CONDITION

1. Fair to high values and well main-tained.

2. Fair to high values, but show-ing a constant tendency towards lower values.

3. Low but well maintained.

4. So low as to be unsafe.

WHAT TO DO No cause for concern.

Locate and remedy cause and check downward trend. the the Condition is probably all right, but the cause of the low values should be checked. Clean, dry out, or otherwise raise the values before placing equipment in service (test wet equip-ment while drying it out).

5. Fair or high Make tests at frequent values, previously intervals until the cause of well maintained low values is located and but showing sud-remedied or until the den lowering. values become steady at a level that is lower but safe for operation or until values become so low that it is unsafe to keep the equipment in operation.

Short-Time or Spot-Reading Tests Several test methods are commonly used. We will discuss the short-time or spot-reading tests.

In this method, you simply connect the megger across the insulation to be tested and operate it for a short, specific time period (60 seconds usually is recommended). As shown in figure 7-28, you have picked a point (to take the reading) on a curve of increasing resistance values; quite often the value will be less for 30 seconds, more for 60 seconds. Bear in mind also that temperature and humidity, as well as condition of the insulation, affect your reading.

If the apparatus you are testing has low capacitance, such as a short run of type NM cable (Romex), the spot-reading test is all that is necessary; however, most equipment is capacitive, so your first spot reading on equipment in your work area- with no prior tests- can be only a rough guide as to how "good" or "bad" the insulation is. For many years, maintenance personnel have used the 1-megohm rule to establish the allowable lower limit for insulation resistance. The rule may be

Figure 7-28.- Typical curve of insulation resistance (in megohms) with time.

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.

Common Test Voltages Commonly used dc test voltages for routine maintenance are as follows:

EQUIPMENT AC RATING DC TEST VOLTAGE

up to 100 volts 440 to 550 volts 2,400 volts

4,160 volts and above

(See selector switch settings on Megger(c) in fig. 7-25.) 100 and 250 volts 500 and 1,000 volts 1,000 to 2,500 volts or higher

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 manufacturer's 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.

Causes of Low Insulation Resistance Readings 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.