How insulation resistance is measured

How an Insulation Resistance Tester Operates 

The Megger insulation tester is a portable instrument that provides a direct reading of insulation resistance in ohms, megohms, gigohms, or teraohms (depending on the model

chosen) regardless of the test voltage selected. For good insulation, the resistance usually reads in the megohm or higher range. The Megger insulation tester is essentially a high-range resistance meter (ohmmeter) with a built-in dc generator.

The instrument’s generator, which can be hand-cranked, battery or line-operated, develops a high dc voltage that causes several small currents through and over surfaces of the insulation being tested. The total current is measured by the ohmmeter, which hasan analog indicating scale, digital readout or both.

Components of Test Current

If we apply a test voltage across a piece of insulation, then by measuring the resultant current and applying Ohm’s Law (R=E/I), we can calculate the resistance of the insulation. Unfortunately, more than one current flows, which tends to complicate matters.

Capacitive Charging Current

We are all familiar with the current required to charge the capacitance of the insulation being tested. This current is initially large but relatively short lived, dropping exponentially to a value close to zero as the item under test is charged. Insulating material becomes charged in the same way as a dielectric in a capacitor.

Absorption or Polarization Current

Absorption current is actually made up of up to three components, which decay at a decreasing rate to a value close to zero over a period of several minutes. 

The first is caused by a general drift of free electrons through the insulation under the effect of the electric field.

The second is caused by molecular distortion whereby the imposed electric field distorts the negative charge of the electron shells circulating around the nucleus toward the positive voltage.

The third is due to the alignment of polarized molecules within the electric field applied, see figure 1. This alignment is fairly random in a neutral state, but when an electric field is applied, these polarized molecules line up with the field to a greater or lesser extent.

Figure 1: Alignment of Polarized Molecules

The three currents are generally considered together as a single current and are mainly affected by the type and condition of the bonding material used in the insulation. Although the absorption current approaches zero, the process takes much longer than with capacitive current. 

Orientational polarization is increased in the presence of absorbed moisture since contaminated materials are more polarized. This increases the degree of polarization. Depolymerization of the insulation also leads to increased absorption current. Not all materials possess all three components and, indeed, material such as polyethylene exhibits little, if any, polarization absorption.

Surface Leakage Current

The surface leakage current is present because the surface of the insulation is contaminated with moisture or salts. The current is constant with time and depends on the degree of ionization present, which is itself dependent on temperature. It is often ignored as a separate current, being included with the conduction current below as the total leakage current.

Conduction Current

Conduction current is steady through the insulation and is usually represented by a very high value resistor in parallel with the capacitance of the insulation. It is a component of the Leakage Current, which is the current that would be measured when the insulation is fully charged and full absorption has taken place. Note that it includes surface leakage, which can be reduced or eliminated by the use of the guard terminal.

The graph in figure 2 shows the nature of each of the components of current with respect to time.

Figure 2: Components of Test Current

The total current is the sum of these components. (Leakage current is shown as one current.) It is this current that can be measured directly by a microammeter or, in terms of megohms, at a particular voltage by means of a Megger insulation tester. Some instruments offer the alternatives of displaying a measurement in terms of current or as a resistance. 

Because the total current depends upon the time that the voltages applied, Ohm’s Law (R = E/I) only holds, theoretically, at an infinite time (that implies waiting forever before taking a reading). It is also highly dependent upon starting from a base level of total discharge. The first step in any insulation test is, therefore, to ensure that the insulation is completely discharged.

Please note: The charging current disappears relatively rapidly as the equipment under test becomes charged. Larger units with more capacitance will take longer to be charged. This current is stored energy and, for safety reasons, must be discharged after the test. Fortunately, the discharge of this energy takes place relatively quickly. During testing, the absorption current decreases at a relatively slow rate, depending upon the exact nature of the insulation. This stored energy, too, must be released at the end of a test, and requires a much longer time to discharge than the capacitance charging current.