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 Open single-pole single-throw (SPST) knife switch | |
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Aknife switch is a type ofswitch used to control the flow ofelectricity in acircuit. It is composed of ahinge which allows ametal lever, orknife, to be lifted from or inserted into aslot or jaw. The hinge and jaw are both fixed to aninsulated base, and the knife has an insulated handle. Current flows through the switch when the knife is pushed into the jaw. Knife switches can take several forms, includingsingle-throw, in which the knife engages with only a single slot, anddouble-throw, in which the knife hinge is placed between two slots and can engage with either one. Multiple knives may be attached to a single handle and can be used to activate more than one circuit simultaneously; this is amulti-pole switch.
Though used commonly in the past, knife switches are now rare, finding use largely in science demonstrations where the exposed mechanics of the switch make its function and state visually apparent. The knife switch is extremely simple in construction and use, but for any dangerous electrical supply its exposed metal parts present a great risk ofelectric shock, and the switch is subject toarcing when opened at highervoltages, which poses a further risk of shock orburns to the operator and can cause fires or explosions under certain conditions.
Open knife switches were supplanted bysafety switches with current-carrying contacts inside metal enclosures which can only be opened by switching off the power. In modern applications, automatic switches (such ascontactors andrelays) and manual switches such ascircuit breakers are used. These devices use snap-action mechanisms which open the switch contacts rapidly and featurearc chutes where the arcs caused by opening the switches are quenched. These devices also prevent injury due to accidental contact, as all of the current–carrying metal parts of the switch are surrounded by insulating guards.
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