International. Field tests can be grouped into three categories: acceptance testing, maintenance, and fault localization. It is done by wire or cable after an installation, but before putting it into service, an acceptance test detects damage to the installation or shipment that can affect the performance of the cable.
Once the cable has been put into service, maintenance tests detect deterioration in service. On the other hand, fault location tests pinpoint the exact site of failure on a cable. Knowing exactly where the cable has failed, allows the cable to be repaired or replaced as needed. Some of the most common test methods are described below:
Megger Tests
A Megger test uses a megohm meter to test the insulation properties of elements such as electrical wiring, motor windings, and high-power antenna mounts. Normally, done with a 600 volt power cord, either for acceptance or maintenance purposes, a megohm meter usually applies 600 to 2,500 V DC for several minutes to the cable. The megohm meter measures the current "leakage" through the insulation and displays the results in resistance units (i.e., megohms or millions of ohms). A reading of less than 100 megohms indicates a possible cable problem. Although Megger is a registered trademark of Megger Limited Group, the term is widely used for all similar tests, regardless of the manufacturer.
Hypot test
The hypot (high potential) test is an acceptance or maintenance test and is typically used on cables of 5 to 35 kV or more. Like Megger's test, this device applies DC voltage to a wire and measures current leaks through the insulation. Unlike the Megger test, the applied voltage is considerably higher (for example, up to 65 kV for a 15 kV cable) and the results are displayed in microampere units (μA). With the hypot test, a high reading (for example, above 100 μA) indicates a possible problem. Due to the high potentially lethal voltages involved, this test should be performed only by qualified personnel.
Continuity testing
Continuity testing, which can be used on virtually all types of cables except fiber optics, is probably the easiest and least expensive field test available. A handheld multimeter performs the test using a resistance setting to verify that the cable or accidental contact between the copper conductors due to damaged or defective insulation. A multimeter can also be used to check if conductors have broken somewhere along the length of the cable. Continuity testing can be used for acceptance, maintenance, or fault localization testing.
Crash tests
A thumper is a fault locator device that applies a high-voltage pulse to a wire to determine the exact location of a cable failure. It works by applying a brief high-energy boost (milliseconds) to the cable. At the point of cable failure, the injected energy is released with a strong blow similar to that of a firecracker. When the test is performed on a cable buried several feet deep, a dull knock on the ground is usually heard. Usually, the cable is struck several times per minute until someone running the length of the cable can locate the point of failure.
Other test methods
- Arc pulse fault location equipment
- Capacitance-based fault locator equipment.
- LAN cable testers (measure attenuation, crosstalk and impedance)
- Partial download online / offline
- Optical time domain reflectometers
- Resistance-based fault location equipment
- AC Test Resonant Series
- Time domain reflectometers
- Very low frequency AC test
