Short Circuit Ratings 2

[timm33333]

I know the difference between momentary and interrupting ratings: momentary is the maximum fault current which the breaker/fuse can withstand, and interrupting (AIC) is the maximum fault current which the breaker/fuse can interrupt. But I have few questions as below, thanks for help!

1. Is "SCCR (short circuit current rating)" and "momentary duty" the same thing?
2. For low voltage breakers and fuses: do we need both momentary duty and interrupting duty, or only the momentary duty is enough?
3. For medium voltage breakers and fuses: do we need both momentary duty and interrupting duty, or only interrupting duty is enough?
4. For the bus-bars of low voltage and medium voltage panels: is the term “SCCR” used for both LV panels and and MV panels?

 

[jraef]

1. Is "SCCR (short circuit current rating)" and "momentary duty" the same thing?
I've never used the term "momentary", I've only heard it referred to as "withstand rating", and yes, SCCR is the newer terminology for that. I guess someone felt that "withstand" was not clear enough. I THINK though that it stems from the fact that a fuse (or breaker) can have a withstand rating that might not be the same as the withstand rating of whatever is holding it, so "SCCR" is always for an entire piece of equipment, not just the interrupting device itself.

2. For low voltage breakers and fuses: do we need both momentary duty and interrupting duty, or only the momentary duty is enough?
"Enough" for what? The withstand / SCCR must ALWAYS, by definition, be as high or higher than the interrupt value,because if the device cannot hold together long enough to interrupt the fault current, the interrupt rating is a moot point. So practically speaking, it's actually the interrupt capacity that is important in selection of a breaker or fuse itself. But SCCR can be extended in a "series" rating if tested for it, based on the let-through of the device up stream and in doing so, you can sometimes get an SCCR series rating that is higher than the individual component IC rating. So really, you need both values.

3. For medium voltage breakers and fuses: do we need both momentary duty and interrupting duty, or only interrupting duty is enough?
Same issue, the voltage is irrelevant. But see below.

4. For the bus-bars of low voltage and medium voltage panels: is the term “SCCR” used for both LV panels and and MV panels?
Technically, it would pertain to both, but the requirement for an SCCR is codified only in LV equipment, not in MV.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[dpc]

Just to add to what jraef said: For ANSI-rated equipment, low voltage breakers and fuses have a single short circuit rating and it is based on the momentary (1/2 cycle) fault current. There is no separate interrupting rating.

Medium-voltage breakers (ANSI) have both a withstand (1/2 cycle, close & latch, crest) rating AND a separate interrupting rating. Medium-voltage fuses have a single short circuit rating.

Equipment generally has a short circuit withstand rating (the duration depends on the equipment type).

But as noted, for low voltage equipment such as panelboards, switchboards, MCCs etc, the UL label for the assembly has an short circuit rating that is based on the lowest rated device in that equipment.

Cheers,

Dave

 

[LionelHutz]

Generally speaking, components which do not protect, contactors for example, have a withstand rating and components that protect, breakers for example, have an interrupt rating.

Low voltage molded case circuit breakers interrupt because there is a built-in instantaneous trip.

Low and medium voltage power circuit breakers can have an interrupting ratings and a short time withstand rating. Typical I've seen is the same number. As as example, 40kA for 2 seconds and 40kA interrupting. This just means you could hold the breaker from tripping for up to 2 seconds during a fault.

I don't see how a fuse could have a withstand rating. You can't selectively decide if a fuse will clear a fault or not.

SCCR is the overall rating of a complete low voltage built panel using multiple components.

 

[timm33333]

IEEE C37.010 is for LV short circuit, and IEEE C37.13 is for MV/HV short circuit. There is a document at:

https://support.skm.com/File/Get/302160

In this document, section II (1st paragraph) states that there are three kinds of short circuit ratings: LV (momentary), MV/HV (momentary), and MV/HV (interrupting).
Section II (3rd paragraph) states that short circuit contribution can be neglected for LV motors up to 50 HP.
Section IX (1st paragraph) states that only momentary rating is enough for LV if X/R is less than 15.

So to summarize:

For LV
1. SCCR (also called momentary) is required for panels and OCPD when X/R<15,
2. Asymmetrical (1.6 * SCCR) is required for panels and OCPD when X/R >15
3. Interrupting is not required for OCPD (for all values of X/R)

For MV and HV
1. SCCR is required for panels and also for OCPD
2. Interrupting is required for OCPD

Is it correct?

 

[dpc]

Not really. There are separate standards for breakers and for what you are calling "panels". And each type of "panel" has a separate standard. It would be nice if the terminology was consistent across all of the IEEE/ANSI standards, but that is just not the case.

Devices that interrupt faults are tested at a certain X/R ratio. The test X/R depends on the type of breaker or fuse and for molded-case breakers, also the short circuit rating.

The term "interrupting rating " has a special meaning when talking about medium and high voltage breakers. It takes into account the fact that the fault current will be highest during the first 1/2 cycle. By the time the breaker contact open, the asymmetrical current will be less. The interrupting rating allows this to be taken into account by the user. Because low voltage breakers can open more quickly, only the momentary current is used for evaluation.

I don't think you're going to be able to boil this down to five bullet points. I'd suggest getting a good text on short circuit calculations. If you have a specific issue with a specific class of device, you might post a separate question.

Good luck,

Dave

 

[LionelHutz]

What does obsessive compulsive personality disorder have to do with fault ratings?

Do you have an actual specific question or are you just trying to define terms?

 

[timm33333]

I was just trying to define terms. It's Ok now.

 

[jraef]

What does obsessive compulsive personality disorder have to do with fault ratings?

LOL, nice.
For some reason I never made that connection before...


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[cuky2000]

OBSERVATION: Harmonization between HV circuit breaker of the ANSI/IEEE and the IEC standards happens recently. Both agree that the time constant of dc decay is standardized at 45 ms, which corresponds to a change from X/R=15 to [highlight #FCE94F]X/R ratio of 17[/highlight] for a 60 Hz system and 14 for 50 Hz.

This also impacting adjustment in the traditional published values for the closing and latching current rating and the momentary current of tested HV circuit breaker.

The X/R ratio of other protective devices is shown in the table below.

 

[dpc]

cuky2000,

This is true, but to clarify: any changes only apply to testing and rating of new circuit breakers. Older breakers must be evaluated based on their nameplates and the standards that they were tested to as I'm sure you know.