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Accu-Guard ?
SMD Thin-Film Fuse
HOW TO CHOOSE THE CORRECT ACCU-GUARD ? FUSE
FOR CIRCUIT PROTECTION
Correct choice of an Accu-Guard ? fuse for a given applica-
tion is fairly straightforward. The factor of pre-arc I 2 t, howev-
er, requires clarification. The proper design for pre-arc I 2 t is
presented by way of example.
DESIGN PARAMETERS
1. Operating Temperature
The Accu-Guard ? is specified for operation in the tempera-
ture range of -55°C to +125°C. Note, however, that fusing
current is sensitive to temperature. This means that the fuse
must be derated or uprated at circuit temperatures other
than 25°C:
5. Switch-on and Other Pulse Current
Many circuits generate a large current pulse when initially
connected to power. There are also circuits which are sub-
ject to momentary current pulses due to external sources;
telephone line cards which are subject to lightning-induced
pulses are one example. These current pulses must be
passed by the fuse without causing actuation. These puls-
es may be so large that they are the determining factor for
choosing the Accu-Guard ? current rating; not necessarily
steady state current.
In order to design for current pulses, the concept of fuse
pre-arc Joule integral, I 2 t, must be understood. Fuse current
rating is defined by the requirement that 2 x I R will cause
actuation in <5 seconds. This rating does not indicate how
Environmental
Temperature
Accu-Guard ?
Current Carrying Capacity*
the fuse will react to very high currents of very short duration.
Rather, the fusing characteristic at very high currents is
F0402E,
F0603E
F0805B, F1206A, F0805B 2.50A
F1206B & 3.00A
F0603C F0612D
specified by I 2 t-t curves (or I 2 t-I).
I 2 t expresses the amount of energy required to actuate the
-55°C to -11°C
-10°C to 60°C
61°C to 100°C
101°C to 125°C
1.07 x I R
I R
0.85 x I R
0.80 x I R
1.07 x I R
I R
0.93 x I R
0.90 x I R
1.07 x I R
I R
0.90 x I R
0.90 x I R
1.07 x I R 1.07 x I R
I R I R
0.90 x I R 0.80 x I R
0.75 x I R 0.75 x I R
fuse. Total I 2 t expresses the total energy which will be
passed by the fuse until total cessation of current flow.
Pre-arc I 2 t expresses that energy required to cause large
irreversible damage to the fuse element (Total I 2 t = pre-arc I 2 t
+ arc I 2 t). If the Joule integral of the switch-on pulse is
*As a function of nominal rated current, I R .
2. Circuit Voltage
Maximum Voltage: Accu-Guard ? is specified for circuits of
up to rated voltage. Accu-Guard ? will successfully break
currents at higher voltages as well, but over voltage may
crack the fuse body.
Minimum Voltage: Accu-Guard ? cannot be used in circuits
with voltage of about 0.5V and less. The internal resistance
of the fuse will limit the fault current to a value which will pre-
vent reliable actuation of the fuse (<2 x rated current).
3. Maximum Fault Current
Accu-Guard ? is fully tested and specified for fault currents
up to 50A. Accu-Guard ? will successfully break currents
above 50A, but such over current may crack the fuse body
or damage the fuse terminations.
4. Steady-State Current
The Accu-Guard ? current rating is based on IEC Specifica-
tion 127-3. In accordance with this international standard,
Accu-Guard ? is specified to operate at least 4 hours at rated
current without fusing (25°C). Engineering tests have shown
that F0805B and F1206A/B Accu-Guard ? will in fact operate
at least 20,000 hours at rated current without fusing (25°C).
larger than the fuse pre-arc I 2 t, nuisance actuation will occur.
In order to choose the proper Accu-Guard ? current rating for
a given application, it is necessary to calculate the I 2 t Joule
integral of the circuit switch-on and other current pulses and
compare them to the Accu-Guard ? I 2 t-t curves. An Accu-
Guard ? fuse must be chosen such that the pulse I 2 t is no
more than 50% of the pre-arc I 2 t of the prospective fuse.
Pre-arc I 2 t of the Accu-Guard ? fuses is well characterized;
I 2 t-t and I 2 t-I graphs are in this catalog. The problem is cal-
culating the I 2 t of the circuit current pulses. This concept is
not familiar to most engineers. Correct calculation of pulse
Joule integral and subsequent choice of Accu-Guard ?
current rating is illustrated by way of the attached examples.
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