Most cables that are rated for use at voltage levels above 600 V are shielded cables. A shielded cable has a conductor in the center, a semiconducting layer over the strands that is surrounded by insulation, a semiconducting layer, and then a metal foil or wire mesh that surrounds the whole assembly.
There is usually another layer over the shield that makes up the outer jacket of the cable. It is a common practice to hi-pot test the cables on initial installation in order to verify that the cables were not damaged when they were pulled into place and that all the splices and/or terminations were installed properly.
The voltage level that is selected usually is lower than factory test levels, frequently 80% of the dc equivalent of the factory test level.
There are normally two considerations that are given to hi-pot testing of cables as a routine maintenance practice. One is a function of the chosen maintenance philosophy [i.e., breakdown maintenance, preventive maintenance, predictive maintenance, or reliability-centered maintenance (RCM)].
The other depends upon the type of operation and how critical it is to have continuous power without interruption.
The debate on whether or not to perform maintenance hi-pot testing centers around the fact that a cable in marginal condition can be caused to fail by the hi-pot test itself. A cable that is in good condition should not be harmed.
People who subscribe to maintenance testing feel that it is much better to have the cable fail under test. Cable maintenance testing frequently is performed at 50-65% of the factory test voltage.
Problems can then be corrected while the circuit is intentionally shut down, thus avoiding an in-service failure that could interrupt production.
It is important to remember that the necessary material, such as splice kits or cable terminations, should be available to facilitate repairs should the cable fail during testing.
HIGH POTENTIAL (HI POT) TESTING BASIC INFORMATION AND TUTORIALS
What is HiPot testing?
High-potential testing, as its name implies, utilizes higher levels of voltage in performing the tests. It is generally utilized on medium-voltage (1000Ð69 000 V) and on high-voltage (above 69 000 V) equipment.
As stated earlier, the leakage current is usually measured. In some cases, such as in cable hi-potting, the value of leakage current is significant and can be used analytically. In other applications, such as switchgear hi-potting, it is a pass/fail type of test, in which sustaining the voltage level for the appropriate time (usually 1 min) is considered "passing."
High-potential testing, as its name implies, utilizes higher levels of voltage in performing the tests. It is generally utilized on medium-voltage (1000Ð69 000 V) and on high-voltage (above 69 000 V) equipment.
As stated earlier, the leakage current is usually measured. In some cases, such as in cable hi-potting, the value of leakage current is significant and can be used analytically. In other applications, such as switchgear hi-potting, it is a pass/fail type of test, in which sustaining the voltage level for the appropriate time (usually 1 min) is considered "passing."
INSULATION RESISTANCE TEST BASIC INFORMATION AND TUTORIALS
What is insulation resistance test? How to conduct insulation resistance test?
Insulation resistance tests are typically performed on motors, circuit breakers, transformers, low-voltage (unshielded) cables, switchboards, and panel boards to determine if degradation due to aging, environmental, or other factors has affected the integrity of the insulation.
This test is normally conducted for 1 min, and the insulation resistance value is then recorded. As mentioned earlier, the electrical properties of the insulation and the amount of surface area directly affect the capacitance between the conductor and ground, and therefore affect the charging time.
With larger motors, generators, and transformers, a common test is to measure the "dielectric absorption ratio" or the "polarization index" of the piece of equipment being tested. The dielectric absorption ratio is the 1 min insulation resistance reading divided by the 30 s insulation resistance reading.
The polarization index is the 10 min (continuous) insulation resistance reading divided by the 1 min reading. Both of these provide additional information as to the quality of the insulation.
Many types of insulation become dry and brittle as they age, thereby becoming less effective capacitors. Thus, a low polarization index (less than 2.0) may indicate poor insulation.
Even though insulation may have a high insulation resistance reading, there could still be a problem, since the motor and transformer windings are subjected to strong mechanical stresses on starting. With the exception of electronic equipment (which can be damaged by testing), insulation resistance testing is normally done on most types of new equipment and is also part of a maintenance program.
It is a good practice to perform insulation resistance testing on switchgear and panelboards after maintenance has been performed on them, just prior to re-energizing them. This prevents re-energizing the equipment with safety grounds still applied or with tools accidentally left inside.
Insulation resistance tests are typically performed on motors, circuit breakers, transformers, low-voltage (unshielded) cables, switchboards, and panel boards to determine if degradation due to aging, environmental, or other factors has affected the integrity of the insulation.
This test is normally conducted for 1 min, and the insulation resistance value is then recorded. As mentioned earlier, the electrical properties of the insulation and the amount of surface area directly affect the capacitance between the conductor and ground, and therefore affect the charging time.
With larger motors, generators, and transformers, a common test is to measure the "dielectric absorption ratio" or the "polarization index" of the piece of equipment being tested. The dielectric absorption ratio is the 1 min insulation resistance reading divided by the 30 s insulation resistance reading.
The polarization index is the 10 min (continuous) insulation resistance reading divided by the 1 min reading. Both of these provide additional information as to the quality of the insulation.
Many types of insulation become dry and brittle as they age, thereby becoming less effective capacitors. Thus, a low polarization index (less than 2.0) may indicate poor insulation.
Even though insulation may have a high insulation resistance reading, there could still be a problem, since the motor and transformer windings are subjected to strong mechanical stresses on starting. With the exception of electronic equipment (which can be damaged by testing), insulation resistance testing is normally done on most types of new equipment and is also part of a maintenance program.
It is a good practice to perform insulation resistance testing on switchgear and panelboards after maintenance has been performed on them, just prior to re-energizing them. This prevents re-energizing the equipment with safety grounds still applied or with tools accidentally left inside.
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