sscr

[mbarry]

We designed and built a custom control panel labeled at 10ka per UL508a. All the power devices in panel are actually rated at 65ka, but since we used a distribution block, panel could only carry 10ka rating. Now panel is at installation and it is discovered that available fault current is 25ka.
Would it be acceptable to install a fused disconnect ahead of panel with current limiting fuses having a peak let through current of less than 10ka @ 25ka?

 

[jim dungar]

We designed and built a custom control panel labeled at 10ka per UL508a. All the power devices in panel are actually rated at 65ka, but since we used a distribution block, panel could only carry 10ka rating. Now panel is at installation and it is discovered that available fault current is 25ka.
Would it be acceptable to install a fused disconnect ahead of panel with current limiting fuses having a peak let through current of less than 10ka @ 25ka?

I believe that UL508A has an exception for some fuses but, at 25kA available I think you will be limited to a 60A max fuse size.

Can't you just change the block? There are distribution blocks available (i.e. Ferraz) with high fault ratings.

 

[mbarry]

Do the Ferraz distribution blocks require fuse protection? Usually distribution blocks with high fault ratings require that they be used in combination with fuses (makes sense since the people that make distribution blocks also make fuseblocks). This panel was built using circuit breakers. there is not room in panel to add fused disconnect, that is why I was hoping it would be o.k. to install one ahead of panel to limit available fault current.

 

[dbaird]

Check Bussman Disconnect:

http://www.bussman.com/pdf/c15588b5-fba0-4b2e-8a05-35d33bd23b89.pdf

You would still have a concern/issue as to how to interlock your enclosure door that inhibits entry unless disconnect turned off. Look at NFPA 79 Chapters 5 and 6 for the rules.

 

[petersonra]

Check Bussman Disconnect:

http://www.bussman.com/pdf/c15588b5-fba0-4b2e-8a05-35d33bd23b89.pdf

You would still have a concern/issue as to how to interlock your enclosure door that inhibits entry unless disconnect turned off. Look at NFPA 79 Chapters 5 and 6 for the rules.

Is there a requirement that ALL disconnects interlock with the door? It presumably already has to have a disconnect that would have the interlock.

 

[jim dungar]

Check Bussman Disconnect:

http://www.bussman.com/pdf/c15588b5-fba0-4b2e-8a05-35d33bd23b89.pdf

You would still have a concern/issue as to how to interlock your enclosure door that inhibits entry unless disconnect turned off. Look at NFPA 79 Chapters 5 and 6 for the rules.

NFPA79 and UL508A have different requirements for door and disconnect interlocking. which one you chose to follow is up to you (neither one is "code") orper your customer's specs.

And providing an interlocked disconnect does not always negate the SCCR problem.

 

[davidr43229]

Do the Ferraz distribution blocks require fuse protection? Usually distribution blocks with high fault ratings require that they be used in combination with fuses

A Power Distribution block by itself has a standard default of 10KA, unless it is in a UL listed combination. You can not install a Fuse ahead of the breaker, due to that combination not being tested.
Additionally a standard Power Distribution block does not have the required spacing within the feeder, as listed in UL508A section 10.
You will have to live with your 10KA, unless you change out the Power Distribution Block.
Try these "Stand-alone" PDB with 200KA SCCR
http://cooperbussmann.com/pdf/6d72da03-c454-4902-b786-f9f5bcfb134c.pdf
http://cooperbussmann.com/pdf/d297539f-08be-4af5-a762-5f6bdcd02e1c.pdf

 

[cbolwerk]

davidr43229,

I looked at the literature at the link you provided. Am I reading this wrong? Doesn't this PDB still require fuses? There is a column that lists the max fuse size, so does that mean that you have to use fuses?

Thanks!

 

[Zoltar]

Couldn't you just put in a 1:1 isolation transformer ahead of your cabinet to limit the available fault current?

 

[rplecha]

If the fused disconnect is not part of the control panel in question, why couldn't he just use the manufacturers data on the current limitiing fuses and consider the fused disconnect a new power source for the control panel. This way the available fault current to the control panel is determined by what is truly available from the load side of the fuses rather than that useless SB2.2 table. My whole angle on this is that adding the fuses outside means that it is not part of the control panel and maybe is not strictly bound by the infamous UL508, so then the manufacturers data for say J fuses could be used, easily obtaining an available fault current to the panel <10ka. Any thoughts on this?

 

[winnie]

rplecha,

You asked this question on its own, and in an old thread. As answered in that other thread, for _overcurrent_ devices in series, if you want to take advantage of the 'current limiting' capability of the fuses, you must use a tested series rating.

However when the device being protected is passive, that is, not actively responding to the overcurrent condition and trying to open the circuit, then I believe that 'current limiting' fuses may be applied without a series rating.

So without testing a current limiting fuse cannot be used to protect a circuit breaker, but can be used to protect a distribution block.

This particular situation is more confused however, because there would be circuit breakers downstream from the fuse and upstream of the distribution block. This is not a 'series rating' situation, since the circuit breakers are fully rated for the available fault current. But it does create a confusing situation: is " 25KA available fault -> current limit fuse -> fully rated circuit breaker -> 10KA rated distribution block" kosher?

-Jon

 

[jim dungar]

This particular situation is more confused however, because there would be circuit breakers downstream from the fuse and upstream of the distribution block. This is not a 'series rating' situation, since the circuit breakers are fully rated for the available fault current. But it does create a confusing situation: is " 25KA available fault -> current limit fuse -> fully rated circuit breaker -> 10KA rated distribution block" kosher?

Hmmm,
If the contacts of the circuit breaker begin to open while the fuse beginning to melt, would the increased dynamic resistance (from the arc) be large enough to slow down the clearing time of the fuse reducing its current limiting effect?

 

[rplecha]

Winnie,
Thanks. Sure, I know how circuit breakers complicate things. I would consider bypassing the main circuit breaker all together and just going fuses to PDB. I cant remember if the branch circuits in this guys application are protected by breakers though, but would that still fall under the series rating constraint?