Terminal (electrical engineering)
A terminal is used in electrical engineering for the detachable connection or connection of wires , cores and lines . Permanent, safe contact must be guaranteed when connected. This is achieved by mechanical fixation (screw or spring) of the connected conductors in a conductive body. There are also screw-on cable connectors. Clamping and disconnecting are derived words that describe the connection and disconnection of a device from the power supply system that is done without a connector .
Types of clamps
- Single terminal (loose, insulated) according to DIN EN 60998-2-1 (VDE 0613-2-1) as screw terminal or DIN EN 60998-2-2 (VDE 0613-2-2) as screwless terminal for use in installation boxes according to VDE 0606-1 (also called junction boxes or junction boxes)
- Bridge terminals are several terminals that are electrically connected to one another.
- A terminal strip either has several electrically interconnected connection points or is used to accommodate a number of terminal blocks.
Terminal blocks are flat terminals that are lined up in large numbers on a mounting rail (e.g. DIN rail TS35 "top hat rail"):
- Feed-through terminal: Single or multi-pole ("multi-level") terminal blocks for wiring complex distribution and switch boxes clearly. (e.g. protective conductor terminals, shield conductor terminals, double through-type terminals, double-tier terminals, motor connection terminals).
Installation terminal: Single or multi-pole ("storey terminals") terminal blocks with special functions for building installation (e.g. N-isolating terminals, equipotential bonding terminals, distribution feed terminals):
- Disconnect terminal : Special forms of terminal blocks for manual disconnection of circuits.
- Bus terminal , further developed terminal blocks with control electronics
- PCB terminal or print terminal : are attached and soldered directly to printed circuit boards or printed circuits and are used to connect flexible cables by screwing, clamping or plugging. PCB stands for the English term for printed circuit board .
Box terminals and appliance terminals used to consist of screws with an exposed, broad head under which the conductor was clamped. Lateral restrictions prevented the ladder from being pushed out when screwing it tight. The common jamming of several conductors under one screw head was problematic. To create a secure connection, an eyelet was bent from the stripped end of the conductor, through which the screw could then be inserted.
For safe and permanent transmission of larger currents, screw terminals must be tightened with a defined minimum force. This is usually only possible without damaging the screw head or thread if the clamp is made of high quality material. In the case of simple clamps without a tongue or pressure plate , which lies between the screw and the conductor, the conductor is also constricted when the screw is screwed in and, in the worst case, severed.
Fine and ultra-fine stranded wires , like aluminum conductors, can only be used with screw terminals if they are either provided with a ferrule or if the terminal has a tongue or bracket that protects the conductor from the rotating end of the screw. Ordinary aluminum conductors may generally only be temporarily connected in screw terminals, as the contact pressure decreases over time as the aluminum flows. The contact resistance increases and the contact heats up. If the screw terminal is not tightened regularly, the terminal can heat up until the insulation melts and surrounding materials catch fire. The oxide layer of the aluminum conductor also increases the contact resistance. The conductor is therefore polished before the connection and protected against renewed oxidation by greasing . The clamp should also be filled with grease.
Terminal blocks and box terminals are simple screw terminals in the house wiring for connecting the circuits in junction boxes are used. Usually several terminals are combined on one insulating body. The term terminal block refers to the insulating body, which used to be mostly made of ceramic.
Luster terminals are also called lamp or candlestick terminals or block terminals . Luster terminals sit side by side in an insulating body made of ceramic or plastic. Longer rows are usually easy to split into smaller units. In contrast to terminal blocks, luster terminals each have two screws so that one wire can be clamped on both sides. This facilitates the assembly as individual wires can be exchanged without loosening the opposite wire. To further facilitate the repeated disconnection of multipole connections, double rows of terminals with three screws per pair of terminals are available. Protruding pins create the connection between the two rows. After loosening the middle row of screws, the rows of terminals can be pulled apart.
Metal tongue terminals are luster terminals with internal tongues. When the screw is tightened, the metal tongue presses on the conductor so that even finely stranded strands can be clamped without damaging them.
Instead of a thin metal tongue, elevator clamps have a solid bracket ("elevator") that presses the ladder together when it is tightened. This means that flexible stranded wires can be clamped directly without a ferrule without the screw damaging the individual wires during the clamping process. A larger number of individual cores can also be clamped together without them being able to evade the screw, as is the case with conventional screw terminals.
Spring terminals also are plug-in terminal , Käfigzugklemme , cage clamp , spring-loaded terminal by a known manufacturer Wagoklemme or generally spring clip called. They are now widely used because they allow a fast and secure connection. Most socket and switch socket inserts as well as many electrical components are now equipped with spring-loaded terminals.
Stripped wire ends inserted into the terminal are clamped in place by a curved spring blade. A loosening of the connection due to vibration is excluded. The increase in contact resistance due to a decrease in pressure, oxidation or other types of contact corrosion almost does not occur because the contact pressure is maintained by the spring tension.
The manufacturers usually also approve spring-loaded terminals for aluminum conductors. However, the conductor should first be sanded bare. In addition, the terminal is filled with grease before the conductor is inserted to prevent the aluminum from oxidizing again.
To release the connection, the tip of a screwdriver is usually inserted through an opening in the housing in order to push back the spring clamping tongue. Copper conductors can usually also be gradually pulled out of the terminal by repeated alternating rotation of the wire.
Insulation displacement terminal
IDC terminals consist of a slotted piece of sheet metal in a holder made of insulating material. The slot widens slightly towards the top and is matched to the diameter of the conductor used. The wire to be connected does not need to be stripped. When the wire is inserted into the slot, the insulation is cut and the contact is made by clamping the sheet metal onto the conductor.
- Fuse clips are used to hold a fuse element against overcurrent.
- Grounding terminals ( PE terminals ) establish a connection to metallic components such as housing, mounting rails, profiles or pipes and are used to connect protective conductors
- Shield clamps (sometimes also shield clamps ) are used to connect external cable shields to the ground potential (e.g. with coaxial cables ) in orderto ensure electromagnetic compatibility .
- Magnetic clamp : Particularly suitable for temporary connections such as experimental set- ups . With this type of clamp, battery holders can be produced in a simple manner.
- Induction clamp for current measurement: In contrast to the other clamps, an induction clamp does not make any electrical contact, but is simply placed around an electrical line in order to measure the magnetic field strength of the conductor in the manner of a current transformer ; the measured values are either transmitted to a current or energy measuring device or, in automotive engineering, are used to record the ignition pulses by an electronic speed indicator.
- 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
- Herbert Schmolke: VDE series 45; "Electrical installation in residential buildings", manual for installation practice . 7th edition. VDE Verlag GmbH, Berlin and Offenbach 2010, ISBN 978-3-8007-3029-2 , p. 364 ff .