There are many different environmental and operational conditions which are likely to influence the longevity of electrical cable's service life. The insulation and sheathing materials of cables may degrade over time when exposed to heat, moisture, UV light, ozone, various corrosive chemicals, excessive flexing, or mechanical action.
Low Voltage Cable typically have a voltage grade of 450/750 V or below. They are made of plain or tinned copper or solid or stranded aluminium, and the wire can have a circular, compacted or shaped configuration. Cables can be either flexible or rigid and sheathed with different thermoplastic material. T The insulation materials used for low voltage power cables are PVC, XLPE and HFFR / LS0H.
Medium / High voltage cables pertain to cables used for electric power transmission at madium and high voltage (usually from 1 to 33 kV are medium voltage cables and those over 50 kV are high voltage cables).
- Conductor Faults
- Reduced Insulation Resistance
- Reduced Dielectric Strength
- Partial Discharge
After a new cable has been installed and before it is energized, acceptance testing, or proof testing, should be performed.
In general, acceptance proof tests are conducted at 80% of final factory test voltage. Also, high voltage tests should be conducted on installed cables as part of routine maintenance Maintenance tests are usually conducted at 60% of final factory test voltage.
There are several accepted methods for testing low voltage and high voltage cables. The following are the most common methods used in industry:
- Insulation Resistance
- DC Hi-Pot Testing
- AC Hi-Pot Testing
- VLF Tests
Insulation Resistance test method, or megger test, is used to test low voltage cables (less than 600 Volts). This method is also used as a preiminary test for medium voltage cables.
This method relies on a source of high DC voltage for the testing of cables and other types of electrical equipment.
DC hi-pot test is a simple, extremely portable, and relatively inexpensive piece of test equipment to purchase and operate. Because it’s a simple pass or fail procedure, low skill sets are required to perform the test, plus it’s the least costly of any of the methodologies.
The DC hi-pot test can be a destructive test when performed on service-aged, MV cable insulation (as per an EPRI study performed in the 1990s). This test methodology is also blind to certain types of insulation defects and induces space charges that can aggravate existing insulation defects in aged extruded cables. The EPRI study also showed that continued DC testing of serviced-aged, MV cables can accelerate the aging process of the cables. Atmospheric conditions, such as moisture and wind, can also affect leakage current readings.
Cables and accessories may also be field tested with 60 Hz AC voltage, although this is normally not done because of the requirement for heavy, bulky, and expensive test equipment that may not be readily available or transportable to a field site. The most common field tests performed on cables are DC hi-pot or VLF tests.
The AC hi-pot test is considered a good test for conductive and high-impedance defects in cables and does not induce space charge. Continued use generally does not create additional aging effects. It is widely used to replicate steady-state AC service conditions and the original factory over-potential test.
The AC hi-pot test requires a much larger power source than its DC counterpart and comes with an inherent need for multiple personnel at times, which makes it the most costly test to run. Although this AC test can accelerate the aging process of the cables, it typically does so at a much slower rate than DC. This test is generally a “go” or “no go” test.
This method uses AC high voltage for the testing of cables and other types of electrical equipment at a level greater than the rated voltage of the tested equipment.
VLF testing of cables is supported in IEC 60502 (up to 35 kV) and in IEEE 400.2 (up to 69 kV). As higher voltage VLF equipment is developed, standards may be adapted to increase the voltage level for application.
Very low frequency (VLF) test is conducted with an AC voltage wave with a frequency ranging from 0.01 to 1 Hz .
The tested cable must withstand an AC voltage for a specified testing time without flashover. This method yields a "pass/fail" statement.
The VLF test is a simple, extremely portable, and relatively inexpensive piece of test equipment to purchase and operate with low power requirements. It is considered a good test for conductive and high-impedance defects in cables and does not induce space charge. Continued use generally does not accelerate or create additional aging effects after the test is administered; however, some existing defects may grow rapidly, making the test time short.
The VLF methodology has a relatively short history of usage and is just becoming commonplace in the field. This methodology can aggravate existing insulation defects in aged extruded cables. IEEE 400.2 standards caution against use on service-aged cables with multiple defects. The elevated voltage levels could cause the defects to bridge together. Generally a “go” or “no go” test, the VLF test does not replicate service conditions or factory tests and is considered a destructive test.