Current limiting fuses

[Vegemite]

Under what circumstances will a current limiting fuses take longer than one half of a power frequency cycle to fully interrupt a short circuit current significantly greater than the current at which the fuse begins to current limit? Are there differing considerations for HV fuses versus LV fuses? The reason I ask is to try and rationalise the ANSI approach with the traditional British approach to grading relays upstream from current limiting fuses. ANSI permits a minimum time margin of 0.12 seconds whereas the British formula calls for a time margin not less than 40% of the fuse's melt time plus 0.15 seconds.

 

[dpc]

For ANSI/UL low voltage fuses there is a specified ratio of available fault current to the fuse rating at which it must clear before the first current peak. Below that value, it may or may not be current-limiting. I have a table of these ratios at work, but I'm still at home. I think the coordination time interval has more to do with fuse pre-loading (heating) than the current limiting effect.

 

[zappedagain]

Where did you find that ANSI standard? It seems like a piece is missing, as a absolute margin 0.12 sec makes no sense when the I2T can vary widely from fuse to fuse. I took a quick pass through the NEC (NFPA 70) and 0.12 doesn't pop out.

Z

 

[dpc]

I believe it is somewhere in ANSI/UL 198 series. The NEC does not require selective coordination except for emergency power systems. And even there, it is not specific about coordination time intervals, etc.

Coordination between low voltage fuses is determined by test for the most part, especially in the current-limiting range. It gets complicated when there are multiple devices in series and all are trying to trip or melt. for lower values of fault current, the fuse minimum melt and total clearing time curves should be pretty accurate.

 

[Vegemite]

Thank you, Z. The ANSI standard Referred to is IEEE 0242-2001 (Buff Book) and Table 15-3 gives 'minimum' CTIs for various combinations of protective devices.
My concern is grading with an upstream relay.

 

[dpc]

Well, the Buff book is not really a standard - just a guide. I'm familiar with the recommendation. The pre-heating of fuses does need to be accounted for and this probably accounts for most of the recommended interval. And there will be some margin of error in the relay curves as well. I guess I'm not understanding your concern.