Current Limiting Fuse Instantaneous Peak vs Panel SCCR symmetrical RMS 2

[Arentsch]

Given:
In an existing installation, a standalone panel has an SCCR label that reads 5kA symmetrical rms. A current limiting fuse mounted outside of this panel and on the line side of this panel has an instantaneous peak let through current of 10kA and an rms symmetrical let through current of 5kA. (Fuse let through currents were derived from the fuse manufacturer charts using available fault current data).

Question:
Does this fuse protect the panel from short circuit damage?

 

[jraef]

Here's how I understand it, the result of being involved in several mistakes along these lines.

Maybe, but more likely no. The more likely no part comes into play in that there is no way to predict how power components will interact with not just the limited current, but the actual RATE OF RISE of the current up until the fuse clears to limit it. This can only be determined via specific series combination testing. So every device in that panel would need to have been tested with that specific fuse.

The maybe part is that if you are a PE, and take the time to evaluate EACH power component in that panel individually as to whether is has a higher SCCR when protected by a specific class and size of CL fuse ahead of it, and when you have all of that info, it turns out you can select a class / size fuse that fits an acceptable profile for each device in the panel individually, then you can certify that the fuse you select can be used with that panel. If you are a PE and are willing to undertake this exercise for free, bless you. But if you are the end user and want to have this done for you by a PE (the only alternative method accepted by code), then the risk is that you will pay for the PE's time regardless of the outcome, where the result may very well be that it will NOT work! So then your only alternative is to install a transformer or Current Limiting Reactor to lower the available fault current to 5kA at the control panel terminals.

So word to the wise for anyone reading this, do NOT allow your control panels to be purchased WITHOUT STATING that the panel have an SCCR commensurate with your available fault current where it is intended to be used. Panel builders bidding on projects will often take the cheapest way out of fulfilling this SCCR labeling requirement by using the "courtesy" 5kA SCCR label that can be applied without testing, unless YOU specify as stated above. That's because you often CAN get a suitable SCCR by using tested listed combinations, but that then limits what components you can use in a panel and generally means all of the power devices will need to come from the same mfr, as opposed to shopping them all against each other to get the lowest cost on individual parts.


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

 

[Arentsch]

Let's assume a specific series combination test was performed and the selected fuse appears to operate satisfactorily with the devices in the panel. Does the panel SCCR symmetrical rms current of 5 kA need to be below the fuse's instantaneous peak let through current of 10ka or the symmetrical rms let through current of 5kA to be considered acceptable short circuit protection?

 

[Arentsch]

Correction: "above" not "below"

 

[jraef]

Well, assuming that were true, SCCR is based on SYMMETRICAL current values.


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

 

[Arentsch]

Thank you jraef! I know it sounds obvious, but I needed someone else to confirm that "like" current units should be compared.

 

[juleselec]

It's probably worth going back to the definitions / intentions of the different SCCR terms.

Usually, symmetrical rms SCCR refers to a thermal rating (heat energy i2t), which is often why there is an associated rated time, e.g. 25kA for 1s, or it is converted to an MVA rating. The rough idea is that the busbars and associated equipment will withstand the thermal energy of a sustained fault within the rated time (e.g. it doesn't melt).

The instantaneous peak rating is more about whether the switchboard can withstand the mechanical forces induced by the peak fault current. So this relates more to the busbar supports, structures, etc.

So when a panel says 5kA symmetrical rms, it is most likely referring to the thermal rating. You should ask the panel manufacturer what the peak rating is. FWIW, the rule of thumb in the IEC world is that the peak rating is 2.5 times the thermal rating for 50Hz and 2.6 times for 60Hz based on some dc time constant that covers most of the cases (there is an IEC standard talking about this, but I can't remember which).

 

[waross]

Is the Short Circuit Current Rating based on the Available Short Circuit Current?


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Arentsch]

Thank you juleselec. So, in the IEC world, you would assume the peak rating of the subject panel is 5kA(rms) x 2.6 = 13kA?

 

[Arentsch]

waross, I hope I understand your question correctly, but the panel SCCR is not based on the Available Short Circuit Current. The panel OEM had know idea what the Available Short Circuit Current would be when designing the enclosure.

 

[jraef]

The term "SCCR" implies a specific requirement added to the US National Electric Code starting in 2008, pertaining initially to "Industrial Control Panels" as defined in Article 409, but now expanded to other equipment. I am unaware of the term SCCR being something associated with IEC rules and regs, although I am by far no expert in that. From and engineering standpoint the concept I'm sure is common to both, but the use of the specific term/acronym "SCCR" to me means someone is asking for an SCCR label as required in the US under article 409.110 SCCR has, as a result, became part of the UL Listing process for custom control panel assemblies under UL-508A, where is is specifically described in Supplement SB. In all of that documentation specific to how one attains an acceptable SCCR listing / label, the process only addresses symmetrical current.


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

 

[No1BEE]

The electrical code requires short circuit rating (SCCR) of an equipment to be selected so that it would not subject the equipment to extensive damage when it has a short circuit. According to the electrical code the SCCR is the prospective symmetrical fault current at a nominal voltage.

For the panel to be protected adequately against short circuit, the available symmetrical short circuit current at the panel must be equal or less than the equipment SCCR (in your case 5 KA). Installation of current limiting fuses at the panels was a past practice to assure that the equipment SCCR was not exceeded by using the fuse's current limiting capability to reduce the available fault current. However, due to changes in low voltage circuit breakers designs, such practice has diminished if not stopped since early 1980.

If the panel has any interrupting devices, then, the application of any current limiting fuse or circuit breaker would require either a combination series listing of the devices or would need a professional licensed electrical engineer with intricate knowledge of the devices in question.

 

[LionelHutz]

In North America, I would say the practice was mostly stopped because people with no clue on how to properly select the series co-ordinated parts were doing it wrong, resulting in changes in the electrical codes stipulating who is allowed to select the parts.

UL508A does have guidelines for doing series co-ordination. One of the stipulations is that the panel SCCR can never be increased above the interrupt rating of the lowest rated protection device. This means the fuses can't be used to protect the panel if the panel has a 10kA rated breaker and upstream of the fuses is 25kA of available fault current. UL508A contains tables for selecting series rated fuses, but the selections are very slim meaning it's almost pointless attempting to put together a series fuse-breaker combination.

So, the question posted is incomplete and can't be answered.

 

[controlsdude]

If you do a google search "WhitePaper SCCr" you will get informed and can ask better informed questions. I find the whole subject a bit confusing since all the panel components are either rated in the xx KA AIC rating or the SCCR rating. Its like comparing apples and oranges.

I thought the xx KA AIC rating would tell you some info when the components could explode on you based on amps flowing thru the device.

I also thought the SCCR rating would give you a comparable chart that you could cross reference tot he proper PPE ratings of all the gear you need to wear when doing electrical work on said panel.
Lower SCCR rating would cross to a lower cal/cm2 rating
Higher SCCR rating would cross to a higher cal/cm2 rating
But this is really simplictic statement on my part since voltage, distance , etc factors come into play here for proper selection of PPE.

Just remember who has to do work on panel and get that SCCR rating down and said worker would be thanking you due to less PPE gear required to work on your panel in future.

 

[djs]

I believe that the PPE level is determined by the available energy which is something different than SCCR. Max energy is not obtained during a short circuit, but rather through a fault that allows substantial voltage across it. Most likely an arcing fault. So the PPE level would be more determined by the time delay curves of the upstream protective device rather than the instantous trip.

I work for a company that has a UL508A panel shop. SCCR calculations can either be very simple, or very complex. Really nothing in between. Usually the complex cases occur when trying to meet a high SCCR requirement. At that point you start to scratch your head, and open up a lot of books and catalog looking for solutions.

 

[LionelHutz]

The electrical code requires short circuit rating (SCCR) of an equipment to be selected so that it would not subject the equipment to extensive damage when it has a short circuit.

To pass a fault current test basically requires that there isn't a failure to ground and that the enclosure door doesn't allow enough of the fireball to blow-out that it catches cotton outside the enclosure on fire. There is no requirement the components inside are function or easily repaired. The components inside can be (and often are) damaged to the point they are garbage yet still pass the test.

Just remember who has to do work on panel and get that SCCR rating down and said worker would be thanking you due to less PPE gear required to work on your panel in future.

Putting a lower SCCR rating on the panel does nothing to actually lower the arc flash hazard since the arc flash hazard depends on the characteristics of the feeder coming into the panel, not the number put onto the panel label.

 

[djs]

To pass a fault current test basically requires that there isn't a failure to ground and that the enclosure door doesn't allow enough of the fireball to blow-out that it catches cotton outside the enclosure on fire. There is no requirement the components inside are function or easily repaired. The components inside can be (and often are) damaged to the point they are garbage yet still pass the test.

Actually the idea behind the idea of SCCR is that the panel can withstand an external short or an internal short whether line to line or ground to ground without damage. In fact that is how components are tested for SCCR rating. I believe that UL requires that a given device must withstand three hits without malfunctioning. What you are talking about is the arc flash containment.

 

[LionelHutz]

Safety is the basis behind the UL standards, not functionality. Why do people still believe UL standards are to ensure functionality?

As an example, from UL508 which basically covers motor starters and overloads.

After each operation, the contacts of the motor control devices, or the entire motor control device, may be replaced and new current elements may be installed in the overload relay. The same sample may be used provided that no additional impedance is introduced. If an overload relay employs noninterchangeable current elements, the entire overload relay may be replaced.

There is a criteria list that says what can and can't be damaged. The things that can't be damaged generally would cause additional safety risks if they were damaged. For example, wires can't come disconnected because you wouldn't want wires ends floating in the cabinet that could be energized if the breaker was turned back on. But, the motor starter and overload can be garbage after one test.