In brief
A circuit breaker is a switching device that switches operating currents and automatically and safely interrupts overload and short-circuit currents.
Contents
What is Circuit breaker?
Circuit breakers protect installations, distribution boards and cables and can interrupt even large short-circuit currents without being destroyed themselves. Unlike fuses, they can be switched on again after tripping. In low voltage, they are covered by the IEC/EN 60947 series of standards. In medium and high voltage, there are separate designs, for example with vacuum or gas quenching.
In low-voltage installations, a distinction is made between moulded case circuit breakers and air circuit breakers for high currents, often in withdrawable design. Tripping is thermal-magnetic or electronic; electronic trip units offer adjustable protection functions against overload, short-time-delayed and instantaneous short circuit and, in some cases, earth fault. Key ratings are rated current, rated voltage, ultimate and service short-circuit breaking capacity, number of poles, trip unit type and accessories such as motor operator, auxiliary switches and shunt trip. Graded settings ensure selectivity between upstream and downstream protective devices.
Circuit breakers are installed in main distribution boards, incoming feeders and large outgoing circuits. For older devices, spare parts such as trip units, operators and contacts have often been discontinued, which is relevant for spare parts planning.
In practice: maintenance and procurement
For a replacement, the rated current, breaking capacity, number of poles, trip unit type with settings, mounting type (fixed or withdrawable) and accessories must be known. A different device can change the selectivity of the installation; the selection should therefore be agreed with the responsible qualified electrician.
Synonyms and abbreviations: MCCB (moulded case circuit breaker) · ACB (air circuit breaker) · power circuit breaker · low-voltage circuit breaker
Frequently asked questions
What is the difference between a circuit breaker and a miniature circuit breaker?
Miniature circuit breakers are small, usually non-adjustable devices for final circuits with limited breaking capacity. Circuit breakers cover higher currents, have greater breaking capacity and often adjustable trip units. They are used for incoming feeders, main distribution boards and large loads. The underlying standards are different.
What do Icu and Ics mean for a circuit breaker?
Icu is the rated ultimate short-circuit breaking capacity, i.e. the highest short-circuit current the breaker can interrupt. Ics is the rated service short-circuit breaking capacity, after which the breaker remains fit for further operation. Both values must at least match the possible short-circuit current at the point of installation, which results from the installation calculation.
What information is needed to replace a circuit breaker?
The type designation, rated current, rated voltage, breaking capacity, number of poles, trip unit with all settings, mounting type, connection type and accessories such as motor operator, auxiliary switches or shunt trip are needed. For withdrawable devices, the withdrawable chassis must also be stated. Photos of the nameplate and trip unit are very helpful.
What should be done if a circuit breaker has been discontinued?
First, clarify whether replacement devices, exchange trip units or retrofit solutions are available that fit into existing panels. Many manufacturers offer successor types with adapters. If this is not possible, a repair or conversion of the panel may be considered. In any case, protection settings and selectivity must be reassessed.