About Non-Resettable Fuses, continued
time-lag
super time-lag
=
=
T
TT
7. Blowing Charts and Tables
This catalog differentiates between two types of blowing charts
and tables: IEC and UL/CSA. Proper understanding of these
differences is essential when one tries to match the fuse
characteristics to the circuit requirements.
Example of fuse markings: T200 mA / 250 V
UL listed and CSA certified fuses are not as easily identifiable
because neither any lettering nor color code is required on the
fuse itself.
a) Chart and Table for IEC fuses: This chart is an interval graph
showing a curve representing the minimum blowing times and
another curve representing maximum values for a set of current
ratings. The tables give the checkpoints or “gates” mandated by
IEC. Please note that only the gate values are tested by the
agencies. Values on the curve between two gates are
geometrically arrived at and are not guaranteed by the
manufacturer.
11. Dimensions
Traditionally the dimensions have been 5 x 20mm for international
type fuses and 1/4 x 11/4" (6.3 x 32mm) for domestic, Northern
American fuses. Today however, IEC 5 x 20mm fuses with UL
recognition and CSA Component Acceptance are becoming
increasingly popular in North America, especially in applications
where saving space is a major concern.
b) Chart and Table for UL/CSA listed fuses: The curves for this
graph represent average values, individually for each current
rating.
It should also be pointed out that the 5 x 20mm fuse is available at
most over-the-counter distributors as well as radio supply shops in
North America.
8. Fusing Integral l2t
The fusing integral is the thermal energy needed to melt the fuse-
element. The fusing integral l2t is used to determine:
- the blowing time for higher overcurrents
IEC fuses of the 1/4 x 11/4" size should not be used in North
America. These fuses were designed for replacement use in
American made equipment located in Europe.
- the aging behavior of a fuse caused by in-rush currents.
12. Interchangeability of IEC Fuses with UL Fuses and
The formula given below is only valid for blowing periods of less
Vice Versa
than 10 ms.
l2t
For general applications the rated current of the fuse to be
converted should be multiplied/divided by a factor 2 depending on
whether the fuse has to be converted from the use in a 250V
circuit to the use in a 125V circuit or vice versa. In this case,
however, the fuse normally doesn't provide anything more than
short current protection. For a more accurate correlation, the time
current characteristic curves of both the IEC and UL fuse must be
compared. As a rule of thumb, a factor of 2.4 to 2.6 can be used
to convert an IEC fuse (used in a 250V circuit) into a UL fuse
(used in a 125V circuit) with the corresponding characteristics
(e.g. a 1 A IEC fuse corresponds to a 2.5 A UL fuse).
t
=
= seconds + 20%
(overcurrent)2
To prevent aging caused by in-rush currents we recommend
staying within the following limits:
l2t of in-rush
(to be determined by user)
less than 30% of fuse l2t for
time delay fuses
less than 40% of fuse l2t for
normal blow fuses
22
20
18
16
14
The proper selection of a fuse requires that attention be given to
this subject. Often times a fuse problem can only be pinpointed
after a thorough study of this issue.
UL
12
I
n
10
8
—
A
9. Power Dissipation
6
Power dissipation heats up the fuse and its surroundings.
Especially when selecting fuseholders, it is important to ensure
that, allowing for the ambient temperature, they are capable of
absorbing sufficient dissipated power. Please refer to the power
dissipation sections on the individual fuse pages when selecting a
fuse.
4
2
0
0
2
4
6
8
10
12
14
I
n
IEC-(DIN)
—
A
13. Quality Control
Details about Schurter’s strict quality control procedures are
available upon request.
Should you need fuses for non-standard applications, please
contact our Engineering Department.
10. Specification of Characteristics
To quickly and easily classify the various fuse types by their time-
current characteristics, the following letter codes are stamped on
IEC, or other 5 x 20mm fuses.
Letter Code
super quick-acting
quick-acting
medium time-lag
=
=
=
FF
F
M
Note: new fuse qualifications have been established for low-
voltage fuses; reference EIA/IS-722.
Schurter, Inc. • Phone 707-778-6311 • Fax 707-778-6401 • E-mail info@schurterinc.com • Website http://www.schurterinc.com
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SCHURTER [ SCHURTER INC. ]
