Still fighting with Ith and Icw

[98mlo]

I have a terrible confusion between the following currents:

-Ith as mentionned in IEC 60909-0:thermal equivalent short-circuit current ,the rms value of a current having the same thermal effect and the same duration as the actual short-circuit current, which may contain a DC component and may subside in time

-Icw as IEC 60947-2: Rated short-time withstand current (Icw),the rated short-time withstand current of a circuit-breaker is the value of short-time withstand current assigned to that circuit-breaker by the manufacturer under the test conditions specified

For a.c., the value of this current is the r.m.s. value of the a.c. component of the prospective short-circuit current, assumed constant during the short-time delay.

Both currents are however subject to a duration (1 second for example)?
Any explanations which can help me?
thanks a lot

 

[che12345]

Dear Mr. 98mlo (Electrical)(OP)30 Jan 23 15:1
" #1. ....-Ith as mentionned in IEC 60909-0:thermal equivalent short-circuit current ,the rms value of a current having the same thermal effect ..... which may contain a DC component and may subside in time....."
In IEC 60909-0 there are two typical curves of short circuit current, i.e. near to or far from gen ....... Take note that the curves are offset due to "A" the initial value of the direct current, which subside in time. The upper envelope 1 and the lower envelope 2 are NOT sinusoidal; due to 3 the decaying direct current.
I am of the opinion that : the Ith concept is to determine the offset into a steady r.m.s. equivalent. . See 2 below.

"#2. ....-Icw as IEC 60947-2: Rated short-time withstand current (Icw).... is the value of short-time withstand current...."
Per IEC 60947-2 states Icw .... is the rated short-time withstand current, for a.c., the value is the r.m.s. value of the a.c. component of the prospective short-circuit current, assumed constant during the short-time delay. The short-time delay shall be.....up to 1 s.

"#3. Both currents are however subject to a duration (1 second for example)?..."
3.1 Per IEC 60909-0 , time is irreverent. See above 1.
3.2 Per IEC 60947-2, short time delay shall be at least 0.05 s, preferred value being 0.05, - 0.1,- 0.25, - 0.5, - 1 s. In addition, the rated short-time withstand current shall be not less than.....
Che Kuan Yau (Singapore)

 

[RRaghunath]

Ith is general for any equipment and speaks of thermal withstand of the conductors used for winding a CT etc.
Whereas, Icw is specifically for the circuit breakers where the short circuit current also (in addition to heating) produces dynamic forces against which the contacts are supposed to remain closed for the specified duration. This applies to Contactors etc. in addition to circuit breakers which are expected to remain latched during faults external to the subject feeder.

 

[98mlo]

Thank you for your answers.
I have the following comments on your contributions:
@che12345
Regarding your hypothesis:

the Ith concept is to determine the offset into a steady r.m.s. equivalent.
Now according to IEC 60909 the Ith through this equation ith=Ik "sqrt(m+n) shows that the influence of the thermal effect dc(m) of the DC component of the short circuit but also AC are taken into account.


@RRaghunath and @che12345
I understand that if Icw is for circuit breakers, contactors, I deduce that when in a single line diagram (SLD) if I see 50kA/1s marked on a HV or LV switchboard it is for Icw of electrical equipment and that Ith does not exist for this equipment (circuit breaker, contactor).
I also understand that there is necessarily a check to be made: Ensure that the Icw>=Ith (since Ith is relevant for busbars, CT etc...).do you agree?
Also in view of your comment(RRaghunath) can Icw have a relationship with Ip as Icw is the current that produces the electrodynamic forces and also includes the thermal release): we would have Ip(all the switchboard)>Icw(breakers contactor>Ith: does this make sense to you both(even if Ip is not subject to a duration)?

I would like to thank you for your precious information and do not hesitate to correct me if my reasoning is not correct.