Inspection and precautions for using megohmmeter

It is also generally called insulation resistance megohmmeter, digital megohmmeter, electric megohmmeter, etc. It is an instrument for measuring high resistance and is often used to measure the insulation resistance value of equipment or power supply lines. The megohmmeter is easy to operate, simple to read, and small in size. It is a portable instrument. The dial scale of the megohmmeter is in megohms (mω).

The megohmmeter is mainly selected according to its voltage and measurement range. High-voltage electrical equipment requires a high-voltage megohmmeter, and low-voltage electrical equipment requires a low-voltage megohmmeter. The general selection principle is: electrical equipment below 500 volts should use a 500-1000 volt megohmmeter; porcelain bottles, busbars, and knife switches should use megohmmeters above 2500 volts. The selection principle of the megohmmeter measurement range is: to make the measurement range adapt to the value of the measured insulation resistance, and effectively avoid large errors in readings. For example, some megohmmeters do not start reading from zero, but from 1 megohm or 2 megohms. The megohmmeter is not suitable for measuring the insulation resistance of low-voltage electrical equipment in a humid environment. Because the insulation resistance of such equipment may be less than 1 megohm, the instrument cannot get a reading, and it is easy to mistakenly believe that the insulation resistance is zero, which can easily lead to wrong results.

When selecting the resistance range, it is necessary to note that there are two small black dots on the dial scale of the megohmmeter. The area between the small black dots is the accurate measurement area. Therefore, when selecting the meter, the insulation resistance value of the device under test should be within the accurate measurement area. When the megohmmeter is working, it will generate high voltage, and the measurement object is electrical equipment, so it must be used correctly, otherwise it will cause equipment accidents or even personal safety accidents.

The following preparations should be made when using the megohmmeter:

(1) Before measurement, the device under test must be disconnected and short-circuited to the ground. Measurement is never allowed while the device is energized to ensure personal and equipment safety.

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

(3) The surface of the object to be measured must be clean to reduce contact resistance and ensure the accuracy of the measurement results.

(4) Before measuring, check whether the megohmmeter is in normal working condition, mainly check its "0" and "∞" points. That is, shake the handle to make the motor reach the rated speed. The megohmmeter should point to the "0" position when short-circuited and the "∞" position when open-circuited.

(5) The megohmmeter lead wire should be made of multi-strand soft wire with good insulation;

(6) For double-circuit overhead lines and busbars that cannot be completely shut down, it is prohibited to measure the overhead lines and electrical equipment connected to the overhead lines when the induced voltage of the measured circuit exceeds 12 volts, or when a thunderstorm occurs;

(7) When using, the megohmmeter should be placed in a stable and firm place, away from large external current conductors and external magnetic fields.

When measuring, please pay attention to the following points:

1. When measuring the insulation resistance of electrical equipment, the power supply must be cut off first. If there are equipment with insulation resistance, such as cables, the lines must be discharged first.

2. The megohmmeter must be placed flat when in use.

3. Before using the megohmmeter, turn it a few times to see if the pointer is at the maximum position. Then short-circuit the "l" and "e" terminals, and slowly turn the megohmmeter handle to see if the pointer is at "zero".

4. The leads of the megohmmeter must be well insulated and the two wires should not be twisted together.

5. When measuring with a megohmmeter, the reading after one minute of rotation shall be taken as the standard.

6. When measuring, the megohmmeter should be rotated at 120 rpm.

7. The range of a megohmmeter is often several thousand megohms, and the minimum scale is around 1 megohm, so it is not suitable for measuring resistance below 100 kilohms.

There are several things to note when using a megohmmeter:

(1) It is prohibited to measure insulation resistance during lightning or near high-voltage equipment. It can only be measured when the equipment is not energized and there is no induced electricity.

(2) During the shaking test, no one is allowed to work on the device under test.

(3) The megohmmeter wires should not be twisted together and should be kept separate.

(4) Do not touch the megger with your hands before it stops rotating or the device under test is discharged. Do not touch the metal part of the lead when removing the wires.

(5) At the end of the measurement, large-capacitance devices must be discharged.

(6) The accuracy of the megohmmeter should be calibrated regularly.

When using a megohmmeter to measure the insulation of electrical appliances, please note:

①The megohmmeter must be kept flat when in use.

②The speed of the megohmmeter is 120 revolutions per minute.

③Self-check: a. Open circuit test: When the megohmmeter reaches 120r/min, the pointer should be at “∝”. b. Short circuit, slowly turn the megohmmeter, the pointer should be at “0”.

④ The insulation resistance between windings, between phases, and between phases and the shell should be ≥0.5mω. Mobile power tools ≥2mω

⑤ When measuring line insulation: phase to phase ≥ 0.38mω, phase to zero ≥ 0.22mω.

⑥ Medium and small motors generally use 500-1000 type.

⑦ If the measured resistance of this phase is zero, it indicates that this phase is short-circuited.

⑧If the measured resistance of this phase is 0.1 or 0.2mω, it means that the insulation resistance performance of this phase has been reduced.

⑨The insulation resistance of electrical equipment should be as high as possible when permitted.

⑩ If the insulation resistance performance of the motor or line is reduced or a short circuit occurs, the megohmmeter needs to be repaired and cannot be used normally.