A screw terminal is a type of electrical connector where a wire is held by the
tightening of a screw. 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 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.
Applications 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.
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 and, when used on consumer electronics, often have a washer with
gripping "teeth". 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 in the UK.
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 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.
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 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 must never be 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
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.
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 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. 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. History
Screw terminals were one of the first types of electrical connectors to be
used, with one of the earliest patents found utilizing a screw "terminal" being
that awarded to William P. Marr of Toronto, Canada - for an "Electric Wire
Connecter" U.S. Patent 1,583,479, Filed March 3 1923, Patented May 4,1926
See also Electrical connector
Wire nut Chocbox
References