Before SCCR was established a few code cycles ago, the requirement for the panel was to be marked with the Interrupt Rating (IR). Was this IR simply the interrupt rating of the main OCPD for the panel? So for an IR the marking would be 300KA for the main disconnects LPJ fuses, and that would be that? Obviously this could be much greater than the SCCR, but I am trying to understand the previous requirement for IR, and how that IR number was derived..
IR VS SCCR
- Thread starter fifty60
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- San Francisco Bay Area, CA, USA
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Interrupt Rating (IR) is assigned to over current protective devices which actually interrupt, like circuit breakers and fuses, and the equipment that contains them. SCCR is mostly what we used to call the “Withstand Rating” (WR) and was assigned to the equipment itself (gear) and any NON-interrupting devices in the power stream. The difference is in that SCCR can now include a series testing of a string of devices including the OCPD and any non-interrupting components along with the gear, to where the overall SCCR might be significantly higher than the WR of some of the devices in the string.
So for example you may have an MCC bucket with a circuit breaker that has an IR of 65kA as a stand alone device, a contactor good for 10kA WR, and an overload relay good for 5kA WR. But instead of the untested maximum of the lowest device (5kA), the entire string has been tested and listed by UL at 100kA SCCR, based mostly on the current limiting aspects of the breaker. BUT, if you plugged that unit into an MCC with busbars that are braced for only 42kA (their WR), then the entire lineup of the MCC gets an overall SCCR of 42kA, because the bucket(s) are down stream of the busbars so they can’t help. There were rules like that under the old system but they were more segmented and difficult to pull together. The change to using SCCR makes it easier for the end user to ensure they are picking the right equipment.
Just to clarify, IR did not go away, breakers and fuses still get an IR (often referred to as “kAIC”). What SCCR displaced was the Withstand Ratings.
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Understood, for the most part. What I am referring to is the actual labelling required on an industrial machine. I believe at one time the labeling requirement was just for the IR. Would this be just the IR of the main disconnect and fuses on the panel, and nothing else? I know that SCCR labeling is the law of the land now, and that not all devices have an SCCR, as current interuppting devices have IR instead of SCCR.
My question, to attempt to clarify, was about the labeling requirement for a machine. When a machine is labelled with a single IR label, what is that actually telling you?
My question, to attempt to clarify, was about the labeling requirement for a machine. When a machine is labelled with a single IR label, what is that actually telling you?
Basically I have a customer asking for me to provide the IR of the machine. I plan on giving them them the IR and the SCCR, but I am not sure exactly how to decide what the "IR" rating of the machine is. Is it just the IR Rating of the main equipment disconnect fuses?
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