SCA protection

[HighVoltage21]

Is it still permisable to series rate devices to meet short circuit current? EX. existing system that has a 14kaic panel with a couple 10kaic panel should be raited for 35kaic. Is it still permisable to put a fusable disconnect on the line side of this panel that will meet the required short circuit rating? Thanks for the input

 

[augie47]

Take a look at 240.86 & 110.22
Basically you can use a manufactured listed combination or engineering data

 

[jim dungar]

Is it still permisable to put a fusable disconnect on the line side of this panel that will meet the required short circuit rating? Thanks for the input

It has never been acceptable (well at least not for the last 36 years) to assume that a fusible disconnect always protects a breaker, although it has been a widely held misconception.

As Augie said, you need to use the breaker manufacturer's series combination.

 

[RoberteFuhr]

The manufacturer of the breakers most likely will have a table listing specific fuse sizes and classifications that have been tested by UL with their breakers.

 

[weressl]

Is it still permisable to series rate devices to meet short circuit current? EX. existing system that has a 14kaic panel with a couple 10kaic panel should be raited for 35kaic. Is it still permisable to put a fusable disconnect on the line side of this panel that will meet the required short circuit rating? Thanks for the input

To elaborate the objective of the series rating. The objective of the series rating is that the upstream device will limit the let-through current to the lower rated downstream device rating. Since most of the time it is done without engineering calculations or review devices in specific combination are tested and listed is the solution. Zone interlocked tripping devices are another method to accomplish this.

 

[jim dungar]

The objective of the series rating is that the upstream device will limit the let-through current to the lower rated downstream device rating.

This is not always true.

The limitation of the fault current, through the combined devices, typically is due to the dynamic impedance which occurs when one of the devices (not always the upstream one) begins to interrupt the fault.

Guaranteeing that the downstream device remains inactive during the time while the upstream device operates is a major problem of using the 'by engineering' option.

 

[RoberteFuhr]

To elaborate the objective of the series rating. The objective of the series rating is that the upstream device will limit the let-through current to the lower rated downstream device rating. ....

No, the objective of the two (sometimes three) devices in series will operate for the higher fault current (usually above one or more of the device interrupting rating) and neither device will sustain substantial damage. They basically have to pass the UL test for series ratings. There are two types of tested combinations. They are fuses and MCCB breakers and the other is MCCB breaker with another MCCB, Please see the UL article.

http://www.ul.com/global/documents/...echnical/ul_PanelboardShortCircuitRatings.pdf