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First, the test phenomenon affect the wire and cable insulation resistance measurement of the factors of the instrument accuracy, environmental conditions and personnel quality in several aspects, the following GB5023.3-1997 general purpose single-core hard conductor non-sheathed cable (Model 227IEC01 (BV )) As an example, talk about several issues that should be noted in the measurement of insulation resistance. According to the provisions of GB5023.3: The test shall be performed on a 5m long insulated wire core with a water temperature of (70 ± 2) °C, an arbitration test of (70 ± 1) °C, a water immersion time of not less than 2h, and insulation resistance shall be applied Measured after 1 minute of voltage. How do you understand these requirements in the standard and how they affect the measurement results?
This experiment was conducted four times:
The first time: 5m long, 70°C insulation resistance, 1 minute reading measured: 6.80×106Ω
The second time: 5m long, 70°C insulation resistance, 1.5 minute reading reading: 7.01×106Ω
The third time: 5m long, 20°C insulation resistance, 1 minute reading measured: 109.6×106Ω
The fourth: 10m long, 70°C insulation resistance, 1 minute reading measured: 3.40×106Ω
Second, the reason analysis The same group of wire insulation resistance in different temperatures, different lengths, different reading times why there is such a big difference? Are analyzed as follows:
Insulation resistance is the ratio between the DC voltage U and leakage current I applied to the insulation.
When the insulation layer is applied with DC voltage, weak current flows along both the insulation surface and the insulation. Resistances corresponding to these two currents are called the surface insulation resistance and the volume insulation resistance respectively. In general, the insulation resistance without special instructions means Volume insulation resistance, only a few products have surface insulation resistance requirements (such as automotive high voltage ignition wire).
After the voltage is applied to the insulation layer, the following four currents flow through the insulation:
1. Capacitance current is generated due to dielectric polarization. Actually, the conductor and the external pole (insulating layer) are used as a pair of electric levels to constitute an electric device's capacitive current. Capacitance current decays exponentially with time quickly, generally in several milliseconds. Close to disappear within time.
2. The irreversible absorption current is generated due to the electrolytic conductance in the insulating material and decays to zero after a few seconds.
3. Absorption current refers to the displacement current of the insulating material, which reaches the maximum value at the instant when the voltage is applied, and then tends to be stable in displacement and tends to disappear after a few minutes.
4. Leakage current Leakage current is generated by free ions in the insulating material and conductive impurities mixed in. It is independent of the voltage application time. Ohm's law is met when the electric field strength is not too high, and the resistance increases with temperature. Its size reflects the quality of the insulation quality. Strictly speaking, only the resistance corresponding to a constant electric conduction current is the volume insulation resistance.
Since there are three currents attenuating with time in the insulation after the voltage is applied, it is theoretically possible to read the value of the leakage current after all the three currents have been attenuated to calculate the insulation resistance, but since the current can be absorbed It takes a few minutes before disappearing, taking into account the long-term stability of the measurement system, the measurement time should not be too long. The same measurement conditions, the reading time will cause great differences, reading a long time, will cause the value is too large, from the first and second data can be clearly seen. Therefore, the standard clearly stipulates that after the current is switched on for 1 minute (ie when the reading is reaching 1 minute), the 1 minute reading ensures that most of the non-leakage current has disappeared and the measurement time is unified, making the value repeatable. And comparable. The first and third data shows that the insulation resistance decreases rapidly with increasing temperature. This is because as the temperature rises, the movement speed of impurity ions in the insulating material is accelerated, making the conductance increase and the insulation resistance decrease. The relationship with the insulation resistance approximates an exponential relationship. Therefore, when measuring, the temperature must be strictly controlled. The length of the insulation resistance measurement is also different. This is because the insulation resistance is inversely proportional to the length. When measuring the length of the wire, the error must be controlled within ±1%.
The value of the insulation resistance is inversely proportional to the length of the product and is closely related to temperature. In order to be uniform and convenient in the product standard, the minimum value of the insulation resistance at a length of 1 km is used as a standard value at a temperature of 20°C (or 70°C) (this standard value can be calculated theoretically), and this product standard has Strict test conditions, so in the test process should be strictly in accordance with the standard, and should not relax the test conditions, so as not to affect the accuracy of the measurement.
Insulation resistance is the main indicator reflecting the insulation properties of wire and cable products. It reflects the cable's ability to withstand electrical breakdown or thermal breakdown, as well as the dielectric loss of insulation and the gradual deterioration of insulation materials under working conditions. Extremely close relationship. The insulation resistance of the product mainly depends on the insulating material chosen, but the process level has a great influence on the insulation resistance. Therefore, the determination of the insulation resistance is a method of supervising the material quality and the technological level. The determination of insulation resistance can be used to identify defects in the process, as well as to study the quality and characteristics of insulating materials, and to study the insulation structure and the performance of the product under various operating conditions. The insulation resistance is One of the important basis for judging product quality changes. The accuracy of the insulation resistance measurement directly affects the determination of the product quality. Therefore, attention must be paid to the measurement of the insulation resistance.