<I2t> of fuses is very confusing. Most non-Japanese brand Fuse specs show <I2t> or more precisely I2t.
I = current, t = time
I have found it confusing after checking several specs of several different vendors and some articles available on the net. This may be due to the short time dynamic and catastrophic nature of fuse.
Up to certain current a fuse (element) must have low resistance and generate little heat (almost as a good conductor with a very little loss of energy) while when certain over current starting flow a fuse (element) must have high resistance and generate enough heat to melt itself and open (dis-function) itself. This change occurs in a very short period , especially Fast-Acting Chip Fuse.
The catalogue specs of Japanese Fuse suppliers (mostly Chip Resistor suppliers as well) like Panasonic, KOA, Hokuriku, Kamaya do not show I2t values and they simply show <Fusing tine> at 100%, 200%, 300% Rated current and show <Fusing time vs Current＞log-log charts.
On the other hand, US companies like Little Fuse, Bel Fuse as well as Taiwan and China companies show I2t values in their spec. Little Fuse, Bel Fuse show some explanations on I2t on their technical notes, separately from their specs.
Little Fuse
438 Series – 0603 Fast-Acting Fuse (Spec)
% of Ampere Rating Ampere Rating Opening Time at 25ºC
100% 0.25A – 6A 4 Hours, Minimum
250% 0.25A – 6A 5 Seconds, Maximum
Nominal Melting I2t (A2tSec.) (Note 3) are shown for each part in the parts list in the spec.
I2t are ranging from 0.0017 for 0.25A part (Nominal Resistance (Note 2) 2.218
Ohm). to 1.3838 for 6.0A part (Nominal Resistance 0.0085 Ohm ).
Note 2 Nominal Resistance measured with < 10% rated current.
Note 3. Nominal Melting I²t measured at 1 msec. opening time.
<opening time> may mean that the time up to the fuse open after over current starting to flow.
They measure I2t at 1 msec and I being the current for opening the fuse. I is not specified
By using this, 0.25A part
I2t = 0.0017 at 1 msec
I shall be
I2 x 0.001 = 0.0017
I2 = 0.0017 / 0.001 = 1.7
then
I = 1.3038A not 0.25A.
This may be what Little Fuse does.
6.0A part
I2t is 1.3838 at 1 msec
I shall be
I2 x 0.001 = 1.3838
I2 = 1.3838 / 0.001 =1383.8
then
I = 37.1994624 (A) not 6A.
Bel Fuse
C2F Series – 0603 Fast-Acting Fuse (Spec)
Minimum Maximum
100% 4 Hrs. N/A
200% N/A 5 Sec
300% N/A 0.2 Sec
Nominal Melting I²T @10 In (A² Sec) are shown for each part in the spec.
I2t are ranging from 0.0003 for 0.5A part (Nominal Cold Resistance 0.43 Ohm). to 0.500 for 8.0A part (Nominal Cold Resistance 0.0075 Ohm ).
They measure I2t at 10 In (10 times Nominal current) (A² Sec) and t (time) not specified.
Nominal current is not defined in the spec either. We use Rated current as Nominal current.
By using this, 0.5A part
as I is 10 In = 10 x 0.5 = 5A
5 x 5 x t = 0.0003
t = 0.0003 / 25 = 0.000012
8A part
as I is 10 In = 10 x 8 = 80A
80 x 80 x t = 0.500
t = 0.5/6400 = 0.000078
Conclusion
The methods of calculating I2t are different. Does I2t has any useful data ?
From Technical notes
Little Fuse - Fuseology
1. I2t
I2t is an expression of the available thermal energy resulting
from current flow. With regard to fuses, the term is usually
expressed as melting, arcing, and total clearing I2t. The units
for I2t are expressed in ampere-squared-seconds [A2s].
Melting I2t: the thermal energy required to melt a specific
fuse element. (melts and becomes open - sptt)
Arcing I2t: the thermal energy passed by a fuse during the
arcing time. The magnitude of arcing I2t is a function of the
available voltage and stored energy in the circuit.
Total Clearing I2t: the thermal energy through the fuse from
overcurrent inception until current is completely interrupted.
Total clearing I2t = (melting I2t) + (arcing I2t).
I2t has two important applications to fuse selection. The first
is pulse cycle withstand capability and the second is selective
coordination.
2. Pulse Cycle Withstand Capability
Electrical pulses produce thermal cycling and possible
mechanical fatigue that could affect the life of the fuse.
For this reason, it is important to know the pulse cycle
withstand capability of the fuse, which is defined as the
number of pulses of a given I2t value that can be withstood by
the fuse without opening, assuming that there is sufficient cool
down time between pulses.
Fuseology - Selection Guide
Nominal melting I2t is a measure of the energy required
to melt the fusing element and is expressed as “Ampere
Squared Seconds” (A2 Sec.). This nominal melting I2t,
and the energy it represents (within a time duration of
8 milliseconds [0.008 second] or less and 1 millisecond
[0.001 second]or less for thin film fuses), is a value that is
constant for each different fusing element. Because every
fuse type and rating, as well as its corresponding part
number, has a different fusing element, it is necessary to
determine the I2t for each. This I2t value is a parameter of
the fuse itself and is controlled by the element material
and the configuration of the fuse element. In addition
to selecting fuses on the basis of “Normal Operating
Currents”, “Rerating”, and “Ambient Temperature” as
discussed earlier, it is also necessary to apply the I2t
design approach. This nominal melting I2t is not only a
constant value for each fuse element design, but it is also
independent of temperature and voltage. Most often, the
nominal melting I2t method of fuse selection is applied to
those applications in which the fuse must sustain large
current pulses of a short duration. These high-energy
currents are common in many applications and are critical
to the design analysis.
(Continues)
Bel Fuse - I2t explained
I2t - TEST METHODS AND DATA PRESENTATION
A mojor concern of the fuse industry and users alike is how different fuse manufacturers arrive at the I2t values published in their catalogs, and what these numbers signify. For the Miniature and Micro fuses included in the catalogs, there are no prescribed test procedures /criteria contained in UL/CSA248 or IEC127 for evaluating I2t. Without a regulatory frame of reference, the user must rely on information provided by the fuse manufacturer. Therefore, to provide the user with meaningful data a fuse catalog should include information that that clearly indicates the specific test methods used in arriving at the published values.
At the IEC Working Group meetings covering Policy for Miniature Fuse Specifications, it was unanimously agreed the published I2t values should be defined as:
Meting I2t measured at 10In, using constant current DC
Every major fuse manufacturer was represented in this working group. Although this agreement represents a starting point, there are additional factors which must be considered. The use of 10In (10 times rated current) was considered expedient, since data fro most IEC Style 5x20mm fuses already exists. However, the use of 10In does not always produce true adiabatic conditions (opening times less than 1/2 cycle).
For example, some slow-blow /time-lag designs require a higher multiple rated current to obtain a true I2t value. Conversely, printed element designs (e.g. chip fuses) have their film-like elements immediately bonded to their substrates and are therefore unable to exhibit the true, adiabatic behavior essential for producing a unique, single value
