Do current limiting fuses allow downstream panels to be braced at the current limit of the fuse?
Do current limiting fuses allow downstream panels to be braced at the current limit of the fuse?
(OP)
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.






RE: Do current limiting fuses allow downstream panels to be braced at the current limit of the fuse?
RE: Do current limiting fuses allow downstream panels to be braced at the current limit of the fuse?
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.
RE: Do current limiting fuses allow downstream panels to be braced at the current limit of the fuse?
RE: Do current limiting fuses allow downstream panels to be braced at the current limit of the fuse?
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
RE: Do current limiting fuses allow downstream panels to be braced at the current limit of the fuse?
RE: Do current limiting fuses allow downstream panels to be braced at the current limit of the fuse?
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.
RE: Do current limiting fuses allow downstream panels to be braced at the current limit of the fuse?
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.
RE: Do current limiting fuses allow downstream panels to be braced at the current limit of the fuse?
You do know that testing them usually involves destroying a few of them?
RE: Do current limiting fuses allow downstream panels to be braced at the current limit of the fuse?
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
RE: Do current limiting fuses allow downstream panels to be braced at the current limit of the fuse?
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.
RE: Do current limiting fuses allow downstream panels to be braced at the current limit of the fuse?
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
RE: Do current limiting fuses allow downstream panels to be braced at the current limit of the fuse?
RE: Do current limiting fuses allow downstream panels to be braced at the current limit of the fuse?
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.