Precautions for the use of the hottest low voltage

Several precautions for the use of low-voltage circuit breakers

① overload long-time delay protection. When thermal type (bimetal element) is used as overload long-time-delay protection, its action source is I2R, and the effective value of AC current is equal to the average value of DC, so it can be used without any modification. However, for large current specifications, if the secondary side current of current transformer is used for heating, the transformer cannot be used because it cannot be used in DC circuit

if the overload long-time-delay tripper adopts the full electromagnetic type (hydraulic type, i.e. oil cup type), the time-delay tripping characteristics should change, and the minimum action current should be increased by 110% - 140%. Therefore, the AC full electromagnetic tripper cannot be used in the DC circuit (if it is used, it should be redesigned)

② short circuit protection

the short-circuit protection of the thermal electromagnetic AC circuit breaker adopts the magnet system, which is used in the filtered rectifier circuit (DC). It is necessary to multiply the set current value of the original AC by a factor of 1.3. The short-circuit protection of full electromagnetic type is the same as that of thermal electromagnetic type

2. Accessories of circuit breaker, such as shunt release, undervoltage release, fashionable appearance, dynamic electric operating mechanism, etc; It mainly distributes all kinds of steel produced by local Angang, Shagang, Xinpu, Fengbao and other enterprises. Shunt and undervoltage are voltage coils. As long as the voltage value is consistent, they can be used in AC system without any change. Auxiliary and alarm contacts, AC and DC common. The electric operating mechanism should be redesigned when it is used for DC

3. Because DC current does not have the characteristics of zero crossing unlike AC, there are two main factors for the opening of DC short-circuit current (even the fault current with small multiple): the short clamping length of the sample and the selection of the jaw of the fixture are not open; It is difficult to extinguish the arc, so the wiring should adopt the method of two or three poles in series to increase the fracture, so that each fracture bears part of the arc energy

II. Undervoltage release

if the line voltage is reduced to 70% of the rated voltage (called collapse voltage), the motor will not be able to start, the lighting fixtures will be dim, and the heating of the resistance furnace will be insufficient; When the working voltage of the running motor is reduced to about 50% (called critical voltage), Locked rotor will occur (unable to drag the load, and the motor will stop rotating), and the current of the motor will rise sharply, up to 6in. If the time is a little longer, the motor will be burned. In order to avoid the above situation, it is required to install an undervoltage release on the circuit breaker. The operating voltage of the undervoltage release is set at (70% - 35%) of the rated voltage. The undervoltage release has instantaneous and time-delay types (1s, 3S, 5S... -....) Two. The time-delay undervoltage release is used in the main line or important branches, while the instantaneous release is often used in general branches. For areas with poor power supply quality, the voltage itself fluctuates greatly, which is close to the upper limit of the action voltage of the undervoltage release, which is not suitable for the use of the undervoltage release