Pre-test guidelines a. Testing equipment Direct current test equipment is commercially available with a wide range of voltages. Accessory equipment necessary to safely conduct high voltage tests such as safety barriers, rubber gloves, non-conducting hard hats, and, where necessary, arc ash protection must be used. Preparation for testing Before conducting any Hi-Pot testing, the following steps should be taken.
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Pre-test guidelines a. Testing equipment Direct current test equipment is commercially available with a wide range of voltages. Accessory equipment necessary to safely conduct high voltage tests such as safety barriers, rubber gloves, non-conducting hard hats, and, where necessary, arc ash protection must be used.
Preparation for testing Before conducting any Hi-Pot testing, the following steps should be taken. All equipment, such as transformers, switches, taps, motors, circuit breakers, surge arrestors, etc. Provide adequate clearance about 2.
Ground all circuit conductors not under test including the cable shields and nearby equipment. Test equipment should be supplied from a stable, constant voltage power source. Do not use the same source that is supplying arc welders or other equipment that may cause line voltage fluctuations! The use of a portable, motor driven alternator to provide power to the test set is recommended. Hi-pot testing procedures The DC test voltage may be applied either continuously or in predetermined steps up to the maximum value in accordance with the applicable specifications.
Some equipment will take longer to reach the maximum test voltage because of the amount of charging current. Step method The test voltage is applied slowly in 5 to 7 increments of equal value up to the maximum specified. Allow sufficient time at each step for the leakage current to stabilize. Recommended testing procedure Maintain the test voltage at the prescribed value for the time specified in the applicable specifications.
After testing, maintain solid grounds on the cable for at least 4 times the duration of the test since DC charges can build up on the cable to potentially dangerous levels if grounds are removed too quickly.
On exceptionally long cable lengths, it may be necessary the increase the grounding time. It is also advantageous to maintain these grounds longer and while reconnecting circuit components.
Acceptance testing Acceptance testing is performed to detect any defects in cable insulation and terminations which may have resulted from poor workmanship or mechanical damage. This proof test confirms the integrity of the insulation and accessories before the cable is placed into service. After installation and before the cable is placed in regular service, the test voltages specified in Table c.
Record the leakage current at one minute intervals for the duration of the test. Table c. Routine periodic DC maintenance testing of cables for the evaluation of the insulation strength is not a common practice.
Some power cable users, particularly in the continuous process industries, have adopted a program of testing circuits during planned outages, preferring breakdowns during testing rather than experiencing a service outage during regular operations. It is nearly impossible to recommend test voltage values for maintenance testing without having a history of the cable circuit. An arbitrary test voltage level could result in a cable failure in a circuit that would otherwise have provided long trouble-free service at normal operating ac voltage.
Periodic off-line partial discharge testing at very low frequencies VLF is a diagnostic method for monitoring the insulation degradation of medium voltage cable. This type of testing is discretionary and generally not recommended.
The DC voltage potential in the windings is rapidly raised to a predetermined level, or raised in steps up to this level, depending on what test method is used. As the voltage is raised, several currents will flow into and possibly out of the windings to ground, and the combined total of these currents are measured by the Hipot tester. The windings have capacitance. Current is required to elevate its voltage potential. This current typically drops to zero within seconds after the test voltage provided by the motor tester is stable. IA— Absorption current: This current is present during the atomic and any molecular polarization of the insulation, and is the current one is interested in during a PI test. This current will drop to zero, or near zero, over a period of time that varies by motor.
How DC Hipot Tests Work
Hipot test is the opposite of a continuity test. Continuity Test checks surety of current flows easily from one point to another point while Hipot Test checks surety of current would not flow from one point to another point and turn up the voltage really high just to make sure no current will flow. Importance of HIPOT Testing: The hipot test is a nondestructive test that determines the adequacy of electrical insulation for the normally occurring over voltage transient. This is a high-voltage test that is applied to all devices for a specific time in order to ensure that the insulation is not marginal. Hipot tests are helpful in finding nicked or crushed insulation, stray wire strands or braided shielding, conductive or corrosive contaminants around the conductors, terminal spacing problems, and tolerance errors in cables.