![DPDT knife switch in closed position](https://upload.wikimedia.org/wikipedia/commons/f/fe/DPDT_knife_switch_in_closed_position.jpg)
A knife switch is a type of
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 of ...
used to control the flow 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 described ...
in a
circuit. It is composed of a
hinge
A hinge is a mechanical bearing that connects two solid objects, typically allowing only a limited angle of rotation between them. Two objects connected by an ideal hinge rotate relative to each other about a fixed axis of rotation: all other ...
which allows a
metal
A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...
lever, or
knife
A knife ( : knives; from Old Norse 'knife, dirk') is a tool or weapon with a cutting edge or blade, usually attached to a handle or hilt. One of the earliest tools used by humanity, knives appeared at least 2.5 million years ago, as evidenced ...
, to be lifted from or inserted into a
slot
Slot, the slot or Slots may refer to:
People
* Arne Slot (born 1978), Dutch footballer
* Gerrie Slot (born 1954), Dutch cyclist
* Hanke Bruins Slot (born 1977), Dutch politician
* Tonny Bruins Slot (born 1947), Dutch association football coach wh ...
or jaw. The hinge and jaw are both fixed to an
insulated base, and the knife has an insulated handle to grip at one end. Current flows through the switch when the knife is pushed into the jaw.
Knife switches can take several forms, including single throw, in which the "knife" engages with only a single slot, and
double throw, in which the knife hinge is placed between two slots and can engage with either one. Also, multiple knives may be attached to a single handle and can be used to activate more than one circuit simultaneously.
Current uses
Though used commonly in the past, knife switches are now rare, finding use largely in science experiments where the position of the switch may be plainly seen in demonstration. The knife switch is extremely simple in construction and use, but its exposed metal parts present a great risk of
electric shock
Electrical injury is a physiological reaction caused by electric current passing through the body. The injury depends on the density of the current, tissue resistance and duration of contact. Very small currents may be imperceptible or produce ...
, and the switch is subject to
arcing
An electric arc, or arc discharge, is an electrical breakdown of a gas that produces a prolonged electrical discharge. The electric current, current through a normally Electrical conductance, nonconductive medium such as air produces a plasma (p ...
when opened at higher
voltage
Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to m ...
s, which poses a further risk of shock or
burns Burns may refer to:
* Burn, an injury (plural)
People:
* Burns (surname), includes list of people and characters
Business:
* Burns London, a British guitar maker
Places:
;In the United States
* Burns, Colorado, unincorporated community in Eagle ...
to the operator and can cause fires or explosions under certain conditions.
Open knife switches were supplanted by
safety switches
Safety is the state of being "safe", the condition of being protected from harm or other danger. Safety can also refer to the control of recognized hazards in order to achieve an acceptable level of risk.
Meanings
There are two slightly dif ...
with current carrying contacts inside a metal enclosure which can only be opened by switching off the power. In modern applications, automatic switches (such as
contactor
:''In semiconductor testing, contactors can also be referred to as the specialized socket that connects the device under test.''
:''In process industries, a contactor is a vessel where two streams interact, for example, air and liquid. See Gas- ...
s or
relay
A relay
Electromechanical relay schematic showing a control coil, four pairs of normally open and one pair of normally closed contacts
An automotive-style miniature relay with the dust cover taken off
A relay is an electrically operated switch ...
s), and manual switches such as
circuit breaker
A circuit breaker is an electrical safety device designed to protect an electrical circuit from damage caused by an overcurrent or short circuit. Its basic function is to interrupt current flow to protect equipment and to prevent the risk ...
s are used. These devices use a snap action mechanism which opens the switch contacts rapidly, and feature an
arc chute
A circuit breaker is an electrical safety device designed to protect an electrical circuit from damage caused by an overcurrent or short circuit. Its basic function is to interrupt current flow to protect equipment and to prevent the risk ...
where the arc caused by opening the switch is quenched. These devices also prevent injury due to accidental contact, as all of the current carrying metal parts of the switch are hidden within or surrounded by insulating guards.
References
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Switches
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