Chip Fuse I2t

<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 I²t, as is typical of wire element constructions. The thermal heat sinking that results from the large contact area between the printed element and the planar base (chip) can cause the fuse I²t to vary as a function of clearing time by one or more orders of magnitudes. In order to generate a meaningful base value for  I²t, a test current that produces clearing in just under 1/2 cycle (8 msec for 60Hz and 10 msec for 50Hz) should be used.
I²t values arrive at using this approach allow for valid comparison between chip fuses of different ratings or from different series. 

Bel I²t Data

 The data included in this catalog represent nominal MELTNG I²t. The value indicatted for wire element designs are those measured at10In as well as thise reorded for melting times less than 1msec (at 50Hz), using constant current DC. 

For chip designs, the values are those measured during clearing times of just under 1/2  cycle. Additional curves displaying chip fuse I²t as a function of clearing time are being developed and will be posted to our Web site and will be printed in future catalog.

 

As ref

https://www.eaton.com/sg/en-us/products/electronic-components/topics/fuse-technology.html

Melting Integral

The melting integral of a fuse, commonly referred to as I2t, is the thermal energy required to melt a specific fuse element. The construction, materials and cross sectional area of the fuse element will determine this value. Each fuse series and ampere rating utilize different materials and element configurations; therefore, it is necessary to determine the I2t value for each fuse. Tests to determine the I2t of a fuse are the rated current with a time constant of less than 50 microseconds in a dc test circuit. High-speed oscilloscopes and integral programs are used to measure very accurate I2t values. I2t data is depicted in a time vs. current graph (Figure 1).

The melting I2t of a fuse is one of the values used to assist circuit designers when selecting and properly sizing a fuse in a specific application. It can be compared to the thermal energy created by transient surge currents in a circuit.

Figure 1. Time vs. Current Curves for Model S505SCH timedelay, high I2t fuse

This may not be＜Time vs. Current Curves＞ but ＜I2t vs. Current Curves＞ (sptt) 

 0603 Fast-Acting Fuse is not high I2t fuse (sptt)

 

Roughly at 10A  I2t are 

 

10 x 10 x t =300 (3.15A fuse)

t = 300/100 = 3 sec

 

10 x 10 x t = 15,000 (5A fuse)

 

t = 15000/100 = 150 sec

 

11 x11 x t = 300 (5A fuse)

t= 300/121 = 2.48 sec

 

10.5 x 10.5 x t = 10,000 (6.3A fuse)

 

 t = 10000/110 = 90 sec

 

 30 x 30 x t = 400 (6.3A fuse)

 

t = 400/900 = 0.444 sec

sptt