Abstract: Regarding the insulation value of 10kV transformers measured by shaking, there are three misconceptions when using ZC series portable megohmmeters. The rated voltage of the megohmmeter is not compatible with the voltage of the tested electrical equipment, and the measurement method of two people cooperating is not adopted. The non-standard practice of using general flower wire and rubber woven wire instead of the dedicated wire of the megohmmeter.

There are three misconceptions when using ZC series portable megohmmeters to measure the insulation value of 10kV transformers during shake testing:

Misconception 1:

The rated voltage of the megohmmeter does not match the voltage of the tested electrical equipment. Different megohmmeters should be used to test the high and low voltage coils of the transformer; Apply AC 2kV as the withstand voltage test voltage to the insulation of the 0.4kV coil, and there should be no abnormal situations such as breakdown or flashover within 60 seconds, while apply AC 42kV as the withstand voltage test voltage for 10kV. Therefore, a megohmmeter with a range of 2500M Ω or more should be used to measure the insulation resistance values between high voltage and low voltage, high voltage and ground, and iron core and ground. The low voltage on the 0.4kV side can only use a 500V or 1000V megohmmeter for grounding. If a 500V and 1000V megohmmeter is used to measure the high voltage side with a rated voltage higher than 500V and 1000V, the measurement results may have errors. If a megohmmeter with a rated voltage too high is used to measure the insulation resistance value on the low voltage side, it may damage the insulation.  

Misconception 2:

There was no measurement method that involved two people working together. After reading the above values, the test sample should be disconnected without stopping the rotation of the handle, and then the shaking should be stopped to prevent damage to the megohmmeter due to the feedback discharge of the accumulated charge in the capacitance of the test sample. This is a common usage method introduced in general user manuals or textbooks, but a considerable number of testers shake the megohmmeter after wiring, so many megohmmeter components are damaged, or the megohmmeter is scrapped and repaired after one or two uses. When using a megohmmeter, a two person cooperative operation method is adopted for measurement. After connecting the test line, one person is responsible for rotating the megohmmeter handle and reading, while the other person holds the test end and aligns it with the test sample, following the operator's command to rotate the megohmmeter handle; The password for "overlapping test samples, pulling apart, and discharging". Due to the use of a megohmmeter in conjunction with the operation, the ZC series megohmmeter is not easily damaged.  

Misconception 3:

Using regular flower thread and rubber woven thread instead of megohmmeter specific thread is an extremely non-standard practice. Because the insulation value of general flower thread and rubber woven thread is much lower than that of specialized test thread. The dedicated test line will not affect the insulation value due to overlapping the test sample with the hand-held end. On the contrary, the general flower wire and rubber woven wire may also injure the operator during shake testing. At the same time, it is necessary to properly keep the dedicated test line, including the dedicated power cord and test probe of ZC48 electronic megohmmeter.  

It is recommended to use an electric shaking meter to avoid the above problems. The HT2671 and HT2500 series megohmmeters are ideal replacement products for ZC series equipment