Fuses, Circuit Breakers, and Fuseholders
Overcurrent protection devices serve two purposes in an electrical circuit:
1. To protect components, equipment and associated wiring from costly damage.
2. To isolate sub-systems from the main system once a current fault has occurred.
Fuses and circuit breakers are commonly selected as the preferred overcurrent devices.
The key is that the overcurrent device should blow before the wire is damaged. It
can also protect some devices, but if the device contains transistors, the transistor(s)
usually blow well before the device. So it'll save the wire going to your stereo,
but probably not the stereo. What it will do is keep the stereo from catching
on fire; this is it's real purpose in life
Fuses are current sensitive devices and selected to be the weakest link in the
circuit. Circuit protection is provided when the fuse link melts and safely interrupts
THERMAL CIRCUIT BREAKERS
The basic components of a thermal circuit breaker are the composite alloy reed,
two precious metal contacts and the interconnecting terminals. When an overcurrent
occurs, heat is generated as the current flows through the reed causing the reed
to deflect and snap open. This separates
the contacts and safely interrupts the current flow.
There are three types of automotive breakers:
Type 1 the most common and cheapest, will automatically reset itself when
it cools off. This means the power come back on, possibly unexpectedly.
Type 2 will stay tripped for as long as power is applied, but it too will
reset if power is removed from the breaker and it cools off.
Type 3 will stay tripped until it is manually reset, usually when a button
on it is pressed, regardless of anything else.
TYPES OF OVERCURRENT
Short Circuit: Short circuit is a current condition that greatly exceeds
the rating of the device. It is caused when a malfunction or accident
creates a break in the normal path allowing electricity to flow directly
This shorter current path bypasses the resistance offered by
the circuit components connected in the normal current path. In this
situation, there is virtually no resistance to impede the current and
will build to a level where the heat generated can cause insulation and/or
Overload: An overload is an overcurrent that is within the normal current
path. Overloads occur when the current exceeds the value for which
the equipment or associated wiring is rated. This typically occurs
are connected to the circuit or when a
device connected to the circuit malfunctions. Sustained overloads may
slowly cause overheating of the wiring and the components. The circuit
device must open before these types of overloads cause damage.
SELECTING OVERCURRENT PROTECTION
During normal conditions, an overcurrent protection device must carry
the current without nuisance openings. However, when an overload or short
occurs, the device must interrupt the overcurrent and withstand the voltage
across the device after arcing. To properly select an
overcurrent device, the following items must be carefully considered:
Voltage Rating: Represents the maximum system voltage present in the
circuit in which the overcurrent device is installed. The system voltage
not exceed this value for proper operation of the device during an overcurrent
Current Rating: This is the amperage value marked on the circuit protection
device. The circuit protection device is designed to handle this value
under steady operating conditions and at ambient temperatures near 25°C. The current
rating of the fuse should not exceed the rating of the device.
Characteristics of equipment to be protected/In-rush characteristics:
During the operation of protected equipment, system current can significantly
vary. This is particularly evident when motor or other inductive loads
in the circuit
cause large current surges during start-up or
shutdown. A time-delay fuse should be used in applications where in-rush
currents are possible.
Because fuses contain no moving parts, they do not generally fail due
to wear-and-tear. When a fuse opens, it’s important that you determine
caused the overcurrent situation in the first place. Otherwise, the
replacement fuse is likely to open, too — and the circuit still will
function as intended.
Fuses are installed in one or more fuse blocks
— which can be located in various locations in the vehicle, depending
on its make and model.
fuse panels can be found in the passenger compartment, below the
steering wheel; in the engine compartment; or in some cases, in
the trunk. See the vehicle owner’s manual for the precise location
of the fuse block(s).
On most vehicles, each clip in the fuse block will be labeled, to
help you identify which circuit the fuse protects (wipers, radio,
The fuse itself is labeled with an amperage rating, and (in the
case of blade fuses) a color coding.
Important: Always replace an open fuse with a new fuse of the same
amperage rating and fuse style/dimension. Otherwise, the fuse may
not be able
to perform its protective function in the circuit. If you are unsure
is required for the circuit, check the vehicle
owner’s manual or the label on the fuse block.