1. Ambient temperature and humidity

    The insulation resistance value of general materials decreases with the increase of ambient temperature and humidity. Relatively speaking, the surface resistance (rate) is more sensitive to ambient humidity, while the bulk resistance (rate) is more sensitive to temperature. As humidity increases, surface leakage increases, and conductor conductance current increases. When the temperature rises, the movement rate of the carriers increases, and the absorption current and conduction current of the dielectric material will increase accordingly. Therefore, when measuring insulation resistance, it is necessary to specify the temperature and humidity at which the sample and the environment reach equilibrium.

2. Test time

    When a certain DC voltage is used to pressurize the material under test, the current on the material under test does not reach a stable value instantaneously, but has a decay process. At the same time of pressurization, a large charging current flows, followed by a slowly decreasing absorption current for a long time, and finally a relatively stable conductance current is achieved. The higher the resistance value being measured, the longer it takes to reach equilibrium. Therefore, in order to correctly read the measured resistance value during measurement, the value should be read after stabilization. It is stipulated in the test method of insulation resistance of communication cables that the reading after charging for 1 minute is the measured value of the insulation of the cable. However, in practice, this method is somewhat inappropriate, because when the DC voltage pressurizes the material to be tested, the current on the material to be tested is a capacitive current. Since it is a capacitive current, it is related to the capacitance of the cable, and the large capacitance needs to be charged. The longer it takes, especially the ointment-filled cable, which takes longer. Therefore, for the same type of cable, due to different lengths and different capacitances, the reading when the charging time is one minute is obviously unscientific, and further research and discussion are needed.

3. Accurate use of test equipment

    Many manufacturers of test instruments generally use high resistance meters and megohmmeters. When working, the instruments themselves generate high voltage, and the measurement object is electrical equipment, so they must be used correctly, otherwise it will cause personal or equipment accidents. Before use, first make the following preparations:

(1) Before the measurement, the power supply of the device under test must be cut off, and the short-circuit discharge to the ground must be performed. 

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

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

(4) Before measurement, check whether the instrument is in normal working condition, mainly check its “0” and “∞” points. The megohmmeter is to shake the handle to make the motor reach the rated speed. The megohmmeter should be in the “0” position when it is short-circuited, and it should be in the “∞” position when it is open.

(5) The instrument should be placed in a stable and firm place, away from large external current conductors and external magnetic fields. After the above preparations are done, the measurement can be carried out. When measuring, pay attention to the correct wiring, otherwise it will cause unnecessary errors or even mistakes.

4. The factors of the cable itself

    When the cable is exposed to heat and moisture, the insulation deteriorates. Its insulation resistance is reduced.