Resistance Testing Part 3

Because many insulation breakdowns occur during operation under the stress of high voltages, more dependable tests are performed with a megger (megohm-meter) and with hi-pot (high potential or high-voltage) equipment. The megger is a tiny, portable instrument that can be battery operated or powered by a hand-cranked generator. Meggers supply from 500 to 5000 volts for the insulation test. They measure leakage current, but the readout is calibrated in insulation resistance (ohms), in ranges to 100 and often 2000 megohms (MQ). The megger can measure insulation resistance between wires, between wires and ground, and between wiring and casings. Normal insulation resistance depends on the wire, insulation, length, and rating by the manufacturer. The greater the leakage, the more defective is the insulation. The wire manufacturer’s rating should be the guide.

Hi-pot testing is more dependable because it applies kilovolt power to come close to replicating real operation. Often, up to over 60 kilovolts (kV) is used depending on the cable. An ac hi-pot more closely replicates real operation, but dc hi-pots are simpler to utilize in the field. A high surge current occurs when the test starts since the long line acts as a charging capacitor, but this settles down to a leakage current. The hi-pot test voltage is usually stepped up in 5- to 10-kV steps, and leakage current is noted at every step to make sure it is within appropriate limits. If the insulation is defective, it will break down under the kilovolt pressure to allow excessive current to flow. Usually, the length of the line is “walked” to inspect it before testing is done. Weather, moisture, and wind affect hi-pot results, so it should be performed on a calm, low-humidity day. The hi-pot test voltages and reading results should be within the wire manufacturer’s specifications to prevent cable damage.

Additionally, hi-pot testing includes impulse testing, in which an impulse capacitor, charged to about 7.5 to 15 kV, applies a burst of energy to the line. These are also called thumper tests.

A good ground is essential to protect people, as well as equipment. The centertap of any service entrance power is referred to as the neutral line and is connected to ground with a metal stake or to other metal equipment which is already at ground potential, such as metal plumbing pipes, particularly the main water line coming in from the outside. In the entrance junction box, the ground and neutral terminal bus bars are the junctions for the neutral entrance line, the system ground wire, and the neutral (white) and ground (bare) wires of every distribution line. Each metal conduit, BX cable, box, and appliance and tool cabinet or cover should be at ground potential (0 V), so no one will be shocked when making contact. With up-to-date wiring receptacles and plugs, ground is made available at the outlet to automatically neutralize a case or cover. The center screw on the outlet plate on such outlets is also at ground. If the equipment is not self-grounding through its plug, then its metal cover or cabinet should be connected to ground via a wire. Ensure the mainline power is off. Utilize an ohmmeter to test the resistance from a metal plumbing pipe to all grounded points. The ohmmeter should read zero ohms.

A plug-in test will assess for proper ground. Special ground-fault circuit-interrupter outlets will disable its outlets when trouble causes current flow to ground through the hot or neutral conductors. For long line testing, special ground resistance meggers are used to make long-line ground measurements. Ground resistance meggers assess for low readings.


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