Need SCCR help

[Chief531]

New fish in the pond here...

Hers is the question about calculating SCCR.. I have a disconnect @ 50kA to the PDB @ 100kA then to the fuse block @ 200kA to the motorstarter and overload block both rated at 5kA RMS Symmetrical @ 600 volt.

Does my "panel" SCCR then become 5kA due to the MS?

Any help appreciated...

Mike
Production Manager
Panel Shop

 

[jim dungar]

New fish in the pond here...

Hers is the question about calculating SCCR.. I have a disconnect @ 50kA to the PDB @ 100kA then to the fuse block @ 200kA to the motorstarter and overload block both rated at 5kA RMS Symmetrical @ 600 volt.

Does my "panel" SCCR then become 5kA due to the MS?

Any help appreciated...

Mike
Production Manager
Panel Shop

You have not calculated any SCCR. You have determined the minimum SCCR of the devices in your panel. Without any further information you would be correct in stating that the maximum SCCR of your panel can not exceed that of the minimum rated device.

It is possible that you may have "listed" combinations of devices (i.e. fuses and starters) that might allow a higher SCCR than any single device. You might want to check with the manufacturer of the starter.

 

[Chief531]

Thanks Jim,

These are the listed SCCR's that are on the device'e themselves. I have not found anything that tells me how to calculate the SCCR based on these ratings. Even UL 508A does not refer to any type of formula, or am I looking in the wrong spot??

Mike

 

[jim dungar]

In general a device's SCCR is determined by actual testing not via formula. Although UL508A does have some methodology for unlabeled devices.

Most starter manufacturers have tested their 5kA starters in series with generic fuses (actually they are called umbrella fuses). This witness testing usually results in an effective combination SCCR which is much higher than 5kA.

 

[davidr43229]

Chief - You can call the Mfg'r of the MS and they should be able to provide you with the SCCR individual or combinational rating, based upon umbrella fuse ratings. Do not accept a withstand rating.
UL508SB states that you can modify (SB4.1.1) (b) an individual rating based upon the upstream fuse Peak let trough in Table SB4.2 to attain a combinational rating if none is available from the MS mfg'r.

 

[petersonra]

Surprisingly, many PDB are only rated at 5kA.

 

[davidr43229]

Bob,
UL508A in table SB4.1 for Power Distribution Blocks that are NOT marked, lists a standard default rating or 10KA, as a maximum.

 

[petersonra]

Bob,
UL508A in table SB4.1 for Power Distribution Blocks that are NOT marked, lists a standard default rating or 10KA, as a maximum.

Many are now listed at very high values as well. You just have to be careful which ones you select.

Some are not rated for anything with CB but are with fuses, so presumably an unlabeled PDB would be good for 10kA with CBs.

 

[ECE2001]

Aside from the issue at hand, be careful of what you are calling a "power distribution block". It could be listed to UL1059 or UL1953. It only has the proper spacings that make it the acceptable choice for use in a feeder circuit if it's listed to UL 1953. UL 1059 is the standard used investigate terminal blocks.

 

[davidr43229]

Aside from the issue at hand, be careful of what you are calling a "power distribution block". It could be listed to UL1059 or UL1953. It only has the proper spacings that make it the acceptable choice for use in a feeder circuit if it's listed to UL 1953. UL 1059

ECE2001,
Now there's a person that knows what he's talking about, within the feeder circuit. A proposal for 2008 was made to maintain the old UL1059 & spacings, however the proposal was rejected. UL508A in Table 10.2 adresses this issue.
ECE- on your 5K issue, why not conside replacing the "MS" to attain a higher SCCR rating?
For right now, used "wisely", UL has allowed PDB's & Terminal Strips to UL1059 in the feeder circuit, but will be disallowed in 2008, due to these spacing requirments within the feeder circuit.
Just my $.02