Disconnect switch Short Circuit Rating

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Grouch1980

Grouch1980

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New York, NY
Hey guys,
If you look at the attachment, I'm trying to understand the table with the heavy duty switches, towards the middle. Let's say I choose an 800 amp switch, in a NEMA 1 enclosure. I'll use either class R, J, or T fuses, as noted on top of the table.

The short circuit ratings of the switch are as follows:
200k at 480V
100k at 600V

My question is: does the table mean I can't use a fuse rated for 250 volts? The fuses have to be rated either rated for 480 or 600 volts? I'm looking to use the switch on a 120/208 volt system.
 

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Grouch1980 said:
Hey guys,
If you look at the attachment, I'm trying to understand the table with the heavy duty switches, towards the middle. Let's say I choose an 800 amp switch, in a NEMA 1 enclosure. I'll use either class R, J, or T fuses, as noted on top of the table.

The short circuit ratings of the switch are as follows:
200k at 480V
100k at 600V

My question is: does the table mean I can't use a fuse rated for 250 volts? The fuses have to be rated either rated for 480 or 600 volts? I'm looking to use the switch on a 120/208 volt system.
Click to expand...
That is a 600 volt rated switch so will need 600V fuses, they are physically different than 250V fuses.
 
electrofelon said:
That is a 600 volt rated switch so will need 600V fuses, they are physically different than 250V fuses.
Click to expand...
I'm confused on how to read the table... so where it says 480V, it's a 480 volt rated switch? therefore needing 480 volt rated fuses? So this depends on whether or not you use 480 volt vs 600 volt rated fuses... it's the fuses that determine the voltage of the switch?
 
Grouch1980 said:
I'm confused on how to read the table... so where it says 480V, it's a 480 volt rated switch? therefore needing 480 volt rated fuses? So this depends on whether or not you use 480 volt vs 600 volt rated fuses... it's the fuses that determine the voltage of the switch?
Click to expand...
Fuses are generally rated at a maximum of 250 volts or 600 volts AC. You can use a fuse rated at 600 volts on a 208Y/120 volt system but as EF noted the fuse gap for 600 volt fuse would be different than that of a 250 volt fuse. If you chose a 600 volt switch you could not use a 250 volt fuse.
 
infinity said:
Fuses are generally rated at a maximum of 250 volts or 600 volts AC. You can use a fuse rated at 600 volts on a 208Y/120 volt system but as EF noted the fuse gap for 600 volt fuse would be different than that of a 250 volt fuse. If you chose a 600 volt switch you could not use a 250 volt fuse.
Click to expand...
I follow that. What I don’t follow is the table. What is the 480 volt switch for then if fuses are either rated 250 or 600 volts?
 
Grouch1980 said:
I follow that. What I don’t follow is the table. What is the 480 volt switch for then if fuses are either rated 250 or 600 volts?
Click to expand...
You're referring to Table 28.1-43 in your link correct? That is telling you that at 480 volts the rating is 200K, at 600 volts the rating drops to 100k.
 
Grouch1980 said:
Yes, that’s the table I’m referring to. So to get that rating of 200k… I have to use 480 volt rated fuses?
Click to expand...
Well you have to use 600 volt fuses because that is what physically fits in the switch. There are no 480 volt fuses. The table is referring to the max voltage of the system the switch is being used on. If you are using it on a 600 volt system the AIC is going to be lower than if you were using it on a 480 volt system.
 
Grouch1980 said:
Yes, that’s the table I’m referring to. So to get that rating of 200k… I have to use 480 volt rated fuses?
Click to expand...
600 volt switch, 600 volt fuses, dual rating of either 200k @ 480 volts or 100K @ 600 volts. The K rating would be based on which system (480 or 600 volt) you were using.
 
Thanks everyone for the feedback. So now that I follow that part... if I use the switch on a 120/208 volt system, what's the K rating of the switch? Or is the switch not rated for that system voltage?
 
Or should I read it as, yes it can be used on a 208 volt system... the short circuit rating of the switch is even higher at 208 volts, but it's not published what that value is since it wasn't tested at that system?
 
Grouch1980 said:
Or should I read it as, yes it can be used on a 208 volt system... the short circuit rating of the switch is even higher at 208 volts, but it's not published what that value is since it wasn't tested at that system?
Click to expand...
I think that you've hit the nail on the head. So if it's 200k @ 480 you can use the 200K rating at the lesser voltage of 208.
 
infinity said:
I think that you've hit the nail on the head. So if it's 200k @ 480 you can use the 200K rating at the lesser voltage of 208.
Click to expand...
Great! Makes sense. Thanks for the help everyone.

One more question. The same table... there's a note 1 at the bottom of that table saying Class L fuse connectors are supplied as standard for the 800 and 1200 amp switches. Does that imply if you use class L fuses, you'll get the short circuit ratings indicated in the table (200k at 480 volts, 100k at 600 volts), or do you still have to use class R, J, or T fuses as mentioned in the table header?
 
Note the ratings you mention on the switch are the ratings for the switch itself.
The different type fuse have unique characteristics and should be selected based on those. For instance, if you are applying series ratings you will find downstream equipment may series rate with one class fuse and not the other.
 
I understand that... those ratings on the table are for the switch itself. But what's the short circuit rating of the 800 amp or 1200 amp switch (200k or 100k) if one applies a class L fuse?
 
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