What is screw on the circuit breaker

What are miniature circuit breakers (miniature circuit breakers)?

1. Basics of miniature circuit breakers (LS switches)


Miniature circuit breakers, also known as miniature circuit breakers for short, are automatic switches that protect cables and lines from the consequences of short circuits and overload currents. In 1924 they were invented by Hugo Stotz's company in Mannheim.

LS switches have two different triggers:

  • a time-delayed thermal release for overload protection
  • an electromagnetic release for short-circuit protection

Figure 1: Miniature circuit breaker type B16 A

Particularly important properties of a circuit breaker are:

Characteristic: B, C, D and E, ...

Rated current I.N: denotes the current that the fuse can hold permanently.

Short-circuit switching capacity: describes the level of the short-circuit current that can still be switched off.

2. Norms

It is recommended to use LS switches according to EN 60898-1 (VDE 0641-11) for:

  • Electrical installation material, circuit breakers for house installations and similar purposes
  • Part 1: Circuit breaker for alternating current (AC)

In principle, the following restrictions must be observed for LS switches:

  • Miniature circuit breakers must not be used to protect motors. This use case rarely occurs in private households.
  • LS switches are suitable for separating circuits; operational switching must not be carried out using LS switches.
  • Use of LS-switches at a temperature range of -5 degrees C to +40 degrees C, whereby an average value of +35 degrees C must not be exceeded within 24 hours.

3. Advantages of an LS switch

  • Can be switched on again: after the cause of the overload has been eliminated, the LS circuit can be switched on again.
  • LS switches have a more compact design: they require less space in the circuit distributor.
  • Other important advantages include multi-pole disconnection - when disconnecting the LS switch, for example for three-phase current (3-pole), it is safe to switch off the remaining phases by mechanically connecting the levers. In addition, circuit breakers do not have any age-related changes in their tripping behavior.

4. Design of a line circuit breaker

Internal structure: A miniature circuit breaker has a thermal and an electromagnetic release that are in series.

Figure 2: Internal structure of an LS circuit

Nominal values:

Figure 3: Nominal values ​​of a circuit breaker

5. Types of tripping of an LS switch (miniature circuit breaker)

The disconnection mechanism of an SL switch takes place in four trigger types:

  • Thermal tripping in the event of overload

The voltage is disconnected by a bimetal in the circuit breaker.

If the rated current is exceeded, the bimetal is heated. This bends the toggle switch to the lower position and also interrupts the circuit.

  • Electromagnetic release in the event of a short circuit

Electromagnetic releases are quick releases. If too high a current flows through the coil, the built-in electromagnet, attached to the toggle switch, pulls into the inside of the coil and therefore tilts the switch to the lower position, thereby interrupting the electrical circuit.

A toggle switch or a release button on the front of the LS switch offers the option of manual shutdown.

  • Triggered by additional modules

An LS switch can be switched again with the aid of additional modules such as attachable undervoltage and shunt releases, FI switches, arc fault protection devices.

A special feature of miniature circuit breakers is that they cannot be influenced Trip-free, this enables the switching contacts to go “off” in the event of an overload or short circuit even when the handle is held “on”, regardless of the handle position.

6. Tripping characteristics and characteristics of a line circuit breaker

As can be seen in Figure 4, LS switches are differentiated according to time-current characteristics. These are manufactured for rated currents from 0.2 A to 100 A. The small values ​​on the tripping time axis indicate the behavior of the delayed bimetal trip, the large values ​​of the instantaneous electromagnetic trip.

Figure 4: Time-current diagram for circuit breakers

The current standardized types of circuit breakers are summarized in Figure 5 according to the types of tripping and application area. LS switches are also built according to the corresponding manufacturing steps for the following nominal reference temperatures.

B, C, D characteristics 30 ° C

K, Z characteristic 20 ° C

LS switches are manufactured in various designs and rated currents. They are commercially available in 1 to 4 poles and have a switching capacity of 6000 A to 35000 A. The production of the new generation can be described as follows:

  • 1-pole miniature circuit breakers are 1 module (TE), i.e. from 17.5 ... 18 mm wide
  • 2-pole miniature circuit-breakers are 2 TE, 1.5 TE or 1 TE wide
  • 3-pole and 4-pole miniature circuit breakers are correspondingly wider

The 4-pole versions are also used as all-pole switches in three-phase circuits.


Figure 5: Tripping characteristics

Figure 6 below shows an example of different types of B characteristic.




253 V ~

72 V –...



440 V ~

125 V -



440 V ~




440 V ~

125 V -


Rated current IN A


40,50,63, 80, 100


40,50,63, 80, 100


40,50,80, 100


40,50,63, 80, 1

Short name

S 201-B 6

S 201-B 63

S 201-B 6

S 201-B 63

S 201-B 6

S 201-B 63

S 201-B 6

S 201-B 63

Figure 6: Examples

7. Assembly and disassembly of a line circuit breaker (LS switch)

The assembly process of a circuit breaker is simple and quick.

There are two connection options:

You can connect two conductors with the same cross-section or one conductor with a cross-section of up to 35 mm2 to one terminal. The clamps have a special property. They are equipped with sliding plastic covers to increase protection against dangerous contact.

Then you connect a busbar to the lower side of the LS switch as shown in Fig. 6. Finally, the snap fasteners are mounted on the top-hat rail.

Figure 8: Connection of two conductors to one terminal

Figure 9: Connection of the power rail

An LS switch can be easily dismantled by hand without tools.

Figure 10: Disassembly of an LS switch

Our line circuit breakers from OEZ are VDE certified!