Where and When to Use Fuses.
Fuses are esential in any electrical system, and are used to protect a circuit's cabling from excessive current that could lead to damage and, very often, an electrical fire.
Excessive current is most likely to be caused by three things:
Excessive current is most likely to be caused by three things:
- Incorrect wiring - wrong connections as a result of human error
- Damage to the circuit - e.g. a wire working loose or insulation wearing through and causing a +ve wire to short to ground.
- Overloading the circuit - e.g. connecting one or more pieces of equipment that draw more current than the circuit is designed to carry or, conversely, using cable of insufficient size for the current draw of the intended equipment.
Where and when to use fuses
In an ideal world each individual circuit would be fused, as this would provide the most protection and make fault finding relatively straight forward as it would allow you to narrow down the problem to a single circuit.
It is important to note that the section of cable coming from the battery, positive terminal to the first fuse or battery distribution box is effectively an unprotected part of the circuit.
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. For this reason the length of cable from the battery positive terminal to the first fuse should be as short as possible so that damage is minimised in the event of an electrical fire.
In an ideal world each individual circuit would be fused, as this would provide the most protection and make fault finding relatively straight forward as it would allow you to narrow down the problem to a single circuit.
It is important to note that the section of cable coming from the battery, positive terminal to the first fuse or battery distribution box is effectively an unprotected part of the circuit.
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. For this reason the length of cable from the battery positive terminal to the first fuse should be as short as possible so that damage is minimised in the event of an electrical fire.
Fusing Exceptions
There are some instances where fuses are not normally used, and one example is for the short, length of cable from the battery to the starter motor.
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.
For this reason it is usually deemed impractical to fuse this length of cable, although some boats do have fusible links which are simply a small section of lower current rating cable encased in a fireproof sleeve. They are installed and are designed to melt and break the circuit in an over-current condition. 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 would be damaged.
To offer an increased level of safety it is required 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 boats electrical system in the event of a problem.
There are some instances where fuses are not normally used, and one example is for the short, length of cable from the battery to the starter motor.
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.
For this reason it is usually deemed impractical to fuse this length of cable, although some boats do have fusible links which are simply a small section of lower current rating cable encased in a fireproof sleeve. They are installed and are designed to melt and break the circuit in an over-current condition. 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 would be damaged.
To offer an increased level of safety it is required 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 boats electrical system in the event of a problem.
Fuse ratings.
Continuous rating
Fuse are marked with the current that they will continuously pass without blowing, known as the continuous rating. It is good practice not to allow the continuous current to exceed 75% of the fuse's rated value to accommodate momentary current surges that might cause the fuse to fatigue over time or blow unnecessarily (nuisance blow).
How much current will it take for a fuse to blow?
In simplified terms the greater the current is above the continuous rating, the faster the fuse will blow.
For example, if a 10A fuse is exposed to 11A then it might take many minutes for it to blow but if it is exposed to 20A then it may blow in a fraction of a second.
Continuous rating
Fuse are marked with the current that they will continuously pass without blowing, known as the continuous rating. It is good practice not to allow the continuous current to exceed 75% of the fuse's rated value to accommodate momentary current surges that might cause the fuse to fatigue over time or blow unnecessarily (nuisance blow).
How much current will it take for a fuse to blow?
In simplified terms the greater the current is above the continuous rating, the faster the fuse will blow.
For example, if a 10A fuse is exposed to 11A then it might take many minutes for it to blow but if it is exposed to 20A then it may blow in a fraction of a second.
Selecting the correct fuse rating
If replacing a blown fuse in a manufacturer designed application, e.g. in a boat fuse panel, then the same type and rating of fuse should be used. If a fuse continues to blow then there must be a fault with the circuit and a higher rating fuse should not be fitted to overcome this, even temporarily.
Doing this creates a high risk of component failure and electrical fire.
When specifying a fuse for an after market application, the key consideration is that the fuse should be the weakest point i.e. lowest rated component so that it always blows before any damage occurs to other parts of the electrical 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:
The fuse rating should lie somewhere between these two values to allow normal operation but blow on overload.
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.
If replacing a blown fuse in a manufacturer designed application, e.g. in a boat fuse panel, then the same type and rating of fuse should be used. If a fuse continues to blow then there must be a fault with the circuit and a higher rating fuse should not be fitted to overcome this, even temporarily.
Doing this creates a high risk of component failure and electrical fire.
When specifying a fuse for an after market application, the key consideration is that the fuse should be the weakest point i.e. lowest rated component so that it always blows before any damage occurs to other parts of the electrical 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:
- The current rating of the smallest cable in the circuit
- The current draw on the circuit under normal expected operating conditions
The fuse rating should lie somewhere between these two values to allow normal operation but blow on overload.
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.
Circuit Breakers
What is a circuit breaker?
Circuit breakersprovide overload protection for the circuit in the same way that a fuse does (by breaking the circuit continuity) but, unlike a fuse, can be reset rather than having to be replaced. These are useful in applications where, due to their nature, an over-current condition is expected to occur now and again under normal use and to have to frequently replace fuses would be inconvenient (e.g. electric motors used for winches, power windows etc.). They are also useful where you might need to reset the circuit quickly and don't want to spend time looking for a fuse.
How do they work?
Most circuit breakers found in 12V or 24V systems operate thermally. The heat generated by the excess current during an overload causes the contacts inside the breaker to come apart and break the circuit.
These are then reset either manually or automatically depending on the design of the circuit breaker. Circuit breakers that have to be manually reset provide an opportunity to check for any problems before using the circuit again and on some types you can manually trip the breaker, which is useful for isolating the circuit and for testing the breaker.
What is a circuit breaker?
Circuit breakersprovide overload protection for the circuit in the same way that a fuse does (by breaking the circuit continuity) but, unlike a fuse, can be reset rather than having to be replaced. These are useful in applications where, due to their nature, an over-current condition is expected to occur now and again under normal use and to have to frequently replace fuses would be inconvenient (e.g. electric motors used for winches, power windows etc.). They are also useful where you might need to reset the circuit quickly and don't want to spend time looking for a fuse.
How do they work?
Most circuit breakers found in 12V or 24V systems operate thermally. The heat generated by the excess current during an overload causes the contacts inside the breaker to come apart and break the circuit.
These are then reset either manually or automatically depending on the design of the circuit breaker. Circuit breakers that have to be manually reset provide an opportunity to check for any problems before using the circuit again and on some types you can manually trip the breaker, which is useful for isolating the circuit and for testing the breaker.
Fuses vs circuit breakers
Whilst circuit breakers are very useful solution in certain applications it should be noted that a good quality fuse will ultimately be more reliable as there are no moving parts, so should be considered first and where sensitive electronic equipment is involved a circuit breaker should never be used.
This is because the time taken for the circuit breaker to operate can be longer than it would take a traditional fuse to blow, potentially exposing the circuit to damaging overload currents for longer.
