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A screw terminal is a type of
electrical connection Components of an electrical circuit are electrically connected if an electric current can run between them through an electrical conductor. An electrical connector is an electromechanical device used to create an electrical connection betwee ...
where a
wire Overhead power cabling. The conductor consists of seven strands of steel (centre, high tensile strength), surrounded by four outer layers of aluminium (high conductivity). Sample diameter 40 mm A wire is a flexible strand of metal. Wire is co ...
is held by the tightening of a
screw A screw and a bolt (see '' Differentiation between bolt and screw'' below) are similar types of fastener typically made of metal and characterized by a helical ridge, called a ''male thread'' (external thread). Screws and bolts are used to fa ...
.


Description

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 American English, a set screw is a screw that is used to secure an object, by pressure and/or friction, within or against another object, such as fixing a pulley or gear to a shaft. A set screw is normally used without a nut (which distin ...
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 A ferrule (a corruption of Latin ' "small bracelet", under the influence of ' "iron") is any of a number of types of objects, generally used for fastening, joining, sealing, or reinforcement. They are often narrow circular rings made from me ...
, 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 Hex keys of various sizes Socket head screws of various sizes A hex key (also, hex wrench, Allen key and Allen wrench) is a simple driver for bolts or screws that have heads with ''internal'' hexagonal recesses (sockets). Hex keys are fo ...
, Torx key, or other tool may be required to properly tighten the connection for reliable operation.


Applications

Screw terminals are used extensively in
building wiring Electrical wiring is an electrical installation of cabling and associated devices such as switches, distribution boards, sockets, and light fittings in a structure. Wiring is subject to safety standards for design and installation. Allowable ...
for the distribution of
electricity Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as describ ...
- connecting electrical outlets, luminaires and
switch In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type ...
es to the mains, and for directly connecting
major appliance A major appliance, also known as a large domestic appliance or large electric appliance or simply a large appliance, large domestic, or large electric, is a non-portable or semi-portable machine used for routine housekeeping tasks such as cooki ...
s such as
clothes dryer A clothes dryer, also known as tumble dryer or simply dryer, is a powered household appliance that is used to remove moisture from a load of clothing, bedding and other textiles, usually shortly after they are washed in a washing machine. Many d ...
s and
oven upA double oven A ceramic oven An oven is a tool which is used to expose materials to a hot environment. Ovens contain a hollow chamber and provide a means of heating the chamber in a controlled way. In use since antiquity, they have been use ...
s drawing in excess of 15 amperes. Screw terminals are commonly used to connect a chassis ground, such as on a record player or
surge protector A 'surge protector'' (or spike suppressor, surge suppressor, surge diverter, surge protection device (SPD) or transient voltage surge suppressor (TVSS) is an appliance or device intended to protect electrical devices from voltage spikes in al ...
. Most
public address A public address system (or PA system) is an electronic system comprising microphones, amplifiers, loudspeakers, and related equipment. It increases the apparent volume (loudness) of a human voice, musical instrument, or other acoustic sound sou ...
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 and, when used on
consumer electronics Consumer electronics or home electronics are Electronics, electronic (Analogue electronics, analog or digital electronics, digital) equipment intended for everyday use, typically in private homes. Consumer electronics include devices used for ...
, 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.


Multiway versions

Multiple screw terminals can be arranged in the form of a barrier strip, with a number of short metal strips separated by a raised insulated "barrier" on an insulating "block" – each strip having a pair of screws with each screw connecting to a separate conductor, one at each end of the strip. These are known as "connector strips" or "chocolate blocks" ("choc blocks") in the UK. This nickname arises from the first such connectors made in the UK by GEC, Witton in the 1950s. Moulded in brown plastic, they were said to resemble a small bar of chocolate. A similar arrangement is common with paired screw terminals, where metal tubes are loosely encased in an insulating block with a
set screw In American English, a set screw is a screw that is used to secure an object, by pressure and/or friction, within or against another object, such as fixing a pulley or gear to a shaft. A set screw is normally used without a nut (which distin ...
at each end of each tube to clamp and thus connect a conductor. These are often used to connect
light fixture A light fixture (US English), light fitting (UK English), or luminaire is an electrical device containing an electric lamp that provides illumination. All light fixtures have a fixture body and one or more lamps. The lamps may be in sockets fo ...
s. 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 In electric power distribution, a busbar (also bus bar) is a metallic strip or bar, typically housed inside switchgear, panel boards, and busway enclosures for local high current power distribution. They are also used to connect high volt ...
for power distribution, and so may also include a master input connector, usually binding posts or
banana connector A banana connector (commonly banana plug for the male, banana socket or banana jack for the female) is a single-wire (one conductor) electrical connector used for joining wires to equipment. The term 4 mm connector is also used, especia ...
s.


Installation

Assembly of a screw connection requires some care in workmanship to ensure proper removal of insulation, containment of all wire strands, and the adequate tightening of the screw. If the wire diameter is small in relation to the size of the screw, the wire may be cut through by the over-tightening of the screw. This is less likely to occur when a wire is clamped between two plates by the action of a screw. Since wire strands may not be contained by the screw head in a basic screw terminal, stranded wires may be first crimped into a
ferrule A ferrule (a corruption of Latin ' "small bracelet", under the influence of ' "iron") is any of a number of types of objects, generally used for fastening, joining, sealing, or reinforcement. They are often narrow circular rings made from me ...
to prevent the bridging of terminals; this partly offsets the simplicity and economy of a "bare" wire termination. While wires may be crimped, they should not be heavily tinned with
solder Solder (; NA: ) is a fusible metal alloy used to create a permanent bond between metal workpieces. Solder is melted in order to wet the parts of the joint, where it adheres to and connects the pieces after cooling. Metals or alloys suitable ...
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.


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. 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 Vibration is a mechanical phenomenon whereby oscillations occur about an equilibrium point. The word comes from Latin ''vibrationem'' ("shaking, brandishing"). The oscillations may be periodic, such as the motion of a pendulum—or random, suc ...
or
corrosion Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engi ...
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).


See also

*
Chocbox Chocbox is a patented plastic box for safely enclosing electrical connections made with screw connector strips. It was invented by entrepreneur Peter Moule, who appeared on the BBC television programme '' Dragons' Den'' in 2007. He secured a deal ...
* Wire nut * Euroblock


References

{{Authority control Electrical connectors de:Klemme (Elektrotechnik)