An inertial switch is a

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 ...

, firmly mounted upon a vehicle or other mobile device, that triggers in the event of shock or vibration. It is a part of

electrical circuits An electrical network is an interconnection of electrical components (e.g., batteries, resistors, inductors, capacitors, switches, transistors) or a model of such an interconnection, consisting of electrical elements (e.g., voltage sources, c ...

that may either enable or disable some function.

Disconnect

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

fuel pump A fuel pump is a component in motor vehicles that transfers liquid from the fuel tank to the carburetor or fuel injector of the internal combustion engine. Carbureted engines often use low pressure mechanical pumps that are mounted outside the f ...

in automotive applications. This functionality is required in some vehicle racing applications, since an electric fuel pump may otherwise continue operating after a

collision In physics, a collision is any event in which two or more bodies exert forces on each other in a relatively short time. Although the most common use of the word ''collision'' refers to incidents in which two or more objects collide with great fo ...

or rollover. 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 Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different eleme ...

) is trapped within a spring-loaded cage. A shock in any direction will cause movement of the mass relative to the cage. If sufficiently shocked, the cage will spring open which actuates 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 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 ...

) to disable the high power circuit of a

battery electric vehicle A battery electric vehicle (BEV), pure electric vehicle, only-electric vehicle, fully electric vehicle or all-electric vehicle is a type of electric vehicle (EV) that exclusively uses chemical energy stored in rechargeable battery packs, ...

upon collision.

Actuation

A normally open switch is used to activate passenger safety equipment early in a collision, typically to pre-tension seat belts and/or to activate air-bags to protect the occupant(s) from collision with the vehicle interior or the steering wheel. Inertia switches are also used in

ordnance Ordnance may refer to: Military and defense *Materiel in military logistics, including weapons, ammunition, vehicles, and maintenance tools and equipment. **The military branch responsible for supplying and developing these items, e.g., the Unit ...

applications for both safety (shell will not explode until it has been subjected to launch acceleration) and fuzing.

References

Safety switches {{Automotive-part-stub