What is Megger Testing? (i.e. Insulation Resistance Testing)

Insulation Resistance Testing Ensures Personal Safety and Power Reliability

Almost all wires carrying electric current have a covering of insulation, whose sole purpose is to protect the wire from the environment and confine the current flow along the wire. A major part of maintenance and testing of electrical equipment involves the evaluation of insulation integrity, which makes insulation resistance testing extremely significant.

Quality of Electrical Insulation

The presence of a voltage across a resistance forces a current flow through it. According to Ohm’s Law, the product of the current and the resistance value equals the voltage applied. This forms the basis of specifying the quality of electrical insulation. Good insulation has high resistance, allowing only small amounts of current flow through it, whereas deteriorating insulation has lower resistance, allowing comparatively higher amounts of leakage current through it.

Causes of Detnerioration of Electrical Insulatio

Although manufacturers are continually improving the quality of insulation they use for wires, cables, motors, and so on, several factors subject the insulation to a constant downward trend of deterioration. Chief among them are the environmental factors and mechanical damage.

Environmental factors exert a significant influence on electrical insulation, starting right from the time it is made. Age and temperature extremes deteriorate its performance, while chemical contamination causes additional damage to the quality of electrical insulation. As this can lead to impaired personal safety and power unreliability, identifying the deterioration in time is essential to take corrective measures.

Mechanical damage caused by vibration and electrical stresses also affect insulation. They create hairline cracks or pin holes through which dust, moisture, and other foreign matter penetrate the surface of the insulation, reducing its resistance.

Necessity of Periodic Insulation Resistance Testing

As deterioration proceeds with the different factors aiding each other, they permit increasing amounts of current through the insulation. The insulation resistance drops gradually, giving adequate warning. Therefore, periodic checking of insulation resistance allows reconditioning before failure of service.

If functioning is allowed without periodic checking, poor insulation may gradually deteriorate to the point of becoming dangerous to touch, leading to electric shock in the presence of applied voltage. Excessive leakage may also cause the insulation to burn out, making it a partial conductor.

Manufacturers use insulation resistance testing for controlling quality during production and as a means to ensure their products meet specifications. Periodic insulation resistance testing also helps preventive maintenance tasks, with engineers and technicians using it as a troubleshooting tool.

Performing an Insulation Resistance Test

Insulation resistance test is essentially a measure of the current flowing along the surface of the insulation on application of a voltage generated by a high direct voltage generator. An ohmmeter measures the small current, computes the resistance (from Ohm’s Law) in megohms, and indicates it either digitally or on an indicating scale. The megohmmeter or the Megger insulation tester combines the DC voltage generator, the ohmmeter, and the display in one convenient, portable instrument. Built-in current and voltage coils allow the Megger to read true ohms directly, independent of the actual voltage the instrument applies to the insulation.

For making relatively accurate comparisons with earlier tests, the industry standard specifies applying the test voltage on the insulation for a period of one minute only. During this interval, the Megger may show a relatively steady reading or a slight drop, with the final resistance value being the reading at the end of the minute.

As electrical insulation exhibits dynamic behavior when testing, consistency during repeated testing is highly recommended. When evaluating multiple test results on the same insulation, conduct the tests in the same way, making sure the environmental parameters remain relatively constant. That is because insulation testing is time and temperature dependent.

Factors Affecting Insulation Resistance Readings

Insulation materials exhibit capacitance. This will charge during the test, influencing the resistance reading. The rate of change of resistance during the test is a useful tool for the seasoned electrician.

As electrical resistance decreases with an increase in temperature and vice-versa, the readings recorded need correction with respect to a base temperature. Refer to the Maintenance Testing Specifications for Electrical Power Distribution Equipment and Systems from the interNational Electrical Testing Association (NETA), for interpretation of test results.

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