CONQUER
Fuce Facts
FUSES
FUSE SELECTION GUIDE
What are fuses? They are protectors that prevent damage
The fuse must carry the normal load current of the
circuit without nuisance openings. However, when an
overcurrent occurs the fuse must interrupt the
overcurrent, limit the energy let-through, and withstand
the voltage across the fuse during arcing. To properly
select a fuse the followings must be considered:
to electronic and electrical equipment. Fuses allow electric
current to pass through them until the amount of current goes
over the fuse's rating. At that time, the fuse "Blows" and stops
the flow of electric current. Fuses are rated in amperes, the
unit of measure for electric current.
FAST-ACTING vs. SLOW-BLOW. Fast-Acting fuses react
quickly to current overloads. Slow-Blow fuses are less sen-
• Normal operating current (The current rating of a fuse is
sitive to start-up surge and protect against long-term
overloads.
typically derated 25% for operation at 25% to avoid nuisance blowing. For example, a fuse with a current rating
A FUSE IS A SAFETY DEVICE. Always replace a blown
of 10A is not usually recommended for operation at more
than 7.5A in a 25% ambient.)
fuse with the same type-replace fast-acting with fast acting,
slow-blow with slow-blow. Always replace with the same
• Overload current and length of time in which the fuse must
current (Ampere) rating and the same (or Higher) voltage
rating.
open.
• Application voltage (AC or DC Voltage).
FUSE TYPE
There are four basic types of fuses: (1) Slow Blow/Time lag
fuses (2) Dual element slow blow fuses (3) Fast acting fuses
(4) Very fast acting fuses
A major type of slow blow fuse is the dual-element fuse.
This fuse consists of a short circuit strip and a soldered joint,
spring connection. During overload conditions, the soldered
joint gets hot enough to melt and the spring shears the junction loose. Under short circuit conditions, the short circuit
element operates to open the circuit. All dual-element fuses
are considered to be slow blow but all slow blow fuses are
not dual-element.
Slow-blow fuses are ideal for circuits with a transient surge
or power-on inrush. These circuits include: motors,
transformers, incandescent lamps and capacitate loads.
This inrush may be many times the circuit's full load amperes.
The inrush can be 20 times the normal current level. When
using fast-acting, single-element type fuses, it is necessary
5
to rate the fuse at 150% to 300% of the circuit's full load
current. With this rating rule inrush may cause nuisance
openings.
Conquer Electronics Co., Ltd.
Slow-blow fuses allow close rating of the fuse without nuisance opening. Typically rate the slow-blow fuses from 125%
to 150% of the circuit's full load amperes.
Fast-acting fuses have no intentional built in slow-blow and
are used in circuits without transient inrush currents.
Very fast-acting fuses often have silver links. Because of
the fuses' current limiting ability, these fuses are frequently
used to protect semiconductor circuits.
• In-rush currents, surge currents, pulses, start-up currents
characteristics.
• Ambient temperature.
• Applicable standards agency required, such as UL, CSA,
and VDE....
• Considerations: Reduce installed cost, ease of removal,
mounting type/form factor, etc.