How does a megger work? How to use it

A megger, also known as a megohmmeter, is an instrument used to measure the insulation resistance and high-value resistance of the equipment under test. It consists of a hand-cranked generator, a meter head and three terminals, because most of them use hand-cranked generators. Power supply, so it is called a megger, which is a measuring instrument commonly used by electricians (there are also some digital megohmmeters). The megohmmeter is mainly used to check the insulation resistance of electrical equipment, household appliances or electrical lines to ground and between phases. It can detect whether the insulation material is damp, damaged, or aged, thereby discovering equipment defects and avoiding accidents such as electric shock, casualties, and equipment damage. Its scale is in megaohms (MΩ).

1. Working principle

When the handle is shaken evenly at a speed of 120 rpm, the DC generator in the meter outputs the rated voltage of the meter. When the resistance being measured is infinite, the current flows from the positive pole diameter R1 of the generator and the red coil of the ammeter to the negative pole. This When the measured resistance is 0, another part of the current flows through the measured resistor R2. At this time, the meter needle points to 0. Due to the limitations of R1 and R2, the current will not be very large. In addition, the generator speed limiter, so Ensure voltage stability.
 

2. A multimeter cannot replace a megger to test insulation.

There are usually two ways to measure ordinary resistance: low voltage measurement and high voltage measurement. Since the insulation resistance generally has a high value (generally in the megohm level), the measured value at low voltage cannot reflect the true insulation resistance value working under high voltage conditions. The insulation resistance meter is the most commonly used instrument for measuring insulation resistance. It has its own high-voltage power supply when measuring insulation resistance. This is the difference between it and other resistance measuring instruments.

3. Safety in use

What the megometer measures when working is electrical equipment, so it must be used correctly, otherwise it may cause personal or equipment accidents. When using the megometer to measure equipment, you must do the following:

1. Before measuring the equipment, first disconnect the power supply of the equipment under test and short-circuit it to the ground for discharge. Never allow the equipment to be powered for measurement to ensure the safety of people and equipment.

2. Remove the external connecting wires of the electrical equipment under test.

3. For equipment that may induce high-voltage electricity, this possibility must be eliminated before measurement can be carried out.

4. The surface of the equipment under test should be clean to reduce contact resistance and ensure correct measurement results.

5. Before measuring the equipment, check whether the megohmmeter is normal. Shake the megohmmeter so that its rated speed is 120 rpm. When the megohmometer is short-circuited, the pointer should point to the "0" position. When the circuit is open, the pointer should point at the "∞" position.

6. The megometer should be placed in a stable place for measurement.

4. Correct wiring

When measuring, the megometer must be wired correctly otherwise it will cause errors and errors. There are three binding posts of the megger: one is "L" (line terminal), one is "E" (ground terminal), and the third one is "G" (shield terminal, also called guard ring). Generally, the object under test is connected between the "L" and "E" terminals. However, when the surface leakage of the insulator under test is serious, the shielding ring or the part that does not need to be measured must be connected to the "G" terminal.
When using the megohmmeter to measure electrical equipment, be sure to note that the "L" and "E" terminals cannot be connected reversely. The correct connection method is: the "L" wire terminal is connected to the conductor of the equipment under test, the "E" ground terminal is connected to the grounded equipment shell, and the "G" shield is connected to the insulating part of the equipment under test. If "L" and "E" are connected reversely, "G" will lose its shielding effect and bring great errors to the measurement.