General
This section discusses the basics of electromechanical switch devices, whose function is to make or break an electrical circuit.  There are three types of switches commonly used in non-manned space flight.  (No solid-state switches are discussed.)

Sensitive and Push (snap action) Microgap Switches are actuated by a reciprocating plunger, operated by physical movement.  They are used to sense deployment of antennas or solar arrays and shut off movement when a pre determined position is reached.  Hermetically sealed switches should be used in lieu of open constructions whenever possible to prevent contamination.  Sealed switches also eliminate increased potential for arcing caused by reduced internal pressure.

Thermostatic Switches are actuated by temperature change.  For space use, they are generally actuated by a bimetallic disc element that uses a snap action.  They are used for temperature protection or temperature control, such as control heater circuits.

Pressure Switches are actuated by pressure changes in liquid or gas.  In satellite use, they are used to monitor pressure vessels such as propellant tanks.  The switch mechanism consists of an electrical snap switch actuated by the displacement of a pressure sensing device.

(For non manned flight, manually operated toggle and rotary switches are not used and are not discussed here.)

It is important to not only consider the effect of the switch on the circuit, but to consider the effect of the circuit on the switch.  Switches are given multiple contact ratings depending on the type of load being switched.  Capacitive loads have high in-rush (surge) currents for short periods of time.  Motor loads have initial surge limited only by the DC resistance of the wire.  Inductive loads are subject to inductive surge when the circuit is opened.  Purely resistive loads are not subject to surge.  Most circuits are a blend of inductive or capacitive and resistive loads.  Excessive surge currents can cause contact overheating, resulting in pitting or other damage to contacts.  In worst case scenarios, surge currents can cause contacts to weld closed.  Consequently, it is necessary to derate their current rating in order to provide for protection from surge currents.

Environmental conditions that have severe shock, vibration and acceleration can loosen or dislodge mechanical components of the switch, and even cause inadvertent plunger actuation.  Low temperatures cause certain materials to contract, while high temperature can cause chemical degradation to non metallic materials such as insulators.

For switches used in space, it is recommended that small particle cleaning and inspection is performed on each device, in order to assure there are no conductive particles are present that could bridge contacts or interfere with mechanical operation.  It is also important to have pre-seal inspection for contact alignment, deformed or loose mechanical parts, or binding parts.