Short Circuit Current

[Zyb]

Do I need a fusible switch, if a typical non fusible is only 10kAIC. 39kA and 18kA are available fault currents?

 

[jim dungar]

Check with the manufacturer.
Most, if not all, nonfusible switches have 100kA series ratings with upstream fuses.
Many also have series ratings with upstream breakers from the same manufacturer.

 

[Jraef]

Correct. Assuming the NF switch has a “series rating” with fuses, those fuses don’t have to be IN the switch, they can be upstream.

 

[Zyb]

@Jraef, so if i use 100kAIC fuse on my first switch then rating on my NF switch will be 100kAIC? just want to make sure i got it right. Thank you!

 

[Deltaforce]

If future changes likely, opting fully rated fusible switch better

 

[Deltaforce]

@Jraef, so if i use 100kAIC fuse on my first switch then rating on my NF switch will be 100kAIC? just want to make sure i got it right. Thank you!

You better not do it, switch manufacturer to certify it

 

[ron]

Correct. Assuming the NF switch has a “series rating” with fuses, those fuses don’t have to be IN the switch, they can be upstream.

I'm not familiar with this concept for safety switches.
I need to dig around, but the ones that I've seen require the fuses to be in the switch enclosure for it to have a higher rating that 5kA or 10kA

 

[Deltaforce]

I'm not familiar with this concept for safety switches.

Testing and listing by manufacturer required

 

[augie47]

Here's a link for Eaton's series ratings for NF safety switches.

https://www.eaton.com/content/dam/eaton/products/low-voltage-power-distribution-controls-systems/swiching-devices/eaton-switch-combination-short-circuit-current-ratings-non-fusible-td00801005e-en-us.pdf

 

[jim dungar]

You better not do it, switch manufacturer to certify it

All it takes is to look in the manufacturer's literature.

 

[steve66]

FWIW: I find it hard to believe that the fuse would protect the switch on its line side, but not the switch on its load side.

But here are SQD ratings for heavy duty switches. (Ratings for their general duty switches max out at 10K when used in series with any molded case breakers.)

https://www.productinfo.schneider-electric.com/nadigest/5c51d645347bdf0001f1f280/Master/17703_MAIN%20(bookmap)_0000060130.xml/$/ULListedMaximumShortCircuitCurrentR-8071BF65

 

[jim dungar]

FWIW: I find it hard to believe that the fuse would protect the switch on its line side, but not the switch on its load side.

I don't know what you are getting at.
SCCR is always dpecified as being at the incoming/line side terminals of a device.

 

[Deltaforce]

All it takes is to look in the manufacturer's literature.

But op asking to replace with 100kA fuse in fusible switch marked with 39kA!

 

[Deltaforce]

FWIW: I find it hard to believe that the fuse would protect the switch on its line side, but not the switch on its load side

No issue, if a manufacturer not listed such combination, a PE can certify the combination

 

[ptonsparky]

If future changes likely, opting fully rated fusible switch better

You may be able to, but I cannot foresee the future.
Common, acceptable and legal, practices change from one code to the next. Occasionally waffling back and forth.

 

[jim dungar]

No issue, if a manufacturer not listed such combination, a PE can certify the combination

No, they cannot.

The NEC makes no provisions for field engineered SCCR series ratings of non-fused switches.
The only NEC allowed engineered series ratings are for circuit breakers without instantaneous trip functions.

 

[jim dungar]

But op asking to replace with 100kA fuse in fusible switch marked with 39kA!

There is no fusible switch rated at only 39kAIC.
The OP needs a 39kA minimum fused switch to protect a non-fused switch which has 18kA available. The manufacturer literature says an upstream fuse can series protect a downstream switch up to 100kA, so the OP's installation meets the NEC, as long as fuse rejection clips (disallowing 10kA fuses) are installed in the upstream switch.