By John Olobri, Technical Director, AEMC ® Instruments

Insulation testing is often misunderstood or not employed at all. Developing a standard practice of testing insulation resistance will provide considerable savings in unnecessary rework or down time. Testing should be completed before putting new equipment into service for the first time and at a periodic schedule (typically once a year) as a preventive maintenance tool. Testing new cable installations is a valuable time saving way to find nicks and shorts that will cause problems later on.

Why have an insulation testing program?
A regular program of testing insulation resistance is strongly recommended to prevent electrical shocks, assure safety of personnel and to reduce or eliminate down time. It helps to detect deterioration of insulation in order to schedule repair work live. It is also helpful when evaluating the quality of the repairs before the equipment is put back into operation and the quality of new wiring.

What causes insulation failure?
Some of the more common causes of insulation failure include excessive heat or cold, moisture, dirt, corrosive vapors, vibration, oil, nicked wires and aging.

What test voltage should I use?
There are two schools of thought regarding the voltage at which insulation should be tested. The first applies to new equipment or cable and can use AC or DC test voltages.

When AC voltage is used, the rule of thumb is 2 x Nameplate voltage + 1000. When DC voltage is used (most common on megohmmeters manufactured today) the rule of thumb is simply 2 x Nameplate voltage.

Equipment/Cable Rating DC Test Voltage

24 to 50V 50 to 100V dc

50 to 100V 100 to 250V dc

100 to 240V 250 to 500V dc

440 to 550V 500 to 1000V dc

2400V 1000 to 2500V dc

4100V 1000 to 5000V dc

It is always advisable to contact the original equipment manufacturer to get their recommendation for the proper voltage to use when testing their equipment.

What tests are used to detect insulation deterioration?
There are numerous maintenance tests for assessing insulation quality. The three tests discussed here are used primarily to test motor, generator and transformer and insulation wire.

What equipment is necessary for conducting insulation resistance tests?
• Megohmmeter

• Temperature meter

• Stopwatch with second and minute hands (if megohmmeter does not have a timer)

Test Currents in Insulation
Total current in the body of the insulation is the sum of three components

• Capacitance Charging Current

• Absorption Current

• Leakage or Conduction Current

Insulation Resistance Readings
Readings are time dependent

• At the start, capacitance is what you see first;

• At or about one minute, absorption; and

• At 10 minutes, reading is mainly leakage current.

Note the deflection of the needle

SPOT READING TEST
Method
For this test, the megohmmeter is connected across the insulation of the windings of the machine or cable being tested. A test voltage is applied for a fixed period of time, usually 60 seconds and a reading is taken. The spot reading test should only be carried out when the winding temperature is above the dewpoint 1.

Test Duration
To obtain comparable results, tests must be of the same duration. Usually the reading is taken after 60 seconds.

Interpretation of Results
Proper interpretation of spot reading tests requires access to records of results from previous spot reading tests. For conclusive results, only use results from tests performed at the same test voltage for the same amount of time, and under similar temperature and humidity conditions. These readings are used to plot a curve of the history of insulation resistance. A curve showing a downward trend usually indicates a loss of insulation resistance due to unfavorable conditions such as humidity, dust accumulation, oil penetration, etc. A very sharp drop indicates an insulation failure.

DIELECTRIC ABSORPTION TEST
Method
This test is based on the comparison of absorption characteristics of good insulation vs. the absorption characteristics of humid or otherwise contaminated insulation. During the test, a test voltage is applied for an extended period, usually 10 minutes. The operator takes a reading every 10 seconds for the first minute, and then every minute up to 10 minutes. A curve is drawn showing the insulation resistance value versus time.

Test Duration
0 to 10 minutes.

Interpretation of Results
T he slope of the curve indicates the condition of the insulation under test. A good insulation will show a continual increase in resistance, as shown in Curve D. Contaminated, moist, or cracked insulation will produce a relatively flat curve similar to that of Curve E.

A ratio known as the polarization index (PI) can be obtained by dividing the value from the 10-minute reading by the value from the one-minute reading. This polarization index is indicative of the slope of the curve.

A low polarization index usually indicates excessive moisture and contamination. On large motors or generators, values as high as 10 are commonly expected.

Test Methods – Time-Resistant Tests Dielectric Absorption Ratio (DAR) (Not a commonly used test)

• The ratio of 60 seconds/30 seconds

• less than 1 = failed

• 1.0 to 1.25 = OK

• 1.4 to 1.6 = excellent

step voltage test
Method
In this test, the operator applies two or more test voltages in steps. The recommended ratio for the test voltage steps is 1 to 5. At each step, test voltage should be applied for the same length of time, usually 60 seconds. The application of increased voltage creates electrical stresses on internal insulation cracks. This can reveal aging and physical damage even in relatively dry and clean insulation which would not have been apparent at lower voltages.

Test Duration
A series of “steps,” each step lasting 60 seconds.

Interpretation of Results
Compare the readings taken at different voltage levels, looking for any excessive reduction in insulation resistance values at the higher voltage levels. Insulation that is thoroughly dry, clean, and without physical damage should provide roughly the same resistance values despite changes in test voltage levels. If resistance values decrease substantially when tested at higher voltage levels, this should serve as a warning that insulation quality may be deteriorating due to dirt, moisture, cracking, aging, etc.

Polarization Index (PI) = 10-minute reading
1-minute reading

The IEEE Std 43-1974 2 (Note 2 ) lists the following minimum values for the polarization index for AC and DC rotating machines:

Class A: 1.5 Class B: 2.0 Class C: 2.0

Utilizing the Guard Terminal
The guard terminal is useful when measuring very high resistance values.

Advantages of DC Testing
The majority of megohmmeters now use DC voltage. There are several clear advantages of DC Testing when using a megohmmeter. Key advantages include: lighter size and weight of test equipment, non-destructive testing and historical data can be compiled.

Transformer Testing
Transformers are tested at or above the rated voltage to be certain there are no excessive leakage paths to ground or between windings. These are conducted with the transformer completely disconnected from the line and load. However, the case ground should not be removed.

(+ = Earth Terminal, - = Line Terminal, G = Guard Terminal)

Single-Phase Transformer
The following 5 tests and corresponding wiring diagrams will completely test a single-phase transformer. Allow at least 1 minute for each test or until the reading stabilizes.

a. High voltage winding to low voltage winding and ground

b. Low voltage winding to high voltage winding and ground

c. High voltage winding to low voltage winding

d. High voltage winding to ground

e. Low voltage winding to ground

 

 

 

 

 

 

Three-Phase Transformer
The following 5 tests and corresponding wiring diagrams will completely test a three phase transformer.

a. High voltage winding to low voltage winding and ground

b. High voltage winding to ground with low voltage winding to guard

c. High voltage winding to low voltage winding

d. Low voltage winding to ground and high voltage winding to guard

• High voltage winding to low voltage winding

 

 

 

Cable Testing
Transformers are tested at or above the rated voltage to be certain there are no excessive leakage paths to ground or between windings. These are conducted with the transformer completely disconnected from the line and load. However, the case ground should not be removed .

 

Single Conductor
Connect as shown in the diagram

a. Conductor to Line (-) terminal and sheath to Earth (+)

Multi-Conductor
a. Single conductor

b. One conductor to all

c. One conductor to earth

• One conductor to others minus ground

 

Motor and Generator Testing
Before testing the above lift the rotor brushes, ground the starter terminal and frame and ground the motor shaft. Discharge the field winding by grounding. Then remove the field winding from ground and connect to the (-) Line connection on the megohmmeter. Connect the (+) Earth terminal to ground. The diagram shows the connection for testing the field insulation resistance. The stator winding may also be measured in a similar manner .

 

 

(Note 1) Dewpoint temperature is the temperature at which the moisture vapor in the air condenses as a liquid.

(Note 2) IEEE Std. 43-1974, “Recommended Practice for Testing Insulation Resistance of Rotating Machinery.” Available from the Institute of Electrical and Electronics Engineers, Inc., 345 E. 47th St., New York, NY 10017.