jcdpk
22 posts · Joined 2011

#65 · Sep 10, 2013

Commonly used DC test voltages for AC equipment are:
AC equipment rating DC test voltage
Up to 100 VAC 100 and 250 VDC
440 to 550 VAC 500 and 1000 VDC
2400 VAC 1000 to 2500 VDC
4160 VAC and above 1000 to 5000 VDC (or higher)

So, what is “good” insulation? Since we know that insulation has a high resistance to current flow, “good” insulation must be able to provide a high resistance to current flow and be able to maintain that high resistance over a long period of time. In order to evaluate the quality of the insulation certain standard tests have been developed which provide a reliable indicator to determine what comprises “good” insulation.

There are two tests that the production technician can easily perform using a battery powered megger like the one shown in Figure 29. The first test is the short time or spot reading test and the second test is the one minute test.

The spot reading test
In the spot reading test you simply connect the “earth” lead of the megger to a good ground and the “line” lead to the conductor and operate the megger for a short time, say for 30 seconds or so. If the apparatus you are testing has a very small capacitance, such as a short run of cable, the spot reading is all that is necessary. However most equipment (like electric motors and long runs of electrical cable) is capacitive, so the very first spot reading can be only a rough guide as to how good or how bad the insulation is.
Bear in mind that the temperature and humidity will affect the readings and electrical circuits do not have to read infinity (perfect insulation on the megger scale) for the circuit to be serviceable. The NEMA standard for minimal insulation resistance is: 1 megohm per rated KV plus 1 megohm. What that means is that if you have a 1000 volt circuit, you should have a minimum of 2 megohms to ground for the circuit to be considered safe to energize and operate. If you have a 4000 volt circuit, you need (5 megohms) and if you have a 460 volt circuit, you should have (1-1/2 megohms), and so on. Insulation that is in good condition will normally have 40 - 50 megohms, or more, to ground.

The one minute reading test
The other test is the one minute reading. This method is fairly independent of the influence of temperature. It is based on the current absorption of good insulation compared to the current absorption of moist or contaminated insulation. A characteristic of good insulation is that it will show a continual increase in resistance (which means less leakage current is flowing) over a period of time. The initial test current (called the absorption current) is absorbed by the capacitance of the equipment being tested and then after that, any current flowing is the leakage through the insulation. If the insulation contains much moisture or other contaminants, the absorption current is masked by a high leakage current which stays at a fairly constant value, keeping the resistance low.
The value of the one minute test is that it can give you a better idea as to the condition of the insulation and alert you to a problem even when the spot reading indicates that everything is OK.
For example, let’s say a spot reading on a 460 volt induction motor was 10 megohms, which at first glance is well above the minimum requirement. Now lets assume that the one minute test showed the resistance quickly climbing to 10 megohms and then from there, holding steady for the rest of the 60 seconds. This means that there may be dirt or moisture on the windings. On the other hand, if the reading gradually increases between the 30 and 60 second time interval, then you can be reasonably certain that the windings are in good condition.
The comparison of the 30 second reading to the 60 second reading is called the dielectric absorption ratio (or D.A.R.). The way the ratio is calculated is to divide the 60 second reading by the 30 second reading. Using that method, the chart in Figure 30 will give you an idea of how to determine if the insulation is good.

Insulation

condition

60/30 second

ratio

Poor

Less than 1

Questionable

1.0 - 1.25

Good

1.4 – 1.6

Excellent

Above 1.6*​

*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.

Dielectric absorption ratio chart
There is another megger test called the “Ten minute test” which is similar to the one minute test. Because this test requires ten minutes to perform, it is better accomplished by line operated (120 volt) equipment. Essentially the test is performed for ten minutes, the ten minute reading is divided by the one minute reading and the resulting ratio is called the Polarization Index. This test is mostly used on larger equipment that has large capacitance and requires longer time to stabilize the absorption test current. The conclusions drawn from the test are the same as those drawn from the dielectric absorption ratio but the actual ratio values of the polarization index are not the same as for the dielectric absorption chart in Figure 30.
Using the some of the symptoms listed in the “Problem” column of the troubleshooting chart, let’s go through the process that should be followed to perform the tests and develop the solution
Breaker trips free when motor start is attempted.

If the circuit breaker trips free (or trips during normal motor operation) it should not be re-closed and another motor start attempted before testing the motor and the power cables for an insulation failure. If there is a problem, additional starts will just cause further damage.
The symptom of the circuit breaker tripping is always caused by overcurrent; either a phase to phase short circuit or a phase to ground short circuit.