Distribution Board short circuit ratings

[Grouch1980]

I understand that when you have a disconnect switch, you have to combine it with integral fuses in order to achieve a short circuit rating (series rated) for the combined device (switch plus fuses).

When you have a distribution board comprised of multiple fusible switches, with a main switch, does each switch have a different short circuit rating depending on which class fuses are installed inside each switch? I would imagine yes. And then the board itself (the copper bus) has its own SCCR rating.

 

[David Castor]

For a factory-assembled switchboard that is UL labeled, there will be a label with a single SC rating for the assembly based on the lowest rated device in the switchboard.

 

[Grouch1980]

For a factory-assembled switchboard that is UL labeled, there will be a label with a single SC rating for the assembly based on the lowest rated device in the switchboard.

So you're free to use whichever class fuses, and that doesn't affect the SC rating of the whole assembly? Or the individual switches?

 

[David Castor]

Didn't say that. Fuses must match the manufacturer's requirements for the switch's rating and the switchboard. If you modify the fuse type. It's no different than a circuit breaker switchboard. If you replace a 65 kA breaker in a board rated for 65 kA with a 10 kA breaker, then you no longer have a 65 kA switchboard

 

[Grouch1980]

Didn't say that. Fuses must match the manufacturer's requirements for the switch's rating and the switchboard. If you modify the fuse type. It's no different than a circuit breaker switchboard. If you replace a 65 kA breaker in a board rated for 65 kA with a 10 kA breaker, then you no longer have a 65 kA switchboard

ah got it.

So when you have an individual disconnect switch (just one switch, no switchboard), the series rated combination between the disconnect switch and its fuses (class dependent) determine the short circuit rating of the assembly.

When you have a fuse and switch switchboard / distribution board, with multiple switches, now it acts similar to a circuit breaker panel... you have the switchboard SCCR rating, and the fuse AIC ratings. Whichever is lower, that's the rating of the assembly. It doesn't matter the class of the fuse.

Do I have it right?

 

[drktmplr12]

ah got it.

So when you have an individual disconnect switch (just one switch, no switchboard), the series rated combination between the disconnect switch and its fuses (class dependent) determine the short circuit rating of the assembly.

When you have a fuse and switch switchboard / distribution board, with multiple switches, now it acts similar to a circuit breaker panel... you have the switchboard SCCR rating, and the fuse AIC ratings. Whichever is lower, that's the rating of the assembly. It doesn't matter the class of the fuse.

Do I have it right?

The MFR specifies which fuse types must be used to maintain the listed rating.

 

[Grouch1980]

The MFR specifies which fuse types must be used to maintain the listed rating.

Great, thanks.

 

[Jraef]

The fused switch has a maximum rating based on the fuse with the highest rating it was listed with. Using it with a lower rated fuse doesn’t change the switch rating, but the fuse rating becomes the limiting factor in APPLYING the switch to an installation. If you have 200kA available and someone installs a fuse rated for 100kA, it’s a problem for the fuse and the application, not for the switch itself. That issue extends to the switchboard rating.