Background

High-potential tests are required in connection with the manufacture, repair, and reconditioning of electric motors. The purpose of the tests is to determine the effectiveness of electric motor insulation. The tests will indicate any defective material and workmanship and permit replacement prior to actual use.

High voltage is used to conduct the test; therefore, the operator must know:
• The safety precautions for using the equipment
• How to calculate the voltage for the unit under test
• How to conduct the test
• How to interpret and record the test results

Safety precautions

•  Ensure that the test area is away from densely traveled areas.
•  Keep personnel who are NOT involved in testing away from the test area.
•  Inspect high voltage leads to ensure that insulation is intact.
•  Do NOT test a unit that has failed a ground test.
•  Do NOT exceed calculated test voltage.

Calculating test voltage

The formula for calculating test voltage for new windings is 2E + 1000, where E is design voltage.

In reconditioned windings, test voltage is calculated by: 2/3 (2E + 1000).

Example:

1. AC VOLTMETER. Three-range meter that allows continuous monitoring of voltage

2. VOLTAGE RANGE. Three-position switch selects maximum output necessary for the test

3. INTER OPEN. The amber indicator light indicates the position of the interlock switch, if installed.

4. AC POWER ON. Toggle switch turns power ON and OFF.

5. AC POWER. When this amber indicator light is lit, power is ON.

6. ZERO START. This is a rheostat position on the RAISE VOLTAGE control; it must be zeroed before the start of each test.

7. RAISE VOLTAGE. This interlocked control allows continuous adjustment of voltage.

8. HIGH VOLTAGE ON. Pushbutton, when pressed, energizes a relay to apply power to the primary of the high-voltage transformer.

9. HIGH VOLTAGE INDICATOR LIGHT. Glows red when high voltage is present.

10. HIGH VOLTAGE OFF. Deenergizes the high-voltage transformer

11. OVERLOAD RESET. This pushbutton resets the overload after the overload trips and extinguishes the OVERLOAD indicator light.

12. OVERLOAD. Indicator light glows red when the overload is tripped.

13. SENS. Control allows sensitivity adjustment of the overload element

14. BACKUP. If the overload fails to trip, this circuit breaker switch provides backup tripping capability.

15. MILLIAMP. This meter provides a readout on current leakage between the high-voltage line and the ground. It is connected in the grounded side of the line.

LEADS. Three-pronged grounded power cord, high-voltage lead, ground straps for the instrument and test component (must be fabricated)

Ensure that the instrument case is grounded before connecting the instrument to the power source. This must be a separate-case ground in addition to the ground in the plug.

Conducting an ac high-potential test

Ensure that the area is clear of personnel who are NOT involved in testing.

Ensure that the power supply is within 7 feet of the test unit. Do NOT use an extension cord.

Conduct an insulation resistance-to-ground test of motor and record results in the appropriate section of the motor data sheet (electrical).

Calculate test voltage as described above.

Ensure that the power switch is OFF.

Ground the instrument case.

Plug the instrument into a 115-volt ac, 60-hertz single-phase (convenience) outlet.

Twist together the line leads of the winding to be tested.

In cases of multiple winding motors, twist together and ground the leads of windings not under test.

Connect the high-voltage lead to the winding leads to be tested.

Connect the ground lead to the instrument and motor.

Check the test set to ensure that the RAISE VOLTAGE control is set at zero.

Select the proper voltage range for the voltmeter.

Set the SENS control to MIN.

Turn the test set ON. The white AC POWER indicator light should come on at this time.

Press the HIGH VOLTAGE ON pushbutton. The red HIGH VOLTAGE indicator light should come on at this time.

Use the RAISE VOLTAGE control to increase the voltage to test level while observing voltmeter.

Insulation failure will be indicated by the red OVERLOAD indicator light coming on and the voltmeter falling to zero.

Apply test voltage for 1 minute as timed by a watch with a sweep second hand.

After 1 minute, decrease the voltage at a rate to lower the value to 25 percent of test voltage in 15 seconds as timed by a watch with a sweep second hand.

From this point, use the RAISE VOLTAGE control to slowly lower voltage to zero.

Press the HIGH VOLTAGE OFF pushbutton.

Short the high voltage lead to ground.

Disconnect the high voltage lead and ground lead from the test component.

When a unit has failed a test, lower the RAISE VOLTAGE control to zero and press the OVERLOAD RESET switch. Check the circuit breaker and reset if necessary before attempting another test.

Record results of the test in the appropriate section of the motor data sheet (electrical).

Conduct another insulation resistance-to-ground test on the motor and record those results in the appropriate section of the motor data sheet (electrical).

Instrument reset after insulation failure

Press the OVERLOAD RESET switch.

Check the BACKUP circuit breaker.

Return the RAISE VOLTAGE control to zero.