Preventive Maintenance test with Insulation Resistance Test, Part 3

Part 1 provides an overview on the insulation resistance test in preventive maintenance. To read part 1, click here.
Part 2 covers the insulation resistance test methods. To read part 2, click here.

Test Voltage Selection
As the insulation resistance test consists of the high DC voltage, the appropriate test voltage has to be selected to avoid over stressing the insulation, which may lead to insulation failure. The test voltage applied to should be based on the product/equipment manufacturer recommendations. If the test voltage is not specified, industrial standards and practices may be applied.   The following guideline for rotating machinery shown in Table 2 may be adopted in the absent of the manufacturer’s data.

Table 2 Guidelines for DC voltage to be applied during insulation resistance test (extracted from IEEE Std 43-2000)

Winding rated voltage (V)1	Insulation resistance test direct voltage (V)
< 1000	500
1000 - 2500	500 - 1000
2501 - 5000	1000 - 2500
5001 – 12000	2500 – 5000
> 12000	5000 - 10000

¹ Rated line-to-line voltage for three-phase AC machines, line-to-ground voltage for single-phase machines, and rated direct voltage for DC machines or field windings.

Table 3 insulation Resistance Test Values Electrical Apparatus and System (extracted from NETA ATS-2007 Acceptance Testing Specifications for Electrical Power Distribution Equipment and Systems)

In the absence of consensus standards dealing with insulation-resistance tests, the Standards Review Council suggests the above representative values.

Test results are dependent on the temperature of the insulating material and the humidity of the surrounding environment at the time of the test. Insulation-resistance test data may be used to establish a trending pattern. Deviations from the baseline information permit evaluation of the insulation.

The test voltage may vary according to the international standards. Consulting the product/equipment manufacturer for the proper test voltage values is recommended.

Determination of Minimum Insulation Resistance

The IEEE Std 43-2000 indicates that the minimum insulation resistance for AC and DC machine stator windings and rotor windings can be determined by:

R_m = kV + 1

Where,

• R_m is the recommended minimum insulation resistance in MΩ at 40 °C of the entire machine winding, and
• kV is the rated machine terminal-to-terminal voltage in kV unit

Table 4 Recommended minimum insulation resistance values at 40 °C (extracted from IEEE Std 43-2000)

Minimum insulation resistance (MΩ)	Test specimen
IR1 min = kV + 1	For most windings made before about 1970, all field windings, and others not described below
IR1 min = 100	For most dc armature and ac windings built after about 1970 (form-wound coils)
IR1 min = 5	For most machines with random-wound stator coils and form-wound coils rated below 1 kV

Safety consideration

As insulation resistance testing involves high DC voltage application, the following safety precautions should be taken:

• Make sure that the device under test is discharged.
• Conduct the test at the de-energized condition to ensure that no test voltage other than that from the insulation resistance tester is applied.
• Restrict personal access when high voltage testing is being conducted.
• Use of personal protective equipment (e.g. protective gloves) where applicable. 
• Ensure suitable test leads are used and that they are in good condition. Using unsuitable test leads not only contributes to errors in readings, they may be hazardous.

After the test, make sure the device is fully discharged. This can be done by shorting the terminal with a suitable resistor. A minimum discharge time of four times the applied voltage duration is recommended. Some insulation resistance testers may have the built in self discharge circuit to ensure a safe discharge after the test. Testers with this feature ensure devices are safely discharged after every test.

Planning for a Maintenance Program

When planning for a maintenance program, equipment that needs maintenance needs to be identified, and priorities set accordingly. A motor or machine that supports the whole line should be a high priority. The frequency of checks to be conducted should also be defined. The frequency can be varied from unit to unit depending on the criticalness of the unit in the environment. Past history will be a good guide for determining when the next maintenance activities will be needed.
The maintenance record should cover the following:

1. Date of the test
2. Test voltage and current
3. Test time
4. Insulation resistance value
5. Temperature of winding/equipment
6. Identification of the equipment/device under test
7. Parts or equipment that were included in the test
8. Relative humidity

As with every preventive maintenance program, record keeping and plotting of consecutive readings can identify trends and enable you to predict and plan for the next action.

Conclusion

Periodic testing is the best approach for preventive maintenance of electrical equipment and charting result values helps in monitoring the trend of the insulation resistance, which helps predict the future need for action.