A Megger is a firm that manufactures megohmmeters, a device used to measure insulators’ level of resistance. Because of the numerous industrial applications, it is a very significant instrument. It offers a very high DC voltage for the resistance measurement. It is a portable instrument that is also known as a megohmmeter. It can be run mechanically or with the help of a battery. Ohms are being used to represent the reading accurately.
Working Principle of Megger:
Megger works on the comparison principle, which compares the insulation’s resistance to a known value of resistance.
- When the insulating resistance is high, the moving coil’s pointer deflects in the direction of infinity,
- When the insulating resistance is low, the pointer indicates zero resistance.
When compared to other devices, the Megger is highly accurate.
What is Insulation Resistance?
Insulation resistance, measured in ohms, is the resistance of wires, cables, and electrical equipment that is used to protect electrical systems, such as electrical motors, from any unintentional harm, such as electrical shocks or unexpected discharges of current leakages in wires.
Why conduct an insulating resistance test?
An electrical system’s insulating resistance decreases with time due to environmental factors as
- Moisture, and
- Dust particles.
In order to prevent a serious electrical shock that could be fatal, it becomes vital to regularly evaluate the equipment’s insulating resistance. This can be a sign that the insulation was damaged.
Construction of Megger:
Deflecting and control coil:
Typically, they are parallel to the generator and mounted at an angle to one another. The polarities of the particles cause the torque they generate to be in the opposite direction.
Permanent magnet with north and south poles that creates a magnetic effect for pointer deflection.
A scale with values ranging from “zero” to “infinity.” It is measured in “ohms,” the SI unit of resistance.
On a scale, a pointer is attached to the coils and end of the pointer float.
Connection to a D.C. Generator or Batteries:
In order to provide testing voltage, manual Meggers use a hand-operated D.C generator, while automatic Meggers use a battery and an electronic voltage charger.
The Pressure Coil Resistance and the Current Coil Resistance:
They are provided to prevent instrument damage in the event of low external source resistance during testing.
Working of Megger:
- The voltage needed for testing is provided by a hand generator included within the instrument, or battery charger, or an electronic voltage regulator. Usually smaller in size, it has a voltage of 250 or 500 volts.
- For testing ship equipment running at 440V A.C., a test voltage of 500V D.C. is suitable. For the onboard high voltage system, test voltages ranging from 1000V to 5000V are used.
- The deflecting coil, which is connected in series, conveys the current drawn by the circuit being tested. The control coil (pressure coil) is wired across the circuit.
- To prevent damage in the event of low resistance from an external source, current limiting resistors (CCR and PCR) are connected in series with pressure and current coil.
- To generate a test voltage using the electromagnetic induction effect, the armature of a hand generator moves in the field of a permanent magnet or vice versa.
- Pointer Deflection:
- The deflection of the pointer increases with an increase in potential voltage across the external circuit and
- The deflection of the pointer decreases with an increase in current
- Therefore, the resultant torque on the movement is directly proportional to the potential difference and inversely proportional to resistance.
- The pointer will display “infinity” when the external circuit is open and the torque caused by the voltage coil is at its maximum. The pointer will display “0” in the event of a short circuit.
How do a megger and a standard ohmmeter differ from one another?
A megohmmeter, which can be used to measure larger resistance levels like numerous megohms, requires a high voltage to operate. An ohmmeter monitors low resistance values.
Types of Meggers:
Megger are classified into 2 different types:
- Hand Operated Megger and
- Electronic type Megger
Hand Operated Megger:
- The analogue display,
- Hand crank, and
leads are essential components of the hand-operated type megger, which is manually operated. The wire leads link the circuit to the megger. The appropriate RPM is attained with a hand crank, and the analogue display is used to show the IR value recorded. The feature that the hand-operated megger does not require an external power source and is also relatively affordable is one of its most significant advantages.
Electronic Type Megger:
The electronic type megger includes a number of significant components including
- Wire leads,
- Selection switches, and
- Digital Displays.
The value of IR is displayed on a computerised screen. The lines link the megger to the outside electric source. The electrical parameter range can be chosen using the selection switches, and the status of the parameters is shown via the indicator. The electric variety is simple to use and may still function in congested places.
Advantages of Megger:
- Megger can deliver an accurate result.
- Megger is really convenient and simple to use.
- Megger is applicable everywhere.
- Results can be obtained very quickly.
- It is secure and more reliable to use.
Disadvantages of Megger:
- Operating a hand-operated type Megger is complicated. Additionally, the result is not accurate.
- An external power source would be necessary for an electronic type Megger.
- The value of the electronic type Megger is high.
Application of Megger:
- Electrical systems and components are tested using megger.
- The electrical resistance of insulator is measured accurately.
- Testing of battery, relay, ground connection…etc
- Megger are used for the insulation of windings.
SOME USEFUL INFORMATIONS ABOUT MEGOHMMETER (MEGGER):
1. How does a megger determine resistance?
Meggers utilises a high-voltage, low-current DC charge to measure the resistance in wires and motor windings in order to analyse insulation. This allows them to identify current leakage and defective or damaged insulation. It’s called as a meggering test.
2. Why does the megger use DC?
When an AC voltage is put across an insulator, the reactance of the ideal insulator is extremely high. As a result, the insulator completely prevents the current. For DC voltage, the accurate reading of the current is provided by the current flowing through the insulator. Consequently, the insulating resistance as tested is acceptable.
3. What is the IR test?
Any two points that are electrically insulated from one another are tested for insulation resistance (IR), which measures the overall resistance between them. As a result, the test establishes the dielectric’s (insulation’s) capacity to resist the flow of electrical current.
4. How are megger values determined?
Measuring Voltage Test:
When using AC voltage,
Test Voltage (A.C.) = (2 X Name Plate Voltage) +1000.
When using DC Voltage,
Test Voltage (D.C.) = (2 X Name Plate Voltage).
5. What are different types of IR test by megger?
Megger performs three different types of IR tests:
- Time-Resistance method,
- Spot reading or short time test and
- Dielectric Absorption Ratio.