These are called tap fuses or piggy back fuses. They are a useful way of obtaining power off the fuse box in a car. They replace an existing fuse and you replace the fuse removed into the tap fuse. They also have a 3 amp fused wire coming off the tap fuse which can then be used to power a reversing camera system. We find a 3 amp fuse is enought as most camera systems draw less than 1 amp. You should use on a circuit that is controlled by the ignition. These are for standard size blade fuses.