An inertial switch is a switch

Switch

In electronics, a switch is an electrical component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another....

, firmly mounted upon a vehicle or other mobile device, that senses shock or vibration. It is a part of electrical circuits

Electrical network

An electrical network is an interconnection of electrical elements such as resistors, inductors, capacitors, transmission lines, voltage sources, current sources and switches. An electrical circuit is a special type of network, one that has a closed loop giving a return path for the current...

 that may either enable or disable some function.

Disconnect

The switch shown to the right is intended to disable an electric fuel pump

Fuel pump

A fuel pump is a frequently essential component on a car or other internal combustion engined device. Many engines do not require any fuel pump at all, requiring only gravity to feed fuel from the fuel tank through a line or hose to the engine...

 in automotive applications. This functionality is required in some vehicle racing

Auto racing

Auto racing is a motorsport involving the racing of cars for competition. It is one of the world's most watched televised sports.-The beginning of racing:...

 applications, since an electric fuel pump may otherwise continue operating after a collision or rollover

Rollover

A rollover is a type of vehicle accident in which a vehicle tips over onto its side or roof. The most common cause of a rollover is traveling too fast while turning.- Dynamics :Vehicles can roll over in several ways...

. If the fuel line is broken or the vehicle is inverted, fuel may be spilled, creating a fire hazard. A small loose weight (called a proof mass) is trapped within a spring-loaded cage. A shock in any direction will cause movement of the mass relative to the cage, and if sufficiently shocked the cage will spring open, actuating an associated switch. The switch is reset by pressing the cage closed through the flexible (red) top cover, retrapping the mass. These switches are also used to open a contactor (a large relay) to disable the high power circuit of a battery electric vehicle

Battery electric vehicle

A battery electric vehicle, or BEV, is a type of electric vehicle that uses chemical energy stored in rechargeable battery packs. BEVs use electric motors and motor controllers instead of, or in addition to, internal combustion engines for propulsion.A battery-only electric vehicle or...

 upon collision.

Actuation

A normally open switch is used to activate passenger safety equipment early in a collision, typically to pre-tension seat belt

Seat belt

A seat belt or seatbelt, sometimes called a safety belt, is a safety harness designed to secure the occupant of a vehicle against harmful movement that may result from a collision or a sudden stop...

s and/or to activate air-bags

Airbag

An Airbag is a vehicle safety device. It is an occupant restraint consisting of a flexible envelope designed to inflate rapidly during an automobile collision, to prevent occupants from striking interior objects such as the steering wheel or a window...

 to protect the occupant(s) from collision with the vehicle interior or the steering wheel

Steering wheel

A steering wheel is a type of steering control in vehicles and vessels ....

.