Why insulation resistance test

Time-resistance tests on large rotating electrical machinery - especially with high operating voltage - require high insulation resistance ranges and a very constant test voltage. A heavy-duty megohmmeter serves this need. Similarly, such an instrument is better adapted for cables, bushings, transformers, and switchgear in the heavier-duty sizes.

In this test, the operator applies two or more test voltages in steps. The recommended ratio for the test voltage steps is 1 to 5. At each step, test voltage should be applied for the same length of time, usually 60 seconds. The application of increased voltage creates electrical stresses on internal insulation cracks. This can reveal aging and physical damage even in relatively dry and clean insulation which would not have been apparent at lower voltages. Compare the readings taken at different voltage levels, looking for any excessive reduction in insulation resistance values at the higher voltage levels.

Insulation that is thoroughly dry, clean, and without physical damage should provide roughly the same resistance values despite changes in test voltage levels. If resistance values decrease substantially when tested at higher voltage levels, this shouldserve as a warning that insulation quality may be deteriorating due to dirt, moisture, cracking, aging, etc. Absorption curve of test conducted on HP Motor: Curve D indicates a good insulation with an excellent polarization index of 5.

Curve E indicates a potential problem. Before and after repair: Curve F shows a downward trend of insulation resistance values as the test voltage is increased. This indicates a potential problem with the insulation. Curve G shows the same equipment after it has been repaired. There are two schools of thought regarding the voltage to test insulation at. The first applies to new equipment or cable and can use AC or DC test voltages.

When DC voltage is used most common on megohmmeters manufactured today the rule of thumb is simply 2 x nameplate voltage except when higher voltages are used. See chart below for suggested values. It is always advisable to contact the original equipment manufacturer to get their recommendation for the proper voltage to use when testing their equipment.

Transformers are tested at or above the rated voltage to be certain there are no excessive leakage paths to ground or between windings. These are conducted with the transformer completely disconnected from the line and load. However, the case ground should not be removed. The following 5 tests and corresponding wiring diagrams will completely test a single-phase transformer. Allow at least 1 minute for each test or until the reading stabilizes. The following 5 tests and corresponding wiring diagrams will completely test a three-phase transformer.

When an electric current is sent through a conducting wire it would reach the destination but due to certain reasons some of the electric current would be lost along the way. So we need to do the insulation testing to check if there is any leakage of current and this current leakage could be harmful.

Insulation resistance testing is really important to prevent damage and injury. This test must be conducted to ensure the reliability of the electrical systems and also the equipment.

The electrical resistance of insulating material is to direct a voltage, and it can be detected by measuring the leakage of current which flows through the insulation.

By doing an insulation resistance test we can determine if the equipment is in proper condition, and we can almost detect the service life of the insulation by this test. The insulation testing must be done when installing a new electrical machinery, so while testing this equipment we can make sure that the insulation is in proper condition for the operation and it would also be useful as a reference for the future testing.

The wire could be copper or aluminum which will be a good conductor and the insulation will resist the current and will keep the current in its path along the conductor. So an important matter which should be considered is that no insulation is perfect and because of that some of the electric currents would flow through the insulation to the ground.

This small current will be one microampere so it is very low and this is the major part of the insulation testing. Insulation could go bad if the conductors are subjected to mechanical stress, the mechanical stress is caused by the short circuit current, thermal expansion, contraction of the conductors and it can also happen because of vibrations.

So we need to check the insulation frequently and the insulation tests must be conducted. The testing of insulation resistance in a cable is done to determine the resistance value to the current flow across it. If there is a high resistance then it means a very low current is escaping and if the measured value of the resistance is low then it means the leakage is high.

We can do the insulation test by pressurizing the conductor with a given voltage and dividing it by the current that escapes the insulation. Portable insulation resistance testers and megohmmeters are designed to help prevent hazards such as electric shock and short-circuits caused when the insulation in electrical devices, parts, and equipment used in industrial plants, buildings, and other settings degrades over long periods of use. Degradation of the insulation between charged and non-charged portions of electrical devices and equipment utilizing the type of structure illustrated in Figure1 poses the risk of a ground fault or electric shock.

Performing such tests at regular time intervals can detect impending insulation failures before they occur and prevent user accidents or costly product repairs. As shown in Figure 15, the 2-wire ungrounded connection is the recommended setup for testing ungrounded components.

This is the most common configuration for testing 2-terminal devices such as capacitors, resistors, and other discrete components. Referring to Figure 16, the 2-wire grounded measurement is the recommended connection for testing grounded components.

A grounded component is one in which one of its connections goes to an earth ground, whereas an ungrounded component is one in which neither connection goes to earth ground. Measurement of insulation resistance of a cable in a water bath is a typical application of a 2-wire grounded connection. An insulation resistance test usually has four phases: charge, dwell, measure, and discharge.

During the charge phase, the voltage is ramped from zero to the selected voltage, which provides stabilization time and limits the inrush current to the DUT. Once the voltage reaches the selected value, the. Once the resistance has been measured for the selected time, the DUT is discharged back to 0V during the final phase.

The output voltage is typically in the range of 50 to Volts DC.