IEC breakers - Icu Significance

[NickParker]

What is the significance of Icu rating in IEC breakers? From what I have read,

Icu - Cannot be to put back into service after interrupting the fault at the rated Icu Short circuit current
Ics - Can be able to put back into service after interrupting the fault at the rated Ics Short circuit current.

What is the meaning of Icu = 100% of Ics? Can the breaker be remain in service or not? after interrupting the fault at rated Icu.

 

[RRaghunath]

You mean Ics = 100%Icu. Icu is the reference here.
If Ics is equal to Icu, this means the CB can be put back in service after it interrupts a fault current equal to Icu.

 

[che12345]

Dear Mr NickParker

Q. What is the significance of Icu rating in IEC breakers? From what I have read ...What is the meaning of Icu = 100% of Ics? Can the breaker be remain in service or not? after interrupting the fault at rated Icu.
A. Reference IEC 60947-2
1. Definition:
a) ultimate short-circuit breaking capacity
a breaking capacity for which the prescribed conditions according to a specified test sequence [do not] include the capability of the circuit-breaker to carry its rated current continuously.
b) service short-circuit breaking capacity
a breaking capacity for which the prescribed conditions according to a specified test sequence [include] the capability of the circuit-breaker to carry its rated current continuously.
Note: [....] is inserted by the writer to stress the difference.
2. Characteristic of circuit-breakers:
a) Rated ultimate short-circuit breaking capacity (Icu)
... is the value of ultimate short-circuit breaking capacity, see above 1.a).
b) Rated service short-circuit breaking capacity (Ics)
... is the value of service short-circuit breaking capacity, see above 1.b). ... it may be expressed as a % Icu.
3. My opinion
I am of the same opinion as learned Mr RRaghunath.

Che Kuan Yau (Singapore)

 

[NickParker]

Since If I use the Ics rating, even after breaking the fault current, the breaker can be in service again. then what is the need of Icu rating? However if I use the Icu rating for selecting the breaker and after breaking the fault current, it can not be in service again, I may use fuse here. I cannot understand the use of Icu?

How Icu and Ics differs for different voltages?

 

[che12345]

Dear Mr NickParker

Q1. Since If I use the Ics rating, even after breaking the fault current, the breaker can be in service again. then what is the need of Icu rating?
A1. All IEC breaker manufacturer ratings stated on their catalogues are Icu values (e.g. 35, 65, 85, 100kA etc). The Ics rating of MCCBs which are (e.g. 50, 75, 100% of Icu) may not be noticed by the designers.
Depending on the importance of the load, it may not be necessary to select all MCCBs based on the Ics rating; which are usually bigger in physical size and higher in cost. The main reason being that most earth-faults, over-loads and even short-circuit trips are usually lower than 50% of the [calculated prospective short-circuit current] at the breaker location.

Q2. However if I use the Icu rating for selecting the breaker and after breaking the fault current, it can not be in service again, I may use fuse here. I cannot understand the use of Icu?
A2. A MCCB (e.g. Ics=50% Icu ) is [capable] of carrying the rated current,if the fault-current does [not] exceed 50% of Icu. See the main reason mentioned above.
A fuse would need a [complete replacement], irrespective of the fault-current value!

Q3. How Icu and Ics differs for different voltages?
A3. a) A MCCB Icu (e.g. at 50-60Hz 380/415V =65kA, at 690V =40kA and at 220/230V =100kA etc)
b) A MCCB Ics in %Icu (e.g. at 50-60Hz 380/415V =75%, at 690V =50% and at 220/230V =100% etc)

Che Kuan Yau (Singapore)

 

[NickParker]

Simply what is the criteria to select the IEC breakers with respect to the IEC 60909 short circuit calculation. From what I have read, all says that all the terms Icu, Ics, Icw should be greater than Ik"

Icu > Ik"
Ics > Ik"
Icw > Ik"

 

[che12345]

Dear Mr NickParker

Q1. Simply what is the criteria to select the IEC breakers with respect to the IEC 60909 short circuit calculation.
A1. IEC 60909-x title "Short-circuit currents in three-phase a.c. systems"
Reference IEC 60909-0 2001-07 attention: replace fig 1 and 2 in page 31 and 33, see corrigendum 1.
Definition: a) initial symmetrical short-circuit current I"k is the r.m.s. value of the a.c. symmetrical component of a [prospective (available) short-circuit current]* applicable at the instant of short circuit if the impedance remains at zero-time value, see fig 1 and 2.
b)* prospective (available) short-circuit current is the current that would flow if the short circuit were [replaced by an ideal connection of negligible impedance] without any change of he supply see ....
Note: 1. [...] and [...]* are inserted by the writer
2. IEC publications use the term I"k, no mention of Ik" .

Q2. From what I have read, all says that all the terms Icu, Ics, Icw should be greater than Ik"
Icu > Ik"
Ics > Ik"
Icw > Ik"
A2. I do {not] accede to the assertion. The main reasons being:
a) there is always impedance (R+X), which is [not negligible] at the up-stream of the breaker.
b) all breakers [do not] open at time zero i.e. at the value I"k. The breaker would likely open when the short-circuit current had been lowered to the steady-state short-circuit current Ik, when
idc would had being =0 some time ago.

Che Kuan Yau (Singapore)

 

[NickParker]

Dear Mr Che,

Can you confirm the below criteria for the breaker selection.

Icu > Ik, if want to be on safer side, Icu > I"k.
Ics > Ib (breaking current at the instant of contact separation)
Icw > I"k for a time till the break opens.

 

[che12345]

Dear Mr NickParker

Q1. Can you confirm the below criteria for the breaker selection.
A1. a)Reference IEC 60909-0 2001-07 page 31/33 attention: replace fig 1 and 2 in page 31 and 33, see corrigendum 1.
Ik = peak short-circuit current.
I"k = initial symmetrical short-circuit current.
b)Reference IEC 60947-2
Icu = rated ultimate short-circuit breaking capacity.
Ics = rated service short-circuit breaking capacity
Icw = rated short-time withstand current.
( ) = rated short-circuit breaking capacity..... stated as Icu and Ics .....

Q1.1 Icu > Ik, if want to be on safer side, Icu > I"k.
A1.1 a) Icu > Ik is fine.
b) if want to be on safer side Icu > I"k is fine but [not] mandatory.
In general, I"k >>> Icu. It would [not] be cost effective to select Icu > I"k.

Q1.2 Ics > Ib (breaking current at the instant of contact separation)
A1.2 It is fine if you mean Ics > Icu. Note: Ib = ?

Q1.3 Icw > I"k for a time till the break opens.
A1.3 Icw > I"k is fine but [not] mandatory.
In general, I"k >>> Icw [even for 0.5s]. It would [not] be cost effective to select Icu > I"k.

Che Kuan Yau (Singapore)

 

[NickParker]

As per IEC-60909; For the short circuits far from Generator, Since I"k = Ik = Ib are approximately equal,

I"k --> RMS value of the initial symmetrical current
Ik --> RMS value of the steady state symmetrical current
Ib --> RMS value of the symmetrical current at the instant of contact separation.

Therefore being on a safer side, can we take the conservative route of,

Icu > Ik"
Ics > Ik"
Icw > Ik"

 

[che12345]

Dear Mr NickParker
Introduction: As per IEC-60909; For the short circuits far from Generator, Since I"k = Ik = Ib are approximately equal,
I"k --> RMS value of the initial symmetrical current
Ik --> RMS value of the steady state symmetrical current
Ib --> RMS value of the symmetrical current at the instant of contact separation.

Q1. Therefore being on a safer side, can we take the conservative route of,
Icu > Ik"
Ics > Ik"
Icw > Ik"
A1. [Agreed in full] with your learned opinion for (far-from-generator circuits where
I"k = Ik = Ib ).
Note:1. Reference IEC 60909-0 Fig.1 and sub-clause 4.5.1
2. Values shown in [brief brochures] are [rated ultimate short-circuit breaking capacity Icu ]. Only in [detailed catalogues] are shown with more detailed ratings, where (in general for MCCBs) Icu > Ics >>> Icw 1s .
Thank you for your kind advice.

Che Kuan Yau (Singapore)