The Tan Delta and Capacity test on cable insulation permits accurately determining the status of the cable with respect to small manufacturing defects.

The test is usually conducted on cable samples 15 m long, at 2kV AC, and test temperature between 5 K and 10 K above the operating temperature of the cable.

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The dielectric rigidity test is useful to detect any changes or deviations in the normal insulation characteristics due to manufacturing or cable installation processes.

The test can be carried out at ambient temperature or at the temperature specified in the product standard. The test voltage is usually applied to cable for 4 hours between conductors and screens. The typical test voltage withstand value is 4Uo.

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Electrical insulation must be able to withstand over-voltage conditions caused by atmospheric conditions (such as lightning strikes). The ray type pulse voltage withstand test evaluates the capacity of the cable to withstand these possible conditions during its working life.

The cable sample size is typically 15 m, on which the two ends are pared leaving the bare ends of the conductor and screen exposed.

Once the cable has been prepared, the temperature of the conductor is raised between 5 K and 10 K above the operating temperature of the cable. The sample must be able to withstand 10 positive pulses followed by 10 negative pulses at the pulse voltage defined in the relevant cable standard.

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A partial discharge is an electrical discharge that partially short-circuits the insulation between two electrodes. The purpose of the partial discharge test is to verify that the cable insulation is free from any defects caused by partial discharges above a specified magnitude, typically 5pC at the voltage specified in the cable standard.

To carry out the partial discharge test, equipment is required that includes a high voltage power supply with suitable capacity for the length of the test cable, a voltmeter suitable for high voltages, a partial discharge measuring device and a discharge calibrator.

The test procedure to test partial discharge involves applying a test voltage between the conductor and the cable screen. The voltages are determined according to the relevant cable standard.

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The conductor strength test with a cable or sample of the conductor of at least 1 m long is measured in accordance with the standard IEC 60228. The test determines the continuous strength of Class 1, Class 2, Class 5 and Class 6 conductors for smooth copper, metal, aluminium and aluminium alloy coated copper, either round or shaped. The cable or conductor standard specifies the area of the nominal cross section in a range from 0.5 mm2 to 2500 mm2, and the results are expressed in ohm/km.

The conductor strength test equipment that is employed is usually a Kelvin Double Bridge or a Wheatstone Bridge, using the principle of balancing voltage between resistances known as determining the unknown resistance. The sample and test room must be at a temperature as near to 20ºC as possible.

The accuracy of the test is typically 0.2% to 0.5%. A formula is used to correct deviations from 20ºC and then to compare the results with the standard for each conductor type and size.

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The insulation strength test is the key test for electrical cables since it is a measure of the effectiveness of the cable insulation. Deficient insulation can lead to electric shocks, short-circuits or fires.

The test is designed to test the insulation on the cable at the maximum operating temperature. This temperature is determined according to the type of materials used in the cable insulation and sheath.

The insulation strength test requires a water bath, a controlled temperature and the ability to maintain the temperature within the tolerances defined in the relevant standard on cables, and the insulation strength measurement equipment.

The test is carried out on cable samples usually 5 metres long. The ends of the insulation are removed and the conductors are spliced together to comprise one of the connection points. Keeping the connected ends out of the water, the rest of the cable is submerged and left in the water at the set temperature for the duration specified in the cable standard.

After two hours, the insulation strength between the ends of the conductor and the water is measured by applying a voltage at the magnitude and for the duration specified in the cable standard, expressing the result in MO/km.

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