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Screw terminal
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A screw terminal is a type of electrical connector where a wire is clamped down to metal by a screw.
The wire is sometimes just stripped of electrical insulation at the end, and is bent in a U or J shape to fit around the shaft of the screw.
Alternatively, a lug can be crimped onto the end of the wire to protect it.
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Encyclopedia
A screw terminal is a type of electrical connector where a wire is clamped down to metal by a screw.
The wire is sometimes just stripped of electrical insulation at the end, and is bent in a U or J shape to fit around the shaft of the screw.
Alternatively, a lug can be crimped onto the end of the wire to protect it. Setscrews are also used, as in a luster terminal or lustre terminal, but lugs will not fit into them. For both lugs and bare wire, the screw is tightened for a secure connection.
Description and usage
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 speakers, and sometimes for other outputs and inputs. They are also used extensively in wiring for electricity, for connecting electrical outlets and switches to the mains, and for connecting major appliances like clothes dryers and ovens to the particular plug installed in a home, or directly hard-wiring to the house, as required by local electrical codes in the US. Screw terminals for earth ground connections are used on most US outlets, even for those that otherwise have "rear-wire" push-in connections, despite the difficulty of bending heavy-gauge solid copper around a screw with pliers. Grounding screws are often marked green, and when used on consumer electronics often have a washer with gripping "teeth".
Multiway version
Multiple screw terminals can be arranged as a barrier strip, with each short metal strip having a pair of screws. This is used for connecting two different components, one on each side of the pairs. This arrangement is common in luster terminals, as pictured at left. These are known as connector strips or chocolate blocks in the UK. Conversely, 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 binding posts and/or banana connectors.
Advantages and disadvantages
One advantage of screw terminals are that no connectors are used, thus no compatibility problems with mismatched sizes or shapes. Additionally, the connections are very secure, both physically and electrically, because they firmly contact a large section of wire. This is also a disadvantage however, because it can take a few minutes to secure or undo a set of connections that could otherwise be simply plugged or unplugged. Another disadvantage is that their use with wire too thin is liable to partly cut through the wire.
Screw connectors sometimes come loose over time if not done up tightly enough at fitting time. In the UK all screw connectors on fixed mains installations are required to be accessible for servicing for this reason.
Overtightening can cut most of the way through wires.
An alternative in some countries is to use wire nuts, usually done where there is no chassis involved.
See also
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