Circuit breaker

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2-pole circuit breaker

A circuit breaker short miniature circuit breaker or English Miniature Circuit Breaker (MCB), also colloquially breaker or shortly machine or fuse called, is an overcurrent protection device in the electrical installation . Miniature circuit breakers are used in low-voltage networks to protect cables from damage caused by heating up as a result of excessive currents . The circuit breaker is a reusable, not automatically resetting fuse element. It was invented in 1924 in Hugo Stotz's company in Mannheim .

A combination of a miniature circuit breaker (MCB) with a residual-current device (RCCB) is used as RCBO ( English R esidual current operated C ircuit- B reaker with O vercurrent protection ), respectively.


 Circuit breakers can - just like a fuse or a circuit breaker - automatically switch off a circuit in the event of an overload or short circuit.

For Germany, the following applies for new installations (according to TAB in conjunction with DIN 18015-1):

  • In the power distribution of apartments only circuit breakers may be used for lighting and socket circuits. Fuses are only permitted for permanently connected devices (e.g. instantaneous water heaters) or as back-up fuses for sub-distributors.
  • Selective circuit breakers (SLS) are used to protect the pre-counter area . NH fuses are only permitted in this area of ​​application if another "unmanageable activation option for the customer system", e.g. B. as a post- meter protection with a Neozed switch disconnector is given.


Opened circuit breaker
  1. Shift lever for manual switching on and off. Also includes the visual display of the switching status
  2. Trip-free
  3. Switch contact
  4. Terminals
  5. Bimetal strips for thermal overload release
  6. Calibration screw, which is used by the manufacturer to determine the thermal shutdown behavior (part of the characteristic).
  7. Coil for electromagnetic tripping for high currents, typically short-circuit currents
  8. Quenching chamber for quenching the arc when a short-circuit current is disconnected. Due to thermal effects, the arc moves from the opening switch contact (3) to the area of ​​the quenching chamber, where it is extinguished due to the division and cooling
Single -pole
circuit symbol

Shutdown mechanism

The shutdown mechanism can be triggered in four ways:

Trip on overload
If the specified nominal value of the current flowing through the miniature circuit breaker is significantly exceeded for a long time, the system is switched off. The time until tripping depends on the strength of the overcurrent; with a high overcurrent it is shorter than when the nominal current is exceeded slightly. A bimetal is used for tripping , which bends when heated by the flowing current and triggers the shutdown mechanism (thermal tripping).
Electromagnetic release in the event of a short circuit
If a short circuit occurs in a system , it is switched off within a few milliseconds by an electromagnet through which current flows .
Manual release
For maintenance work or for temporary shutdown, circuits can be switched off manually at the line circuit breaker. There is also a toggle switch or a release button on the front.
Triggered by additional modules
For most circuit breakers renowned manufacturers there is next auxiliary switches also attachable undervoltage and shunt release, residual-current device (English Residual Current Device , shortly RCD ), arc-fault protection devices (English Arc Fault Detection Device , shortly AFDD ) and motor drives (reclosing devices) with the help of which the circuit breaker can be switched. The additional modules are plugged in on the right or left, depending on the circuit breaker, or wired accordingly in the distribution.


An important feature of miniature circuit breakers is that they cannot be influenced by trip- free release . It ensures that in the event of a short circuit, an immediate trip occurs even if the switch lever is operated or held in the on position.

Turn back on

After an overload release, the bimetal strip must have cooled down before it can be switched on again. The manual switching action required for switching on again draws the user's attention to a possible error and prevents automatic switching on again ( fail-safe ). This means that the uncontrolled restart of overloaded systems or the uncontrolled restart of defective devices / installations is excluded.

Tripping characteristic

A distinction is made between miniature circuit breakers in addition to rated current and design according to the tripping characteristics.

Usage and remarks Tripping current
(multiple of the nominal current)
Overload release (thermal) Short-circuit
release (magnetic)
AC 50 Hz DC
A. Siemens (not standardized); Semiconductor protection; with high network impedance ; similar to Z 1.13–1.45
[30 ° C, 1 hour]
(over 63 A: 2 hours)
2-3 × 1.5
B. Standard line protection 3-5
C. for higher inrush currents (machines, lamp groups), standard line protection in Italy 5-10
D. highly inductive or capacitive load: transformers , electromagnets , capacitors , switched-mode power supplies 10-20
E. " E xakt", SLS main line circuit breaker 1.05–1.2
[30 ° C, 2 hours]
Z Semiconductor protection; with high network impedance

Circuit breaker according to EN 60947-2 (VDE 0660-101)

[20 ° C, 2 hours]
[30 ° C, 1 hour]

2-3 × 1.5
R. Moeller ; " R apid", out of date; identical to Z
K " K raft", for high inrush sensitive overload trip 8-14
S. Moeller (not standardized); " S expensive transformers"; similar to D 13-17
H H aushalt”, until around 1977; with high network impedance; similar to A or Z ; Replacement type in the household: B. 1.5–2.1 (up to 4 A)
1.5–1.9   (6–10 A)
1.4–1.75 (12–25 A)
1.3–1.6 (over 25 A)
[25 ° C, 1 hour]
2-3 3-5
L. " L eitungsschutz" (originally " L maybe") until 1990; Replacement type: B ;
still standardized as an automatic screwdriver
approx. 3.5-5 Max. 8th
U U niversal”, until approx. 1993 (e.g. ABB , Moeller , Schrack ); often in Austria, forerunner: HG ; Replacement type: C 5.5-12
U second variant (less often, e.g. AEG ): overload release similar to G 1.05-1.35 [1 hour] 6-10 × 1.5
G " G eräteschutz" (international " g eneral") obsolete; Replacement type: C
Time-current characteristic with tripping times

The two values ​​for overload tripping denote the non-tripping current (small test current) and the tripping current (large test current). The maximum tripping time applies to the tripping current.
Some manufacturers specify closer tolerances for the tripping currents in the event of overload and short circuit.

The currently standardized types B, C, D, E, K and Z are highlighted in the table.

As a rule, circuit breakers with characteristic B are used in living rooms or offices. The C characteristic is used for lines to loads with a high inrush current (line and device protection).

Miniature circuit breakers with characteristic B are available as standard for the following rated currents:
6 | 10 | 13 | 16 | 20 | 25 | 32 | 40 | 50 | 63 | 80 | 100 | 125 amps.
Other values ​​are also available, depending on the manufacturer. Type C and D as well as type K and Z miniature circuit breakers are available in a large variety of types with values ​​well below 1 A.

The H characteristic has been used for household electrical circuits since the 1950s in order to reliably achieve rapid tripping in the event of a short circuit in networks with high impedance or protective grounding.
With today's network conditions, the sensitive short-circuit triggering can respond undesirably. Affected are z. B. Consumers with a switching power supply (computer, television) or motors (vacuum cleaner). In such cases, it is advisable to replace it with a B line circuit breaker. H10A can usually be replaced by B13 (same overload characteristics).

Particular attention must be paid to the selection of miniature circuit breakers to protect electronic loads (electronic ballasts, switched-mode power supplies), as their high inrush currents must be observed.

Tripping in the event of a short circuit

Miniature circuit breakers must be able to switch off high short-circuit currents. The breaking capacity, referred to as the rated short-circuit breaking capacity, is graded as follows:

Switching capacity
(230/400 V AC 50 Hz)
03000 A Not permitted in Germany and Austria
04500 A Standard in Italy for single-phase consumers
06000 A Minimum value in Germany (according to TAB ) and Austria.
Standard for residential and office buildings, small businesses
10,000 A Standard in industrial plants
15000 A high industrial standard, for special cases
25000 A High performance machines and selective circuit breakers

switching capacity,
energy limitation

There are also requirements for short-circuit current limitation. According to TAB, energy limitation class 3 (selectivity class 3, "high requirements") applies to circuit breakers up to 32 A in Germany .
In the event of a short circuit, the current ("prospective short-circuit current") determined only by the network impedance (internal resistance) is very high. The circuit breaker limits the short-circuit current to a lower value due to its design. A high energy limit results in a high degree of selectivity for upstream fuses and protects the system from electromagnetic effects.


Transparent miniature circuit breaker for training purposes. Contact against a gray background.
Stotz automatic socket machine, 1950s (characteristic K)

Miniature circuit breakers have a plastic housing . Older designs were cylindrical and were inserted into the Edison screw thread instead of the screw locks that were customary up to that time , or screwed onto a thin metal rail. Modern miniature circuit breakers have rectangular housings and can be mounted close together on a mounting rail (top hat rail).

Single-pole miniature circuit breakers are mostly 1 module wide today  . Two-pole versions are manufactured with 2 HP, 1.5 HP or 1 HP width. Three- and four-pole machines are correspondingly wider. There are also miniature circuit breakers with a width of 1.5 TE per pole. Mostly these are those for nominal currents of 80 A - 125 A and / or with a very high breaking capacity. The selective circuit breakers are 1.5 HP wide, older types 2 HP. They are mounted on a busbar (40 mm center-to-center distance). Alternatively, the selective miniature circuit breakers are also mounted on normal DIN rails, but they do not fit in conventional small distribution boards.

If the line circuit breaker also switches the neutral conductor , the contact for the neutral conductor must open with a lag and close with a lead. This ensures that the phase is never switched through without a neutral conductor.

Norms and standards

  • DIN VDE 0100-430: Setting up low-voltage systems - Part 4-43: Protective measures - Protection against overcurrent
  • DIN EN 60898-1 (VDE 0641-11): Electrical installation material - Miniature circuit breakers for house installations and similar purposes, Part 1: Miniature circuit breakers for alternating current (AC)

See also

Specialist literature

  • Günter Springer: Expertise in electrical engineering . 18th edition, Verlag - Europa - Lehrmittel, Wuppertal 1989, ISBN 3-8085-3018-9
  • Theodor Schmelcher: Handbook of Low Voltage, project planning information for switchgear, switchgear and distribution boards . 1st edition, Siemens Aktiengesellschaft (Abt. Verlag), Berlin and Munich 1982, ISBN 3-8009-1358-5
  • Ernst Hörnemann, Heinrich Hübscher: Electrical engineering specialist training in industrial electronics . 1st edition. Westermann Schulbuchverlag GmbH, Braunschweig 1998, ISBN 3-14-221730-4
  • Alfred Hösl, Roland Ayx, Hans Werner Busch: The electrical installation in accordance with regulations, residential construction, commercial industry . 18th edition, Hüthig-Verlag, Heidelberg 2003, ISBN 3-7785-2909-9
  • Data sheet miniature circuit breaker from ABB (for tripping characteristics): tripping characteristics for automatic circuit breakers in comparison (PDF; 263 kB)
  • Technical appendix Schupa tripping characteristics miniature circuit breakers (PDF; 316 kB)
  • Data sheet SLS from Möller (for tripping characteristics of SLS) Selective main line circuit breaker LSHU-KL (PDF; 1.9 MB)

Web links

Commons : Circuit Breakers  - Collection of images, videos, and audio files