MCB
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Circuit breaker switch - fuse elements with clever technology
Whether in operation, in the office or at home: A circuit breaker has certainly been seen or operated by everyone. Because if the current has failed due to a short circuit or overload, one inevitably searches for the fuse elements. We show you which clever technology is in a circuit breaker and what was otherwise necessary to observe.
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What was a miniature circuit breaker?
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Why is a miniature circuit breaker required?
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How is a miniature circuit breaker structured?
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How does a miniature circuit breaker work?
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Which different circuit breakers are available?
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What was a selective circuit breaker?
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What was the trigger characteristic for an miniature circuit breaker?
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What was the difference between the circuit breaker and the RCD?
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What was to be observed when replacing a circuit breaker?
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Frequently asked questions about miniature circuit breakers
What was a miniature circuit breaker?
As the name implies, an miniature circuit breaker or a miniature circuit breaker is a safety device that reliably protects downstream installation equipment, devices and electrical systems against overload.
In colloquial terms, miniature circuit breakers are also referred to as circuit breakers or only as automatic circuit breakers.
Before the introduction of automatic fuses, fuses took over this task. In case of overload due to excessive current, the wire in the fuse burned through and the fuse had to be replaced. This provided some protection, but it also had disadvantages. If no new fuse was available, defective copies were sometimes patched in quite adventurous ways.
In contrast to the fuses, a circuit breaker can be activated again and again by hand if the cause of the tripping has been eliminated.
In addition, the miniature circuit breakers are also approved as operational switches in accordance with VDE regulations.
Single-pole miniature circuit breaker for DIN rail mounting.
Why is a miniature circuit breaker required?
Thanks to the voltage of 230 V, the wires are sufficient with 1.5 mm².
In order to keep the costs of the "electrical wiring" of a house affordable, it was decided long ago to set the "operating voltage" for the house installation to 230 V AC voltage. This makes it possible to use cables with a cross-section of only 1.5 - 2.5 mm 2.
If one assumes a maximum permissible current of 16 A for a cable with 1.5 mm2, power ratings of up to 3680 W (16 A x 230 V) can be transferred. This is more than adequate for most applications or consumers.
However, the high voltage also has disadvantages which must be minimized by appropriate measures:
Touch protection:
It must always be ensured that direct contact with live conductors and parts is avoided.
Overload protection:
The installation equipment operated on the mains voltage must be reliably protected against overload. A small example illustrates this indispensable necessity:
If the power line between a socket and the fuse box is approximately 15 m long, the line resistance for the phase and the neutral wire (return and return) is approximately 0.36Ω . The calculated value refers to a copper cable with a cable cross-section of 1.5 mm 2 .
If a massive short circuitΩ with assumed 0 is now created at the socket, the Ohm's law would lead to a staggering 638.89 A instead of the max. permissible 16 A (I = U : R, i.e. 230 V : 0.36Ω ) flow over the two connection cables of the socket .
If the power is calculated according to the formula P = U x I, the 146 944.44 watts or about 147 kW, which are converted into heat. Without a fuse that responds in this case and switches off the voltage in time, the wires in the wall would glow in seconds .
How is a miniature circuit breaker structured?
A circuit breaker is provided for mounting on top hat rails or DIN rails. For this purpose, it has the necessary locking lugs (9).
The connection cables are inserted via the cable openings (3) and fastened with the clamping screws (2).
When the shift lever (1) is moved to the "ON" position, the shift contact (6) is closed.
The LS switch is activated when either the electromagnet (7) or the bimetal strip (4) activates the trigger mechanism (8). The spark extinguishing chamber (5) protects the switch contact from fire.
Note:
The gear lever is not mechanically locked to the trigger mechanism. This means that even if the shift lever is held in the "ON" position by hand, the triggering mechanism can still open the switch contact .
The basic design is the same for all automats.
How does a miniature circuit breaker work?
The current flow through the machine is marked with a red line.
The mechanical design varies from manufacturer to manufacturer, but the function is always the same.
To understand the function correctly, you need to know how the current flows through the circuit breaker:
The left-hand terminal (10) is connected to the moving part of the switch contact (6) via a flexible cable (11).
The fixed part of the switching contact is in contact with the left connection of the electromagnet (7). The right-hand connection of the electromagnet coil is firmly connected to the lower end of the bimetal strip (4).
At the upper moving part of the bimetal strip, a flexible cable (12) leads to the right terminal clamp (13).
Thermal tripping in the event of overload
If more and more consumers are being operated on a socket or circuit, the current may slowly but steadily exceed the limit value of the circuit breaker . This can happen very quickly if extension cables and multiple sockets are connected in series without thought.
In this case, the thermal release via the bimetal strip (4) is triggered. The bimetal stiffener consists of two metals with different expansion coefficients.
This means that the higher the current that flows over the strip, the more the strip bends in a certain direction.
Since the stiff is firmly anchored in the lower area, the upper part of the strip bends to the right . As a result of a certain deformation, the strip actuates a locking lever (14) and thus triggers the locking mechanism (8) of the switching contact. The circuit is interrupted.
In order to reactivate the circuit breaker, you have to wait until the bimetal strip has cooled down again.
Magnetic triggering in case of short circuit
In the event of a massive short circuit, the triggering by the bimetal strip would be much too slow . Therefore, an electromagnet (7) is installed in a circuit breaker.
This is rated so that currents up to 16 A are not sufficient to activate the electromagnet. Only by a high short-circuit current , the magnetic field of the coil is so strong that the coil core (15) is tightened. A rocker (16), which is directly against a pin (17) of the latch lever (14), is used to actuate the latch lever and the locking mechanism (8) is triggered. The quick opening of the switch contact is supported by a plastic part (18). As soon as the coil core is tightened, the plastic part presses against the movable part of the switching contact and presses it.
Since an arc is very often present at the switch contact when switching high residual currents, a fire chamber (5) is mounted in parallel to this. This chamber consists of several metal plates which absorb the arc, cool down and clear at the next zero crossing . This effectively protects the switch contact from fire.
Once the cause of the fault has been eliminated, the LS switch can be activated again immediately.
Which different circuit breakers are available?
LS-switches are used in fuse boxes and sub-distributions and are intended for mounting on top hat or DIN rails. Depending on the power requirement of the downstream installation or system, there are LS switches with a wide range of nominal currents . The values can be less than 1 A but also up to 100 A. Circuit breakers with a nominal current of 10 - 16 A are usually used for domestic installations. The switching capacity must be at least 6000 A. betragen.
The different trigger characteristics are described in more detail below.
Another distinguishing feature is the number of switchable lines . Most circuit breakers are 1polig designed and interrupt the current-carrying line (phase) in the respective circuit. For the electric stove or for large electric motors, which are operated at three phases, there are 3polige LS-switches . With these switches, all three current-carrying lines are always disconnected, even if there is a fault only on one line.
In some cases, 2polige circuit breakers are also offered, which switch off the neutral conductor in addition to the phase. Some of these miniature circuit breakers are also available as combination switches, which also act as residual current circuit breakers (FI).
Circuit breaker switch in 1poliger and 3poliger design.
What was a selective circuit breaker?
In the event of a fault, the 16 A fuse triggers before the 63 A fuse.
Selectivity refers to the fact that in the event of a fault, only the overcurrent protection device that is located directly in front of the fault source triggers.
This means that only the faulty circuit is disconnected from the power supply . . Intact circuits remain connected to the supply grid.
In practice, this is achieved by the fact that selective miniature circuit breakers have delayed tripping. Since the downstream miniature circuit breaker activates immediately , a selective switch-off of the voltage can only be achieved in the faulty circuit.
What was the trigger characteristic for an miniature circuit breaker?
LS switches are available for different current strengths and trip characteristics . The current intensity depends on the cross-section of the following power lines or the current consumption of the electrical system .
The behavior of a circuit breaker is described under tripping characteristics . Early and rapid activation , which is desirable in the area of building technology, can cause problems very quickly in workshops or machine shops. Because large electric motors have a very high current requirement at the moment of switch-on, which is many times higher than the current consumption in continuous operation. Of course, it would be a hindrance if the circuit breaker were to be triggered each time the machine was switched on.
The X axis describes the multiple of the rated current.
In office and residential buildings, LS switches with B characteristics are used as standard. LS switches with C characteristics are used for consumers with hohem starting current such as machine tools, fan motors or lamp groups. For consumers with very high capacitive or inductive loads such as capacitors, switching power supplies, transformers or electromagnets, miniature circuit breakers with D characteristics are used.
The attached diagram shows the characteristics of the different trigger characteristics in more detail. The upper range (1) corresponds to the thermal release and the lower range (2) shows the magnetic release .
Other characteristics are:
"E" for "exact" selective circuit breaker
"Z" for semiconductor protection and "
K" for "force" with hohem inrush current and sensitive overload tripping
Overview of trip currents
| MCB | Overload current in case of thermal tripping* | Overload current with magnetic tripping* |
|---|---|---|
| B trip rating | 1.13 – 1.45 | 3 - 5 |
| C trip rating | 1.13 – 1.45 | 5 - 10 |
| D characteristic | 1.13 – 1.45 | 10 - 20 |
| E-characteristic | 1.05 - 1.20 | 5 - 6.25 |
| Z-characteristic | 1.05 - 1.20 | 2 - 3 |
| K characteristic | 1.05 - 1.20 | 8 - 14 |
* (multiple of nominal current)
What was the difference between the circuit breaker and the RCD?
A circuit breaker only monitors the current on the line and disconnects the circuit if the limits are exceeded. A 1poliger or 3poliger LS switch cannot detect whether the current flowing through the phase also flows back over the neutral conductor or flows out to earth (PE protective conductor) in the event of a fault.
An earth leakage circuit breaker (FI) monitors the circuit. The current flowing through the phase must also flow back via the neutral conductor .
If a live line is violated with a drill in the event of a fault, a more or less high current flows over the drill, the drill housing and the earth conductor of the drill in the direction of earth .
In this case, the FI detects that more current is flowing into the circuit than is coming back via the neutral wire . If the limit value specified in the residual current switch is exceeded, the faulty circuit is immediately disconnected from the mains voltage.
In contrast to the LS switch (li), the FI has an N connection.
What was to be observed when replacing a circuit breaker?
Anyone working in a control cabinet must know what was to be observed.
The replacement of a defective circuit breaker is a massive interference in the electrical installation and should only be carried out by specialists who have the necessary expertise. In addition, the relevant safety rules, such as:
- Switch off
- secure against switching on again
- determine voltage-free
- ground and short-circuit
- cover adjacent and live parts or take into
account barriers.
If you do not have the necessary specialist knowledge, you should in any case ask a specialist or ask for help.
Important!
A circuit breaker is always labeled, from which the nominal current and the tripping characteristic e.g. 16 B can be read. As a result, an LS switch of category 16 B must be used again as a replacement.
Frequently asked questions about miniature circuit breakers
Why does the LS switch drop off and on when ich my computer system is switched on?
Often the PC, two monitors, the printer, external hard disks, the sound system and other components are connected to a switchable socket. This has the great advantage that all devices can be put into operation with a switch click. However, the current can be so high at the moment of switch-on, so that the LS switch interprets and triggers the as an error.
What was to be done if the LS switch was triggered?
Experience has shown that an LS switch always triggers when a faulty consumer is connected to a socket or a defective device has been switched on. In this case, the last connected device must be disconnected from the mains socket or switched off again. Now the LS switch can be switched on again. If the LS switch is activated again when a device is put into operation again, you know the cause and can start with more detailed troubleshooting.
If the LS switch seems to have tripped without reason, you have to check which consumers are no longer supplied with power. Gradually you can disconnect them from the mains or switch them off and then check whether the LS switch is held or triggered again.
How can I check a LS switch?
Whether an LS switch functions correctly and reliably triggers even when the limit values are reached can only be checked with special measuring devices. Only the mechanical function can be tested without much effort. In the absence of voltage, a LS switch must always be switched on. If required, the contact resistance between the two terminals can still be measured.