You know that good insulation has high resistance and poor insulation relatively low resistance. The actual resistance values can be higher or lower, depending upon such factors as the temperature or moisture content of the insulation (resistance decreases in temperature or moisture).
The Megger insulation tester is a small, portable instrument that gives you a direct reading of insulation resistance in ohms or megohms. For good insulation, the resistance usually reads in the megohm range.
However, let’s talk now about three basic types of insulation resistance tests using Megger tester:
In this method, you simply connect the Megger instrument across the insulation to be tested and operate it for a short, specific time period (60 seconds is usually recommended). As shown schematically in Figure 1, you’ve simply picked a point on a curve of increasing resistance values.
Quite often the value would be less for 30 seconds, more for 60 seconds. Bear in mind also that temperature and humidity, as well as condition of your insulation affect your reading.
If the apparatus you are testing has very small capacitance, such as a short run of house wiring, the spot reading test is all that is necessary. However, most equipment is capacitive and so your very first spot reading on equipment in your plant, with no prior tests, can be only a rough guide as to how good or bad the insulation is.
For many years, maintenance professionals have used the one-megohm rule to establish the allowable lower limit for insulation resistance.
The rule may be stated: Insulation resistance should be approximately one megohm for each 1,000 volts of operating voltage, with a minimum value of one megohm.
For example, a motor rated at 2,400 volts should have a minimum insulation resistance of 2.4 megohms. In practice, megohm readings normally are considerably above this minimum value in new equipment or when insulation is in good condition.
Equally true, as long as your periodic readings are consistent, they may be ok, even though lower than the recommended minimum values.
The curves of Figure 2 show typical behavior of insulation resistance under varying plant operating conditions. The curves were plotted from spot readings taken with a Megger instrument over a period of months.
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. you simply take successive readings at specific times and note the differences in readings (see curves, Figure 3).
Tests by this method are sometimes referred to as absorption tests.
If the insulation contains much 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 (remember: R = e/I).
The time-resistance test is of value also 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, therefore, compare several motors and establish standards for new ones, regardless of their horsepower ratings.
Figure 4 shows how a 60-second test would appear for good and perhaps bad insulation. When the insulation is in good shape, the 60-second reading is higher than the 30-second reading.
A further advantage of this double-reading test, as it is sometimes called, is that it gives you a clearer picture, even when a spot reading says the insulation looks fine.
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 Megger test set, line-operated, serves this need. Similarly, such an instrument is better adapted for large cables, bushings, transformers and switchgear.
The ratio of two time-resistance readings (such as a 60-second reading divided by a 30-second reading) is called a dielectric absorption ratio. It is useful in recording information about insulation. If the ratio is a 10-minute reading divided by a 1-minute reading, the value is called the polarization index.
With hand-cranked Megger instruments, it’s a lot easier for you to run the test for only 60 seconds, taking your first reading at 30 seconds. If you have a line-operated Megger instrument, you’ll get best results by running the test 10 minutes, taking readings at 1- and at 10-minutes, to get the polarization index.
Table I gives values of the ratios and corresponding relative conditions of the insulation that they indicate.
TABLE I – Condition of Insulation Indicated by Dielectric Absorption Ratios (1)
|Insulation condition||60/30 – second ratio||10/1 – minute ratio
|Dangerous||–||Less than 1|
|Questionable||1.0 to 1.25||1.0 to 2 (3)|
|Good||1.4 to 1.6||2 to 4|
|Excellent||Above 1.6 (2)||Above 4 (2)|
- These values must be considered tentative and relative – subject to experience with the time-resistance method over a period of time.
- In some cases, with motors, values approximately 20% higher than shown here indicate a dry brittle winding which will fail under shock conditions or during starts. For preventive maintenance, the motor winding should be cleaned, treated, and dried to restore winding flexibility.
- These results would be satisfactory for equipment with very low capacitance such as short runs of house wiring.
Reference // The Complete Guide to Electrical Insulation Testing by MEGGER