They push-fit into fuse holders or boxes and are held in place by friction. The 30mm & 32mm fuses are generally interchangeable because the springs in the holders accommodate the small difference in length.īlade fuses are now the most commonly used category of fuses with almost every new vehicle featuring one or more of the different types. These fuses are normally used in fuse holders that screw or twist-lock together. Generally now used in lower current applications such as audio head units and accessories (cigar lighter socket plugs etc.) 20mm fuses are 5mm in diameter and 30 & 32mm fuses are 6.4mm in diameter (1/4'). Older style fuses with a glass body and metal end caps joined to the fusible link inside, with the number referring to the overall length of the fuse. There are several types of fuse currently in use in the automotive market and the table below gives a brief description of each: For example, if the normal expected current draw is 10A and the cable size is 25A, then a fuse rated at 15A would be appropriate. >The fuse rating should lie somewhere between these two values to allow normal operation but blow on overload. The current draw on the circuit under normal expected operating conditions.The current rating of the smallest cable in the circuit.However, you also do not want the fuse to keep blowing under normal operation (known as a nuisance blow), so the two elements to consider are: lowest rated component) so that it always blows before any damage occurs to other parts of the electrical circuit. When specifying a fuse for an after-market application, the key consideration is that the fuse should be the weakest point (i.e. Doing this creates a high risk of component failure and electrical fire. If a fuse continues to blow then there must be a fault with the circuit and a higher rating fuse should never be fitted to overcome this, even temporarily. in a vehicle fuse panel, then the same type and rating of fuse should be used. If replacing a blown fuse in a manufacturer-designed (factory) application, e.g. To offer an increased level of safety it is common in many race car, kit car, custom car and leisure vehicle builds to fit a battery cut-off or master switch that can be manually operated to isolate the main battery or auxiliary batteries from the rest of the vehicle's electrical system in the event of a problem. The other reason for not fusing the starter circuit is that if the battery is disconnected from the alternator whilst it is turning (as would be the case if a fuse blew) the diodes in the alternator's rectifier can be damaged. They are installed with the cable being protected and are designed to melt and break the circuit in an over-current condition. For this reason it is usually deemed impractical to fuse this length of cable, although some vehicles do have fusible links which are simply a small section of lower current rating cable encased in a fireproof sleeve. Starter motors are normally the highest current draw electrical item on a vehicle as they have to crank the engine, and the current can reach several hundred Amps, especially with large diesel engines that have a high compression ratio.

There are some instances where fuses are not normally used and one example is for the, normally short, length of cable from the battery to the starter motor. For this reason the length of cable from the battery +ve to the first fuse should be as short as possible so that damage is minimised in the event of an electrical fire.

If there is a short anywhere along this length then it is very likely to catch fire as the first fuse will not experience the excess current. It is important to note that the section of cable coming from the battery +ve terminal to the first fuse (or battery distribution box) is effectively an unprotected part of the circuit. A good compromise is that every individual circuit should be fused as this provides a good degree of protection and at least allows you to narrow down the problem to one circuit. Having said that this is ideal it is nearly always impractical as it would lead to many fuses fitted throughout an electrical system. In an ideal world each individual section of positive cable would be fused as this would provide the most protection and make fault finding relatively straight forward, because it would allow you to narrow down the problem to a single section of cable (i.e.

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