Cascading current limiting fuses for SCCR

Is the concept of 2 incoming fuses in series entering a panel legal means for raising SCCR of a control panel? In this global shortage of higher kA rated breakers, I'm looking at unorthodox options since I'm not even able to get a timeline on the breakers I'd otherwise need. If I have a fuse that has peak let-thru of 20kA when 100kA is available, then another fuse that would then be 10kA when that 20kA is applied, would that be acceptable means? Obviously these are hypothetical and I have to do the true values, I'm just trying to gauge if this is an option, or a waste of my time. Thoughts?
 

jim dungar

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Staff member
Location
Wisconsin
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Retired Electrical Engineer - Power Systems
Cascading of fuses to protect molded case circuit breakers is not generally accepted in our industry.

For all intents the NEC requires series ratings of protective devices to be tested.

Control panel components can have series ratings determined using procedures outlined in the UL508A standard.
 
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petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Is the concept of 2 incoming fuses in series entering a panel legal means for raising SCCR of a control panel? In this global shortage of higher kA rated breakers, I'm looking at unorthodox options since I'm not even able to get a timeline on the breakers I'd otherwise need. If I have a fuse that has peak let-thru of 20kA when 100kA is available, then another fuse that would then be 10kA when that 20kA is applied, would that be acceptable means? Obviously these are hypothetical and I have to do the true values, I'm just trying to gauge if this is an option, or a waste of my time. Thoughts?
not unless it is listed that way by UL.

HOWEVER, if you are a UL 508a panel shop, take a look at SB4.3. You might find something helpful there. It won't help you with breakers with a low AIC rating though.

There are a lot of things you can do to help yourself, but without knowing a LOT more about your situation it is real hard to give any solid advice.

One possibility is to put a transformer in front of the control panel as a means of reducing the available short circuit current.
 
not unless it is listed that way by UL.

HOWEVER, if you are a UL 508a panel shop, take a look at SB4.3. You might find something helpful there. It won't help you with breakers with a low AIC rating though.

There are a lot of things you can do to help yourself, but without knowing a LOT more about your situation it is real hard to give any solid advice.

One possibility is to put a transformer in front of the control panel as a means of reducing the available short circuit current.
Thank you. We are a UL508A panel shop, however, not all of our panels that go out are UL labeled. This one in particular is NOT UL. We have large loads so our customer would essentially need on the range of 700kVA transformer. I don't think that'll go over well.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Thank you. We are a UL508A panel shop, however, not all of our panels that go out are UL labeled. This one in particular is NOT UL. We have large loads so our customer would essentially need on the range of 700kVA transformer. I don't think that'll go over well.
you cannot use the procedure in UL508a to determine the sccr of the panel unless it is listed, so it is somewhat of a moot point.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
SUPPLEMENT SB – SHORT CIRCUIT CURRENT RATINGS FOR INDUSTRIAL CONTROL PANELS
SB1 Scope
SB1.1 These requirements cover industrial control panels provided with a short-circuit current rating.
These requirements supplement and in some cases modify the requirements contained elsewhere in this
standard.

Pretty clear.

Incidentally, I have not experienced a lot of trouble getting 65 kAIC breakers as long as they get ordered well before needed Sometimes I have to switch brands. I have also had to switch ratings sometimes within the same frame size, but that is usually not much of a problem.
 

jim dungar

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Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
It does indeed get to a gray spot in design. I was looking at 409.110(b):
Short-circuit current rating established utilizing an approved method
This too is vague, but essentially what I'm driving towards.
First you have the SCCR issue in 409, then you have the issue of series ratings for breakers in 240.

Our industry generally accepts factory/manufacturer tested combinations and lookup tables in standards. About the only self certified or engineered values are used with older style (.i.e > 40 years old) power circuit breakers.
 

Macbeth

Member
Location
Livonia NY
Are you trying to Raise the interrupt rating of a breaker? if so, You Can Not Use any device to raise the interrupt rating of Any OCPD. You can only raise the SCCR of an component (Non-OCPD)
Are you trying to raise the SCCR of a panel externally because the internal breakers? Then the breakers must be removed and the panel documented as requiring external fuses.
Don't use breakers use fuses instead.

We are a UL508A panel shop, however, not all of our panels that go out are UL labeled. This one in particular is NOT UL.
It is true you do not have to label you control panels but you still have to build and document them to UL standards, if within the US. If you don't build to UL standards you put the company at risk. If your willing to put your company at risk why concern your self with the interrupt and SCCR rating? Also a field evaluation is $5k the sticker is like $2.65.
 
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