Among the objects of circuit breaker maintenance, there are some electrical equipment that generate several times its rated current in a short period of time during the start-up process
(1) The setting of the operating current of the instantaneous overcurrent tripper
Among the objects of circuit breaker maintenance, there are some electrical equipment that will generate several times the current of its rated current in a short period of time during the start-up process, so that the circuit breaker will bear a large peak current in a short period of time. The operating current lop(o) of the transient overcurrent tripper must avoid the peak current Ipk of the line, i.e. IOP(o) ≥ krel ipk. In the meantime, Krel is the reliability factor. When selecting a circuit breaker, attention should be paid to making the setting current of the instantaneous overcurrent tripper of the circuit breaker avoid the peak current, so as to avoid the malfunction of the circuit breaker.
(2) The setting of the operating current and operating time of the short-delay overcurrent tripper
The operating current lop(s) of the short-time overcurrent tripper should also avoid the peak current of the line, Ipk, i.e., IOP(s) ≥ krel ipk. In the meantime, Krel is the reliability factor. The action time of short-term overcurrent tripping equipment is generally divided into three types: 0.2s, 0.4s and 0.6s, which are confirmed according to the maintenance selectivity of the front and rear maintenance equipment, so that the action time of the previous maintenance is one time difference longer than the action time of the latter maintenance.
(3) The setting of the action current and action time of the long-delay overcurrent tripper
The long-delay overcurrent tripper is mainly used for overload maintenance, so its operating current Iop (l) only needs to avoid the maximum load current of the line, that is, the accounting current I30, that is, IOP (L) ≥ Krel I30. In the meantime, Krel is the reliability factor. The action moment of the long-term overcurrent tripper should avoid the promised short-term overload duration time to avoid the malfunction of the low-voltage circuit breaker.
(4) The requirements for the operation current of the overcurrent tripper and the line to be maintained
In order to avoid overheating, damage and even fire of the insulated cable due to overload or short circuit, the circuit breaker does not trip, and the action current of the circuit breaker overcurrent tripper should meet the requirements of the formula, lop ≤ Kol Ial. In the meantime, Ial is the promised ampacity of the insulated cable; Kol is the promised short-time overload coefficient of the insulated cable, which is generally 4.5 for instantaneous and short-delay overcurrent trippers, and 1.1 for short-circuit maintenance and 1 for load maintenance only. If the above matching requirements are not met, the tripping action current should be changed, or the cross-sectional area of the conductor or cable should be appropriately increased.
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