Understanding Medium Voltage Switch Short Circuit Ratings

[pitt123]

I'm looking at a particular manufacturers 15kV Metal Enclosed Fused Medium Voltage Switch with the following ratings:

Switch Ratings
Fault Close and Momentary Asymmetrical - 61kA
Short Time (2 sec) 38kA
Fused Fault Close - 101kA

Switchgear Assembly rating
Short Circuit Interrupting Rating(SYM) - 40kA
Fault Close & Momentary (ASYM) - 64kA

I am trying to understand how the manufacturer comes up with the overall "Switchgear Assembly Rating" that is shown?

I understand the switch ratings above of 38kA/61kA apply to just the switch mechanism itself without consideration of any other elements of the structure. If the switch is fused as it is in this case then the switch/fuse combination can have a higher Fault Close (momentary) rating which in this case is 101kA. It also has a listed 63kA sym interrupting rating based on the fuse.

What I don't understand is how the manufacturer comes up with the Switchgear Assembly ratings shown above. I guess I understand all the parts and pieces of the switch ratings but don't know how they all fit together to come up with the assembly rating. I saw somewhere that said with the increased fault close ratings with the fuse the manufacturer can place increased numbers on the structure. So in this case with a 101kA maximum fault close with the fuse the manufactuer stated that they are allowed to choose 40/64kA rating for the switch assembly.

Can anyone explain how the Assembly ratings are derived?

 

[7anoter4]

In my opinion, IEC Publication 60298 A.C. metal-enclosed switchgear and controlgear for rated voltages above 1 kV and up to and including 52 kV could enlighten it, in a way.
In ch. 4.6 Rated peak withstand current- it is stated:
“NOTE - In principle, the rated short-time withstand current and the rated peak withstand current of a main circuit cannot exceed the corresponding rated values of the weakest of its series connected components. However, for each circuit or compartment, advantage may be taken of apparatus limiting the short-circuit current, such as current-limiting fuses, reactors, etc.”
Also, in ch. 5.9 Nameplates:
“The removable parts, if any, shall have a separate nameplate with the data relating to the functional units they belong to, but this nameplate need only be legible when the removable
part is in the removed position.”
That means, in my opinion, you may replace the existing switchgear with another keeping the rated currents -or better.

 

[Pitt03]

7another 4

Thanks for the response. Looking at what you posted and looking more into this it seems that manufacturer provides a "series rated" or "combination" listing for the entire structure (in this case just the switch and switch structure) based on combination testing with the fuse and switch. So in the case of my original example the manufacturer is able to give the structure a 40/64kA rating based on testing they have done for the particular fuse and switch combination with their associated ratings?

I imagine that he switch itself has various damage points (instantaneous, 3-cycle, 3-second, etc...) and as long as the fuse current limits below the instantaneous point the fuse would protect the switch. I'm guessing that as part of this testing and determination they somehow arrive at the structure 40/64 rating rather than being able to use the 63/101 rating of the fuse itself?

For equipment evaluation of this switch when comparing it to available fault currents would you use the listed structure rating (40/64kA) as opposed either the individual switch or fuse ratings?

 

[LionelHutz]

It actually sounds like the switch is 38kA (40kA) and the fuse is 63kA so they just called the combination 40kA or the lower of the two.

I'm not exactly sure why, but you can find switches and breakers rated 38kA or 49kA yet be able to call the combination 40kA or 50kA. My guess would be that it's due to testing/calculating the device using it's maximum voltage rating vs the rating combination at it's applied rating. In this case it might be something like 38kA @ 17.5kV for the switch vs 40kA @ 15kV for the package.

 

[Pitt03]

LionelHutz

That makes sense what you said about using the lower of the two.

I guess for the momentary rating of 64 they took a similar approach using the lower of the two (61kA) but somehow through testing/calculating they are able to use 64kA for the momentary rating.

So when comparing to fault duties it seems as if you would use 40kA for the interrupting duty and 64kA for the momentary duty?

It doesn't seem like the fuse provides much in the way of increasing the equipment rating?

 

[Pitt03]

In a follow-up to the last sentence in my previous post I want to compare the fused switch combination rating with that of a fused contactor combination rating. In the above example it seems that the fuse switch tested combination only allows the switch structure to be rated slightly higher than the switch alone (38/61 vs 40/64) but in the case with a fused contactor the fuse allows the contactor to have a much larger rating (from 5kA interrupting to 50kA interrupting and 80kA momentary).

Is it due to a different test procedure or other factors that the fuse in the contactor allows it to achiever a much larger rating than does a fuse located in a switch?