stated thus: Insulation resistance should be approximately 1 megohm for each 1,000 volts of operating voltage with a minimum value of 1 megohm. For example, a motor rotated at 2,400 volts should have a minimum insulation resistance of 2.4 megohms. In practice, megohm readings normally are considerably above this minimum value in new equipment or when insulation is in good condition.
By taking readings periodically and recording them, you have a better basis for judging the actual insulation condition, Any persistent downward trend is usually fair warning of trouble ahead, even though the readings may be higher than the suggested minimum safe values. Equally true, as long as your periodic readings are consistent, they may be all right even though lower than the recommended minimum values.
Commonly used dc test voltages for routine maintenance are as follows:
|EQUIPMENT AC RATING||DC TEST VOLTAGE|
|(See selector switch settings on Megger© in fig. 7-25.)|
|up to 100 volts||100 and 250 volts|
|440 to 550 volts||500 and 1,000 volts|
|2,400 volts||1,000 to 2,500 volts or higher|
|4,160 volts and above||1,000 to 5,000 volts or higher|
CAUTION Use care in applying test voltage to the component to be tested. Do not use a high-test voltage on low-voltage equipment or components.
Do not exceed the commonly used test voltages mentioned above unless you are following the equipment manufacturers instructions to do so. On the other hand, a test voltage lower than the operating voltage of the component to be tested may not reveal a problem that the test should indicate. If the test voltage is too low, you may get no more than a resistance reading such as you would get with an ohmmeter.
Insulation resistance varies with the temperature. The effect of temperature depends on the type of insulation, the amount of moisture in and on the insulation surface, and the condition of the surface.
The amount of moisture in insulation has a great effect on its resistance. For meaningful results, tests of insulation resistance should be made under as nearly similar conditions as practical. Long cables can be exposed to a variety of conditions along the cable route at the same time. A comparison of readings may not indicate a change in insulation condition.
An accumulation of things like dust, dirt, and moisture can cause low-resistance readings. A motor stored or kept idle for a while may have to be cleaned and dried out before being installed and placed in service.
Records should be kept where tests are performed periodically. The frequency of the tests should be based on the importance of the circuit. One test each year is usually adequate. Records of each circuit or component may be compared. Trends may indicate a future problem, and corrections may be made in time to prevent future problems in cables or components, like motors or transformers.
If you want to make reliable comparisons between readings, you should correct the readings to a base temperature, such as 20°C (68°F), or take all your readings at approximately the same temperature (usually not difficult to do). We will cover some general guidelines to temperature correction.
One rule of thumb is that for every 10°C (50°F) increase in temperature, you halve the resistance; or for every 10°C (50°F) decrease, you double the resistance; for example, a 2-megohm resistance at 20°C (68°F) reduces to 1/2 megohm at 40°C (104°F).
Each type of insulating material will have a different degree of resistance change with temperature variation. Factors have been developed, however, toContinue Reading