Figure 1: A circuit breaker in operation.
The circuit breaker is an automatic device for stopping the flow of current in an electric circuit as a safety measure.
Circuit breakers are divided into high voltage circuit breakers and low voltage circuit breakers according to their scope of use. The boundary between high and low voltage is relatively blurred, and generally those above 3kV are called high-voltage electrical appliances.
Power distribution is an extremely important link in the generation, transmission and use of electricity. The power distribution system includes transformers and various high and low voltage electrical equipment, and the low voltage circuit breaker is a widely used electrical appliance.
There are different types of circuit breakers to meet the different meets. The common types are residual current circuit breaker (RCCB), miniature circuit breaker, molded case circuit breaker (MCCB), and disconnector.
The RCCB is a switch that can automatically act when the residual current in the circuit exceeds a predetermined value. Commonly used RCCBs are divided into two types: voltage type and current type. It has short-circuit current limiting function and high rated short-circuit breaking capacity. It has overload protection, short-circuit residual and voltage protection devices, complete protection functions, and convenient and reliable wiring.
In industrial power distribution systems, miniature circuit breakers are used for short circuit and overload protection. Its rated current is 63A-125A, with high rated short-circuit breaking capacity and a short-circuit current limiting structure.
It has complete protection functions, overload and short-circuit protection devices, safe and reliable wiring, "frame" wiring structure, and simple function expansion. It can be equipped with various accessories: residual release, auxiliary contact, alarm contact, shunt release, undervoltage release, busbar.
Figure 2: A molded case circuit breaker.
MCCBs can automatically cut off the current after the current exceeds the trip setting. The molded case refers to the use of plastic insulators as the outer casing of the device to isolate the conductors and the grounded metal parts. MCCBs typically contain a thermal-magnetic trip unit, while larger molded case circuit breakers are equipped with solid-state trip sensors.
Usually, their rated insulation voltage of the circuit breaker is 800V, suitable for alternating current (AC) 50Hz and 60Hz; the rated working voltage is up to 690V, and the rated working current is from 6A to 1250A in the distribution network circuit.
The MCCB is used to distribute power and protect the circuits and power equipment from overload, short circuit, undervoltage and other faults. At the same time, it can also be used as infrequent starting and overload short circuit undervoltage protection of the motor. It has the characteristics of small size, high breaking, short arcing (or no arcing), etc. It is an ideal product for users. The circuit breaker can be installed vertically or horizontally.
The disconnector, also known as the disconnect switch or the isolator switch, is one of the most used electrical appliances in high-voltage switching appliances, and it functions as an isolator. Its working principle and structure are relatively simple, but it is widely used and requires high reliability. It has a great impact on the design, establishment and safe operation of substations and power plants.
Figure 3: Disconnectors applied in power distribution system.
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