Is the neutral disconnected in an AC disconnect?

[ggunn]

That SCCR still needs to be larger than the fault current available at that point, IMHO.

Does the NEC say that? I can see why the KAIC rating is important; if the fuses on a line side connection don't clear a fault then you burn the building down.

 

[iwire]

Does the NEC say that? I can see why the KAIC rating is important; if the fuses on a line side connection don't clear a fault then you burn the building down.

It is in 110.10


In the moment between the fault and the fuse opening the unfused switch is subjected to as many amps as the circuit can provide. If the switch is not rated for it the switch could become the fuse and blow apart.

 

[pv_n00b]

Well, unfused disconnects don't really have a KAIC rating, because they cannot automatically do the function which starts with the letter I in that initialism. They have an SCCR rating instead, which is usually 10 kA.

It is very counterintuitive that an unfused disconnect would have such a weak SCCR rating, because they are usually built with exactly the same main components as their fused counterparts, which have a 200kA SCCR rating.

I had to go back to my trusty Eaton catalog to look this up and at least for their unfused disconnects they only have a fault current rating if there is some kind of OCPD ahead of them and then it's only 10kAIC. I never noticed that little gem before:huh:. If used as a service entrance disconnect there is no fault current rating that I can find for them. I'd be interested in a pointer to a reference to an SCCR rating of an unprotected disconnect because I can't find it, at least for Eaton.

Folks who think a circuit breaker or fuse opens up like a switch when the rating is exceeded need to look at the inverse time graphs that show how these devices respond to fault current. Unless it's a current limiting device that opens on the upslope of the fault current the full available fault current from the utility, and any connected motors, will flow through that device until it opens, and that can be for several cycles depending on the device. It's also why a disconnect with a standard class H fuse has a 10kAIC rating but changing to a class R fuse will give it a 200kAIC rating. Because the class R fuse will limit the amount for current going through it to less than 10kA even if 200kA of fault current are available from the utility.

There are very few current limiting circuit breakers used so it's pretty unlikely anyone will encounter one unless they install it.

Here a couple of fuse data sheets with inverse time graphs to look at:

For you everyday non-current limiting class H fuse: http://www.cooperindustries.com/con...duct-datasheets-b/Bus_Ele_DS_1030_NON_NOS.pdf

For a current limiting class R fuse: http://www.cooperindustries.com/con...tasheets-b/Bus_Ele_DS_1004_LPN-RK_70-600A.pdf

Notice the class R fuse has a current let through rating, that's the maximum current that the fuse will let through before it opens and that protects down stream equipment from large fault currents. The class H fuse does not have that rating because it will let through whatever current is available for the time it takes to open. The real fun is doing series ratings so a lower AIC rated branch OCPD can be used in a circuit with higher AIC rated main OCPD.