A GUIDE TO FUSES & CIRCUIT BREAKERS
The primary purpose of both fuses and circuit breakers is to respond when a fault condition occurs and prevent a circumstance where excessive current may cause overheating of cables and appliances which may lead to damage or even initiate a fire in the vehicle.
FUSES:
Composed of a thin wire in a housing and sized to continually carry a selected maximum current, it will do so until the current increases above its rating and will then melt thus opening the circuit and stopping the current flow. They are simple, inexpensive and generally reliable devices which only act whenever a fault condition such as a short circuit occurs. They are not intended or effective in stopping the current when a circuit is simply overloaded by connecting too many or oversized appliances. Following activation they are not re-useable so spares need to be carried for replacements.
TYPES: Fuses are available in several types from the Automotive Blade type Mini, Standard and Maxi rated from 1Amp to 100Amp. Bolt-in types such as ANG and ANL from 100-400Amps and Class-T ceramic High Rupture type from 200 to 600Amp with an Interrupt Capacity of 20,000Amps. These Class T fuses are applicable to applications where large batteries and very large cables are employed as high short-circuit fault currents may cause flash-over within fuses of lower interrupt capacity.
CIRCUIT BREAKERS:
A circuit breaker is essentially a switch which trips to the off condition when the current rises above its design rating. This current rise may be due to a fault such as a short circuit or it may be due to overload. Most commonly they are resettable so can be placed back into service following event activation. They also provide the useful function of a manually activated isolation switch.
TYPES: Breakers can be internally activated by magnetic action and by thermal action. Magnetic action is more rapid and suited to short circuit faults whilst thermal action is a little slower but simpler and cheaper. Some breakers incorporate both magnetic and thermal action but are expensive. The slow response of the thermal type suits applications where there are brief high-current events such as motor starts. The magnetic function is usually set
well above the continuous current rating to avoid nuisance trips and respond only to high short-circuit currents.
Breakers are available in various mounting forms and in current rating from 10 Amps to 500 Amps. For battery and solar applications it is important to select breakers rated for DC operation and to suit the voltage. Domestic 230vac circuit breakers are NOT rated for DC operation.
Self resetting breakers are available for 12vdc application. They are small and inexpensive but generally of cheap construction and often unreliable. They operate by a bi-metal
bridge across the two contacts deforming with heat produced by the electrical current causing the
bridge to open. After a short time the bi-metal cools and the breaker automatically closes to restore the circuit. This action will continue indefinitely and may eventually raise the cable to a dangerous temperature so they are inappropriate for application in a vehicle which may be unattended.
SUMMARY:
A fuse is appropriate where is no requirement for manual switching and protection is only needed for possible short-circuit faults. They are cheaper and more compact.
Circuit breakers are appropriate where it is possible to have overload conditions yet still protect against short circuits. They also provide for manual switching and generally dispense with the need to carry spares.