Record Keeping 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.

Effects of Temperature 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, to simplify the correction of resistance values. Table 7-3 gives such factors for rotating equipment, transformers, and cable. You multiply the reading you get by the factor corresponding to the temperature (which you need to measure).

For example, assume you have a motor with Class A insulation and you get a reading of 3.0 megohms at a temperature (in the windings) of 131 F (55 C). From table 7-3 you read across at 131 F to the next column (for Class A) and obtain the factor 15.50. Your correct value of resistance is then

3.0 megohms x 15.50 = 46.5 megohms (reading at (C o r r e c t i o n (C o r r e c t e d 131 F) factor for reading for 68 F Class A in-or 20 C) sulation at

131 F) Note that the resistance is 14.5 times greater at 68 F (20 C) than the reading taken at 131 F. The reference temperature for cable is given as 60 F (15.6 C), but the important point is to be consistent-correcting to the same base before making comparisons between readings.

Effects of Humidity We mentioned in this chapter about the presence of moisture in insulation and its marked effect upon resistance values. You might expect that increasing humidity (moisture content) in the surrounding (ambient) air could affect insulation resistance. And it can, to varying degrees.

If your equipment operates regularly above what is called the "dew-point" temperature (that is, the temperature at which the moisture vapor in air condenses as a liquid), the test reading normally will not be affected much by the humidity. This stability is true even if the equipment to be tested is idle, so long as its temperature is kept above the dew point. In making this point, we are assuming that the insulation surfaces are free of contaminants, such as certain lints and acids or TEMP.

Table 7-3.- Temperature Correction Factors (Corrected to 20 C for Rotating Equipment and Transformers; 15.6 for Cable) ROTATING EQUIP.

salts that have the property of absorbing moisture (chemists call them "hygroscopic" or "deliquescent" materials). Their presence could unpredictably affect your readings; they should be removed before tests are made. In electrical equipment we are concerned primarily with the conditions on the exposed surfaces where moisture condenses and affects the overall resistance of the insulation. Studies show, however, that dew will form in the cracks and crevices of insulation before it is visibly evident on the surface. Dew-point measurements will give you a clue as to whether such invisible conditions may exist, altering the test results.

As a part of your maintenance records, it is a good idea to make note at least of whether the surrounding air is dry or humid when the test is made and whether the temperature is above or below the ambient. When you test vital equipment, record the ambient wet-and dry-bulb temperatures from which dew point and percent relative or absolute humidity can be obtained

Preparation of Apparatus for Test NOTE: Before interrupting any power, be certain to check with your seniors (crew leader, project chief, or engineering officer, as appropriate) so that any necessary notification of the power outage may be made. Critical circuits and systems may require several days or even weeks advance notice before authorization for a power outage may be granted.