April 201 Electrical Apparatus

April 201 Electrical Apparatus

This is a summary of the Electrical Apparatus April 2015 featured technical article,  by Richard L. Nailen, P.E.   

In using the megohmmeter to measure insulation resistance (IR) of electrical equipment, and properly interpreting the results, misunderstandings often arise. A d-c test voltage is applied across the insulation involved, for a specified time period, and the measured leakage current is converted into an ohmic resistance. The test is not considered destructive, it is not a “pass-fail” or “go-no go” procedure, and it does not yield a specific measure of future insulation life.

The proper use of IR testing is to establish a trend. Repeated tests, typically once a year, will show whether insulation condition is beginning to deteriorate (most often involving contamination) so that corrective measures can be taken before failure is imminent. Because IR is dependent upon temperature (of the insulation itself, not of the surroundings or the associated conductors), each reading must be corrected to a common reference temperature according to the variation appropriate for the insulation system.

IEEE Standard 43 prescribes test procedures and IR limits for rotating apparatus. Little standard guidance exists for other apparatus, including switchgear and transformers. Apparatus manufacturers can be consulted for specific products. Cable is normally assigned a minimum IR limit for a length of 1,000 feet (305 meters). Because the test voltage is applied between the insulated conductor and the outer surface of the insulation, the shorter the length of cable, the smaller that surface, and the higher the minimum IR. A common method of test is immersion of the cable in water so that the test voltage can be applied between the water and the conductor.

Whatever the procedure, test voltage is too low to constitute an “overpotential” application that threatens insulation life. Typical values are 500 for insulation voltage ratings below 600, up to 5,000 for ratings of 6 kV or more.

How much accelerating torque will be needed depends upon the load. Unless that is coupled through a centrifugal clutch, or an adjustable-speed drive is added to provided high torque, acceleration needs careful evaluation. A possible alternative is either a high resistance bar or double-cage rotor design; however, these tend to increase losses and heating as well as requiring a design study.

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