1. When using a megohmmeter to measure insulation resistance, what should be noted:

When measuring the insulation resistance of a device, the power must be turned off. Equipment with large capacitance (such as capacitors, transformers, motors, and cable lines) must be discharged first; (2) Megohmmeter should be placed in horizontal position. Before wiring, first shake the Megohmmeter to see if the pointer is At the “∞” position, short-circuit the (L) and (E) terminals, and slowly shake the Megohmmeter to see if the pointer is at “zero”. For semiconductor Megohmmeters, it is not appropriate to use a short-circuit check; ( 3) Multi-strand cords shall be used for megohmmeter leads, and shall have good insulation; (4) Double-backed overhead lines and busbars, which cannot be completely powered off, when the induced voltage of the circuit under test exceeds 12 volts, or when thunderstorms occur The overhead lines and electrical equipment connected to the overhead lines at the time of occurrence are prohibited from measuring; (5) When measuring capacitors, cables, large-capacity transformers and motors, there must be a certain charging time, the larger the capacity, the charging time should be The longer. Generally, the reading after one minute of rotation of the megohmmeter shall prevail; (6) When the insulation is measured, the megohmmeter shall be kept at the rated rotation speed. Generally 120 rpm, when the measured object capacitance is large, in order to avoid pointer swing, the rotation speed can be properly increased (for example, 130 rpm); (7) The surface of the measured object should be wiped clean and free of dirt. , in order to avoid leakage affect the accuracy of the measurement.

2. When measuring insulation with a megohmmeter, why is it specified that the shaking time be 1 minute:

When measuring the insulation resistance with a megohmmeter, it is generally prescribed that the reading after one minute of shaking be prepared. Because of the DC voltage applied to the insulator, the current flowing through the insulator (absorbing current) will gradually decrease over time. The DC resistance of the insulation is determined based on the steady-state conduction current, and the insulators of different materials also have different decay times for the insulation absorption current. However, tests have shown that the insulation absorption current of most materials tends to stabilize after one minute, so the insulation resistance value after one minute of pressurization is specified to determine the insulation performance.

3. How to use megohmmeters:

The selection of megohmmeter is mainly to select its voltage and measurement range. High-voltage electrical equipment needs to use a high voltage megohmmeter. Low-voltage electrical equipment requires a megohmmeter with low voltage. The general selection principle is: 500-1000 volt megohmmeters for electrical equipment below 500 volts, and 2500 volt megohmmeters for porcelain vases, busbars, and knife switches.
The selection principle of the megohmmeter measurement range is: to make the measurement range adapt to the value of the insulation resistance to be measured to avoid large errors. For example, some megohmmeter readings do not start from zero, but start from 1 megohm or 2 megohm. This type of meter is not suitable for measuring the insulation resistance of low-voltage electrical equipment in a wet environment. Because the insulation resistance of this device may be less than 1 megohm, making the meter not read, it is easy to mistakenly think that the insulation resistance is zero, and draw the wrong conclusion.

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