Product Code Database
A screw terminal is a type of electrical connection where a wire is held by the tightening of a screw.
Depending on the design, a flat-blade screwdriver, a cross-blade screwdriver, hex key, Torx key, or other tool may be required to properly tighten the connection for reliable operation.
Screw terminals are commonly used to connect a chassis ground, such as on a record player or surge protector. Most public address systems in buildings also use them for Loudspeaker, and sometimes for other outputs and inputs. and building sensor and control systems have traditionally used large numbers of screw terminations.
Grounding screws are often color code green and, when used on consumer electronics, often have a washer with gripping "teeth", to ensure better connections.
Printed circuit board (PCB) terminal blocks are specially designed with a copper alloy pin of suitable size and length, and can be inserted into printed circuit boards for soldering in place. Some designs provide features that allow the flow of molten solder to ensure a better connection between the circuit traces of the board and the electrical equipment which is meant to be controlled or fed appropriate power.
A similar arrangement is common with paired screw terminals, where metal tubes are loosely encased in an insulating block with a set screw at each end of each tube to clamp and thus connect a conductor. These are often used to connect .
Alternatively, terminals can also be arranged as a terminal strip or terminal block, with several screws along (typically) two long strips. This creates a bus bar for power distribution, and so may also include a master input connector, usually or .
While wires may be crimped, they should not be heavily tinned with solder prior to installation in a screw terminal, since the soft metal will cold flow, resulting in a loose connection and possible fire hazard. Screw connectors sometimes come loose if not done up tightly enough at fitting time. Verifying adequate tightening torque requires calibrated installation tools and proper training. In the UK, all screw connectors on fixed mains installations are required to be accessible for servicing, for this reason.
When properly tightened, the connections are physically and electrically secure because they firmly contact a large section of wire. The terminals are relatively low cost compared with other types of connector, and a screw terminal can easily be integrated into the design of a building wiring device (such as a socket, switch, or lamp holder).
Disadvantages include the time taken to strip a wire and, in basic terminals, properly wrap it around a screw head, since it is essential that any wire installed under a screw head be "wound" in the correct direction (usually clockwise,) so that the conductors are not forced outwards when the screw is tightened. This procedure is more time-consuming than using a plug-in connector, thus making screw connections uncommon for portable equipment, where wires are repeatedly connected and disconnected.
However, with the clamping plate type of screw terminal this time is reduced, since it is necessary only to insert the stripped wire between the terminal and the rear clamping plate then tighten the connection, using the screw to clamp the wire between the terminal and the clamping plate, without any need properly to wrap it around the screw head.
The screw mechanism limits the minimum physical size of a terminal, making screw terminals less useful where large numbers of connections are required.
It is difficult to automate multiple terminations with screw connections.
Vibration or corrosion can cause a screw connection to deteriorate over time.
The use of screw terminal "chocolate blocks" in building wiring installations has sharply declined in favour of crimp, push, and twist type connectors which are and easier to fit, and less vulnerable to working loose. In the UK, chocolate blocks are no longer approved for connections that are not accessible for inspection (such as under floors).
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