How to Use a High-Voltage Insulation Resistance Tester
(1) Before measurement, the power supply of the device under test must be disconnected, and the device must be short-circuited and discharged to ground. It is absolutely not allowed to measure while the device is energized, to ensure the safety of personnel and equipment.
(2) For equipment that may induce high voltage, measurement must only be carried out after eliminating such possibility.
(3) The surface of the object under test should be clean to reduce contact resistance and ensure the accuracy of measurement results.
(4) Before measurement, check whether the high-voltage insulation resistance tester is in normal working condition, mainly checking its "0" and "∞" points. That is, shake the handle to make the motor reach rated speed. The megger should point to "0" when short-circuited, and to "∞" when open-circuited.
(5) The high-voltage insulation resistance tester should be placed on a stable and firm surface, away from large external current conductors and external magnetic fields.
After completing the above preparations, measurement can be carried out. During measurement, attention must also be paid to the correct wiring of the high-voltage insulation resistance tester; otherwise, unnecessary errors or even mistakes may occur. Vortex flowmeter
The high-voltage insulation resistance tester has three terminals: one is "L" (line terminal), one is "E" (earth terminal), and the other is "G" (guard terminal, also called guard ring). Generally, the insulation resistance under test is connected between "L" and "E". However, when surface leakage of the insulation is severe, the guard ring of the object under test or the part not to be measured must be connected to the "G" terminal. In this way, leakage current flows directly back to the negative terminal of the generator through the guard terminal "G", forming a circuit, without flowing through the measuring mechanism (moving coil) of the megger. This fundamentally eliminates the influence of surface leakage current. It is especially important to connect the guard terminal "G" properly when measuring the insulation resistance between the cable core and the outer sheath, because when the air humidity is high or the cable insulation surface is not clean, the surface leakage current will be large. To prevent the influence of leakage on the internal insulation measurement, a metal guard ring is usually added to the cable outer sheath and connected to the "G" terminal of the megger.
When using a megger to measure the insulation resistance of electrical equipment, it is important not to reverse the "L" and "E" terminals. The correct connection is: "L" line terminal connects to the conductor of the device under test, "E" earth terminal connects to the grounded equipment shell, and "G" guard terminal connects to the insulation part of the device under test. If "L" and "E" are reversed, the leakage current flowing through the insulation and its surface will be collected to ground via the shell, then flow into the measuring coil through "L" from ground, causing "G" to lose its shielding effect and introducing significant errors. In addition, because the insulation between the internal lead of the "E" terminal and the shell is lower than that between the "L" terminal and the shell, when the megger is used on the ground, with correct wiring, the insulation resistance between "E" and the instrument shell and between the shell and ground is equivalent to a short circuit, causing no error. However, when "L" and "E" are reversed, the insulation resistance between "E" and ground is in parallel with the measured insulation resistance, resulting in a lower measurement value and introducing significant errors.
