The shaking meter, also called megohmmeter, is mainly used to measure the insulation resistance of electrical equipment. It consists of an alternator, a voltage doubling rectifier circuit, a meter head and other components. When the meter is shaken, a DC voltage is generated. When a certain voltage is applied to an insulating material, an extremely weak current flows through the material, which consists of three components, namely, capacitive current, absorbed current and leakage current. The ratio of the DC voltage generated by the meter to the leakage current is the insulation resistance, and the test to check whether the insulation material is qualified by the meter is called the insulation resistance test, which can find out whether the insulating material is damp, damaged or aged, and thus discover the defects of the equipment. The rated voltage of megohmmeter is 250, 500, 1000, 2500V and so on, and the measuring range is 500, 1000, 2000MΩ and so on.

Insulation resistance tester, also known as megohmmeter, shaking table, Megger table. Insulation resistance meter is mainly composed of three parts. The first is a DC high-voltage generator, used to generate a stream of high-voltage. The second is the measurement circuit. The third is the display.

(1) DC High Voltage Generator
Measurement of insulation resistance must be applied to the measuring end of a high voltage, the value of this high voltage in the insulation resistance meter national standard for 50V, 100V, 250V, 500V, 1000V, 2500V, 5000V...
There are three general methods of generating DC high voltage. The first hand-cranked generator type. At present, about 80% of the megohmmeters produced in China are using this method (source of the name of the shaking table). The second is through the utility transformer step-up, rectification to get DC high voltage. General utility megohmmeter method. The third is the use of transistor oscillator or special pulse-width modulation circuit to generate DC high voltage, the general battery-type and utility-type insulation resistance meter using the method.

(2) Measuring Circuit
In the previously mentioned shaking table (megohmmeter) in the measurement circuit and the display part of the combined into one. It is completed by a current ratio meter head, which consists of two coils at an angle of 60 degree (or so), one of which is parallel to the voltage terminals, and the other coil is strung in the measuring circuit. The angle of deflection of the pointer of the head is determined by the ratio of the currents in the two coils. Different angles of deflection represent different values of resistance; the smaller the measured resistance, the higher the current in the coils of the measuring circuit, and the greater the angle of deflection of the pointer. Another method is to use a linear ammeter as the measurement and display. As the magnetic field in the coil is non-uniform, when the pointer is at infinity, the current coil is exactly at the place where the magnetic flux density is the strongest, so even though the measured resistance is very large, the current flowing through the current coil is very small, and the angle of deflection of the coil at this time will be larger. When the measured resistance is small or zero, the current flowing through the current coil is large and the coil is deflected to a place where the magnetic flux density is small, so the resulting deflection angle will not be very large. The resulting deflection angle is not very large, and the non-linearity is thus corrected. Normally the resistance of a megohmmeter head is displayed over several orders of magnitude. However, when a linear ammeter head is directly connected to the measuring circuit, this is not possible, as the scales are squeezed together at high resistance values and are indistinguishable. In order to achieve the non-linear correction as well, non-linear components must be added to the measuring circuit. In order to achieve non-linear correction, a non-linear element must be added to the measuring circuit. This results in a shunt effect at small resistance values. At high resistances, no shunt is generated, so that the resistance value can be displayed by several orders of magnitude.