Virtual time for current limiting fuses

[etorres]

I have no clear the definition of virtual time for current limiting fuses. Is it the time provided by the I-t characteristic of the fuse? Is it the prearcing time or the total operation time?

Thanks in advance

 

[cuky2000]

Virtual time
The virtual time is the value of Joule integral divided by the square of the value of the prospective current. Usually stated for a MV-fuse link are the values of pre-arcing time and of operating time.

 

[mykh]

Fuse literature shows pre-arcing (melting) and clearing i2t. I have never seen fuse specs showing actual fuse -pre-arcing- and clearing time. Also, I wouldn't calculate fuse operation time based on i2t value and prospective fault current value. i2t is test value, for specific test setup, voltage, wave shape and power angle etc.

4ms is being typicaly taken as a fuse operating time if the fuse is exposed to fault current above the total clearing time at the bottom of time-current characteristic (0.01 seconds). This is the amount of time it takes to melt fuse element before the elements starts arcing and clearing the fault. In arc flash analysis, if the arcing fault current is above the total clearing time at the bottom of the curve (0.01 seconds), IEEE 1584 uses 10 miliseconds for the arc duration (clearing) time.

 

[cuky2000]

mykh,

Perhaps IEC standard 60269 “Low Voltage Fuses” may help to clarify this issue.

See also the enclose glossary from Bussmann:

http://www.bussmann.co.uk/High Speed Applications Guide/Appendix 4 Glossary of Terms.pdf