Editing Switch (section)

===Contact terminology===
[[File:Tpst.jpg|upright=1.4|thumb|Triple-pole single-throw (TPST or 3PST) [[knife switch]] used to short the windings of a three‑phase wind turbine for [[Dynamic braking|braking]] purposes. Here the switch is shown in the open position.]]

In electronics, switches are classified according to the arrangement of their contacts. A pair of contacts is said to be "''closed''" when current can flow from one to the other. When the contacts are separated by an [[Insulator (electric)|insulating air gap]], they are said to be "''open''", and no current can flow between them at normal voltages. The terms "''make''" for closure of contacts and "''break''" for opening of contacts are also widely used.

The terms '''pole''' and '''throw''' are also used to describe switch contact variations. The number of "''poles''" is the number of electrically separate switches which are controlled by a single physical actuator. For example, a "''2-pole''" switch has two separate, parallel sets of contacts that open and close in unison via the same mechanism. The number of "''throws''" is the number of separate wiring path choices other than "open" that the switch can adopt for each pole. A single-throw switch has one pair of contacts that can either be closed or open. A double-throw switch has a contact that can be connected to either of two other contacts, a triple-throw has a contact which can be connected to one of three other contacts, etc.<ref>[http://www.herley.com/index.cfm?act=app_notes&notes=switches RF Switch] {{webarchive|url=https://web.archive.org/web/20110423212637/http://www.herley.com/index.cfm?act=app_notes&notes=switches |date=2011-04-23 }} Explanation by [[Herley Industries|Herley&nbsp;– General Microwave]]</ref>

{{anchor|Break-before-make|Make-before-break}}
In a switch where the contacts remain in one state unless actuated, such as a [[push-button]] switch, the contacts can either be '''normally open''' (abbreviated "'''n.o.'''" or "'''no'''") until closed by operation of the switch, or '''normally closed''' ("'''n.c.'''" or "'''nc'''")<ref group="nb" name="NB_NC"/> and opened by the switch action. A switch with both types of contact is called a ''changeover switch'' or ''double-throw switch''. These may be "'''make-before-break'''" ("'''MBB'''" or shorting) which momentarily connects both circuits, or may be "'''break-before-make'''" ("'''BBM'''" or non-shorting) which interrupts one circuit before closing the other.

These terms have given rise to abbreviations for the types of switch which are used in the [[electronics]] industry such as "''single-pole, single-throw''" (SPST) (the simplest type, "on or off") or "''single-pole, double-throw''" (SPDT), connecting either of two terminals to the common terminal. In [[Mains electricity|electrical power]] wiring (i.e.,&nbsp;house and building wiring by [[electrician]]s), names generally involve the suffix ''"-way"''; however, these terms differ between [[British English]] and [[American English]] (i.e., the terms ''two&nbsp;way'' and ''three&nbsp;way'' are used with different meanings).

{{Anchor|Table of switch comparisons|Table of switch types}}
{| class="wikitable"
|-
! Electronics specification and abbreviation
!valign=top| Expansion <br />of<br />abbreviation
! British<br />mains <br />wiring<br />name
! American <br />electrical<br />wiring<br />name
! Description
! Schematic
! Symbol IEC 60617
|-------
| '''SPST'''<br/>(1P1T) || Single pole, single throw || One-way || Two-way || A simple on-off switch: The two terminals are either connected together or disconnected from each other. An example is a [[light switch]]. || rowspan="2" | [[File:SPST-Switch.svg|100px]] || [[File:Symbol circuit breaker (one-pole).svg|50px]]
|-
|'''SPST-NO'''
'''Form A'''<ref name=":0">{{Cite web|url=http://www.esterline.com/powersystems/DesignReference/RelayHandbook.aspx|title=Engineer's Relay Handbook, 5th edition, Chapter 1.6 by RSIA (formerly NARM)|archive-url=https://web.archive.org/web/20170705143411/http://www.esterline.com/powersystems/DesignReference/RelayHandbook.aspx|archive-date=2017-07-05|url-status=dead}}</ref>
|Single pole, single throw, normally open
|
|
|A simple on-off switch. The two terminals are normally disconnected (open) and are closed when the switch is activated. An example is a [[Push switch|pushbutton switch]]. || 
|-
|'''SPST-NC'''<br/>'''Form B'''<ref name=":0" />
|Single pole, single throw, normally closed
|
|
|A simple on-off switch. The two terminals are normally connected together (closed) and are open when the switch is activated. An example is a [[Push switch|pushbutton switch]].
|[[File:SPST-NC-Switch.svg|frameless|100x100px]] || 
|-------
| '''SPDT'''<br/>'''Form C'''<ref name=":0" /> 
| Single pole, double throw || Two-way || Three-way || A simple break-before-make changeover switch: C (COM, Common) is connected either to L1 or to L2. || rowspan="2" | [[File:SPDT-Switch.svg|100px]] || [[File:Symbol change over switch.svg|50px]]
|-------
| '''SPCO<br />SPTT, c.o.''' || Single pole changeover<br />''or''<br />single pole, centre off ''or''<br />single pole, triple throw|| &nbsp; || &nbsp; || Similar to ''SPDT''. Some suppliers use ''SPCO/SPTT'' for switches with a stable off position in the centre and ''SPDT'' for those without. || &nbsp;
|-------
| &nbsp; || &nbsp; || &nbsp; || &nbsp; || Serial switch or two-circuit switch{{citation needed|date=January 2024}} || &nbsp; || [[File:Symbol Series switch.svg|50px]]
|-------
| '''DPST'''<br/>(2P1T) || Double pole, single throw || Double pole ||Double pole || Equivalent to two ''SPST'' switches controlled by a single mechanism. || [[File:DPST-symbol.svg|100px]] || [[File:Symbol circuit breaker (two-pole).svg|50px]]
|-------
| '''DPDT'''<br/>(2P2T) || Double pole, double throw || || || Equivalent to two ''SPDT'' switches controlled by a single mechanism. ||rowspan=2| [[File:DPDT-symbol.svg|100px]] || [[File:Symbol change over switch (two-pole).svg|50px]]
|-------
| '''DPCO''' || Double pole changeover <br /> ''or'' double pole, centre off || &nbsp; || &nbsp; || Schematically equivalent to ''DPDT''. Some suppliers use ''DPCO'' for switches with a stable center position and ''DPDT'' for those without. A DPDT/DPCO switch with a center position can be "off" in the center, not connected to either L1 or L2, or "on", connected to both L1 and L2 at the same time. The positions of such switches are commonly referenced as "on-off-on" and "on-on-on" respectively. || &nbsp;
|-------
| &nbsp; || &nbsp; || Intermediate switch || Four-way switch || ''DPDT'' switch internally wired for polarity-reversal applications: only four rather than six wires are brought outside the switch housing. Also called cross switch, crossover switch or reversing switch.{{citation needed|date=January 2024}}
|| [[File:crossover-switch-symbol.svg|100px]] || [[File:Symbol cross switch.svg|50px]]
|-------
| '''2P6T''' || Two pole, six throw || || || Changeover switch with a COM (Common), which can connect to L1, L2, L3, L4, L5, or L6; with a second switch (2P, two pole) controlled by a single mechanism. || [[File:2P6T-symbol.svg|100px]] || &nbsp;
|}
Switches with larger numbers of poles or throws can be described by replacing the "S" or "D" with a number (e.g. 3PST, SP4T, etc.) or in some cases the letter "T" (for "triple") or "Q" (for "quadruple"). In the rest of this article the terms ''SPST'', ''SPDT'' and ''intermediate'' will be used to avoid the ambiguity.