For arc-flash analysis the fuse ‘total clearing time’should be used.

IEC Fuses

In the IEC world, fuse manufacturers publish fuse time-current characteristics which plot the ‘pre-arcing time’ as a function of rms prospective current. IEC 60269-1 states that for times longer than 0.1 sec, the difference between ‘pre-arcing time’ and ‘operating time’ is negligible.

The ‘operating time’ is the total clearing time which is the sum of the ‘pre-arcing time’ plus the arcing time.

American Fuses

In Amercian fuse catalogs I have seen fuse time-current characteristics plotted as :

-‘minimum melting time’,

-‘minimum melting time’ and ‘total clearing time’.

as a function of rms prospective current.

[SIZE=3]Questions[/size]


IEC Fuses

·What +/- tolerance should be used for the ‘pre-arcing time’ ?

·What additional time should be added for times below 0.1 sec for the ‘operating time’ ?

American Fuses

·What +/- tolerance should be used for the ‘minimum melting time’ ?

·Where only the ‘minimum melting time’ is published, what time should be added for the ‘total clearing time’?

If the fuse has any current limiting properties at all, then you can't just blindly go about this. Current limiting reduces the thermal flux from the arc by reducing the energy but also extends the fuse opening time. That is why IEEE 1584-2002 has a series of empirical equations to apply to fuses. For fuses that don't follow the fuse classes that are given, I'm not sure what the correct answer is. Mersen at least has been heavily involved in research on this topic and can probably give a very detailed answer for their fuse products.

Thank you PaulEngr for your helpful response.

Where fuses other than those for which IEEE 1584-2002 provide empirical formula for arc-flash energies ( Class L and Class RK1 ) are installed , PTW arc-flash software recommends following fuse total clearing time for generic current limiting fuses :

a) The current-limiting range is assumed to start where fuse clearing curve drops below 0.01 sec.

b) Fuses operating in the current limiting range are assumed to clear in ½ cycle for currents 1 to 2 times the current where the current-limiting range begins, and ¼ cycle for currents higher than 2 times the current where the current-limiting range begins.

For Class L and Class RK1 fuses, PTW arc-flash software has the option of using the equipment empirical formulas.

My concern is for generic current limiting fuses where only the average melt time or average pre-arcing time is published and where the current is below the range defined in a) and b) above.

I am assuming the following fuse total clearing time:

For average melt times below 0.10 sec, add 15% to average melt ( pre-arc ) time

For average melt times above 0.10 sec, add 10% to average melt ( pre-arc ) time.

PTW documentation recommends :

For average melt times below 0.03 sec, add 15% to average melt ( pre-arc ) time

For average melt times above 0.03 sec, add 10% to average melt ( pre-arc ) time.

If you are using PTW and you are up on your service fees, then I suggest an even better approach: call them. I have found that SKM has huge amounts of library data on nearly anything you can imagine. The default libraries that it comes with only scratch the surface. Their modelling department is really good at finding out information and sending you a model for free if you just ask instead of trying to figure it out yourself. At least that's what I do. Especially when some fuses like the medium voltage power fuses are just downright confusing as to what the correct times are.

I did some research on fuse clearing and found the following information:

A quote from IEEE 1584 - 2002 (4.6) should answer your questions about what and how fuse time-current characteristics are used for determining the duration of the arcs:



Typically, fuse time-current characteristics tolerance is +/- 5%. I believe the curve tolerance has been factored in into above recommendation. I would research the web or contact fuse manufacturers or service providers like SKM to find old / discontinued fuse data.

Most arc flash software come with built in protection device libraries. Some of them also allow end user to add more fuses and breakers in case they are not already listed in the supplied libraries. As an example, please find below a screen shot from iAFA V1.0 mobile app showing old Brush 500MT fuse and GE F 225 Line 250A molded-case breaker information and characteristics being added to the program protection library:



The program would than determine arc duration based on selected protection device characteristics and predicted arcing current. You can download your free copy of iAFA V1.0 online at http://www.arcadvisor.com/arcflash/iafa.html