Understanding Medium Voltage Equipment short circuit ratings

[philly]

I am trying to understand the differences between the different Medium Voltage short circuit ratings for different medium voltage equipment.

For MV breakers I understand there are 3 different ratings called the Close & Latch (momentary), interrupting, and short time short circuit currents. I understand the close & latch to be the maximum first cycle asymmetrical short circuit current which is usually listed as a total RMS value or a peak value. I understand the momentary rating to be the short circuit rating at 3,5,8 cycles etc.. where the breakers interrupting time is at which the breaker interrupts the current. This is a maximum permissable value of current the breaker can interrupt at a given cycle time. I'm not quite sure if I understand what the short time rating is? Is this a rating that the breaker can withstand for a given time while a device upstream, or downstream is responsible for clearing the fault?

For MV Load Break switches I understand these switches to have (2) different ratings. For older switces I have seen them listed as "fault closing" and "momentary" however for newer switches I have seen them listed as "Fault Closing (momentary)" and "Rated Short-Time current" Can someone please explain these two different ratings for an MV switch and why newer switches seem to have different terminology for the ratings?

Are there any good references for understanding these ratings and how they are applied?

 

[templdl]

This is a copy of a section of a typicfal specifical for a typical MV breaker. As you can see they are not what you typicall see for LV breakers where you have numerous KAIC ratings for a given voltage. With the LV breakers you will not that the KAIC of a specific breaker that has multiple voltage ratings goes up as the voltage goes down and that there are a multiple of interupting ratings for a given frame size. With MV breakers a specific breaker is rated for its maximum symmetrical interrupting rating at its voltage rating.
F. Circuit Breakers:
1. The circuit breakers shall be rated [ ] nominal volts, [4760] [8250] [15000] maximum volts, 60 Hz, with a continuous current rating of [1200] [2000] [3000] amps and a maximum symmetrical interrupting rating* of [36kA/250mVA - 4.76 kV system] [49kA/350MVA - 4.76 kV system] [41kA/500MVA - 8.25 kV system] [23kA/500MVA - 15 kV system] [36kA/750MVA - 15 kV system] [48kA/1000MVA - 15 kV system].

 

[philly]

This is a copy of a section of a typicfal specifical for a typical MV breaker. As you can see they are not what you typicall see for LV breakers where you have numerous KAIC ratings for a given voltage. With the LV breakers you will not that the KAIC of a specific breaker that has multiple voltage ratings goes up as the voltage goes down and that there are a multiple of interupting ratings for a given frame size. With MV breakers a specific breaker is rated for its maximum symmetrical interrupting rating at its voltage rating.
F. Circuit Breakers:
1. The circuit breakers shall be rated [ ] nominal volts, [4760] [8250] [15000] maximum volts, 60 Hz, with a continuous current rating of [1200] [2000] [3000] amps and a maximum symmetrical interrupting rating* of [36kA/250mVA - 4.76 kV system] [49kA/350MVA - 4.76 kV system] [41kA/500MVA - 8.25 kV system] [23kA/500MVA - 15 kV system] [36kA/750MVA - 15 kV system] [48kA/1000MVA - 15 kV system].

Thanks for the info. Between what you have posted and some research on my own I now have a more firm understanding of MV breaker ratings.

One thing I am still unsure about however are the ratings of MV switches. When I look at data sheets for older switches vs new switces I see different terminology. For instance when I look at older switches I see ratings listed as "Fault Closing" and "Momentary" ratings. However looking at a newer switch I see ratings of "Fault Close & Momentary amps" and "Rated Short Time Current"

Did the terminology for switch ratings change at some point? Do different manufacturers use different terminology for switch ratings?

Can someone explain to me what the different switch ratings refer to?

 

[kingpb]

MV breakers are not all rated the same. It depends on what year they were built, or to what rating/standard.

In 1999 IEEE C37.09 was revised which defines the SC tests required to certify a MV breakers interrupting rating. The old rating used a K factor to define the interrupting rating as a function of voltage. This was done from 1968 to 1999. Prior to 1968 they were all based on an MVA rating.

So you may see a breaker rated 250MVA, 350MVA for 5kV class, and 500, 750, or 1000MVA for 15kV class. Then when the k factor was introduced the ratings changed depending on voltage applied although they still retained the "MVA" rating. The K factor was based on the properties of the oil and air technologies.

For example: A 5kV class breaker rated 250MVA operated at 4.76kV had a rating of 29kA, but if operated at 4.16kV then it had a 32kA rating. There are limits to the maximum capability.

In 1999 When it changed, it was to basically recognize the vacuum and SF6 technology that was constant regardless of voltage applied. The change essentially changed k=1 for all breakers. So, now that 5kV breaker does not have an MVA rating anymore, it has a rating that is good for all ranges of voltage in it's class. The industry standardized pretty much on 25KA, 40KA, 50KA, and 63KA. It took a while before manufacturers got on board. If you have an older line-up, you can still get the breakers with the older ratings, I believe it's just a nameplate thing.

Anyhow, that should give you some info on what you may be looking at as one job could be different from the next.

 

[philly]

Thanks kinbpb this is what I have discovered in my research as well.

For example: A 5kV class breaker rated 250MVA operated at 4.76kV had a rating of 29kA, but if operated at 4.16kV then it had a 32kA rating. There are limits to the maximum capability.

Shouldn't it be rated at 33kA since (4.76/4.16) x 29kA = 33.18kA?

Also what does the 3-second short time current capability refer to? I notice this appears to be the same as the interrupting current? Is this simply a current that the breaker is able to withstand and not interrupt for through faults which are interrupted by other devices downstream?

 

[zog]

Here is a paper from Siemens that may clear some things up.