Accu-Guard
®
SMD Thin-Film Fuse
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, how ev er, that fusing
current is sensitive to temperature. This means that the fuse
must be derated or uprated at circuit temperatures other
than 25°C:
2. Circuit Voltage
Maximum Voltage: Accu-Guard
®
is specified for circuits of
up to rated voltage. Accu-Guard
®
will suc cess ful ly 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 ter mi na tions.
4. Steady-State Current
The Accu-Guard
®
current rating is based on IEC Spec i fi ca -
tion 127-3. In accordance with this in ter na tion al 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).
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 cur rent.
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
the fuse will react to very high currents of very short duration.
Rather, the fusing characteristic at very high currents is
specified by I
2
t-t curves (or I
2
t-I).
I
2
t expresses the amount of energy required to actuate the
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
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 char ac ter ized;
I
2
t-t and I
2
t-I graphs are in this catalog. The prob lem 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 sub se quent choice of Accu-Guard
®
current rating is il lus trat ed by way of the attached examples.
HOW TO CHOOSE THE CORRECT ACCU-GUARD
®
FUSE
FOR CIRCUIT PROTECTION
Environmental
Accu-Guard
®
Temperature
Current Carrying Capacity*
F0402E, F0805B, F1206A, F0805B 2.50A
F0603C F0612D
F0603E F1206B & 3.00A
-55°C to -11°C 1.07 x I
R
1.07 x I
R
1.07 x I
R
1.07 x I
R
1.07 x I
R
-10°C to 60°C I
R
I
R
I
R
I
R
I
R
61°C to 100°C 0.85 x I
R
0.93 x I
R
0.90 x I
R
0.90 x I
R
0.80 x I
R
101°C to 125°C 0.80 x I
R
0.90 x I
R
0.90 x I
R
0.75 x I
R
0.75 x I
R
*As a function of nominal rated current, I
R
.
