A screw terminal is a type of electrical connector where a wire is held by the tightening of a screw.
The wire may be wrapped directly under the head of a screw, may be held by a metal plate forced against the wire by a screw, or may be held by what is, in effect, a set screw in the side of a metal tube. The wire may be directly stripped of insulation and inserted under the head of a screw or into the terminal. Otherwise, it may be either inserted first into a ferrule, which is then inserted into the terminal, or else attached to a connecting lug. which is then fixed under the screw head.
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 used extensively in building wiring for the distribution of electricity - connecting electrical outlets, luminaires and switches to the mains, and for directly connecting major appliances.
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. Alarm systems and building sensor and control systems have traditionally used large numbers of screw terminations.Grounding screws are often color-coded green.
Screw terminal block has its advantages and disadvantages:
Screw terminals are low in cost when compared to other types of connectors, and can be readily designed into products for circuits carrying currents of from a fraction of an ampere up to several hundred amperes at low to moderate frequencies. The terminals easily can be re-used in the field, allowing for the replacement of wires or equipment, generally with standard hand tools. Screw terminals usually avoid the requirement for a specialized mating connector to be applied to the ends of wires.
Disadvantages include the time wasting. Since it is essential that any wire installed under a screw head be "wound" in the correct direction (usually clockwise,) so 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.
What' more, it is difficult to automate multiple terminations with screw connections and vibration or corrosion can cause the screw connections to deteriorate over time.
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