A screw terminal is a type of electrical connecting where a wire is held by the force tightening of a screw.
The wire may be stripped directly under the head of a screw, or held by a metal plate forced against the wire by a screw.The wire can be directly stripped of insulation and inserted under the head of a screw or into the terminal. Otherwise, for stranded wire need to be inserted first into a ferrule,then inserted into the terminal, or can also be attached to a lug. which can be fixed under the screw head.
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 such as clothes dryers and ovens drawing in excess of 15 amperes.
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 and, when used on consumer electronics, often have a washer with gripping "teeth".
Printed circuit board (PCB) terminal blocks are specially designed with a copper alloy pin of suitable size and length and can be inserted in printed circuit boards to be soldered to allow electrical signals and current to flow to and from PCBs and electrical equipment. Some designs provide features that allow the flow of molten solder to endure a better connection between the circuit traces of the board and the electrical equipment which is meant to be controlled or fed appropriate power.
Two paired screw terminals, typically used for connecting ceiling-mounted light fixtures
Multiple screw terminals can be arranged in the form of a barrier strip (as illustrated at the top right), 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 nick-name 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 at each end of each tube to hold and thus connect a conductor. These are often used to connect light fixtures and are shown at the right.
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.
Assembly of a screw connection requires some care in workmanship to ensure proper removal of insulation, containment of all wire strands, and the tightening of the screw. If the wire diameter is small compare to the size of the screw, the wire may be cut through by the over-tightening of the screw. This is less to occur when a wire is clamped between two plates by the pressure of a screw. Since wire strands may not be contained by the screw head in a basic screw terminal, stranded wires usually crimped into a ferrule to prevent the bridging of terminals; this partly offsets the economy of a "bare" wire termination.
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.
Advantages and disadvantages
Two types of screw terminals showing a significant development in design. Basic, with the wires held between the screw heads and the terminals (left) and with the wires held, reversely, between the metal clamping plates and the rear of the terminals (right).
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.
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 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.
In vibration environment, the screw easily can be loose after long time using.
The use of screw terminal in building wiring installations has sharply declined in favour of crimp, push and twist type connectors which are not prone to working loose and easier to fit.