Choosing the Circuit breaking capacity of CB

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TMMB

Senior Member
Location
EGYPT
Occupation
Electrical Manager
i have distribution board with short circuit level 30 KA as shown on the single line diagram during choosing of the circuit breaker, which breaking capacity i have to use Icu (ultimate short circuit or Ics (Service short circuit) if the shown levels on the circuit breakers as follow: Icu =30 KA and Ics= 50% of Icu it mean equal 15 KA
 

TMMB

Senior Member
Location
EGYPT
Occupation
Electrical Manager
dear all

112 viewers and no single reply, please it is is very simple question

when we choose breaking capacity of circuit breaker it will be depending on the ultimate (Icu) or the service (Ics) short circuit level

regards
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
dear all

112 viewers and no single reply, please it is is very simple question

Your question deals with IEC applications, it is not surprising no one has responded. This is a forum where most members use the US National Electrical Code.
 

JRW 70

Senior Member
Location
Eastern Central Missouri
Occupation
Testing and Engineer
CB Interrupting Capacity

CB Interrupting Capacity

If your SCC (line to line or line to ground/neutral)
really is 30kA, I would be afraid the breaker would
fail in a maximum short circuit situation. I have seen
similar catastrophic failures where the equipment is
rated (more or less) at the exact Interrupting value
of the protective device. Is there another breaker that
will fit your application that has a 45kA or better yet
a 60kA interrupting value. I only say this because the
30kAIC breaker may clear the fault, but not survive
to be reset and reliably protect into the future.
So in summary, this seems to be too close on the
interrupting capacity compared with that available.
Try to see if there is a more suitable C.B. or
use some other short circuit reduction techniques.

Just seems a little close in my perspective.

JR
 

JRW 70

Senior Member
Location
Eastern Central Missouri
Occupation
Testing and Engineer
One Line

One Line

How accurate was the development of the one line?
Cabling over a surprisingly short distance can make
a quite surprising change in available short circuit
current. If the one-line is the true and exact value,
then there is not much that can be done, other than
going to a higher interrupting capacity set of gear.

On our equipment we make sure that the fault can be
cleared and the C.B. can be rebuilt to our testing
standards.

I hope you have success in your project.

JR
 
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jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
it also shows how little most of us know about AIC ratings :D
Yeah, kind of.

But it also points out that there is a difference between applying IEC values versus UL values. Like for most items NEMA products have historically been overbuilt so that the person applying them did not need much training to select them, whereas IEC products are usually require a fairly detailed analysis of the desired application and the device's ratings.
 

kwired

Electron manager
Location
NE Nebraska
dear all

112 viewers and no single reply, please it is is very simple question

when we choose breaking capacity of circuit breaker it will be depending on the ultimate (Icu) or the service (Ics) short circuit level

regards
Must be complicated enough that you had to ask though:happyyes:

Your IEC terms are kind of like reading a foreign language to some of us, NEC uses different terms for these items, and NEMA equiment is designed to different standards as has been mentioned. Down stream devices can possibly be "series rated" for use with certain upstream devices.
 
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