used for long periods of time may become damaged by movement. This can be caused by the natural effects of aging of the materials used for the insulation and jacket, which can become brittle over time.

For flexible applications
Flexible cables are made up of conductors consisting of small, fine strands and are either twisted or bunched. These cables meet either Class 5 or Class 6 of HD 383 and DIN VDE 0295.

Flexible cables should be used in operating equipment with flexing requirements. The length of the cable must be chosen such that the response of the short-circuit protection device is ensured and long enough to reduce the risk of mechanical damage. In applications where flexible, PVC-jacketed cables are permissible, the use of spiral cables can be considered for reducing the overall length. However, in some applications spiral cables are not necessarily an adequate replacement for flexible, PVC-jacketed cables. Multi-conductor control cables should be protected against permanent bending stress, abrasion, notches and tight bends.

Except if permanently installed in operating equipment, flexible cables should not be permanently attached (with the exception of heavy-duty cable designs for permanent installation in temporary facilities) unless they are enclosed in some form of mechanical protection. For a fixed installation, a non-high-flex cable should be used for “normal” stresses.

Flexible cables should not be subjected to excessive strain from tensile forces, compression, twisting or bending. This applies in particular to the point of entry into the machine and where the cable is attached. The use of connectors, bushings or clamping devices should not damage the cable.

Flexible cables should not be placed under floor coverings or carpets because there is the danger that this can cause insulation to heat up, leading to increased temperatures, or that the weight of furniture or foot traffic can damage the cables.

Flexible cables not suited for high-temp applications should not be in contact with, or close to, hot surfaces nor extend into the immediate vicinity of such, as they are not suitable for this purpose.

Due to their characteristics, the following also applies in particular to PVC-jacketed cables. The suitability of flexible cables for outdoor applications, either for short periods or continuous operation, is defined in the tables of HD 516 and in DIN VDE Part 300. Flexible PVC-jacketed cables are not suitable for permanent use in outdoor applications. Some types of PVC-jacketed cables are suitable for short-term use in outdoor applications. However, they should not be used in conditions other than specified, e.g. at temperatures lower than the allowable operating temperatures.

Read part 1 in this series here and part 2 here.

Contributed by Kevin Siegel, Communications Manager
Helukabel USA
www.helukabel.com