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Qualified "gatekeeper" - wire and cable insulation resistance

Wire and cable insulation resistance

Insulation resistance is the main index of reflecting the characteristics of insulated wire and cable products, it reflects the cable products bear the size of the electric breakdown or thermal breakdown, and insulation dielectric loss and insulating material in the working state of gradual degradation, etc. There is a very close relationship. Products mainly depends on the selecion of insulation materials, insulation resistance but the process has much effect on the insulation resistance, thus the determination of insulation resistance is to monitor the materials quality and technological level of a method. Measuring insulation resistance can be found that the process of defects, as well as research quality and characteristics of insulation materials, insulation structure and products under various operating conditions using performance and so on various aspects of the important means, for product has been put into operation, insulation resistance is one of the important basis for judging product quality changes. Insulation resistance measurement of the determination of accurate or not directly affect the product quality and therefore should pay attention to the measurement of insulation resistance.

A, experimental phenomenon

Factors that affect the wire and cable insulation resistance measurements are instrument accuracy, environmental conditions, and personnel quality and so on several aspects, the following GB5023.3-1997 in general use single core hard without sheathed cable conductor (model 227 iec01 (BV), for example, talk about several problems that should be paid attention to in measuring insulation resistance. According to the rules of GB5023.3: test should be conducted on 5 m long insulation wire core, the temperature of (70 + 2) ℃, arbitration test for (70 + 1) ℃, soaking time should not less than 2 h, insulation resistance should be applied voltage measurement after 1 minute. How to understand these requirements of the standard, they have any influence on the measured results? The following examples.

The tests were conducted four times:

First: 5 m long, 70 ℃, 1 minute reading measuring insulation resistance value is: 6.80 x 106 Ω

Second: 5 m long, 70 ℃, 1.5 minutes reading measuring insulation resistance value is: 7.01 x 106 Ω

Third time: 5 m long and 20 ℃, 1 minute reading measuring insulation resistance value is: 109.6 x 106 Ω

Fourth: 10 m long, 70 ℃, 1 minute reading measuring insulation resistance value is: 3.40 x 106 Ω

Second, the cause analysis

With the same set of wire insulation resistance at different temperature, different length, different reading time, why is there so much difference? Now the analysis is as follows:

Insulation resistance refers to the dc voltage on the insulation and the ratio of between U and leakage current (I)

R =

When insulating layer with dc voltage, along the surface and internal insulation insulation through have a weak current, corresponding to the resistance of the two current respectively referred to as the surface insulation resistance and insulation resistance volume, generally do not add special instructions are refers to the volume of insulation resistance, insulation resistance only a handful of the requirement of products surface insulation resistance (e.g., automobile high voltage ignition line).

Internal insulation insulation layer and the voltage, the current has the following four:

1. The capacitive current

Due to dielectric polarization, in fact, conductor and the outer pole (insulation layer) as a pair of electrode form a electrical capacitance current, capacitance current according to the index law soon attenuation with time, generally within a few milliseconds time close to disappear.

2. The irreversible absorption current

Due to electrolytic conductivity of insulation materials, after a few seconds the decay to zero.

3. Current can be absorbed

Refers to the displacement current, insulation material at the instant of the applied voltage reaches the maximum, and then tends to stable displacement, tend to disappear after a few minutes.

4. The leakage current

Leakage current is refers to the free ions in the insulation material and mixed with conductive impurity produced, has nothing to do with the voltage on time, in the electric field intensity is not too high in accordance with ohm's law, the resistance increased with the temperature increasing. It reflects the size of the insulation quality, strictly speaking, only corresponding to a constant electrical current resistance is the volume of insulation resistance.

Due to the applied voltage, there are three types of decay over time in the insulation of electrical current, so in theory should be after all of these three kinds of current attenuation, just read out the value of leakage current, insulation resistance to calculate, but due to the absorption current tends to disappear after a few minutes, considering the stability of the measurement system for a long time, measuring time shoulds not be too long. The same measurement conditions, readings will cause a big difference in different time, reading a long time, will cause the slants big, from the first and second data can be clearly seen. So the standard specified in the current is switched on for 1 minute after reading (i.e., up to 1 minute reading), 1 minute reading not only ensure the most has disappeared, the leakage current and measuring time have a unified, make the numerical repeatability and comparability. First and third data shows that with the increase of temperature insulation resistance drops rapidly, this is because with the rise of temperature, the movement of the impurity ions in the insulation material faster, makes the conductivity increases, insulation resistance to drop, the temperature and the relationship between the insulation resistance approximate relation to index. Measurement, therefore, must strictly control the temperature, the length of the different measuring insulation resistance value is different, this is because the insulation resistance is inversely proportional to the length of the gauge length, the wire error to control within the plus or minus 1%.

The value of insulation resistance and inversely proportional to the length of the product, and have close relationship with temperature. For unity in the product standard and convenience, to 20 ℃ (70 ℃), the length of 1 km insulation resistance minimum limit value as a standard (the standard values can be calculated by theory), there are strict in the product standard for this test conditions, so in the process of testing should be performed in accordance with the standard, not relax test conditions, so as not to affect the accuracy of measurement.


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