Miniature Circuit Breaker, as known as MCB, is one of the most commonly used terminal protection appliances in building electrical terminal distribution devices. It refers to a mechanical switching appliance that can make, carry and break current under normal circuit conditions, and can also make, carry and break current for a certain period of time under specified odd circuit conditions. It is used for single-phase and three-phase short circuit, overload and overvoltage protection below 125A, including 1 pole, 2 pole, 3 pole, and 4 pole.
A miniature circuit breaker is an electrical switch designed to protect an electrical circuit from damage caused by overcurrent/overload or short circuit. Its basic function is to interrupt current flow after protective relays detect a fault. A circuit breaker can make, carry, and break the current under standard circuit conditions. Hence it is called as an on load device.
A circuit breaker can be used as a protective device that can control the flow of the current just like a fuse. This is a benefit you might not expect from a switch disconnector. It breaks the circuit in case of fault conditions like short circuit and overload. Unlike a switch, a circuit breaker automatically does this and shuts off the power immediately. In this way it works as an automatic service protection device.
Miniature circuit breakers are composed of operating mechanisms, contacts, protection devices (various releases), and arc extinguishing systems. Its main contacts are manually operated or electrically closed. After the main contact is closed, the free trip mechanism locks the main contact in the closed position. The coil of the overcurrent release and the thermal element of the thermal release are connected in series with the main circuit, and the coil of the undervoltage release is connected in parallel with the power supply. When the circuit is short-circuited or severely overloaded, the armature of the overcurrent release is pulled in to make the free trip mechanism act, and the main contact disconnects the main circuit. When the circuit is overloaded, the thermal element of the thermal release will heat up and curve the bimetal, pushing the free release mechanism to act.
MCB is divided into A, B, C, D curve trip characteristics.
The design and use of MCB is for the 50~60Hz AC power grid. Since the electromagnetic force of the magnetic release is related to the power frequency and operating current, when the MCB used under AC voltage is used for the protection of DC circuits or other power frequency occasions, the operating current of the magnetic release is different. Generally, it should be converted according to the magnetic tripping action current provided by the manufacturer and the coefficient of variation of the power supply frequency.
The overload protection of miniature circuit breaker relies on the thermal release. Usually, the rated current of the thermal release of the MCB is set at the base temperature of 30℃. The working temperature of it is generally recommended to be -25℃~+55℃. The thermal release is composed of a bimetallic strip, which trips the MCB when the passing current reaches a certain set value and maintains it for a certain period of time. If the ambient temperature changes, the working temperature of the MCB will change, and the working characteristics of the thermal release will change accordingly. Since the miniature circuit breaker is usually installed in the distribution box, the ambient temperature cannot be kept constant at 30°C. Therefore, the actual working temperature of the MCB is always about 10C~15℃ higher than the ambient temperature. Generally speaking, when the ambient temperature is higher or lower than the correction value 10℃, the rated current value of the MCB must be reduced or increased by about 5%.
|1P||Single pole switch of circuit breaker: Only one connecting plug, it can only break one phase line, this switch is only suitable for controlling one-phase live wire.|
|2P||Two pole switch of circuit breaker: Two connecting plugs, one is connected to the phase line and the other is connected to the zero line.|
|3P||Three pole switch of circuit breaker: Three connecting plugs, which are connected to the live wires. This switch is suitable for controlling three-phase 380V circuit.|
|4P||Four pole switch of circuit breaker: Four connecting plugs, three are connected to the live wires and the remaining is connected to the zero line. This switch is suitable for controlling three-phase four-wire system circuit.|
|1P+N||Single pole switch with zero terminal (the zero line doesn’t break)|
|3P+N||Three pole switch with zero terminal (the zero line doesn’t break)|