Recently, a net user in the background asked the problem of wire cable resistance detection and calculation. The method for detecting resistance is as follows, some common types of wire 90 AAAC cables can be obtained by consulting an electrician’s manual.
(1) DC resistance detection.
According to the standards of the countries concerned, the conductor resistance per 1,000 meters is used as the standard for comparison of the DC resistance of electric cables. It is clearly stipulated that it must After converting the measured DC resistance value into the DC resistance value under the condition of 20°C, if the value is less than the predetermined reference value, the electric wire/cable sample is accepted, and conversely, it is rejected.
At present, domestic relevant departments generally adopt two methods, the bridge method and the current method, to measure the DC resistance of wire cables. The measurement range of the bridge method is relatively narrow. If the resistance value of the electric cable is less than 1, the two-arm bridge method is adopted. Current method is also called micro-European metering method, its principle is based on the magnitude of the resistance value of the wire cable, using a constant current source to output different constant current, and then the measured voltage across the wire cable can be measured accurately and the measured data can be calculated according to Ohm’s law to obtain the measured DC resistance of the cable. Current method can output different currents, so its measuring range is relatively wide.
(2) Insulation resistance detection.
Insulation resistance measurements of electric cables shall be converted into insulation resistance values per 1,000 meters. Unlike DC resistance, insulation resistance is inversely proportional to cable length. There are four types of measurement voltages for detecting the insulation resistance of low-voltage cables: 100 V, 250 V, 500 V, and 1000 V. Of these, the detection voltages of 100 V and 500 V are widely used in quality inspection departments. . The length of the measured wire cable is not clearly defined, but it is convenient to measure and calculate, so generally 10 m is taken and measured. The charging time before measurement is typically 1 minute.
Insulation resistance detection of electric cables generally adopts the voltage-current method, which is also called the high-resistance meter method. Some wire cables have a metal protective cover and have some shielding function, and many insulation resistance measurements of such wire cables measure the insulation resistance between the conductor and the metal cover or shield layer or armor layer. On the other hand, when measuring the insulation resistance value of electric cables without a metal sheath, first immerse the measured electric cable in water, then measure the insulation resistance between the conductor and water, and test The sample must be kept set with the water temperature.
At present, the domestic DC resistance insulation resistance tester ZZZJ 3D has been developed. This tester is easy to operate, the whole measurement process can be controlled by computer, and the accuracy and stability are much higher than the traditional detection equipment.
(3) Frequency withstand voltage detection.
Frequency withstand voltage is generally detected using AC voltage. According to national standards, the AC voltage used is an approximate sine wave with a frequency between 49 Hz and 61 Hz. For products with cable rated voltage of 450/750 V, adopt 1500 V high voltage when insulation thickness ≤ 0.6 mm. If the insulation thickness is ≧0.6 mm, a high voltage of 2000 V is applied for 5 minutes. For example, there is a standard of 60227 IEC 53 (RVV) 300/500 V 32.5 samples must withstand. Then connect the 1st core to water with high pressure, connect the 2nd core to water with high pressure, connect the 3rd core to water with high pressure, finally connect all 3 cores to water with high pressure once each need to do it. It must withstand 4 times in total.
(4) Machine performance inspection.
Mechanical properties mainly refer to the magnitude of tensile force before and after deterioration of electric cables. Relevant national standards dictate that aged wire and cable specimens be produced using a forced draft aging box to be as close as possible to the unaged part during inspection. Mechanical performance detection is commonly measured directly using an electronic tensile force meter. First, the width and thickness of the middle part of the measured wire cable are accurately measured with a measuring instrument, then the sample is placed in a blow drying box for artificial aging, measured with an electronic tension machine, and the tensile fracture of the wire cable is measured. The elongation distance and the magnitude of the maximum tensile stress are recorded, and the obtained data can be used to calculate the tensile strength and breaking elongation rate of the measured wire cable before and after aging. It is possible to judge whether or not the product is acceptable by comparing it with the product standard of the product.
(5) other inspection items and inspection methods;
In addition to the above main detection items, there are other items such as insulation thickness detection, size and label detection, sheath thickness detection, etc., which generally can be done by adopting simple measuring instruments or artificial inspection. . Insulation thickness is the thickness after removing all the protective layers on the insulation layer, measured with a projector and a reading microscope, the measured data is compared with the product standard specification, and the average measured value is the specified value. If it is not larger, it is not acceptable. The outer dimensions can be measured around the projector or bandage, the ellipticity measurement method is to measure the outer diameter of any two points on the same cross section of the circular sheath cable, take the difference, then the difference and the cable standard shall not exceed 15% of the measured average outside diameter ratio. The failure rate of China’s wire and cable markings is very high. As for abrasion resistance, wipe lightly 10 times with medical absorbent cotton soaked in alcohol, and the letters are clear.