Low-voltage voltage is divided into two branches and trunk lines. A branch is a starter-to-motor cable that powers a single motor; a trunk is a cable that connects a diverter switch to a starter and supplies power to multiple motors. The choice of low voltage cable is to determine the type, number of cores, length and cross section of each low voltage cable.
Determination of the type, number of cores and length of low-voltage cables
1. Selection of low voltage cable model
2. Determine the number of cores of the cable
3. Determine the cable length
Selection of main core wire section of low voltage cable The main core wire section of low voltage cable must meet the following conditions:
1) During normal operation, the actual temperature of the cable core should not exceed the long-term allowable temperature of the cable, so it should be ensured that the maximum long-term operating current flowing through the cable must not exceed its allowable continuous current;
2) During normal operation, ensure that the terminal voltage of all motors of the power supply network is in the range of 95% to 105% of the rated voltage, and that individual special far-end motor terminals are allowed to shift by 8% to 10%; 3) Distance is long and power is large The motor should be guaranteed to start normally under heavy load conditions, and ensure that its starter has enough suction voltage.
4) The selected cable cross-section must meet the mechanical strength requirements. When the cross-section of the main core wire of the low-voltage cable is selected according to the above conditions, the branch cable is generally selected according to the minimum cross-section of the mechanical strength, and after confirming the allowable continuous current, it can be determined. When selecting the main cable of trunk cable, if the trunk cable is not long, it should be selected according to the allowable continuous current of the cable; when the trunk cable is longer, the primary voltage should be selected according to the normal allowable voltage loss.
General maintenance of low voltage cables
1. Line Inspection: Regular inspections should be conducted on the entire cable route, including: whether there is excavation marks on the cable trench and nearby ground, whether the soil layer covered by the trench is normal, whether the trees are planted and piled, whether or not there is Stack heavy objects, have no excretion such as acid and alkali, whether the roads and building facilities passed by cable circuits are being overhauled and repaired.
2. Operation check: The cable line should be checked regularly for operation, including:
(1) Measurement of voltage and current: The voltage must not exceed 15% of the rated voltage of the cable and the current must not exceed the design specification.
(2) Cable temperature check: Generally, the temperature of the inner core wire is estimated by the cable jacket temperature. The empirical formula for estimation is: cable core temperature = skin temperature +15-12 degrees Celsius
(3) Insulation performance test: For power cables below 1KV, the insulation resistance per kilometer cable (ambient temperature 20 degrees Celsius) shall not be less than 50 megohms with a 1KV megohmmeter.
(4) Withstand voltage test: Generally, 4 to 5 times the DC voltage of the rated voltage of the cable, continuous voltage for 5 minutes for pressure test (not commonly used).
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