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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 set screws in the side of a metal tube. The wire may be directly stripped of insulation and inserted into the terminal, or may be inserted first into a connecting lug which is then inserted in the terminal.
|This section requires expansion. (August 2013)|
Screw terminals were one of the first types of electrical connectors to be used.
Description and usage
Screw terminals are used extensively in building wiring for electricity, connecting electrical outlets and switches to the mains, and connecting major appliances like clothes dryers and ovens to the particular outlet installed in a building.
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.
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. These are known as connector strips or chocolate blocks ("choc blocks") in the UK.
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 binding posts or banana connectors.
Advantages and disadvantages
Screw terminals are low in cost compared to other types of connectors, and can be readily designed into products for circuits carrying currents of a fraction of an ampere up to several hundred amperes at low to moderate frequencies. The terminals can be easily re-used in the field, allowing for replacement of wires or equipment, generally with standard hand tools. Screw terminals avoid the requirement for a specialized mating connector to be applied to the wire ends.
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 properly wrap it around the screw head, in basic terminals. This procedure is slower than a plug-in connector, making screw connections uncommon for portable equipment where wires are repeatedly connected and disconnected.
Assembly of a screw connection requires some care in workmanship to ensure adequate tightening of the screw, proper removal of insulation, and containment of all wire strands. If the wire is too small, it can be cut through by an overtightened screw head, in both basic and compression terminals. Wire strands may not be contained by the screw head in basic terminals, so sometimes stranded wires are crimped into a ferrule to prevent bridging of terminals; this partly offsets the economy of a "bare" wire termination.
In addition, the screw mechanism limits the minimum physical size of a terminal, making screw terminals less useful where very many 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.
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.