Megger is a measuring instrument used for the measurement of insulation resistance of an electrical system. An electrical system degrades its quality of insulation resistance with time and various environmental conditions including temperature, moisture, dust particles & humidity. Even mechanical and electrical stress affects the insulation resistance which adds to the necessity of checking insulation resistance at regular intervals so as to avoid fatal errors or electrical shocks. Megger is used for measuring the electrical leakage in wires, electrical insulation level in generators, motors etc.
Megger Test
Megger test is carried out by the following steps,
Power is removed from the circuit that is being tested. In a circuit, when these types of high-voltage tests are carried out, there should be no voltage connected to it.
All wires are uninstalled that is meant for testing. To check the insulation resistance of any wire, it should be completely removed and disconnected from the circuit at both ends. A piece of tape is used to cover bare copper ends to avoid shock. While testing motors, wires that feed the motor must be completely disconnected.
One lead of the Megger is connected to the earth ground or electrical frame of the electrical system. This lead in case of motor windings is connected to motor’s metal frame.
The other megger lead is attached to one of the terminals of motor or the bare copper end. The other end of the wire that is being tested should be covered with a tape insulator or must be in free air.
Now turn on the meter. Around 2 to 5 seconds are taken for the build-up of high voltage inside the motor windings or the wire.
Now note down the reading of the meter. A reading value greater than 999megohms is considered as a near-perfect resistance value in case of a new motor or wire. There may be problems with used motors or old wires when the resistance values are less than 1.5 megohms. Reading value between this range is fine provided there is no trouble.
Continue the test with the remaining terminals of the motor or other wires. The motor leads or wires are safe to use when meter’s high voltage source is ceased.
Construction of Megger
The main parts involved in the construction of megger are deflecting coil, control coil, permanent magnets, pointer, scale, DC generator or battery connection, Pressure coil resistance and Current coil resistance.
Deflecting & Control coil: These coils are placed at right angles to each other and are connected to the generator in parallel. These coils maintain its polarities so as to produce torque in the opposite direction.
Scale: This marks the value of insulation resistance from zero to infinity and is placed in the front-top of the device which helps us to read the values.
Pointer: Pointer place represents the value of the insulation resistance, moving from the range of zero to infinity. Pointer’s one end is connected to the coil and the other end deflects on the scale.
Permanent magnet: These magnets produce the essential magnetic field used for the deflection of the pointer with the North-South pole magnet.
D.C generator or Battery connection: Electronic voltage/ battery charger produces testing voltage in case of automatic type megger whereas hand operated DC generator is used for the same purpose in manual operated megger.
Pressure coil resistance and Current coil resistance: This protects the megger from any sorts of damage due to the low external electrical resistance that is under test.
Working Principle of Megger
Testing voltage produced by the rotation of crank in hand operated megger and by the battery in case of electronic type megger. For testing a range up to 440V, equipment requires 550V DC is sufficient. Current coil or deflecting coil is series connected and allows the electric current to flow through the circuit being tested. Control and deflecting coil has current limiting resistor connected in series so as to protect the external circuit in case of damages caused due to very low resistance. Testing voltage is produced by electromagnetic induction in case of hand operated megger and by battery in case of electronic type megger. Deflection of pointer increases with the increases in voltage in the external circuit and also decreases with the increase in current. That is, resultant torque is inversely related to current and directly related to voltage. While the electrical circuit that is being tested is open, the resultant torque caused due to voltage coil is maximum and the deflection pointer shows the value of ‘infinity’ which means the circuit does not have any shorting present and resistance is maximum within the circuit being tested. In case of short circuit, deflection pointer shows ‘zero’ which indicates ‘no’ resistance in the circuit being tested.
Megger Types
There are two types of megger:-
Manual Type Megger
Electronic Type Megger
Manual Type Megger
The important parts of manual type or hand operated megger are analog display, hand crank and wire leads. Analog display is situated on the tester’s front face which is used for IR value recording. Hand crank is used for the rotations so as to achieve the desired RPM which is required for the generation of voltage which flows through the electrical system. Wire leads are mainly two in number for the connection of megger with the electrical system to be tested.
Advantages
Oldest method used for the insulation resistance calculation but still is an important one.
External sources are not required for the operation
Cheaper and easily available in market
Disadvantages
Labor requirement if more, that is, 2. One for connecting the megger with the electrical system that is to be tested and other for the rotation of the crank
Accuracy level varies with the rotation speed of crank
Analog display shows the result
High safety and care is required for the use of the equipment.
Placing of equipment at work sites is an issue since it requires a stable position for operation and unstable placing affects the results.
Electronic Type Megger
The important parts in electronic type megger are digital display, wire leads, selection switches and indicators. The digital display shows the value of insulation resistance in digital form. Wire leads are two in number used for connecting megger for the testing of electrical system. Selection switches are used for the selecting the ranges of electrical parameters. Indicators are used for indicating various statuses of electrical parameters, i.e. On/Off.
Advantages of Electronic Type Megger
Accuracy level is very high.
Digital display enables easy readability of insulation resistance values.
Instrumentation operation can be carried out by a single person.
Even when used in a congested place this device works perfectly and is very handy and safer.
Disadvantages of Electronic Type Megger
External source of energy is required like dry cell.
