1. Ambient temperature and humidity
The insulation resistance value of general materials decreases with the increase of environmental temperature and humidity. Relatively speaking, surface resistivity (rate) is more sensitive to environmental humidity, while volume resistivity (rate) is more sensitive to temperature. As the humidity increases, the surface leakage increases, and the conductor current will increase. As the temperature increases, the rate of movement of carriers increases.
The absorption current and electrical conduction current of the dielectric material will increase accordingly. According to relevant reports, the resistance value of the general dielectric at 70°C is only 10% of that at 20°C. Therefore, when measuring the insulation resistance, the temperature and humidity at which the sample and the environment are balanced must be specified.
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. While pressurizing, a relatively large charging current flows, followed by a relatively long-term and slowly decreasing absorption current, and finally a relatively stable electrical conduction current is achieved. The higher the measured resistance value, 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 it stabilizes. It is stipulated in the test method for the insulation resistance of communication cables that the reading after 1 minute of charging is the measured value of the insulation of the cable.
But in fact, this method is a bit wrong, because when the DC voltage pressurizes the material to be tested, the current on the material to be tested is capacitive current. Since it is capacitive current, it is related to the capacitance of the cable. Large capacitance needs to be charged. It takes a long time, especially for cables filled with grease.
Therefore, due to the different lengths and different capacitances of the same type of cables, the reading when the charging time is one minute is obviously unscientific, and further research and discussion are needed.
3. the cable's own factors
When the cable is heated and damp, the insulation material ages. The insulation resistance is reduced.