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What is the difference between a constant and switched fuse?

Welcome Fellow OPT7-ite,

Your vehicle uses electrical circuits, just like the ones in your home. These circuits supply power to the electrical components located throughout your car, like your headlights, your radio, and your door locks.

In modern cars and trucks, there are generally two types of circuits that run through the vehicle: circuits that are live all the time and circuits that are only live when the engine is running. A circuit that is live or hot all the time means that it constantly receives power, even when your car is parked and the engine is turned off. We’ll refer to this type of circuit as constant. We’ll refer to cold circuits, or circuits that are only live when the engine is running as switched.












Inside your fuse box, these constant and switched circuits tend to live right next to each other, with each circuit protected by an individual fuse. The fuse serves as a “test point” to determine which type of circuit it may be.

There’s no consistent design across vehicle manufacturers or truly reliable guidelines in owner’s manuals to know which circuits are constant and which are switched. 

Identifying this type of circuit is more difficult since in many cases the switched circuits. In this case, you should consult your owner’s manual for more information on your vehicle or get in touch with a professional who can help


OPT7 Recommends to use a 12V source when tapping into a fuse- tap box. Such examples of a 12V source are radio, dome light or those that are marked as accessory in your owner's manual or on your fuse box cover diagram. 


Thank You,
The OPT7 Customer Experience Team

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