Modelling HRC fuses for arc flash analysis to IEEE 1584-2002

[EddyWirbelstrom]

IEEE1584-2002 Section 4.6 Step 5 states that if only fuse average melt time is published, add 15% up to 0.03 sec and 10% above 0.03 sec to determine total clearing time.

Where:
- Average melt time is equal to pre-arcing time.
- Total clearing time is equal to pre-arcing time plus arcing time.

Some LV fuse manufacturers publish a +/- 10% current tolerance for their pre-arcing time curve.
The above IEEE1584-2002 Section 4.6 Step 5 does not take this current tolerance into account.
The effect of i2t due to d.c. and a.c. current decrements is also not mentioned in IEEE 1584-2002.

What is the established practice when modelling HRC fuses for arc flash analysis to IEEE 1584-2002 ?

 

[Skythian]

The IEEE 1584 4.6 Step 5 reads "For fuses, the manufacturer's time-current curves may include both melting and clearing time. If so, use the clearing time. If they show only the average melt time, add to that time 15%, up to 0.03 seconds, and 10% above 0.03 seconds to determine total clearing time. If the arcing fault current is above the total clearing time at the bottom of the curve (0.01 seconds), use 0.01 seconds for the time". It is the established practice when modelling HRC fuses for arc flash analysis to IEEE 1584-2002.

The opening time tolerances differ between fuse designs and the percentage of the overload relative to the amp rating. Incrementing the time according to IEEE 1584 guidelines should effectively compensate for the margin of error in published time-current curves in most cases.

Typical arc flash software program would automatically apply the required 15% and 10% time increments per IEEE 1584 when calculating incident energy and arc flash boundary. Check for instance Arc Flash Analytic software calculation example. The program also provides for adding fuses and circuit breaker data including time-current characteristics for the devices that are not already featured in the built-in protection device library. This feature allows factoring in the actual time-current curve margin of error and/or temperature and altitude derating factors on discretion of the designer.