Circuit breaker types
Summary
Section titled “Summary”Thermal-magnetic circuit breakers are electrical protection devices that use a bimetal (thermal effect) and an electromagnet (magnetic effect) to detect overloads and short-circuits in low-voltage circuits. The rated current ranges from 0,5 A / 0,5 A in miniature models up to 6300 A / 6300 A in power circuit breakers. The following table summarizes the main families.
| Type | Rated current | Typical rated voltage | Breaking capacity |
|---|---|---|---|
| MCB (Miniature) | 0,5 A – 125 A / 0,5 A – 125 A | 230/400 V AC / 230/400 V AC | 3 kA – 25 kA / 3 kA – 25 kA |
| MCCB (Molded case) | 10 A – 2500 A / 10 A – 2500 A | 690 V AC / 690 V AC | 25 kA – 150 kA / 25 kA – 150 kA |
| ACB (Open) | 630 A – 6300 A / 630 A – 6300 A | 690 V AC / 690 V AC | 50 kA – 150 kA / 50 kA – 150 kA |
Operating principle
Section titled “Operating principle”The mechanism combines the thermal deformation of a bimetallic strip by Joule effect and the magnetic force generated by a trip coil. A persistent overload causes the bimetal to heat up, and when it bends it releases a mechanical latch. The short-circuit induces an intense magnetic field in the coil, whose core strikes the same latch, opening the contacts instantaneously. The magnetic force follows the law:
F = k · I²
where F is the force on the mechanism, I is the fault current, and k is a constant that depends on the geometry of the device.
Types of thermal-magnetic circuit breakers
Section titled “Types of thermal-magnetic circuit breakers”They are classified according to their construction, interrupting capacity, and application:
MCB (Miniature Circuit Breaker): compact design for DIN rail mounting, currents up to 125 A / 125 A, breaking capacity from 3 kA to 25 kA / 3 kA to 25 kA. Used in residential and small distribution boards. Detection completely contained within the device itself.
MCCB (Molded Case Circuit Breaker): enclosure made of molded insulating material, currents between 10 A / 10 A and 2500 A / 2500 A, breaking capacity up to 150 kA / 150 kA. Incorporates adjustable electronic relays in the most advanced models.
ACB (Air Circuit Breaker): open construction with air contacts, used in low voltage up to 690 V AC / 690 V AC and currents from 630 A / 630 A to 6300 A / 6300 A. They are used in main industrial distribution switchboards.
Enclosures according to NEMA and IEC: circuit breakers are housed in standardized enclosures. The following table establishes the correspondence between NEMA types (used in the Americas) and IP classification according to IEC 60529.
| NEMA Type | IEC Designation | Description |
|---|---|---|
| 1 | IP10 | Protection against solid objects up to 50 mm / 1,97 in, e.g., accidental contact with hands. |
| 2 | IP11 | Protection against solid objects up to 50 mm / 1,97 in and against vertical dripping of water, such as condensation. |
| 3 | IP54 | Limited protection against dust (no harmful deposit) and against water jets from any direction. |
| 3R | IP14 | Protection against solid objects up to 50 mm / 1,97 in and against water jets from any direction (limited ingress). |
| 3S | IP54 | Limited protection against dust and against water jets from any direction. |
| 4 | IP56 | Limited protection against dust and against powerful water jets. |
| 4X | IP56 | Same as Type 4, plus corrosion resistant. |
| 5 | IP52 | Limited protection against dust and against direct water spray up to 15° from vertical. |
| 6 | IP67 | Total protection against dust and against immersion between 15 cm / 5,9 in and 1 m / 39,4 in. |
| 6P | IP67 | Total protection against dust and against prolonged immersion (same depth as Type 6). |
| 12 | IP52 | Limited protection against dust and against direct water spray up to 15° from vertical. |
| 12K | IP52 | Same as Type 12, but with removable cable glands. |
| 13 | IP54 | Limited protection against dust and against water jets from any direction. |
Note: NEMA standards meet or exceed IEC requirements; the conversion is not valid in the reverse direction.
Advantages and disadvantages
Section titled “Advantages and disadvantages”Compared to fuses, thermal-magnetic circuit breakers present the following differences:
- Automatic disconnection in case of overload and short-circuit, detectable by the position of the lever.
- Higher sensitivity and repeatability in detecting abnormal currents.
- Quick reset by manual reset; no replacement required.
- Greater electrical safety during handling.
- Possibility of remote control in some models.
- Main disadvantage: higher initial cost than an equivalent fuse.
Typical applications
Section titled “Typical applications”- Residential and commercial: MCB in distribution boards for lighting and power outlets.
- Light industrial: MCCB in machine feeders, unit substations.
- Main distribution: ACB in general low-voltage switches with high breaking capacity.
- Motor protection: thermal-magnetic circuit breakers coordinated with contactors for starting and control.
Applicable standards
Section titled “Applicable standards”- IEC 60898 (MCB): Circuit-breakers for household and similar installations.
- IEC 60947-2 (MCCB, ACB): Low-voltage switchgear and controlgear – Circuit-breakers.
- NEMA AB 1: Molded Case Circuit Breakers (American market).
- IEC 60529: Degrees of protection provided by enclosures (IP Code).
Frequently Asked Questions (FAQ)
Section titled “Frequently Asked Questions (FAQ)”What is the maximum current of an MCB type thermal-magnetic circuit breaker?
Section titled “What is the maximum current of an MCB type thermal-magnetic circuit breaker?”The maximum rated current of an MCB is 125 A / 125 A, according to IEC 60898 standard.
What is the breaking capacity of a typical residential miniature circuit breaker?
Section titled “What is the breaking capacity of a typical residential miniature circuit breaker?”Domestic MCBs offer a breaking capacity between 6 kA and 10 kA / 6 kA and 10 kA, sufficient for installations with moderate short-circuit currents.
In what time does the thermal trip act for a 45 % overload?
Section titled “In what time does the thermal trip act for a 45 % overload?”The thermal trip operates in a range from 0,1 s to several minutes; with an overload of 1,45 times the rated current, the opening time can reach up to one hour.
What is the rated ambient operating temperature of an MCB?
Section titled “What is the rated ambient operating temperature of an MCB?”Most MCBs maintain their calibration between -5 °C and 40 °C (23 °F and 104 °F). Outside this range, correction factors apply.
How many electrical operating cycles does a 2500 A MCCB withstand?
Section titled “How many electrical operating cycles does a 2500 A MCCB withstand?”A high-current MCCB typically reaches 10 000 operations under rated load, according to electrical life tests documented by manufacturers.
What is the difference between a 6 kA and a 10 kA circuit breaker?
Section titled “What is the difference between a 6 kA and a 10 kA circuit breaker?”The 10 kA model can interrupt short-circuit currents up to 10 000 A / 10 000 A, while the 6 kA model is limited to 6 000 A / 6 000 A, making it suitable for networks with a lower fault level.
References
Section titled “References”- engineeringtoolbox.com: https://www.engineeringtoolbox.com/nema-iec-enclosure-standards-d_920.html
- electrical4u.com: https://www.electrical4u.com/miniature-circuit-breaker-or-mcb/