Insulation resistance Test ( IS 4722 : 1992)

The insulation resistance after the high-voltage test is applied shall be not less than one megohm.

(When it is required to dry out windings at site to obtain the minimum value of insulation resistance, it is recommended that procedure for drying out as specified in IS 900 : 1992 should be followed.

What is insulation resistance testing?

Basically, you're applying a voltage (specifically a highly regulated, stabilized DC voltage) across a dielectric, measuring the amount of current flowing through that dielectric, and then calculating (using Ohm's Law) a resistance measurement. Let's clarify our use of the term "current." We're talking about leakage current. The resistance measurement is in megohms. You use this resistance measurement to evaluate insulation integrity.

Current flow through a dielectric may seem somewhat contradictory, but remember, no electrical insulation is perfect. So, some current will flow.

How do you perform an insulation resistance test?

Generally, you connect two leads (positive and negative) across an insulation barrier. A third lead, which connects to a guard terminal, may or may not be available with your tester. If it is, you may or may not have to use it. This guard terminal acts as a shunt to remove the connected element from the measurement. In other words, it allows you to be selective in evaluating certain specific components in a large piece of electrical equipment.

Obviously, it's a good idea to have a basic familiarity with the item you're testing. Basically, you should know what is supposed to be insulated from what. The equipment you're testing will determine how you hook up your meghommeter.

After you make your connections, you apply the test voltage for 1 min. (This is a standard industry parameter that allows you to make relatively accurate comparisons of readings from past tests done by other technicians.)

During this interval, the resistance reading should drop or remain relatively steady. Larger insulation systems will show a steady decrease; smaller systems will remain steady because the capacitive and absorption currents drop to zero faster than on larger systems. After 1 min, you should read and record the resistance value.

Why has an insulation testing?

A regular program of testing insulation resistance is strongly recommended to prevent electric shocks, assures safety of personal and to reduce or eliminate down time.

What causes insulation failure?

Excessive heat or Cold

Principle of Insulation Resistance Measurement

The method used to measure insulation resistance is based on Ohm’s law. A high voltage is applied across the resistance; the current that flows through the insulation is measured. The ratio of voltage and current gives the resistance. The value of the insulation resistance is usually in the order of mega ohms

MEGGER

Total Currents in Insulation

Total Currents in Insulation is the sum of three components

Capacitance charging Current

Absorption Current

Leakage or Conduction Current

Method of Insulation Resistance Measurement

The insulation resistance shall be measured between windings and windings and frame. A DC Voltage of 500 V shall be applied for sufficient time for the reading of the indicator to become practically steady. An instrument like hand operated insulation resistance tester having a DC voltage of 500 V may be used

The winding to be tested should first be isolated. The other windings of the machine which are not being tested should be connected to the ground. The voltage is applied to the winding and the reading is taken after about 60 seconds. The reading is noted. After the test is over, the winding needs to be “discharged”. This is because the insulation acts as a dielectric forming a capacitor between the winding and the earth. This can store charge and can deliver a shock if not discharged. Discharging can be done by connecting to

the ground.

How does it work?

The construction and connections are shown below. The moving system consists of two coils, the "control coil" and the "deflecting coil"-rigidly mounted at an angle to one another and connected, in parallel across a small generator, with polarities such that the torques produced by them are in opposition. The coils move in the air gap of a permanent magnet. The control coil is in series with a fixed control circuit; the deflecting coil is connected in series with a fixed deflecting circuit resistance and the resistance under test. If this last is infinitely high no current flows in the deflecting coil and the control coil sets itself perpendicular to the magnetic axis, the pointer indicating "Infinity." A lower test resistance allows current to flow in the deflecting coil and turns the movement clockwise. The control torque produces a restoring torque which progressively increases with the angular deflection, and the equilibrium position of the movement is attained when the two opposing torques balance.

The control coil is actually in two parts, in series, the outer part being a compensating coil. The two parts are arranged with numbers of turns and radii of action such that, for external magnetic fields of uniform intensity, their torques cancel one another thus giving an a static combination.

The instrument has a small permanent magnet d.c. generator developing 500 V DC. (Other models have 100, 250, 1,000 or 2,500 V generators). The generator is hand-driven, through gearing and a centrifugally controlled clutch which slips at a predetermined speed so that a steady voltage can be obtained.

The guard terminal (if fitted) acts as a shunt to remove the connected element from the measurement. In other words, it allows you to be selective in evaluating certain specific components in a large piece of electrical equipment. For example consider a two core cable with a sheath. As the diagram below shows there are three resistances to be considered.

The instrument used to measure Insulation Resistance is known as the Megger. It is similar in principle to the ohmmeter except for the fact that a higher voltage is used. The typical meggars have a test voltage of 500V, 2500V or 5000V. The Meggar has a high internal resistance hence, there it is safe to use despite the high voltage generated. The meggar has 3 terminals. Line, Earth and Guard.

In Our industry we are testing these tests in RMD Section as per IS standards