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White Paper Fuse Selection
Find the Right Fuse!
Criteria for Correct Fuse Selection
Circuitry overcurrent protection rarely receives the attention it deserves. An inadequately
thought out selection of fuses can lead to the breakdown of equipment and installation,
resulting in high replacement costs and dissatisfied customers. This technical article
focuses on the correct selection criteria for fuses and fuseholders, and should help you
take the more important aspects into consideration.
The function of a fuse is to interrupt an uncon-
trolled fault current or overcurrent before seri-
ous damage can occur, such as the overheat-
ing of equipment. Because a fuse is designed
using a fusing element, it is particularly suited
for reliable interruption of short-circuits.
With overcurrents up to 2x or 3x the rated cur-
rent, a fuse becomes less accurate and, as
such, not so well suited for these conditions.
Other overcurrent protection measures such
as electronic protection, thermal overload ele-
ments or additional fuses are then necessary.
Normal operation after switching ON is ex-
plained under point 1. This point should always
be taken into consideration.
Point 2 is only necessary with capacitive loads,
present when the charging of capacitors after
switching ON, leads to high in-rush current
peaks and the rated current of the fuse is ex-
ceeded by many multiples.
Important facts with regard to fuseholders are
given under point 3, where the correct selec-
tion of fuse and fuseholder combinations is
explained.
Point 1:
Normal Operation After Switching ON
Under normal operating conditions, a fuse
is subjected to a maximum operating cur-
rent and a maximum operating temperature.
A «derating» of the rated current is therefore
usually necessary since a fuse is rarely oper-
ated at the set ambient temperature of 23 ˚C.
As an example, let’s look at a scenario using a
time-lag SMD fuse such as the UMT 250 from
SCHURTER. With an operating temperature of
60 ˚C, in accordance with fig. 1 the fuse needs
to be derated by 17%, i.e., when the operat-
Fig. 1: shows the de-rating curve of the
time-lag SMD fuse UMT 250.
Fuses | 01 |
40%
30%
20%
10%
0
-10%
-20%
-30%
-40%
-40°C -20°C 0°C 20°C 40°C 60°C 80°C 100°C
Ambient Temperature T a °C
Derating of rated current
in %
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