Testing times for industrial power cables

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Christian Cornelissen, engineering manager at Nexans, Germany, outlines a stringent test regime for power cables operating in demanding industrial applications

Many industrial applications such as automation robots, cranes, drills and so on involve some kind of movement. The dominant forces in this case are:

Tensile stresses
If a cable is hanging free, such as inside a mine shaft, it will be subjected to tensile forces due to its own weight. Any friction between the cable and contact surface will also create tensile stresses when the cable is pulled, either when the machine it powers is moving or when it is being reeled.

Tests are carried out using a setup with a spindle drive to pull the cable until break and a load cell to measure the corresponding force. Since this test is quite laborious, in many cases the well established maximum tensile loads of the cable elements including appropriate safety margins are used to set the safe limits for cable operation.

Bending
Bending forces are mainly caused by reeling processes, such as in supplying power for a ship-to-shore crane. Even for cables in fixed installations, bending stresses can occur during the installation phase. To avoid the risk of damage, minimum bending radii are defined for the different cable types and applications.
To test the resistance to continuous bending, various test setups are available which simulate operational conditions. In a typical test around 30,000 bending cycles will be carried out.

Torsion
In general, all cable reeling processes also involve a slight, but continuous, torsional stress which may in the long term result in the cable deforming or ‘corkscrewing' if the materials and design are not correctly specified.

To test resistance to torsional stress, a length of cable length is clamped at both ends, and one of them is rotated through a certain angle.

The relevant standards will stipulate no material cracking should be observed within a certain number of torsion cycles, while the number of broken wires (conductor, screen) may not exceed a given limit.

Abrasion
Friction can lead to abrasion. So, if the cables need to move, direct contact with any surface (including other cables, such as in a cable bundle) should be avoided. The use of spacers or rolls should be considered, and appropriate sheathing materials have to be selected.

To test abrasion resistance, tests are performed either on cable samples or on samples of the outer sheath material, for example according to DIN 53516.

Shock and vibrations
Shocks (i.e. single or occasional mechanical impacts with high amplitude) and vibrations (mechanical oscillations with high frequency, but small amplitude) can occur in heavy machinery areas (punching machines, centrifuges, etc.) or in military equipment. Usually, they do not present a risk for the cable itself, but rather for the accessories mounted on the cable. Hence, tests covering the whole cable system have to be applied

In addition to these mechanical forces, cable tests must also consider a number of electrical and environmental factors:

Electrical effects
Overvoltages have probably the most important electrical effect on power cables. They usually occur either with nominal frequency or as a short-term impact with a broader frequency spectrum (e.g. lightning strike, switch failure). The ability of cables to withstand overvoltages is mainly influenced by the insulation system. Standard tests are described in IEC 60502 and 60840.

Electrical overcurrents - currents which exceed the rating of the cable - cause heating effects which can cause damage.

Another aspect which is becoming increasingly important is electromagnetic compatibility (EMC).

Extreme temperatures
The effects of both very high and very low ambient temperatures on the polymeric materials used in cable construction need to be analysed.

High temperature effects can be both internal - from high electrical current flowing through the cable, or external - due to high ambient temperature. These may lead to different temperature profiles across the cable cross-section, but the effect on their polymeric materials is the same as it causes them to degrade.

Since this degradation process can be very slow, long term tests exceeding one year would be necessary, but are not appropriate for standard type tests of cables. Hence, it is usual for accelerated tests to be carried out with a shorter time period and higher temperature

Usually, the elongation at break (EB) of the polymeric material is used as to indicate if any degradation has taken place. So a typical short time test is to measure EB and tensile strength (TS) prior to testing, expose samples to hot air for a certain time (1 day to 1 week) and measure both parameters again.

Low temperatures
Low temperatures can cause the cable materials to become stiff, and this can cause serious problems, especially if the cable is exposed to mechanical impacts, resulting in cracking or other damage to the polymer sheath or insulation.

To obtain information about low temperature behaviour, the EB is usually the most relevant parameter. In many standards, a minimum value of EB is specified at temperatures between -15°C and -40°C.

For cables with small cross-sections, low temperature standards may also specify bending tests.

Chemical effects
Industrial power cables can be exposed to a variety of potentially damaging chemicals such as oils, petrol, diesel, acids, solvents and mud.

To evaluate chemical resistance, a similar procedure to thermal ageing resistance is adopted. Again TS and EB are suitable parameters to evaluate the effect of chemicals on cables.

Further chemical effects result from exposure to ozone and UV radiation. These are tested in special setups where the samples are exposed to a defined intensity of ozone or UV inside a test chamber.

Fire
The behaviour of cables in case of fire is a very large, complex topic. Usually, after a fire cables are severely damaged and will need to be replaced. Nevertheless, there are good reasons why fire tests are very important:

s To assure the fire is not propagated along the cable
s To assure excess smoke is not produced (e.g. for tunnels, public areas)
s To assure the cable will continue to operate for a certain time (for cables used for safety equipment like emergency lights etc.)
To test these requirements, a huge number of national and international standards exist.

Loop cables for wind turbines
An excellent example of a power cable engineered to suit a particularly demanding industrial application is the range of loop cables for wind turbines launched recently by Nexans.

The loop cables connect the low or medium voltage electrical equipment located in the nacelle of the turbine (generator, converter or transformer) with equipment in the base of the tower (e.g. switchgear). The nacelle can revolve a number of times (typically two to five) in both directions.

The cable has to follow this rotation at its upper end, while the lower end is fixed to the tower. To reduce this stress, part of the cable is allowed to hang free inside the tower, but the cable is still subjected to extreme torsional forces of about 100°/m. In addition the free hanging part places an additional tensile load on the cable.

The cable is frequently subjected to low ambient temperatures. It also has to show a good level of resistance to the various fluids used in the nacelle like hydraulic oils, gearbox oils and cooling liquids. Furthermore, for safety, the cable needs to be halogen free and show good fire properties.

To meet these requirements a new sheathing material based on EVA was developed. The new range of loop cables passed a rigorous test regime with flying colours, such as completing over 5000 torsion cycles with ±100°/m at -40°C.