SCHURTER White Paper: Find the Right Fuse!0 pages

Fuse Selection

White Paper

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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.
Normal operation after switching ON is explained 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 exceeded by many multiples.
Important facts with regard to fuseholders are
given under Point 3, where the correct selection of fuse and fuseholder combinations is
explained.

Derating of rated current
in %

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 Ta °C

Fig. 1: shows the de-rating curve of the
time-lag SMD fuse UMT 250.

The function of a fuse is to interrupt an uncontrolled fault current or overcurrent before serious damage can occur, such as the overheating 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 current, 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 elements or additional fuses are then necessary.

Fuses | 01 |

Point 1:
Normal Operation After Switching ON
Under normal operating conditions, a fuse
is subjected to a maximum operating current and a maximum operating temperature.
A «derating» of the rated current is therefore
usually necessary since a fuse is rarely operated 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