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Is there a need for LV breaker Icu rating?

Is there a need for LV breaker Icu rating?

Is there a need for LV breaker Icu rating?

(OP)
In the IEC-60947.2 standard there is a ultimate short-circuit breaking capacity, Icu, rating as well as a service short-circuit breaking capacity, Ics.

Ics is usually expressed as a % of Icu (25%, 50%, 75%, 100%)

They are tested as follows:

Icu  =  rated ultimate short-circuit current
This is the maximum symmetrical short-circuit current the breaker can interrupt and is tested as follows, O – t – CO.

O    =  breaking operation
t    =  time separating two operations equal to 3 minutes or the length of time needed to reset      
                 the breaker, whichever is longer.
CO    =  making operation followed by a breaking operation
  
After the test, the breaker is tested to show that it is electrically safe. The dielectric test involves applying twice rated voltage, the minimum being 1000V.

Ics  =  rated service short-circuit current
It is tested as follows, O – t – CO – t – CO. Following the test a temperature rise, dielectric and tripping test is carried out. This is to verify that the breaker can be returned to service. In essence, Ics is the maximum current the breaker can interrupt three times and be returned to service with its operational integrity intact.

I always use Ics when specifying a circuit breaker as I think it's safer to do so. Surely, the intention is that a breaker must be able to carry load current again after interrupting a fault.

My question is thus what is the use of having the Icu rating? I'm a bit puzzled as to what purpose it serves.ponder

Thanks.
 

RE: Is there a need for LV breaker Icu rating?

(OP)
7anoter4

A very useful website indeed but I still do not quite see the use of Icu.

Thanks anyway.

RE: Is there a need for LV breaker Icu rating?

It is not always possible to select for Ics > maximum available fault current.

Icu is the actual interrupting rating. Yes, the breaker will not work afterward. But it cleared the fault.  Not all faults will draw the maximum.

Good on ya,

Goober Dave

Haven't see the forum policies?  Do so now: Forum Policies
 

RE: Is there a need for LV breaker Icu rating?

(OP)
Hi DRWeig

"It is not always possible to select for Ics > maximum available fault current."

Could you perhaps site an example please?

"Icu is the actual interrupting rating. Yes, the breaker will not work afterward. But it cleared the fault."

What purpose does it serve to have a breaker installed at a given location knowing that it possibly could not be returned to service depending on the fault current? Might as well use a fuse.

I agree that not all faults are at maximum current but how would you know what the fault current was unless you have some measurement functionality somewhere.

Regards.

 

RE: Is there a need for LV breaker Icu rating?

If you need the functionality of a breaker for coordination, shunt trip, or other logic, and the fault currents are larger than the Ics that the manufacturers offer, you can still choose to use them if the Icu is above the fault current level.

It's safe. It will still function and coordinate with downstream devices, and can be part of a control scheme.

It will only turn into an expensive fuse when the worst-case fault happens.

I'm not arguing that it's ideal. If the manufacturers offer a breaker with Ics > fault current, of course that one is preferred. I'm just showing that there is a purpose for the two ratings, in my opinion.
 

Good on ya,

Goober Dave

Haven't see the forum policies?  Do so now: Forum Policies
 

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