Megger is also called an insulation tester. It’s used to measure the insulation resistance of any device. They are work on the principle of comparison and the insulation resistance compared with a known resistance value. If the insulation resistance is high, the arrow of the moving coil deviates towards infinity, and if it is low, then the arrow shows zero resistance.
They are more accurate and very important meter than other instruments. The insulation resistance of the electrical system surpasses IR standards in terms of time, environmental conditions, i.e., temperature, humidity, humidity, and dust particles.
It also has a negative effect due to the presence of electrical and mechanical stresses. It becomes very important to periodically check the Insulation Resistance (IR ) equipment to measure death or electric shock.
A Megger meter consists of a movable coil of uniform length, fixed to a balanced piece. The Megger balances the control coil of one side with the movable coil of the opposite side. As the incoming electrical current passes through the Megger meter, it measures the electrical resistance, which changes due to current through the control coil.
When the change in the surrounding conductivity changes the conductivity of a wire, it changes its magnetic field, which is measured with the help of a pointer attached to it. A typical Megger meter comprises a pair of balanced conductors and a control coil.
One end of this pair of balanced conductors forms an infinity control coil, and the other ends form a reference electrode. The control coil controls the motion of the pointer so that it points towards the infinity point. The infinity point is usually indicated by a zero-point source, which is a permanent magnet.
Working Principle of Megger
The Working principle of Megger is based on the principle of moving coil devices, which states that when a conductor with current is placed in a magnetic field, a mechanical force acts. It is an instrument used to measure the high and low resistance of the order of megohms and to check the insulation resistance. The tool generates its EMF when the handle is turned.
The testing voltage is usually 500, 1000, or 2500 and 5000 VDC, generated by a manual or automatic generator. The generator has one centrifugal clutch, due to which the generator supplied a constant for insulation testing. DC voltage is used to test low resistance insulation.
It has two terminals labeled “Line” and “Earth.” The crank rotates at a moderate speed until the needle deflects continuously. The pointer deflection is directly proportional to the voltage applied to the external circuit.
When the test circuit is passed through the megohmmeter, and there is no short circuit across the entire insulation, the needle tilts toward infinity. It shows that the resistance has high insulation. At low resistance, the arrow approaches zero.
Insulation Resistance Test Or IR Test of Meggers
The Insulation Resistance test or (IR test)measures the total resistance or insulation between any two points separated by electrical insulation. Insulation resistance test is carried out using meggers. Thus, the test determines how effectively the dielectric (insulation) resists the passage of electric current.
Electrical cables and machines, transformers are tested for insulation before they are used for the first time. Such tests are useful for checking the quality of the insulation, not only during the first production of a product but also over time as it is used.
Insulation testers typically have three output leads, ground, positive (+), and negative (-), allowing them to be used in a wide variety of applications. The output voltage is typically in the range of 250 to 5000 VDC.
Insulation resistance testing usually consists of four stages: charging, holding, measuring, and discharging. During the charging phase, the voltage rises from zero to the selected voltage. Which provides stabilization time and limits the inrush current of the device under test.
And wait for the peak of the voltage value as the voltage reaches the selected value. The voltage can then be allowed to withstand or be held at that level until measurements are started. After measuring the resistance for the selected time, the meter again discharges to 0 V in the final phase.