SCCR for ICP using Fused disconnect

[DougPLC]

Need help with SCCR details. As an OEM building an Industrial Control Panel, ICP with 480v 3 phase input: I don't know the available fault current. I have an AB fused disconnect switch with AJT-50 fuses. According to UL, let-thru currents would be 22kA rms and 9kA peak, based on a 100kA availability.

If everything in my system is rated at least at 9kA, can I rate the SCCR for the ICP as 100kA?

The rest of the feeder circuit contains 3 MCPs (65kA rated) and a single phase 7.5kVA xfmr with class CC fuses on pri and sec circuits: 20A pri and secondary of 120v (15A fuses) and 240v (20A fuses)

Any help?

 

[wbdvt]

Fused disconnect switches are rated higher than non fused disconnect switches. Typically fused disconnect switches are 200kA while unfused are 10kA. So you are good to 200kA. You can verify this by looking at the manufacturers technical spec.

 

[jim dungar]

If everything in my system is rated at least at 9kA, can I rate the SCCR for the ICP as 100kA?

You can not rate the panel above 9kA unless there is some type of documentation that the fuses are 'series rated' with the components. UL508A has some methodology that is often employed for this purpose. Manufacturers, both fuse and devices, also publish this information.

When evaluating any current limiting effect, you need to consider both Ip (peak current) and I2t ( the time the fault current flows).

 

[wbdvt]

You can not rate the panel above 9kA unless there is some type of documentation that the fuses are 'series rated' with the components. UL508A has some methodology that is often employed for this purpose. Manufacturers, both fuse and devices, also publish this information.

When evaluating any current limiting effect, you need to consider both Ip (peak current) and I2t ( the time the fault current flows).

Sorry but I was not referring to series rated. That would apply to non fused disconnect but once it is a fused disconnect the ratings change. As an example see the attached Square D catalog sheet which has two tables, one for fused disconnects and the other non fused with 'series ratings' For some reason I cannot attach the cut sheet even though it is 64kB pdf

 

[Sahib]

I don't know the available fault current. Any help?

A knowledge of available fault current is necessary: The current limiting fuse does not limit the current, if the fault current is low.

 

[wbdvt]

You can figure out what the maximum amount of fault current that can be delivered by the transformer by taking the transformer full load amps and dividing by the impedance.

 

[Jraef]

The SCCR of the assembly is not determined BY the available fault current.

The SCCR is by definition a series rating if there is more than one device in a power circuit, because unless they are SPECIFICALLY tested in series, the SCCR becomes the lowest rating of the power devices themselves as stand alone. So even though the fused disconnect may have an SCCR of say 100kA, the CPT fuses are 100kA and the "MCPs" are 65kA, if those are the only power devices, your SCCR would be 65kA.

Now, let's discuss these "MCPs". If by that you mean the Motor Protection Circuit Breakers (MPCB) that are essentially IEC manual motor starters, then those almost always REQUIRE another OCPD ahead of them to have an SCCR, and the series SCCR may even be higher than the MPCB itself. You have a fuse, and that's likely fine, but you must read the data sheet because it may require a SPECIFIC fuse and you must assure that you used that one. Then if, as I suspect, you also have a contactor down stream of each of those then they too are power devices in the stream and must be included in the SCCR. If so, and they are the same mfr as the MPCB, then it's likely they have a series rating with them. You will have to get that value and if it's lower than 65kA, that becomes the new SCCR of the panel. If the MPCB and contactors are different mfrs, most likely there is no series testing done, so the stand-alone rating of the contactors, likely 5-10kA, becomes your new SCCR for the entire panel.

If my "MCP" you mean a magnetic-only molded case circuit breaker, those can ONLY be used in factory assembled listed combination motor starters, they cannot be used stand-alone at all. So your SCCR would be irrelevant for an unsanctioned use of a device.

 

[wbdvt]

True but available fault current is used to determine that the equipment is not overdutied.

 

[topgone]

Know how big a transformer is supplying your panel, the conductors used and the distance of the panel from the transformer! Do your fault calcs and you're done with the job. If you need assistance from us here, please post a diagram with the specs of your system.

 

[Jraef]

True but available fault current is used to determine that the equipment is not overdutied.

Correct. You use the Available Fault Current to determine the NECESSARY SCCR needed for the equipment. But that has no bearing on HOW the SCCR of the equipment is determined.