How to use megohmmeter in series

How to use megohmmeter in series

The megohmmeter (also known as the shaking meter) is a commonly used instrument for detecting the insulation strength of electrical equipment. However, in rural or remote and backward areas, there are generally only 500V or 1000V megohmmeters. When measuring the insulation of high-voltage electrical equipment, the output voltage of the megohmmeter is often insufficient, the sensitivity is low, and even the purpose of detecting local insulation defects cannot be achieved. Because sometimes the insulation resistance value of the test product measured by a 500V megohmmeter is higher, while the insulation resistance value measured by a 2500V megohmmeter is so low that it cannot be used. Insulation is relative. Basic insulators are insulated to low voltage, but not necessarily to high voltage. Therefore, the insulation resistance values ​​measured by megohmmeters of different voltages are different. For example, when a defective test product is measured for insulation resistance at the same time and at the same ambient temperature using megohmmeters of different grades, the test results are shown in Table 1. As
can be seen from Table 1, the insulation resistance value of the test product decreases with the increase of the voltage of the megohmmeter used. This situation can easily cause electrical maintenance personnel to make misjudgments. For this reason. For test products with suspected insulation resistance, especially high-voltage electrical equipment, a megohmmeter with corresponding high output voltage should be used for measurement to facilitate the exposure of insulation defects.

In order to solve the problem of low output voltage of megohmmeter, we can connect two 1000V or 500V megohmmeters in series (as shown in the attached figure).
According to the principle of series voltage superposition, by connecting two 1000V and one 500V megohmmeter in series, a DC voltage of 2500V can be obtained on the test object. However, two points should be noted when using:

(1) The ground potential of the first megohmmeter has been raised, so the megohmmeter housing should be insulated from the ground, and the operator should take safety measures such as wearing gloves. (2) The megohmmeters used in series should be produced by the same manufacturer as much as possible, so that the characteristics are roughly the same.