How to Select a Miniature Circuit Breaker?
Miniature circuit breaker, short for MCB, is one of the most widely used terminal protection appliances in building electrical terminal distribution devices. And this article shows the right selection of MCB based on its electrical parameters.
The rated breaking capacity of MCB is the maximum short-circuit current value that can be broken without any damage to the circuit breaker. The miniature circuit breakers currently on the market generally have several rated breaking capacities, including 4.5kA, 6kA and 10kA according to the relevant technical data and design manuals provided by various manufacturers. When we choose a MCB, we should calculate the maximum short-circuit capacity in this application like MCCB (molded case circuit breaker) and ACB (frame circuit breaker). If the rated breaking capacity of the miniature circuit breaker is less than the short-circuit fault current within the protected range, when a fault occurs, not only can it fail to break the faulty line, but also cause the explosion, endangering the safe operation of the personal and other electrical equipment lines.
The short-circuit current of the low-voltage distribution line is related to the electrical parameters including the wire cross-section of the power supply line, the wire laying method, the distance between the short-circuit point and the power source, the capacity of the distribution transformer, and the impedance percentage. Generally, the voltage on the low-voltage side of the distribution transformers in general industrial and civil buildings is 0.23/ 0.4LV, and the capacity of the transformer is mostly 1600kVA and below. The short-circuit current of the low-voltage side line increases with the increase of the distribution capacity.
For distribution transformers of different capacities, the short-circuit current at the low-voltage feeder end is different. Generally speaking, for civil residences, small shopping malls and public buildings, because the power supply is supplied by the low-voltage power grid, the MCB with breaking capacity of 4.5kA and above is sufficient. For users with dedicated supply or 10kV substations, MCBs with rated breaking capacity of 6kA and above should be chosen because the cables of the power supply lines are thicker and the power supply distance is short.
MCB is divided into A, B, C, D curve trip characteristics.
- The A curve characteristic is generally used in applications that require fast and non-delayed tripping, that is, for a lower peak current value (usually 2-3 times the rated current/n) to limit the allowable short-circuit current value and the total breaking time. Therefore, MCBs can replace fuse as overcurrent protection of electronic components and protection of mutual inductance measurement circuit.
- The B curve characteristic is generally used in applications that require faster tripping and the peak current is not very large. Compared with the A characteristic, the allowable peak current of the B characteristic is less than 3In, which is generally used for resistive loads such as incandescent lamps, electric heaters, etc.
- The C curve characteristic is generally suitable for most electrical circuits, which allows the load to pass a high short-term peak current without MCB action, and the C characteristic allows the peak current to pass <5In generally used for fluorescent lamps, high pressure gas discharge lamps, line protection of power distribution system.
- The D curve characteristic is generally suitable for switchgear with very high peak current (10In), and is used for the protection of primary circuits and solenoid valves of control transformers and local lighting transformers under AC rated voltage and frequency.
For various lines of different nature, be sure to select the appropriate MCB. If there is a circuit of a gas discharge lamp, a large surge current appears when the lamp is started. If the MCB is only used according to the rated current of the lamp, it will cause the MCB to trip by mistake at the moment of turning on the lamp.
In terms of protection characteristics, MCB cannot be used to protect the motor, and can only be used as a substitute for fuses to protect distribution lines (such as wires and cables). At the moment of starting, the motor has a starting current of 5-7In with a duration of 10s. Even if the C characteristic is set to (5-10)In when the electromagnetic tripping current is set, it can ensure that the inrush current is avoided when the motor starts, but the thermal In terms of protection, the action value of its overload protection is set at 1.45In, that is to say, the MCB can only trip when the motor has to withstand an overload current of more than 45%. It is very easy to damage the insulation between the windings, but it is bearable for wires and cables. Therefore, in some occasions, if it is really necessary to use MCB to protect the motor, the MCB external heating relay can be used to protect the motor from overload and short circuit.
Frequency use of MCB
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.
Ambient temperature of MCB
The overload protection of MCB 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. Therefore, thermal releases are temperature dependent. 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 MCB (miniature circuit breaker) is usually installed in the distribution box, the ambient temperature cannot be kept constant at 30°C. In actual use, the MCBs in the terminal distribution box are installed closely together, and in most cases they are embedded in, Installation in the wall will lead to poor heat dissipation effect, and the temperature rise in the distribution circuit will rise greatly. Therefore, the actual working temperature of the MCB is always about 10C~15℃ higher than the ambient temperature. Therefore, when the ambient temperature is higher or lower than the calibration temperature value, we must adjust the rated current value of the MCB according to the temperature and current carrying capacity correction curve provided by the relevant manufacturer. 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%.
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