Switch

Switches are an assembly which produces an electrically conductive connection between two electrically conductive materials or a semiconductor device or separate (→ switching contact). Ideally, it operates on the all-or -nothing principle; that is, an operation always leads clearly to a switching state open or closed. In the switching state should open from safety aspects, the modern semiconductor switches caused "electronic separation" are always distinguished from a mechatronic " galvanic isolation ", the protection can provide of electrical voltage.

  • 2.1 switch assembly generally
  • 2.2 actuating unit
  • 2.3 Switching function
  • 2.4 Switching kinematics
  • 3.1 designations according to the purpose
  • 3.2 use categories according to IEC / EN 60947
  • 3.3 Contact Categories 3.3.1 forcibly guided contacts
  • 3.3.2 Contacts with positive opening
  • 3.3.3 Mirror Contacts
  • 3.4.1 Load Types
  • 4.1 Close
  • 4.2 Power lead
  • 4.3 Opening
  • 4.4 Isolate the open state

Basics

Switch, also known as switching devices can be distinguished by various features. For example, according to the type of operation, the type and design features or usage characteristics. For users on the most important are the electrical parameters (rated data), describing the suitability of a switch for specific voltage and current ranges, and environmental conditions. The suitability has to be given in all operating states of the switch: The contact operation, the current run, the contact separating and isolating safe in the open state.

Subdivided according to the behavior of a switch after an operation was carried out, there are switches that remain in their switching state stable after operation. These are, for example, bistable toggle switches, rocker switches, selector switches, pushbutton switches, multiple - choice or switch. Buttons and keys, however, return after removal of the actuation return to their rest position.

According to the type of application, for example, distinguished:

  • Signal Switches, which are mostly used in equipment control in safety extra-low voltage circuits;
  • Main switch, as they are used as power switches in home appliances, which are normally used from sockets;
  • Power tool switches, which are used primarily in motor-operated electric tools and machines;
  • Service switching devices that are permanently mounted in structures and not, for example, pulling the power plug are energized;
  • Circuit breaker (circuit breaker ).

Definition of the term switch

When a switch is on and off a current in the foreground, and that directly or indirectly as a result of actuation.

Is not changed by the operation of the switching state, but by a control signal, such as a rush current to a coil, is referred to as a relay or contactor. A transistor is also enabled by a control signal, a semiconductor switch, and thus no switch in the proper sense.

The mechanical actuation is used to describe switch is not mandatory, even over

  • Magnetic actuated reed switch,
  • Heat -induced bimetallic etc.

Are switches.

When closed, the switch is differentiated as opposed to regulation and control units: switches can also contain control and controls that provide the all-or -nothing behavior with specific intermediates, such as the speed control of motor units. The delineation of the switch from a controller or regulator is often blurred. If the switching function is considered to be limiting, the switch engages definition, otherwise a regulating and control unit.

The terminology switch is marked differently in different languages ​​: Is etymologically in German with switch and English with switch more mechanical sliding or folding of an actuating element basis, then, the Romanesque room with French interrupteur or the Italian interruttore the divide in the fore, in contrast with the Danish contact that connected them.

Essential elements of a switch

Switch assembly generally

Perhaps the most important element of a switch, the contacts. They consist of chemically noble metals, mostly silver. This other chemical elements are alloyed or admixed to increase contact life. When the signal switches, the contact surfaces are often coated with gold to make the surface with high electrical conductivity chemically corrosion resistant.

Silver has the disadvantage to form with the sulfur from the air sulfides. These salts are chemically resistant and electrically very poorly conductive. Either mechanically by abrasion or by arc erosion, the contact surfaces have to be cleaned to ensure sufficiently low transition resistance.

The contact carrier elements are often made of non-ferrous metals or spring steels to combine according to good electrical conductivity with mechanical strength and elasticity depending on the application. The connections are also formed from these materials and in many variants: screw terminals, plug-in connections for prepared and unprepared wire ends, spades, spade elements and much more are common.

As housing materials insulators are required, mostly plastics which are sufficiently thermally stable and heat-and fire-resistant.

Operating unit

Mechanically operated switches can be actuated by devices either manually, in more general terms by the people, or. The main reason to distinguish this, is that the man can act as a control circuit: Turns a man to a switching device, and does not enter the desired effect in the following one, he will take appropriate action depending on the situation. In mechanized switching (eg switching off a roller shutter in its final position ) structural elements in case of error should ensure that no hazardous condition arises.

The switch operation can be carried out both directly and indirectly. The standard for equipment switches to DIN / EN 61058-1 formulated such that the switching operation of an actuating member or by operation of a sensor may take place may be arranged separately with operating member of the switch or sensor. For the transmission between the operating member and the switching element of optical, acoustic or thermal signal lines are given.

In the mechanically directly actuated switches are categorized according to actuator:

  • Toggle switch
  • Rocker
  • Pressure switch
  • Pushbutton switches
  • Slide switch
  • Rotary switch
  • Step switch
  • Pushbutton
  • Footswitch
  • Key switch
  • Cord switch
  • Pull cord switches
  • Dip or coding

Mechanical devices over or indirectly actuated switches:

  • Cam switches, limit switches
  • Through media operated switch, eg actuated by pneumatic control signals switch, oil pressure switch, pressure switch, float switch ( liquid level control )

Using sensors actuated switches:

  • Twilight switches, motion detectors, wireless switches

Switching function

Upon actuation produce power switch or open contact connections while off switch or break connections separate. Change-over switch ( switch, changer ) and rotary switches connect a contact with one of two or more other contacts. Here are switches that establish the new connection before the old is disconnected, as bridging (also shorting, engl. Make before break ), and those who first disconnect the old connection when not bridging (not shorting, engl. Break before make) respectively.

Switches can be distinguished:

After the switching function

  • The already mentioned NC, NO, CO ( bridging or non -bridging )
  • Series switch ( for switching two circuits with a knob )
  • Intermediate switches ( for switching an electric circuit by more than two switches )
  • Group switches ( shutter switch for alternately switching two circuits with three settings - the eponymous blind can only upload or shut down, but not both simultaneously)

Type and number of contact channels are used for small toggle switches often described with English abbreviations ( SPST and DPST are off switch, SPDT and DPDT switches are AC ).

Switching kinematics

With the flick of a switch the expectation of a change in the status of is " closed " to " open" or vice versa. Especially due to arcing effects during opening and closing is a defined sequence of movements - independent of that of the operation - desired for the operation and service life of a switch. At a certain point, the so-called pressure point, the switching operation is to be initiated irreversible. This can be constructively implemented by all types of fine mechanical snap elements. The switch design constructed in this way is called accordingly snap switch or snap-action switch.

Especially for switches for DC circuits a short time to open the circuit is important. Upon separation of the contacts is above the arc voltage limit and a minimum current danger of a standing electric arc that the switch is overloaded very quickly thermally at longer focal time. To prevent this, the snap systems perform the operation started from the pressure point automatically ended. In AC circuits, any existing arc goes out mostly by himself at the next current zero crossing.

Switch for alternating current over 25 A or for DC have partially horn-shaped contacts and arcing chambers which serve to extend the arc to cool and bring so extinguished. Especially in medium-and high -voltage switches, which operate, for example, under oil or vacuum, special design measures to control emerging switching arc to be made. Often, these switches have a spring-loaded in order to separate the contacts quickly and close.

In contrast to, for example, tipping, tilting or latching switches which are operated fully with each gear change, the change from the working state must be automatic to sleep in touch and in particular with snap switches. To this end, the stored spring energy is used in the system. Depending on the design of a switch than in normally closed (NC, n c. (Normally closed) ) or open (NO, n o (normally open) ), these can be constructed as a positive break from the rest position. This possibly resulting Einschaltverschweißungen can be broken at micro switches or snap-action switches for high inrush currents.

Categories

Names according to the purpose

Further differentiation results from the purpose of fulfilling the switch in a circuit. Major applications are: main switch, emergency stop switches, maintenance switches, circuit breakers, Selective circuit breaker, light switch, deadman switch, circuit breaker, circuit breakers, etc.

Disconnector form a large insulating air gap between the open contacts, but can hardly turn power. They are used mainly in substations for voltages above 1000 V and are used to separate parts of the system for maintenance.

Load switch for switching on the power grid can be mechanically ( eg light switch) or electronic ( solid state relay, SSR ( solid state relay ) ) are operated. They are widely used as power switch on (household ) appliances and can be used only up to their specified rated current.

Disconnector ( see High- voltage switch) combine the properties of circuit breakers and disconnectors.

Circuit breakers can short-circuit currents off (see high voltage switch).

Control switch can no large load currents, but only smaller control switch currents are driven with which, for example contactors. Examples are key switch or button industry. Typical switching voltages are 48 V DC or AC or 230 V AC.

Signal switches usually have contacts from precious metals and particularly to low transition resistance, they are designed specifically for switching low voltages and currents.

Analog switch is a designation for digitally-controlled semiconductor switches, which are suitable for the switching of analog signals (such as audio signals). They typically work with MOSFETs and are built as integrated circuits. As electrically controlled components they are no switches in the strictest sense, but among the electronic switches.

Use categories according to IEC / EN 60947

The Utilization of Low-voltage switchgear defines the characteristic conditions for switching devices. These devices are designed for different electrical loads and for different operating conditions.

The switching to the property or to be controlled load determines the requirements for the switching devices and their correct selection for the intended application. Specifically, the stress of the switching paths by current and voltage during switching on and off are of enormous importance.

See Article Utilization category for detailed information on the topic.

Contact Categories

Forcibly guided contacts

In a relay / contactor with positively driven contacts NC and NO contacts of the auxiliary contacts are guaranteed never closed simultaneously (IEC / EN 60947-5-1, Appendix L).

Contacts with positive opening

A switch ( eg emergency off switch) with positive opening is constructed so that the switching movement inevitably separates the contacts. Case of welded contacts this possibly broken (IEC / EN 60947-5-1, Appendix K).

Mirror Contacts

When a contactor with mirror contact this guarantees open when any main contact is closed ( IEC / EN 60947-4-1, Appendix F).

Rated data according to EN 61058-1

To allow users to use appropriate switch measurement details are specified for each switch, these are ( list not exhaustive):

  • Current and voltage;
  • Type of network: Gleichstrom/Wechselstrom/Gleich- and alternating current;
  • Load type (see below );
  • Ambient temperature;
  • Number of switching cycles;
  • Degree of protection ( IP ) when the switch is mounted;
  • Protection against electric shock when using different types of devices;
  • Degree of contamination of the environment;
  • Resistance to heat and fire;
  • Overvoltage protection;
  • Quality of the insulation capacity.

Load types

A switch for switching capacity is of great importance, as the current-time response of the load acting on the back- torque of the closing and opening. A distinction is made here:

  • Purely resistive load: Only consumers with a strict proportionality between current and voltage. This load type is A 250 V AC marked with the nomenclature eg 16. According to IEC 60947-5-1 she is referred to as AC12 (resistive and semiconductor loads).
  • Motor -resistive load: Since engines briefly have a significantly higher power requirements at start-up, the performance for this type of load is particularly noted: 12 (8 ) A 250 V AC means that the switch motors with 8 A Rated current can be switched on, which by convention with the six times the current is indicated in parentheses overloaded during start-up: 48 A switch on and off with the current value before the brackets. Specified as AC3 ( power factor pf = 0.6 )
  • Capacitive load: load capacitors to evanescent characteristic, which is why the current at switch on is particularly high. Switches, which are particularly efficient here are referred to with the notation eg 5/100 A 250 V AC. This means making capacity up to 100 A, with a typical time constant of 2.5 ms, and a breaking capacity of 5 A at an AC voltage of 250 V.
  • Lamp load: Inrush current of incandescent lamps is by a factor 13 to 15 higher than the rated current, ie by a 100 -W incandescent lamp flows when switching a current of 6 A, while in the closed state flow only 0.4A. This takes similar to a capacitive load of a switch when switched on. According to IEC 61058-1 Lamp rated current is given in square brackets.
  • Strongly inductive load with pf = 0.3 (according to IEC 60947-5-1 called AC15 ): here it comes to increased inrush currents (defined as ten times the rated current ) and a strong turn-off arc

See also under utilization category for low-voltage switchgear.

In the North American region are still called TV ratings in use, which are also taken into account when television sets a high inrush current.

Particularly problematic is the elimination of direct current at voltages above about 30V, because then the switching arc not absolutely extinguished. Switches can therefore (if they are specified for it) turn only comparatively small DC currents at higher DC voltages.

Effects and correlations when switching

Two real contacts moving toward each other at a certain minimum distance, the dielectric strength of the existing air gap is maintained. The magnitude of the minimum distance is dependent on the prevailing field strength and therefore the voltage potential between the switch contacts. As a consequence, above the minimum voltage and current, a spark or so-called Vorzündlichtbogen can occur. This claims the contact surfaces (contact discharge ), they eventually melts and can cause contact welding. In such a damaged switch can not be opened.

At the moment of contact, the rules of the shock mechanics apply: Elastic and plastic deformation of the contact surfaces occurs with the consequence of possible withdrawals, the effect of the bounce ( counter-measures see there). The contacts strike together and feathers short apart again, so that additional glitches can occur.

Carry current

Ideally, a closed switch allows the electric current no series resistance. The current can be performed with minimal losses in the real case, when a metallic contact on a large cross -sectional area is. , The contact resistance of a switch depends on the resistivity of the materials themselves as well as the contact surface, which in turn is directly determined by the contact force and the hardness of the materials used. A significant influence form dirt and oxide layers. The heat produced by the contact resistance heats on contact.

The switch is additionally loaded in the closed state by the vibrations that can cause a relative movement of the two contacts, in addition, there is the risk of fretting corrosion. The smallest metal particles are abraded by the motion and can oxidize and reduce the metallic contact surface. Consequence of this is an increase of the contact resistance and an increase in temperature in the contact zone.

When opening a switch, the contact force decreases initially, the metallic contact areas are smaller. Thus, the electrical resistance increases, the contact point is heating up, the contact material melts at higher currents. Tear down the connecting bridge of material will form at voltages above about 12 ... 20 V and pour over about 2 A, an arc, that is, the air is ionized, which manifests itself in a bluish glow. The current can continue to flow through the arc. It produces very high power losses, leading to the melting and evaporation of contact material. If the distance between the contacts is sufficiently large or (in AC) current passes through zero, the arc can be extinguished.

Isolate in the open state

Ideally, flows across the open switch no power, but offer real insulating materials or dirt leakage current path. In addition to the insulation resistance in the open state, the withstand voltage of the contacts is a separate parameter.

Switch for the safe separation must also withstand overvoltages of the electricity network and have, for example in circuit breakers also a visible power interruption.

Safe Galvanic isolation to the actuator or control circuit is guaranteed for hand-operated power switches by insulating materials and solid-state relays, for example, by optocouplers. Although Integrated semiconductor switches such as low-side or high-side switches have monitoring of the switching state and the overload, but have no isolation to control circuit.

Certified switch, certification

The security of switches is evaluated by which conformity is valued at exposures that have been developed in corresponding switch - safety standards (CB Scheme). For switches, these are firstly the standard for the power switch is based on the globally harmonized IEC 61058-1 and the standard for low- voltage switchgear and controlgear IEC 60947-1. This compliance investigations and confirmations are numerous European certification bodies carried out and forgiven ( in Germany, inter alia, the VDE). In the present conformity switches using the ENEC mark, followed by a code of the issuing CA are marked. Furthermore, it has permanent electrical performance with an indication of the current, the voltage, the type of network (AC or DC) are indicated on the switch, so the user can make the right selection.

In North America certifications lead to switches, mainly, the American UL and Canadian CSA. Please rate until 2015 in accordance with UL 1054 and CSA 22.2 No.. 55 1986 edition, but are just about to take over the IEC standard in their countries.

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