Switchgear 2-Sec vs 0.5-Sec Short Circuit Rating

[rockman7892]

I am used to seeing Switchgear with a 2-second Short Circuit Withstand rating but recently came across a MV MCC (not really switchgear I guess)that had a 50kA 0.5s rating?

I wasn't aware of this 0.5s rating on MV equipment. Is this common among MV MCC's and Switchgear as well?

I'm assuming that it depends on application and cost as to weather to use the 0.5s rating vs 2s rating? With the 0.5s rating I'm guessing that you have to have a main or upstream relay with a maximum of 0.5s delay as well as accounting for breaker/contactor opening time?

 

[wroggent]

http://w3.usa.siemens.com/us/intern...ocs_MV/TechTopics/ANSI_MV_TechTopics22_EN.pdf

The principle standard applicable to metal-enclosed power
circuit breaker switchgear is ANSI/IEEE C37.20.1-2002. As it
relates to the bus ratings, this standard defines several ratings:

...
...

Rated short-time withstand current – This is the average
symmetrical current that the switchgear must be able to
withstand for two periods of 0.5 second

 

[RRaghunath]

Quote "main or upstream relay with a maximum of 0.5s delay as well as accounting for breaker/contactor opening time" Unquote - True.
When the MCC is rated for 0.5s, the incomer should be able to isolate any bus fault in not more than 0.5s. That includes protection operating time as well as the CB opening time.
If not, it could affect the integrity of the MCC.

Rompicherla Raghunath

 

[krisys]

Raghunath,

It depends on the application also. At the point of application, if the available fault current is 50%, then the switchboard rating can be uprated by 4x. Thus 50kA, 0.5s rated switchboard will be re-rated to 25kA, 2 s.

This way the risk can be managed.

If the basis of design is available, this can be ascertained.

 

[dpc]

wroggent: The standard ANSI C37.20.1 is actually for low voltage switchgear, I believe.

 

[wroggent]

dpc, thanks you are correct. OP should find the answer in C37.20.3 which I do not have access to at the moment.

 

[rockman7892]

Raghunath

You bring up an interesting point regarding fault current and duration.

What if for instance on two second rated Switchgear the fault current exceeds the 2-second withstand fault current but the upstream devices clears it faster than 2 sec?

For instance Lets say that on 50kA gear rated for 2-sec the available fault current is 60kA but would be cleared within one second?

This may not be a practical example because the withstand current in switchgear matches the interrupting rating of devices but I have seen this be the case on switches or busway that does not have an interrupting device but only a withstand rating.

The equipment withstand rating times I am accustomed to seeing are

Switchboards and MCC's - 3 cycle
LV Switchgear - .5 seconds
MV Switchgear - 2 seconds (And not I guess .5 seconds as well).

It sounds like these withstand times are based on the withstand times of the breakers that are located in this equipment?

 

[rockman7892]

raghun

Following up on your point I guess the application of the .5s rated switchgear would be application specific based on the clearing time of upstream devices? So if you know that your upstream device will clear fault in less than .5s then you can get away with lesser rated .5sec switchgear as opposed to 2sec switchgear which I'm guessing is cheaper?

What differs in the construction of the switchgear for the 0.5s and 2s ratings? Simply a matter of bus bracing?

 

[RRaghunath]

When we speak of short circuit withstand capability of switchgear, it is always two types - Dynamic and thermal.
25kA, 3s etc. is about thermal withstand. If the actual fault current is more than 25kA, but cleared in less than rated duration (3s in this case), the switchgear may survive as long as Isquared-t values are not exceeded.
Dynamic is different. The dynamic withstand current is 2.55xthermal withstand current typically for HV systems (the factor can be different based on system X/R value). Thus, 25kA switchgear is designed to withstand dynamic forces created by 63kA current. This also called momentary withstand, i.e. at the point of switching on. This can be improved in case of busbars, by improving the bracing/supports etc.
Generally speaking, if you have a switchgear where all the outgings are fused feeders, there is no reason to have thermal withstand >0.5s. This is because the fuses are expected to clear all high short circuit current faults in <0.2s, leaving a margin of 0.3s (0.5-0.2) for the incomer protection.
Hope I have answered your query.

Rompicherla Raghunath

 

[FreddyNurk]

raghun, nice explanation.
I've always found myself having to explain the same, particularly when people only look at the thermal rating, and think 25kA for 3 seconds, that means I can do 50kA for 1, and ignore the dynamic forces.



EDMS Australia

 

[rockman7892]

I took a look at ANSI C37.20.3 and did not see any reference to anything less than 2 seconds for the duration of the short-time withstand. (It stated that 2 seconds was the preferred duration).

Since the MV MCC I referenced in my OP would be classified as metal enclosed switchgear and therefore fall under C37.20.3 I don't see how it could have a 0.5s rating if this is not a listed rating?

I see from rauhgn's explanation that this duration rating (short time rating) relates to the thermal withstand of the switchgear. So perhaps with an MV MCC that has fuses that clear in a fast manner the gear can have a 0.5s rating and therefore be built to less of a thermal withstand since the switchgear withstand is based on the withstand rating of the switching an protective devices?

I have often heard people refer to a "bus bracing" analysis. It is my understanding that this analysis is in reference to the dynamic forces associated with the momentary rating. My understanding has been that this analysis is required when you want to increase the short circuit rating of the switchgear and are also changing and increasing the interrupting rating of the breakers?

 

[krisys]

The breaker design calls for the dynamic current (Peak or close & latch current as the region of use)will be generally 2.5 times the Isc in the case of medium and high voltage. It will be 2.5 times the Isc in the case of LV system. So the ratio of peak current to the Isc depends on the value of X/R ratio.

For the application far from the source, the X/R ratio is low. When the X/R ratio is low, the dynamic current can be as low as 1.5 times the Isc. This is a classical example where there is a scope to engineer the specifications, thus resulting in some cost saving.

Friends,
Gone are the days of liberally designed electrical systems. You decide whether you need the continuity of your job or liberally designed electrical system. It is difficult to have both in these days. There could be a smart guy who might tell I would have saved some cost to the Company!