Reasons for inaccurate testing of cables during insulation resistance measurement:
First, the influence of 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 resistance (rate) is more sensitive to temperature. As humidity increases, surface leakage increases, and conductor conductance current increases. As the temperature rises, the movement rate of carriers increases, and the absorption current and conduction current of the dielectric material will increase accordingly. According to relevant data reports, the resistance value of the general dielectric at 70°C is only 10% of that at 20°C. Therefore, when measuring insulation resistance, it is necessary to specify the temperature and humidity at which the sample and the environment reach equilibrium.
Second, the influence of 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.
Third, the cable itself: when the cable is heated and damp, the insulating material will age. Its insulation resistance is reduced.
Fourth, improper use of the insulation resistance measuring instrument.