Switch (electrical engineering)
Switches are assemblies that use two electrically conductive materials or a semiconductor component to establish or separate an electrically conductive connection ( switching contact ). Ideally, a switch works according to the all-or-nothing principle , that is, an actuation always clearly leads to an open or closed switching state .
In the open switching state , the "electronic separation" caused by electronic switches is differentiated from a mechatronic " galvanic separation ", which represents an isolating distance and can therefore offer protection against dangerous electrical voltage if it is adequately dimensioned .
Switches, also called switching devices, can be differentiated according to numerous characteristics. For example, according to the type of actuation, type and constructional features or usage features. The most important for users are the electrical parameters (ratings), which describe the suitability of a switch for certain voltage and current ranges as well as ambient conditions. The suitability has to in all operating states of the switch to be fulfilled: the contact is, the current lead, the contact separation and safe isolation in the open state.
If one subdivides according to the behavior of a switch after it has been actuated, on the one hand there are switches which remain stable in their switching state after actuation. These are e.g. B. bistable toggle switches, rocker switches, step switches, latching switches, multiple or selector switches. Pushbuttons and buttons, on the other hand, return to their rest position or starting position when the actuation is removed .
A distinction is made, for example, according to the type of application:
- Signal switches which are mostly used in device controls in safety extra-low voltage circuits;
- Appliance switches, such as those used as power switches in household appliances that are normally operated from sockets;
- Power tool switches used in predominantly motor-operated electric tools and machines;
- Installation switching devices that are permanently installed in buildings and are not de-energized by pulling the mains plug, for example;
- Circuit breaker ( miniature circuit breaker ).
Delimitation of the term switch
With a switch, the switching on and off of a current is in the foreground, directly or indirectly as a result of an actuation.
If the switching status is not changed by manual or mechanical actuation, but by a control signal, e.g. B. a current surge to a coil , one speaks of a relay or contactor , but also of a surge switch . A field effect transistor (FET) or a bipolar transistor is also a semiconductor switch activated by a control signal and therefore not a switch in the mechanical sense described here.
When closed, the switch is differentiated as opposed to regulation and control units : switches can contain rules and controls that provide the all-or-nothing behavior with targeted intermediates for. B. the speed control in motorized devices. The delimitation of the switch from an actuator or regulator is often fluid. If the switching function is classified as determining, the switch definition takes effect, otherwise that of a regulation and control unit.
The terminology switch is also coined differently in the different languages: If etymologically in German with switch and in English with switch it is based more on the mechanical pushing or flipping of an actuating element, the Romance space with the French interrupteur or the Italian interruttore represents the separating element in the Foreground, in Danish, on the other hand, contact is what connects them.
Essential elements of a switch
General switch design
Probably the most important element of a switch are the contacts . They consist of chemically noble metals, mostly silver. Other chemical elements are alloyed or mixed with this in order to increase the contact life. The contact surfaces of signal switches are often coated with gold in order to make the surfaces chemically corrosion-resistant when the electrical conductivity is high .
Silver has the disadvantage of forming sulfides with the sulfur in the air . These salts are chemically resistant and have very poor electrical conductivity. Either mechanically by abrasion or by arc erosion, the contact surfaces must be cleaned to a sufficiently low contact resistance ensured.
The contact carrier elements are often made of non-ferrous metals or spring steels in order to combine good electrical conductivity with mechanical strength and elasticity , depending on the application . The connections are also made from these materials and are made in many different ways: screw connections, plug connections for prepared and unprepared cable ends, cable lugs , flat plug elements and much more are common.
As housing materials are insulators necessary, mostly plastics which are sufficiently thermally stable and heat and fire resistant.
Mechanically operated switches can either be operated manually , in more general terms by humans, or by devices . The main reason for differentiating between these is that humans can act as a control circuit: If a person switches a switching device and the desired effect does not occur, he will take appropriate measures depending on the situation. In the case of mechanized switching (e.g. switching off a roller shutter in its end position), structural elements should ensure that there is no danger in the event of a fault.
The switch can be operated either directly or indirectly . The standard for device switches DIN / EN 61058-1 states that the switching process can take place via an actuating part or by operating a sensor, whereby the actuating part or sensor can be arranged separately from the switch. Optical, acoustic or thermal signal paths are given for the transmission between the actuation part and the switching element.
With the mechanically directly operated switches, a distinction is made according to the operating element:
Mechanically via devices or indirectly operated switches :
Switches operated by sensors :
Upon actuation provide power switch or normally open contact connections forth while off or break contact connections separate. Change-over switch (change-over switch, changeover) and rotary switch connecting a contact with one of two or more other contacts. Here are switches that make the new connection before the old one is separated when bridging (also shorting, English. Make before break ,) and those that separate the old connection first, as a non-bridging ( not shorting, English. Break before make ).
Switches can be further distinguished:
After the switching function
- the already mentioned NC contacts, NO contacts, changeover contacts (bridging or not bridging)
- Series switch (for switching two circuits with one knob)
- Cross switch (for switching a circuit using more than two switches)
- Group switch (blind switch, for alternating switching of two circuits with three switch positions - the blind that gives it its name can only be raised or lowered, but not both at the same time)
description symbol IEC 60617 SPST "Single pole, single throw" A single pole on / off switch: The connection between the two connections can be closed or open. Example light switch . SPDT "Single pole, double throw" A single-pole changeover switch (on-on): Connection C is optionally connected to L1 or L2. The middle position is not mechanically stable. SPCO
"Single pole, center off" or
"Single Pole, Triple Throw"
(On-off-on). These switches differ from the SPDT in that they have a mechanically stable middle position in which no connection is connected. DPST "Double pole, single throw" Two-pole on-switch. A lever operates two on-switches at the same time DPDT "Double pole, double throw" Two-pole changeover switch. A lever operates two changeover switches at the same time. DPCO "Double pole, center off" Two-pole changeover switch with stable middle position.
- For switches that switch three poles mechanically coupled, the first letter of the designation is replaced by a "T" for triple . For switches with four poles, it is a "Q" for quadruple. An alternative designation replaces the first letter with the number of the mechanically coupled poles (e.g. 3PDT).
When a switch is operated, there is an expectation of a status change from "closed" to "open" or vice versa. Because of arc effects in particular when opening and closing, a defined sequence of movements - independent of that of actuation - is desirable for the function and service life of a switch. At a certain point, the so-called pressure point , the switching process should be irreversibly initiated. This can be implemented structurally by all variants of precision mechanical snap elements. The thus constructed switch design is in accordance with snap-action switch , or snap switch called.
In the case of switches for DC circuits in particular, opening the circuit as briefly as possible is important. When the contacts are separated, there is a risk of a standing arc above the arc limit voltage and a minimum current, which will quickly thermally overload the switch if it is burning for a long time. To prevent this, the snap-on systems automatically complete the actuation that has started from the pressure point. In AC circuits, a possibly existing arc usually extinguishes itself at the next current zero crossing.
Switches for alternating current over 25 A or for direct current have horn-shaped contacts and arcing chambers which serve to lengthen the arc, to cool it and thus to extinguish it. Especially with medium and high voltage switches that z. B. work under oil or vacuum, special constructive measures are taken to control switching arcs . Such switches often have a spring-loaded mechanism to quickly separate and close the contacts.
In contrast to, for example, toggle, rocker or latching switches, which are actuated with every switch change, the change from the working state to the idle state must take place automatically with pushbutton switches and especially with snap switches. The spring energy stored in the system is used for this. Depending on the design of a switch as closed in the rest position (NC contact, n. C. (Normally closed)) or open (NO contact, n. O. (Normally open)), these can be designed as positively opening from the rest position. In this way, any switch-on welds that may have occurred on microswitches or snap switches for high inrush currents can be broken.
A switch lock is a mechanical safeguard that prevents a switch from being switched on or off unplanned. It can be attached in the form of a main switch / circuit breaker interlock or a padlock.
Designations according to the purpose
Another differentiation arises from the purpose of the switch in a circuit. Main applications are: main switches , emergency stop switches , repair switches , circuit breakers , selective circuit breakers , light switches , dead man's switches , load switches , circuit breakers , etc.
Disconnectors create a large insulating air gap between the open contacts, but can hardly switch any current. They are mainly used in switchgears for voltages above 1000 V and serve to separate system parts for maintenance work.
Load switches for switching in the power grid can be operated mechanically (such as light switches ) or electronically ( semiconductor relays , SSR (solid state relay) ). They are widely used as power switches on (household) appliances and may only be used up to their specified rated current.
Switch disconnectors (see high-voltage switches) combine the properties of load switches and disconnectors.
Circuit breakers can switch off short-circuit currents (see high-voltage switches ).
Control switch can no large load currents, but only small control currents switch with which, for example, contactors are controlled. Examples are key switches or industrial buttons. Typical switching voltages are 48 V direct or alternating voltage or 230 V alternating voltage.
Signal switches usually have contacts made of precious metals and particularly low contact resistance; they are used specifically to switch small voltages and currents.
Analog switch is a designation for digitally-controlled semiconductor switches for switching of analog signals (z. B. Audio signals are suitable). They mostly work with MOSFETs and areinstalledas integrated circuits . As electrically controlled components, they are not switches in the strictest sense, but are considered electronic switches .
The utilization category for low-voltage switchgear defines the characteristic operating conditions for switching devices. These devices are dimensioned for different electrical consumers and for different operating conditions.
The properties of the load to be switched or controlled determines the requirements for the switching devices and their correct selection for the planned application. In particular, the stress on the switching paths caused by current and voltage when switching on and off are of enormous importance.
See Article Usage Category for detailed information on the subject.
Forcibly guided contacts
In the case of a relay / contactor with forcibly guided contacts, the NC and NO contacts of the auxiliary contacts are guaranteed never to be closed at the same time (IEC / EN 60947-5-1, Appendix L).
Contacts with positive opening
A switch (e.g. emergency stop switch) with positive opening is designed so that the switching movement inevitably separates the contacts. If the contacts are welded, they may be broken (IEC / EN 60947-5-1, Appendix K).
In the case of a contactor with mirror contact, this is guaranteed to be open if any main contact is closed (IEC / EN 60947-4-1, Appendix F).
Dimensions according to EN 61058-1
In order to enable users to use suitable switches, rating data are specified for each switch ; these are (list is not complete):
- Current and voltage ;
- Type of network: direct current / alternating current / direct and alternating current;
- Load type (see below);
- Ambient temperature;
- Number of switching cycles;
- Degree of protection (IP protection) when the switch is mounted;
- Protection against electric shock when used in different types of devices;
- Degree of pollution of the environment;
- Resistance to heat and fire;
- Surge resistance;
- Quality of the insulation capacity.
For the switching capacity of a switch, it is of great importance how the current-time behavior of the consumer affects the moment of closing and opening. A distinction is made here:
- purely resistive load: only loads with strict proportionality between current and voltage. This type of load is identified with the nomenclature z. B. 16 A 250 V AC. According to IEC 60947-5-1, it is referred to as AC12 (ohmic and semiconductor load).
- Motor-resistive load: Since motors briefly have a significantly higher current requirement when they start, the capacity for this type of load is particularly noted: 12 (8) A 250 V AC means that the switch can switch on motors with 8 A rated current, which is conventionally done with the Six times the current specified in brackets is overloaded during start-up: Switch on 48 A and switch off with the current value in front of the brackets. Also indicated as AC3 ( power factor cos φ = 0.6)
- Capacitive load: Capacitors charge with a decaying characteristic, which is why the current increases particularly quickly when switched on. Switches that are particularly effective here are denoted by the notation z. B. 5/100 A 250 V AC. This means a making capacity of up to 100 A with a typical time constant of 2.5 ms and a breaking capacity of 5 A at an alternating voltage of up to 250 V.
- Lamp load: The inrush current of incandescent lamps is 13 to 15 times higher than the rated current, i. H. A current of 6 A flows through a 100 W general service lamp when it is switched on, while only 0.4 A flows when it is closed. This stresses a switch when it is switched on in a similar way to a capacitive load. According to IEC 61058-1, the nominal lamp current is given in square brackets.
- Strongly inductive load with cos φ = 0.3 (referred to as AC15 according to IEC 60947-5-1): this leads to increased inrush currents (defined as ten times the nominal current) and strong cut-off arcs
See also under utilization category for low-voltage switchgear .
In North America, so-called TV ratings are still used , which also take into account a high inrush current in television sets.
Switching off direct current at voltages above approximately 30 V is particularly problematic, since the switching arc then does not necessarily extinguish. Switches can therefore (if they are specified for this) switch only comparatively low direct currents at higher DC voltages.
Effects and connections when switching
If two real contacts move towards each other, the dielectric strength of the existing air gap is below a certain minimum distance . The amount of the minimum distance depends on the prevailing field strength and thus on the voltage potential between the switching contacts. As a result, a spark or a so-called pre-ignition arc can occur above the minimum voltage and current . This stresses the contact surfaces (contact erosion), possibly melts them and can lead to contact welding. Such damaged switches can no longer be opened.
At the moment of contact, the rules of impact mechanics apply : elastic and plastic deformation of the contact surfaces occurs with the consequence of possible lift- offs , the effect of bouncing (see countermeasures there). The contacts hit together and briefly spring apart again, so that additional interference pulses can arise.
Ideally, a closed switch offers no series resistance to the electrical current. In the real case, the current can be conducted with as little loss as possible if there is a metallic contact over the largest possible cross-sectional area. The contact resistance of a switch depends on the specific resistance of the materials themselves and the contact surface, which in turn is directly determined by the contact force and the hardness of the materials used. Dirt and oxide layers have a significant influence. The heat loss generated at the contact resistor heats up the contact.
If the switch is also subjected to vibrations when it is closed, which can cause relative movement of the two contacts, there is also the risk of fretting corrosion. The smallest metal particles are rubbed off by the movement and can oxidize and reduce the metallic contact surface. The consequence of this is an increase in the contact resistance and an increase in the temperature in the contact zone.
When a switch is opened, the contact force initially decreases and the metallic contact surfaces become smaller. This increases the electrical resistance , the contact point heats up, and the contact material melts at higher currents. If the connecting material bridge tears off, an arc will form at voltages above approximately 12 ... 20 V and currents above approximately 2 A , i. H. the air is ionized, which manifests itself in a bluish glow. The current can continue to flow through the arch. There are very high power losses, which lead to the melting and evaporation of contact material. If the distance between the contacts is sufficiently large or (with alternating current) the current goes through zero , the arc can extinguish.
Isolate in the open state
Ideally, no current flows through the open switch, but in reality, insulating materials or contamination offer a leakage current path. In addition to the insulation resistance when open, the dielectric strength of the separated contacts is also a parameter.
Switches for safe disconnection must also withstand overvoltages in the power grid and, for example, disconnectors must also have a visible power interruption.
Safe galvanic isolation from the actuating element or control circuit is ensured with manually operated mains switches by insulating materials and with semiconductor relays, for example, by optocouplers. Integrated semiconductor switches such as low-side or high-side switches have monitoring of the switching state and the overload, but have no potential separation from the control circuit.
Certified switches, test marks
The safety system of switches is assessed by assessing the conformity with requirements that have been developed in the corresponding switch safety standards ( CB Scheme ). For switches, these are on the one hand the standard for device switches, based on the globally harmonized IEC 61058-1 and the standard for low-voltage switchgear IEC 60947-1. These conformity tests and confirmations are carried out and awarded by numerous European certification bodies (in Germany, among others, the VDE ). If they are in conformity, the switches are marked with the ENEC symbol followed by a code from the issuing certification body. Furthermore, the electrical performance with information on the current, the voltage, the type of network (AC or DC) must be permanently identified on the switch so that the user can make the right selection.
In North America, switches are mainly certified by the US UL and the Canadian CSA . You will still evaluate according to UL 1054 or CSA 22.2 No. 55 1986 edition, but are in the process of adopting the IEC standard in their countries.
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