CLF's & NFPA 70E

[wbd]

Hello all,

I would like get some opinions on the following statement provided to me by a power system analysis software manufacturer.

"IEEE-1584 only tested a couple of fuses and a couple of sizes with one test per current value. Not a valid testing group. Therefore the equations are not valid for all RAKE or Class L fuses or sizes. Technically they are not valid at all since there was only one fuse test at a given fuse size with only one manufacturers fuse type. This is a huge problem. In our opinion, the data is too limited to be safe."

My problem is that, if the above is true, why are the equations for CLF's included in NFPA 70E Annex D? Are these equations not really supposed to be used?

 

[MJJBEE]

I agree that the IEEE 1584 is not based on testing anywhere near complete. However, what do your salesmen sugest you do? Are they trying to sell you somthing you don't need? Also what do they sugest you do about this I would use the equations from the IEEE standard not because they are corect but because they are the best guess we have.

 

[wbd]

They're saying that don't use the CL feature and treat as a normal fuse, as that will be conservative.

 

[MJJBEE]

They are right that a normal fuse would be conservative but that is totaly forgeting about the CL feature. I would check and see if it makes a huge difference in required protection and only if it does would I try to find out more information from the fuse maker. I dont' really have an answer because I have never done an arc flash analysis that includes CL fuses.

 

[dpc]

The testing of CLFs done by IEEE-1584 was very limited.

You are much better off to just use the fuse TCC and compute the arc duration using that.

A current-limiting fuse limits current by **melting** before the first peak current can occur. Internally, the fusible element is broken into numerous sections and the fault current quickly creates numerous arcs within the fuse. The arc-quenching material in the fuse enables it to extinguish the arc before the current passes through a current zero. The CLF has no magic properties. If the arc is not cleared with 1/2 cycle, the fuse is not in a current-limiting mode.

If you use the fuse TCC, you will find that the arc energy will remain low as long as the fault is clear within 1/2 cycle. It is the speed of clearing that gives the CLF its advantage in limiting arc fault energy, not its "current-limiting" capability. If you are using large size fuses (800A) at 480V, it is unlikely that they will be current-limiting unless you have a huge amount of available fault current.

The CLF helps with arc-flash in some situations, but not every situation.