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I have always wondered as to why does a circuit breaker in the IEC World have so many short circuit breaking

capacities such as Ics(service short circuit breaking capacity), Icu(ultimate short circuit breaking cap), Icw (short circuit

breaking capacity). After running an ETAP Programme we reach an initial symmetrical short circuit current (say 40 kA)

which in turn determine the short circuit rating of the switchgear for 1sec/3 sec( I..E 40kA for 1 second). Some of the

suppliers consider it as the prospective short circuit current and match the Icu rating of the breaker( 40kA) to be used

on such a switchgear . Shouldn't they select Icw rating of the breaker to match the switchgear short circuit withstand

current for 1 sec. Your suggestions will be of great help.

Hi Uno,

Breaking capacity of the CB must always exceed the short circuit withstand current of the switchgear. So the

requirement of the switchgear will determinate the Short circuit breaking capacity of your CB. Standard (depending

the application) the Icw rating of the breaker should exceed the switchgear rating.

regards,

Danny

Please excusse my English. Can you re-phrase the sentence "which in turn determine the short-circuit rating of the

switchgear for 1sec/3sec (i.e. 40kA for 1 second)"? I don't understand. Thank you.

For Icw (selection of CB based on the shc current value at time 1 second): No. The breaking capacity/current Icw of CB

is to be based on the value of short-circuit current calculated at the time when the CB's contacts start to separate,

which in most cases is abt. tens of msec not 1 second; this time is to be obtained from CB data sheet.

For the rest of currents, please give 1-2 days.

(I would suggest you to visit the question "system stability" of this Forum).

Thanks Danny and Davrom for your responses.

Davrom

The 1 second or 3 second is to specify the short time withstand rating of the fault current during which the prospective

fault current(I..E 40kA) is assumed to be constant.

I agree with Danny in that the breaking capacity of the breaker should be Icw and should match the switchear short

circuit withstand level. On some more investigation I also found that some breakers ( in utilization category A) need not

have Icw rating since they do not have intentional time delay under short circuit condition. Then how do we confirm

that the breaker selected by the supplier is ok?

Hi uno.

A. Please forget my first message.

7

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B. Sorry for this delayed response. My attempt to understand the differences between the three currents took me more

than 1-2 days that I expected. I have been checking for and thinking to the three currents over and over again. I have

come to some conclusions, but, as I am not 100% sure, I recommend you to take my below notes as suggestion only.

1. The name of Icw is "rated short-time withstand current" and has nothing to do with breaking capacity (this confused

me). The physical meaning of Icw is more close to the making current Icm than to breaking current(s).

2. The making current Icm must be biger than the peak value ip of shc current and specifies the maximum curent

which the CB withstand to in respect to the electro-dynamic forces produced by the inrush current of shc.

3. For CBs with intentionally time delay and resp. with Icw, the CB must be able to withstand to the shc current (el-

dynamic forces) for the duration of the set delay (e.g. 100 msec) and then to break/trip the circuit (Icu) if the shc is still

present. This is why I afirm that the Icw is more close in meaning to the Icm. The Icw is to be bigger than r.m.s. value

of iksym assumed to be constant from the begining of shc (i.e. 40 kA for 1/3 seconds in your case).

4. Utilization category A means that the CB is not intended to be used in series with another protection device in supply

line; this would be coresp. to the motor protection CBs of Siemens.

5. Utilization category B means that the CB is intended to be used in series with another protective device in supply

line; this would be coresp. to the line protection CBs of Siemens. Consequently, in order to ensure the selectivity of

protection, the CB is to be provided with and intentionally delay release time (which can be adjusted) and, hence, with

the Icw.

6. Based on notes 2 and 3 it results that the Icw is only defined/specified for CBs used in category B.

7. The difference between Icu (ultimate) and Ics (service) currents consists in the qualitative and quantitative

conditions established for testing the CBs. I cannot tell exactly what is this diference.

8. The CBs are constructed in four cases, depending of relationships between the three currents (equal, >, <, <>), i.e.

Ics=Icw<>Ics, Ics=Icw=Icu etc.

9. I have checked the Siemens products catalogue and I haven't found specified the Icw value. I assume that all

Siemens CBs are constructed with Ics=Icw. On the other hand, the Siemens offers CBs provided to "azn" electronic

device; some CBs are only with "an" device - coresp. to motor protection while others are provided with "azn" device.

The Icw and its delay time can be adjusted on "z" portion of tripping characteristic, resp. from Icw=0...8*In of CB and

td=0...300 msec; t=0 msec coresp. to motor protection CB; t>0 sec coresp. to line protection CB (categ. B); the CB can

be used for both categories.

10. The Siemens states that back-up fuses are only required if the shc current at the point of installation exceeds the

Icu (ultimate) value of CB. Cumulated with the fact that Ics is in almost cases smaller or equal with Icu, it result that

the base for selection the breaking capacity of CB is the Icu, but as I said above, I cannot tell for sure what is the

difference between Ics and Icu. A manufacturing engineer may tell this difference; I am only an installation engineer.

11. The breaking capacity/curent of CB (Icu or Ics) is to be bigger that the shc current evaluated at the time when the

CB's contacts start to separate. This value is to be based on the r.m.s. value of the iksym at the time when the contacts

start to separate; I personally prefer to calculate the instantaneous value of shc current (ikasym+iksym) at the

respective moment.

12. As far as I am concerned, I "split" the short-circuit process into three sequences:

a) t=0...topening: the CB must be able to withstand to the el-dynamic forces produced by the inrush current of shc;

therefore, Icm > ip (please read the ">" sign as "must be bigger than"); for CB of category B this additional condition

must be met: Icw > iksym(t=0) - acc. to note 3.

b) t=topening: the CB must be able to break/trip the shc current at this time; therefore, Icu > Ik(r.m.s. of iksym, t=0

(assumed constant)) or, in my theory, Icu > ik(t=topening).

c) t=0...tend: the CB must be able to withstand to the heat effects produced by the shc current in the interval

t=0...topening and by electric arc between opened contacts in the interval t=topening...tend; therefore, Ilt > Ith

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(thermal current).

I hope my notes would be of a help to you, but as I said before, I recommend you take this scheme as suggestion only.

Correction to my previous message:

At note 6 please read "based on notes 4 and 5 ...".

Thanks a ton Davrom

A star to your post. I don't know how I reached the conclusion that Icw is the breaking capacity. I guess then the

manufacturer's do the right thing in considering Icu for the selection of breakers for a switchgear.

Meanwhile I also checked The IEC 947-2 and reached the following conclusion -

When the breaker is tested for Icu , after which it cannot be expected to carry the rated current continuosly which is

not the case when tested for Ics.

Thanks a lot for all your responses.

Uno,

sorry for the late reply, but I saw your post just today. It has allready been perfectly tackled by davrom, theefore just

my understanding as additional information as I had the same problem in the past:

the result of your ETAP program should be:

- Ik": Initial short circuit current (rms value): Maximum value of the 3 pole short circuit current. Occurs directly after

the initiation: Network feeders, generators and motors contribute to this value

- ip: Peak short circuit current (peak value, not rms value). Maximum value (peak value) of the short circuit current.

Occurs within the first sin half wave directly after the initiation.

furthermore, you can also obtain these values from such programs, that are both derived from Ik":

- Ik: Steady state short current (short circuit current after the decaying of the d.c. component). Derived from Ik". Only

network feeders and generators contribute to this value

- Ib: Symmetrical short circuit breaking current: rms value of the short circuit at the moment when the breaker opens.

Derived from Ik". The influence of motors and gnerator is reduced compared to Ik"

Generally speaking, Ik", Ik and Ib are referring to the thermal stress of your equipment, while ip is referring to the

mechanical stress of your equipment.

You do not need Ik and Ib in any case. You can use Ik" instead to be on the safe side.

Rated values of a SCB:

- ICM: (peak value): Max value that the SCB can carry. The greatest stress to be managed is during closing on a short

circuit current. There, the mechanical stress has to be managed.

- ICU: (rms-value): Max current that the short circuit breaker can break without beeing in a dangerous condition

afterwards

- ICS: (rms-value): Same as ICU. The difference is, that the SCB might be defective after the operation. Therefore,

this value is often expressed in % of ICU (0..100%). The higher this value is, the better is the quality of the SCB.

- ICW: (rms value): The maximum value that the SCB can carry (without breaking) of a specified time (very often 1sec.

or 3sec.)

For the correct choice of your short circuit breaker applies:

Mech. stress: ICM > ip

Thermal stress: ICU >= ICS > Ib and ICW > Ik"

or instead of this : ICU >= ICS >= ICW > Ik"

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As far as I know, the BS EN 60947-2 (derivative of IEC 947-2) only defines the following standard terminology

concerning the current rating for MCCB/ACB:

In – Rated current

The maximum continuous current a CB will carry under healthy circuit conditions. I shall not elaborate it further.

Icu – Rated ultimate short circuit (sc) breaking capacity

The maximum prospective fault current level a CB can withstand.

Ics – Rated service sc breaking capacity

The maximum level of fault current operation the CB will withstand and still remain in full performance after the fault.

Icw – Rated short time withstand current

The current the CB can withstand for the maximum short time delay time.

I do not know whether , Icm, the so-called making capacity of CB is defined in any IEC standard. But I did find Icm

rating in a CB manufacturer catalogue. From what I know the term “prospective fault current” level in the IEC standard

always refer to symmetrical steady state fault level and the IEC 947-2 does not define any standard test for

asymmetrical fault current rating for CB. As such I do not know on what basis of method each CB manufacturer defines

the Icm rating

Actually the differences of Icu and Ics rating lies between the method the CB is tested in accordance to the IEC

standard.

Icu – The CB under test must subjected to a test sequences of O-CO with no performance loss at the end of test.

Ics – The CB under test must subjected to test sequences of O-CO-CO with no performance loss at the end of test.

Where O – Opening operation under fault conditions.

C- Closing operation on to a fault.

As can be seen from above, CB under Ics test undergoes more steps. Therefore the we could always expect the fault

level a CB can achieve under the Ics test condition to be equal or lower than the fault level it can go through under the

Icu test condition. Ics is usually indicated as a percentage of Icu (say 100%, 75%, 50% etc.)

Icu capacity indicates the maximum theoretical fault level of the CB and this has to be matched (equal or higher) with

the calculated prospective fault (symmetrical) value of the installation at the point of connection (i.e. the switchboard

kA rating).

For example a CB of Icu = 50kA and Ics = 50% Icu means that that CB can withstand maximum fault up to 50kA and

up to 25kA fault for continuation of service. In other words, if fault levels are between 25kA to 50kA, the CB still able to

cut out the faulty circuit safely but its further operation afterward is not assured. If the fault level is 25kA and below

then the CB is able to cut out the faulty circuit safely and its further operation afterward is assured.

It can be seen that Ics applies to short circuit faults that could occur in practice. In my country, the specification usually

called for Ics be 100% of Icu for incoming feeder and 50% of Icu for outgoing feeder.

I have never specify the value for Icm (neither does I know what IEC definition for it). It is my opinion that Icm shall

only be taken into account in special installation which consist majority of heavy motors, generators etc. Perhaps

<Area> can elaborate the definition of “thermal stress” and “mechanical stress” of the CB as well as Icm with

relevance to any IEC standard or equivalent.

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As for the Icw it is more to coordination curve of CB for downstream consideration and bear no relevance to Ics or Icu.

To Voo:

1. A star to your post. Real thanks for info given upon the Icu and Ics currents. I knew the test sequences of each

current (qualitative conditions) and the capability of CB to carry, resp. not to carry its rated current (quantitative

conditions), but I couldn't understood the physical meaning of these conditions. Now all is clear. Thanks again.

2. The definition of making current Icm can be found in the IEC 60977-1 "LV Switchgear and Controlgear. Part 1:

General rules" and IEC 60977-2 "LV Switchgear and Controlgear. Part 2: Circuit breakers".

The making current Icm is reffered as the peak value ip (instantaneous value, in kA) of the short-circuit current to

which the CB withstand, in respect to the electro-dynamic forces (or "mechanical stress" - area).

I believe that area's terms "mechanical stress" resp. "thermal stress" are the same with my terms "dynamic stability"

resp. "thermal stability" and reffer to the physical phenomena occured during a short-circuit.

A current which flows through a conductor or a current path (e.g. CB's contacts) produces an electro-magnetical field

around the conductor. The physical expression of this field is the force which the conductor interracts with the

elements in its vecinity. The force value is directly proportional with value of current.

Also, the same current causes the conductor to get heated; the heat is being transfered to the medium in which the

conductor is placed and determine the temperature to rise in the respective medium/space; this has impact on the

dielectric properties of the estinguishing medium (the a.m. IEC Part2 specifies a dielectric test to be performed for

CBs). The temperature rise is directly proportional with the value of current.

The short-circuit current produces an inrush force on the CB's contacts and determine a rapidly increase of the

temperature in medium/space of contacts. The maximum instantaneous value of the force is reached for the peak

value of the short-circuit current. Based on this, the making current Icm indicates in fact the maximum force to which

the CB withstand (and keep its contacts closed).

In order to make easier the selection of a CB based on the short-circuit current value (expressed in kA), it has been

choosen to indicate the values of the currents that produce the repective force resp. temperature rise, instead to be

given the values of force and temperature to which the CB withstand.

I hope you will understand something from my notes. I am sorry, but I cannot explain better.

Davrom,

thanks for giving the link to the IEC concerning the definition of Icm. I also appreciate your description of the Icm -

could not do it better. Maybe it was not fully clear to what I was referring with mechanical stress and thermal stress,

but it is exactly what you wrote:

Mechanical stress = dynamic stability

Thermal stress = thermal stability

As you described, during a short circuit you have two phenomenoms: A 'mechanical force' that is caused by the current

and that is directly proportional to the value of the current and a thermal loss (I 2*t), that is depending on the value of

the current and on the duration time of the short circuit. If you have e.g. a big short circuit current fort a short time

(e.g. 20 msec), you will have a big mechanical stress of your equipment (SCB, switchgear) but a minor thermal stress,

as the duration time is limited. If you have a big short circuit for a long time (let's say 300 msec), you have both

problems: A big mechanical stress at the inition of the short circuit and a big thermal stress during the short circuit.

Voo, if you did not find the Icm rating in any manufacturers catalogue, you might want to try this link (hope it works):

http://138.227.174.121/GLOBAL/SCOT/SCOT209.nsf/VerityDi...$File/Emax_Overview.pdf

If it does not work, access the ABB website and search for the description of the Emax series for example.

In fact you are right not to look at the Icm value, as it is usually directly linked to the Icu value of the short circuit

breaker (e.g. the ratio for the Emax series is 2,2, meaning a SCB rated Icu 65kA is rated Icm 143kA), as the ip value in

your short circuit calculation is directly linked to the Ik" value. Therefore, it is usually sufficient to make sure that Icu >

Ik". However, I prefer to check the Icu rating to be on the safe side.

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Suggestions:

1. The various current limit definitions are not including the current waveform classification, e.g. root-mean-square

(rms) value, peak, symmetrical rms, asymmetrical rms, asymmetrical peak, etc.

2. Low voltage circuit breakers have different operating duties then medium and high voltage circuit breaker.

Therefore, not all current limits are posted by the manufacturer, and some of them may coincide.

3. Visit

http://www.schneider-electric.com.gr/pdf/Technical_Libr...

for and example of current limits included on rating plate, on page 2.

4. Low voltage circuit breakers, medium voltage circuit breakers, etc. are normally covered by their own industry

standards, respectively.

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