Fuse Pre-Arc rating 1

[rovineye]

I don't have TCC for a high speed fuse, but I do have I^2t (A^2s) characteristics for Pre-Arc, and clearing @ 660 volts.

What is Pre-Arc? Will being close to this value at starting be detrimental to the fuse over time?

How should I treat the clearing value when at a lower voltage than the 660 volts given?

I also noticed that the clearing and Pre-Arc value can be well above the interrupting rating for the higher amperage rating of this fuse family. How can that be?

 

[rovineye]

I did find a curve for this fuse, as well as a adjustment for voltage. What I don't know is how I apply the voltage adjustment. Do I move the entire curve over to the left along the entire length?

I also thought about the higher values for clearing and pre-arc, and believe it is because the fuse will clear a higher fault, but may break or the connection may fail.

 

[rovineye]

The voltage correction

 

[7anoter4]

Fuse current rating is defined by the requirement that 2xIR 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 I2t-t curves [TCC] (or I2t-I).
I2t expresses the amount of energy required to actuate the fuse. Total I2t expresses the total energy which will be passed by the fuse until total cessation of current flow. Pre-arc I2t expresses that energy required to cause large irreversible damage to the fuse element (Total I2t = pre-arc I2t + arc I2t).
The arcI2t [arc energy] is direct proportional with U [Voltage]
Usually K=TotalI2t/pre-arcI2t>1. But, it seems to me, your K is only for arcI2 as it is direct proportional with voltage. So, TotalI2t could be= (1+K)* pre-arcI2t.

 

[rovineye]

"Pre-arc I2t expresses that energy required to cause large irreversible damage to the fuse element"

That is the part that really throws me. In the attached electrical characteristics, the first fuse listed shows a Pre-arc value of 40 for a 40 amp fuse. How could that fuse be used even at 75% of rating if Pre-arc damage occurs at that low a level?


Is that information of any value other than determining total I2t?

 

[7anoter4]

First of all you may have the TCC curves of any fuse from this Table of characteristics if you change the bif# no with the required bif# in the address:

http://www.bussmann.com/library/bifs/17056314.PDF

Second: the pre-arc I2t shown in the table is only for the shortest time and the highest current. Less current, longer time to pre-arc and higher the pre-arc I2t.
From the TCC for 40 A rated current 40 A2.sec will be only for 633A and 0.0001 sec.For 80 A[the same 40A rated fuse] the pre-arc I2t= 80^2*10=64000 A2sec.
For a certain case the calculated I^2.t [according to shape of the wave-sine, or else-triangle, rectangle, trapezoidal and so on] has to be less than total I2t indicated in table.
I revised also my opinion about using the voltage factor K. For a voltage less than rated you have to multiply the totalI2t by K in order to adjust the new position.

 

[rovineye]

7anoter4

Perfect! Thanks for the bit I couldn't find anywhere, that "the pre-arc I2t shown in the table is only for the shortest time and the highest current".

The TCC for my fuse didn't explain the dotted lines and horizontal A line, but your link does. Thanks for that also.