Do current limiting fuses allow downstream panels to be braced at the current limit of the fuse? 3

[bdn2004]

See the attached sketch. This question came up after a power study was performed. The software shows the available short circuit current is 13kA at the panel, but the panel is only rated at 5kA. The fuse manufacturer is saying this situation is ok because of the 5kA cutoff of the fuse. Is this correct? Does anyone know of am NEC or other standard that spells this out? Thanks.

 

[wroggent]

See NEC 240.86

 

[cranky108]

The purpose of a current limiting fuse is to limit the current to a safer level, by forcing a current zero.

The current limiting fuse would need to be installed before the box, as a fault before the fust, but still in the box would have the same result as a full fault.

A current limiting fuse has a problem, in that coordination with it can not be achieved. Example a fault on a circuit feeder that would normally exceed the current limit would be cleared by the current limiting fuse and not the feeder breaker.

This is not an application for a new panel, but where an older panel is being used with a new source that exceeds it's capacity. It is normally more cost effective (cheaper) than replacing the panel.

 

[ZeroSeq]

A current limiting fuse does not automatically limit current to a set value. The fuse must operate within its current limiting range, which can be solved graphically using the manufacturers technical specifications. A current limiting fuse limits current only by clearing prior to the first half-cycle asymmetrical peak. If the X/R ratio is very low, or the fuse is vastly oversized in relation to the available symmetrical fault current, it will not provide the result you are looking for.

 

[jraef]

The important (critical) parts of 240.86 is that the series combination rating be either a listed combination series rating by the manufacture (which can only be attained via expensive testing), or is done under the auspices of a registered PE and stamped by them. This issue comes up quite a lot now since the issuance of Article 409 in the 2005 NEC, which requires an SCCR (Short Circuit Current Rating) on anything deemed as an "Industrial Control Panel". UL issues SCCR ratings on tested listed series combinations, but panel builders can extend those ratings to custom assemblies, as long as they use the SPECIFIC series combinations that UL has tested. If not, then the builder can only apply what is commonly referred to as the "courtesy" rating of untested components at 5kA. That's what it appears you have here, because of it were an industry standard distribution board, I seriously doubt it would only carry a 5kA listing.

Fuse mfrs make a big deal about how their Current Limiting fuses can be used to satisfy this SCCR requirement for untested equipment, but often leave out the part about how it MUST be performed and stamped by a PE. Then what often happens is that a PE looks at the entire setup, discovers the issues previously raised here, and refuses to stamp it (but sends them the bill for his time nonetheless). It's a mess.

The real cure is for buyers to be aware of this issue and demand, in their specifications, that anything supplied for a project be suitable for being installed in the system, i.e. has the SCCR commensurate with the available fault current. It's just too risky to assume it can be "fixed" after the fact. It's been a steep learning curve on this issue however.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[bdn2004]

I have a follow up to this question. See the attached from Bussman on their current limiting fuses. The control panel equipment is labeled 5kA RMS- same as the original attachment. But Bussman states that the peak and the RMS values must be less than the rating of the panel to avoid damage. It only has an RMS rating. So that doesn't make sense to me. Why don't they just say peak current if that's the case?

 

[LionelHutz]

I don' get the question. The document you posted says there is 13kA symmetric available at the power leads entering the panel yet you want to know if a 5kA rated panel is OK?

UL508A SB2 goes into some detail on using a feeder fuse or breaker to increase the current rating of a panel. It's meant to be used when figuring out the fault rating for the panel label, but could still be used for guidance as to what can or maybe rather should be done. FYI, with <50kA available ahead of the fuses, there are very few fuses that can be used to protect a 5kA rated panel and possibly allow a higher rating.

 

[bdn2004]

We are doing an arc flash study for a Plant using one of the well known softwares. This isn't a new design - everything exists. The Client has HVAC units that have been marked 5kA SCCR. The short circuit study revealed 13,000 Amps symettrical amps is available at these units. Its obvious the panels weren't ever really tested and were slapped with the minimum marking. The fuse people are telling them current limiting fuses upstream of the panels will do the trick. That doesn't appear to be right per their own literature, which is ambiguous.

In hindsight the original design should have had a spec that demanded it meet this criteria. But here we are. I don't know if it can be done but perhaps the better suggestion than a current limiting fuse is to have the vendor actually perform a test on these panels and re-certify them, or find out what has to be done to bring them up in rating.

 

[cranky108]

To test these panels. How many do you have? Are they still made?

You do know that testing them usually involves destroying a few of them?

 

[jraef]

When you say it isn't new, everything exists, that COULD be a trigger to enable a PE to crunch the numbers and certify it, as outlined in 240.86 (A)

(A) Selected Under Engineering Supervision in Existing
Installations. The series rated combination devices shall be
selected by a licensed professional engineer engaged primarily
in the design or maintenance of electrical installations.
The selection shall be documented and stamped by
the professional engineer. This documentation shall be
available to those authorized to design, install, inspect,
maintain, and operate the system. This series combination
rating, including identification of the upstream device, shall
be field marked on the end use equipment.
For calculated applications, the engineer shall ensure
that the downstream circuit breaker(s) that are part of the
series combination remain passive during the interruption
period of the line side fully rated, current-limiting device.

There's more to this in 110.22 (B) as well.

But the operative issue, and the usual stumbling block, is the last sentence above:
"For calculated applications, the engineer shall ensure that the downstream circuit breaker(s) that are part of the series combination remain passive during the interruption period of the line side fully rated, current-limiting device."
That means the protective devices in your 5kA listed panel must NOT try to open during a fault if that up-stream fuse has not cleared. What generally happens is, you hire the PE, he starts the project, finds that the devices in your panel will NOT remain passive and he cannot certify it. Then he sends you his invoice, because he spent the time. Another potential pitfall has to do with the fact that you said this is HVAC equipment, which implies it has motors, which then triggers 240.86 (C). That limits your TOTAL motor load to 1% of the LOWEST rated protective device, in SPITE of the expected series rating, no matter how it is accomplished. So what I've seen happen is that an OEM uses the cheapest little "DIN ail" circuit breakers they can buy off the internet, and those only have an Interrupting Capacity of something like 4kA (I've seen some as low as 1kA). A 4kA rating means your TOTAL motor load in that panel cannot exceed 40A.

Most of the time people I have consulted with on this find out that it is less expensive to take their lumps, change their specs for the next time, and scrap the panels to have someone build them again, this time TO their new spec requiring a proper SCCR.



"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[wroggent]

Would you all consider the following scenario an application of 240.86(A) or (B)?

Scenario:

Brand X panelboard model # 789jkl with Brand X circuit breaker model # xyz123 has a listed series rating with Brand Y OCPD model # 123abc. Panelboard 789jkl is existing and a change in the distribution system results in the interrupting rating of xyz123 breaker being lower than available short circuit current at that location. Panelboard was manufactured and installed prior to time that the series rating was tested (i.e. there are no labels on panelboard referring to a bulletin referencing the series rating). Engineer decides to have 123abc OCPD installed ahead of the panel to achieve a series rating. Application of (A) or (B)?


Note: 110.22(C) requires that tested series combinations be marked by the manufacturer.

 

[cuky2000]

In my opinion, the propose fuse rated and tested for 5kA is not suitable for a SC available above its rating. Two element should be considere
1)Interrupting of SC current beyond 5kA will met the fuse
2) The thermal and mechanical stresses for SC ~13 kA could exceed the brazing (forces) or create and explosion on the fuse cartridge.
We suggest to consult with the application engineer with the fuse manufacturer to see if thee is a recommended alternative

 

[bdn2004]

As stated in a previous post....what's in it for a PE to stamp an existing installation? Not much. Every PE knows they can be held personally liable for screw ups on anything they stamp. There are probably 100 of these units on site.

 

[LionelHutz]

Instead of generalities, why don't you provide some actual details on the various components might help. At least the fuse you are using and the protection device in the panel (including it's interrupting rating if it's a breaker)

I think this will be a decision if you want it to meet code or you just want documentation that shows the panel/system is coordinated for a fault.

The protection device remaining passive part of the code will likely be a big issue. Most low end breakers have a simple instantaneous overcurrent trip and will start to trip before the fuse can clear. UL508A gets around this by saying the panel can only be rated at the interrupting level of that downstream device. Say you have a 100kA fuse in series with a 14kA breaker, the best you can rate the panel is 14kA. Of course, we don't even know if you have a breaker because you haven't posted any details.

Still, even if you decide the passive part can be ignored because the breaker of fuses in the panel are high enough rated, only 13kA of fault current flowing means you might have the issue that ZeroSeq pointed out, where the feeder fuses doesn't operate in their current limiting range and won't actually current limit to 5kA.