About Non-Resettable Fuses, continued
needs to be given to the fact that the current carrying capacity of a
fuse is affected by changes in ambient temperature. IEC and UL/
CSA tests are performed at 23˚C and 25˚C respectively. In
practical applications the fuse’s ambient temperature may be
significantly higher, especially if the fuse is used in an enclosed
type fuseholder or mounted near other heat generating
components. The effect of changes in the ambient temperature is
shown in the chart below.
6. Breaking Characteristic
The breaking characteristic is shown in the respective time-
current blowing charts for each fuse type. The breaking
characteristic is the melting time of a fuse given a defined load.
The melting time is a function of the fuse wire length and diameter
as well as its base material and alloy.
IEC fuses are classified as follows:
40
30
Quick-Acting Fuses
Application: Protection of semiconductors and for very
sensitive instruments. This fuse type tolerates small
overcurrents for a short period of time but breaks very quickly
at higher current values. It limits short circuit currents at a
very early stage.
20
M+T+TT
10
0
FF+F
-10
-20
-30
-40
Time-Lag Fuses
Application: Protection of devices subjected to moderate to
high in-rush currents and/or overcurrent peaks, such as
transformers and motors. This type of fuse also tolerates
higher overcurrents during a short period of time.
-40
-20
0
20
40
60
80
100
Ambient Temperature ˚C
UL/CSA fuses are divided into:
In addition to the effect of ambient temperature conditions, it is
recommended to also de-rate UL listed fuses by approximately
25% of the original current ratios. This is, however, not required
for IEC fuses (See Para. 10).
Non Time Delay Fuses
These fuses are sometimes also referred to as Normal Blow
types.
Time Delay Fuses
4. Rated Voltage
These fuses are sometimes also referred to as Slow Blow or
Surge Proof types.
The rated voltage of the fuse should be in no case lower than the
operating voltage. At low operating voltage, the inherent
resistance must be considered.
For certain applications neither of the above described types may
prove usable. Since the writing of Standards by IEC/UL/CSA
does not always keep pace with the latest technological advances
various fuse manufacturers have developed fuses outside the
realm of such standards.
Please note that UL and CSA require the use of 250V rated fuses
in Power Entry Modules.
5. Breaking Capacity/Short Circuit Rating
Generally the agencies allow the use of such fuses if a particular
application dictates it. The OEM’s risks are that it has to rely on
manufacturer’s own specifications that are not routinely checked
by a safety agency. Schurter offers the following use types for
such purposes:
The breaking capacity is the short-circuit current which the fuse
can break at the rated voltage under the advanced conditions
without being destroyed or causing permanent arcing.
Under IEC, miniature type fuses are classified into two categories:
Super Quick-Acting Fuses
Fuses with Low Breaking Capacity
Application: Protection of semiconductors at the base of S1
and GE (thyristors, triacs, diodes). This fuse type tolerates
small overcurrents only during a short period of time and
limits the current at small short-circuit currents.
Typically, the fuse-element of a fuse with low breaking
capacity is visible. The insulation tube consists of
transparent material, normally glass. There is no
extinguishing medium, the arc is quenched in air. The
breaking capacity at 250 V and a power factor of 1 is 35 A.
Medium Time-Lag Fuses
Applications: Protection devices subjected to moderate to
high in-rush currents and/or overcurrent peaks, such as
transformers and motors. This fuse type also tolerates higher
overcurrents during a short period of time. Due to its
conformance with DIN Standard 41571, this fuse is widely
used in Germany; mainly in government related applications.
Fuses with High Breaking Capacity
The fuse-element of a fuse with high breaking capacity is not
visible. The insulation tube normally is of ceramic material or
glass. To quench the arc, there is always an extinguishing
medium. The breaking capacity at 250 V and a power factor
of .7 to .8 is 1500 A.
Super Time-Lag Fuses
UL’s and CSA’s short circuit requirements are similar, but different
as relates to IEC. At 125 V a UL listed fuse has to interrupt
10,000 Amps AC, whereas at 250 V the range may vary from 35
Amps up to 1500 Amps depending on the specific current rating of
the fuse.
Application: Protection of devices subjected to longer lasting
in-rush currents and/or high overcurrent peaks like
transformers and motors. This type tolerates higher
overcurrents during a longer period of time.
125
SCHURTER [ SCHURTER INC. ]
