310.2

Calculate the meter reading, using one of the following formulas.

Reading x MULTIPLIER = Insulation Resistance Reading

Example 1: "M" MULTIPLIER setting

2 x 100M = 200M (200 megohms)

Example 2: "G" MULTIPLIER setting

2 x 10G = 20G (20 gigaohm)

Example 3: "T" MULTIPLIER setting

0.9 x 1T = 0.9T (0.9 teraohms)

Enter the reading from the "M" setting (example 1) directly in section 2, 3, 4, or 5 of the motor data sheet (electrical). Readings from "G" or "T" scales will have to be converted to megohms as follows:

Conversion of readings from the "G" MULTIPLIER setting to megohms
(Note: One megohm {M} = 10⁶.)

Use Example 2, "G" MULTIPLIER setting.

G = 10⁹ 20G = 20 x 10⁹

Convert 20 x 10⁹ to 10⁶.

Place a decimal point behind the 20.

20. x 10⁹

Move the decimal point right one place at a time and reduce the exponent by one for each place the decimal point is moved.

x 10⁹ = 200. x 10⁸ = 2,000. x 10⁷ = 20,000 x 10⁶

Stop moving the decimal point and dropping exponents when 10⁶ is reached.

Enter 20,000 in the appropriate insulation resistance-to-ground (in megohms) section of the motor data sheet (electrical).

Insulation-to-ground readings are usually required to be converted to megohms for entry into work packages.

Conversion of readings from the "T" MULTIPLIER setting to megohms (Note: One megohm {M} = 10⁶.)

Use Example 3, "T" MULTIPLIER setting.

T = 10¹² 0.9T = 0.9 x 10¹²

Convert 0.9 x 10¹² to 10⁶.

Move the decimal point right one place at a time and reduce the exponent by one for each place the decimal point is moved.

9,000 x 10⁸ = 90,000. x 10⁷ = 900,000. x 10⁶

Stop moving the decimal point and dropping exponents when 10⁶ is reached.

Enter 900,000 in the appropriate insulation resistance-to-ground (in megohms) section of the motor data sheet (electrical).

The minimum allowable insulation resistance-to-ground reading is 100 megohms for new winding and 25 megohms for reconditioned windings. The motor will be reconditioned or rewound if minimum insulation resistance-to-ground readings are not attained.

A polarization index is conducted to measure the cleanliness and dryness of the winding. Connect the megohmmeter in the same manner as previously described.

Observe the 1-minute meter reading and record it on the motor data sheet (electrical). The 1-minute reading is also used as insulation resistance-to-ground test. Observe the 10-minute meter reading and record on the motor data sheet (electrical).

Use the insulation resistance values obtained at 1 minute and 10 minutes to calculate the polarization index.

To calculate the polarization index

Divide the 10-minute value by the 1-minute value. If the polarization index is less than 2, or the 1-minute insulation resistance measurement (corrected to 25°C) is less than 2 (1 + E)1000) megohms where E is the machine's rated voltage, the winding is not suitable for application of the DC high-potential test and should be reconditioned.

Securing and stowing the Type 1863 Megohmmeter

Set the function switch to DISCHARGE. Set the POWER switch to OFF. Unplug the instrument and short the winding under test to ground with the shorting probe. Disconnect the instrument leads from the motor. Disconnect the instrument leads from the instrument. Stow the leads in the instrument cover. Rotate the MULTIPLIER counterclockwise to the minimum position.

Balance the instrument with one hand. Depress the handle with the other. When the instrument is clear of the gasket, tilt the instrument until it is square with the gasket, and the instrument is in the horizontal position, face down. Let the handle rise slowly until the instrument case seats on the gasket. Coil the line cord into the back of the case. Secure the line cord by plugging it into the dummy receptacle on the back of the instrument.

Typical Polarization Index Graph