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 such as: vacuum cleaning, steam cleaning, drying and rewinding. It is also helpful when evaluating the quality of the repairs before the equipment is put back into operation.
Some of the more common causes of insulation failure include: excessive heat or cold, moisture, dirt, corrosive vapors, oil, vibration, aging and nicked wiring.
There are numerous maintenance tests for assessing insulation quality. The three tests discussed here are used primarily to test motor, generator and transformer insulation.
Total current in the body of the insulation is the sum of three components
Readings are time dependent
These changing readings are best seen with analog bargraphs on digital instruments or needle movement on analog instruments.
For this test, the megohmmeter is connected across the insulation of the windings of the machine 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 dew point1. The operator should make a note of the winding temperature, so that it will be possible to correct the reading to a base temperature of 20°C.
To obtain comparable results, tests must be of the same duration. Usually the reading is taken after 60 seconds.
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, etc. a very sharp drop indicates.
Example of the variation of insulation resistance over a period of years:
At A, the effect of aging and dust accumulation is shown by decreasing values.
At B, the sharp drop indicates an insulation failure.
At C, the insulation resistance value after the motor has been rewound.
an insulation failure. See Figure 1.
(1) Dew point temperature is the temperature at which the moisture vapor in the air condenses as a liquid.
This method is fairly independent of temperature and often
can give you conclusive information without records of past
tests. It is based on the absorption effect of good insulation
compared to that of moist or contaminated insulation.
Simply take successive readings at specific times and note
the differences in readings (see curves, Figure 2). Tests by
this method are sometimes referred to as absorption tests.
Good insulation shows a continual increase in resistance (see curve D) over a period of time (in the order of 5 to 10 minutes). This is caused by the absorption; good insulation shows this charge effect over a time period much longer that the time required to charge the capacitance of the insulation.
If the insulation contains moisture or contaminants, the absorption effect is masked by a high leakage current which stays at a fairly constant value - keeping the resistance reading low (R = E/I) (see curve E).
The time-resistance testing is of value because it is independent of equipment size. The increase in resistance for clean and dry insulation occurs in the same manner whether a motor is large or small. You can compare several motors and establish standards for new ones, regardless of their horsepower ratings.
Figure 2 shows how a 60-second test would appear for good and bad insulation. When the insulation is in good shape, the 60-second reading is higher that the 30-second reading. A further advantage of this two reading test is that it gives you a clearer picture, even when a "spot reading" says the insulation looks ok.
Time-resistance tests on large rotating electrical machinery - especially with high operating voltage - require high insulation resistance ranges and a very constant test voltage.
A heavy-duty megohmmeter serves this need. Similarly, such an instrument is better adapted for cables, bushings, transformers, and switchgear in the heavier-duty sizes.
Test Methods - Time-Resistant Tests Dielectric Absorption Ratio (DAR)
Note: This is not a commonly used test
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.
A series of "steps," each step lasting 60 seconds.
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 shouldserve
as a warning
quality may be deteriorating due to dirt, moisture,
cracking, aging, etc.
Polarization Index (PI) = 10-minute reading ÷ 1-minute reading
The IEEE Std 43-2000 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
Absorption curve of test conducted on 350 HP Motor:Curve D indicates
a good insulation with an excellent polarization index of 5. Curve E indicates a
potential problem. The polarization index is only 140/95, or 1.47.
(2) IEEE Std. 43-2000, "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.
Before and after repair:
Curve F shows a downward trend of insulation resistance values as the test voltage is increased. This indicates a potential problem with the insulation. Curve G shows the same equipment after it has been repaired.
The guard terminal is useful when measuring very high resistance values.
There are two schools of thought regarding the voltage to test
insulation at. 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 except when higher voltages are used. See chart below for suggested values.
|Equipment/Cable Rating||DC Test Voltage|
|24 to 50V||50 to 100VDC|
|50 to 100V||100 to 250VDC|
|100 to 240V||250 to 500VDC|
|440 to 550V||500 to 1000VDC|
|2400V||1000 to 2500VDC|
|4100V||1000 to 5000VDC|
It is always advisable to contact the original equipment manufacturer to get their recommendation for the proper voltage to use when testing their equipment.
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.
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.
The following 5 tests and corresponding wiring diagrams will completely test a three-phase transformer.
Cables 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.
Connect as shown in the diagram
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.