MCCBs - Icw less than Ics - Why?

A third of a second at 10kA is a heck of  lot of energy to handle - why do you call it low ??


Given the I squared t nature of thermal damage like melting contacts, 0.3 sec and 10kA should do fairly similar damage to something like 50kA for  0.3sec / 25, a respectable 12msec disconnection time. Given the natural breaking time in ' instant' mode of these devices is  typically 10msec, but is specced at never more than 30msec,  these two specs tell you how to derate if you add a deliberate delay to the breaking time using the 'STD'  = Short Term Delay..

(given that the 'instant' part of  the breaker can be held off to ~ 12 times the rated current, 12* 630A is  more like a worst case of 8kA  for 300msecs for that specific 630A model, but I supspect the 0.3 second  spec is a standard limit tested to for that whole family of breakers to allow you to scale expectation when using the STD- it tells you something about the thermal mass of the contacts and arc traps  - the 630A is the small model. data sheets here  I note the 800A one is also rated at 10kA for 0.3 secs.)

So, use STD, but check the PSSC.

The  other specs,

ultimate short-circuit current - it will safely break this current once but may never work again, and will need replacement

and Ics = 50kA rated service short-circuit current -  it will break this current a few times and still work, but may be degraded after multiple operations.


You may decide to install it in a system with the higher fault level if the chances of a dead short fault close enough to get the full PSSC are so remote, that the cost and pain of replacing the breaker in the unlikely event of the worst possible  fault occuring , is acceptable.


Normally we'd back up with a big fuse to limit the let through energy in the event of a large catastrophe. These devices are big enough that even modest lengths of cable will get the PSSC low enouhg not to be an issue.

A third of a second at 10kA is a heck of  lot of energy to handle - why do you call it low ??


Given the I squared t nature of thermal damage like melting contacts, 0.3 sec and 10kA should do fairly similar damage to something like 50kA for  0.3sec / 25, a respectable 12msec disconnection time. Given the natural breaking time in ' instant' mode of these devices is  typically 10msec, but is specced at never more than 30msec,  these two specs tell you how to derate if you add a deliberate delay to the breaking time using the 'STD'  = Short Term Delay..

(given that the 'instant' part of  the breaker can be held off to ~ 12 times the rated current, 12* 630A is  more like a worst case of 8kA  for 300msecs for that specific 630A model, but I supspect the 0.3 second  spec is a standard limit tested to for that whole family of breakers to allow you to scale expectation when using the STD- it tells you something about the thermal mass of the contacts and arc traps  - the 630A is the small model. data sheets here  I note the 800A one is also rated at 10kA for 0.3 secs.)

So, use STD, but check the PSSC.

The  other specs,

ultimate short-circuit current - it will safely break this current once but may never work again, and will need replacement

and Ics = 50kA rated service short-circuit current -  it will break this current a few times and still work, but may be degraded after multiple operations.


You may decide to install it in a system with the higher fault level if the chances of a dead short fault close enough to get the full PSSC are so remote, that the cost and pain of replacing the breaker in the unlikely event of the worst possible  fault occuring , is acceptable.


Normally we'd back up with a big fuse to limit the let through energy in the event of a large catastrophe. These devices are big enough that even modest lengths of cable will get the PSSC low enouhg not to be an issue.