Thermal effects of short circuit currents according to IEC standards

[MikiBg]

Hi all!
Couple of questions about the subject.
Low voltage: Relatively new IEC 61439 standard defines rated short time withstand current Icw. It is the current which equipment or assembly can withstand for period of time given by manufacturer. Icw shall be equal to or higher than the prospective RMS value of the short circuit current Icp. It is explicitly stated that Icw is RMS value of AC component.
Medium/high voltage: There is a definition of Ik within IEC 60694 standard, practically the same as Icw, but without restriction to AC component, at least I didn't find any.
I am trying to make it clear which current within old (but precise and academic related) standards for calculation corresponds to particular values defined above. In other words, after short circuit calculation for given application, how to prove that some equipment or assembly may be safely installed regarding thermal effects of short circuit.

1. By my understanding of given definitions, Icp is closest to definition of initial symmetrical s.c. current Ik" given within IEC 60909. If so, then Icw/Ik shall be equal to or higher than calculated value of Ik", but then what is the purpose of duration of Icw/Ik, since Ik" doesn't depend on time?

2. There is a thermal equivalent short time current Ith defined within IEC 60865, as a value which is taking into account a thermal effect of DC component as well, and it depends on fault duration time, as it is physically logical. But what is the purpose of calculation of Ith if there isn't any standard value defined for equipment it may be compared with?

3. How can someone show (using IEC terms) that decreasing of fault duration by protection setting change will decrease thermal stresses as well and that same equipment can cope with s.c. for different protection setting, while it may fail for initial setting?

Back in old days, Ith and its duration for equipment or Joule integral was used, and everything was much more clear. Now I'm quite confused with all these values, but they are of course given as equipment data by manufacturers so I must comply. Please help.

 

[che12345]

Dear Mr MikiBg

Q0.1 Icw shall be equal to or higher than the prospective RMS value of the short circuit current Icp. It is explicitly stated that Icw is RMS value of AC component.
A0.1 Reference IEC 60947-2. Rated short-time withstand current (Icw)...the [short-time delay] associated with the rated [short-time current] shall be.... 0.05...1 s.
i.e. a CB Icw shall comprising of [two components] i.e. (current in kA and duration in s) e.g. the [same] breaker may be rated 100kA 1s or 75kA 3s. Note: when the duration is not stated, it is taken as 1 s.
A0.2 Reference IEC 60909-0, the symmetrical short-circuit current is the r.m.s. value of the symmetrical component of a prospective (available) short-circuit current, the aperiodic component of current, if any, being neglected. Note: i.e. It has only one current value in kArms but there is [no] time duration e.g. 100kArms.
A0.3 A0.1 and A0.2 having different definitions, therefore they are [not] comparable.

Q0.4 Medium/high voltage: There is a definition of Ik within IEC 60694 standard, practically the same as Icw, but without restriction to AC component..... In other words, after short circuit calculation for given application, how to prove that some equipment or assembly may be safely installed regarding thermal effects of short circuit.
A0.4. Reference IEC 60947-2. See characteristics of CBs. Rated short-time withstand current (Icw) is the value of short-time withstand current under the [test condition specified] IEC 60947-1.

Q1.1 By my understanding of given definitions, Icp is closest to definition of initial symmetrical s.c. current Ik" given within IEC 60909.
A1.1 Reference IEC 60909-0. If you take (Icp) to mean "prospective RMS value of the short circuit current" then (Icp) = (I"k).
Q1.2 If so, then Icw/Ik shall be equal to or higher than calculated value of Ik", but then what is the purpose of duration of Icw/Ik, since Ik" doesn't depend on time?
A1.2 Reference IEC 60909-0. Icw is not comparable with Ik, see above A0.3.
I"k = initial symmetrical short-circuit current, Ik = steady-state short-circuit current. I"k > Ik , they have [nothing] to do with the Icw, which includes a duration.

Q2. There is a thermal equivalent short time current Ith defined within IEC 60865, as a value which is taking into account a thermal effect of DC component as well, and it depends on fault duration time... But what is the purpose of calculation of Ith if there isn't any standard value defined for equipment it may be compared with?
A2. All breakers including the assembly shall state the [current and duration] withstand rating. e.g. the breaker and the busbar assembly are rated for 50kA 1s. It can be used for lower kA with longer duration within limit, but [not] higher kA with shorter duration without going through Icw testing.

Q3. How can someone show (using IEC terms) that decreasing of fault duration by protection setting change will decrease thermal stresses as well and that same equipment can cope with s.c. for different protection setting, while it may fail for initial setting?
A3. In IEC, it is accepted the rule of thumb formula i*i*t=k (where k is a constant) within limit of 3s. With a known rating of xkA y s , it is possible to calculate any value of kA or s. However, see condition in above A2. must be respected. See also IEC 61439 series.

Che Kuan Yau (Singapore)
\

 

[MikiBg]

Dear Mr Che Kuan Yau,
Thank you for feedback and be free to comment further.
Ik which I mentioned is not steady state current from IEC60909, it is short time withstand current from IEC60694, related to duration time tk and, as I said, practically the same as Icw, only for MV applications, and without a note that only AC component is taken into account.
I agree that Ik" has nothing to do with Icw, and exactly that is one of confusions. Because (as you noticed) according to their names, we can conclude that Icp=Ik" but then IEC61349 requires for Icw to be compared with Ik"!
The question remains: How to take into account thermal effect of DC component and be sure that equipment can withstand this effect as well, since only thermal current Ith from IEC60865 cares about DC component? So you can calculate it, but which characteristic of circuit breaker can you use for comparison, having in mind that Icw, not Ith is defined for circuit breakers?

 

[che12345]

Dear Mr MikiBg

Q1. Ik which I mentioned is not steady state current from IEC60909, it is short time withstand current from IEC60694, related to duration time tk and, as I said, practically the same as Icw, only for MV applications, and without a note that only AC component is taken into account.
A1. Reference IEC 60694 ed 2.1 2001-05.
A1.1 Rated short-time withstand current (Ik) is the r.m.s. value of the current which the "gear" can carry in the closed position during a specific short time .... The (Ik) should be selected from the R 10 series..i.e 1-1.25-1.6-2... e.g. 25kArms.
FYI a) Only the [current kA] value, [without] the [time duration s].
b) there is [no] duration and it should [not] be taken as 1s.
A1.2 Rated duration of short circuit (tk) is the interval of time for which the "gear" can carry, in closed position, a current equal to the (Ik). The (tk) values i.e. 0.5, 1, 2 and 3s
e.g. 1s.
FYI Only the [time duration s] value, [without] the [current kA] value.
A1.3 (Icw) should have [two] components i.e. [current and time duration] e.g. 25kA 3s.
FYI Note The duration is taken as 1s if it is not stated.

Q2. I agree that Ik" has nothing to do with Icw, and exactly that is one of confusions. Because (as you noticed) according to their names, we can conclude that Icp=Ik" but then IEC61349 requires for Icw to be compared with Ik"!
A2.1 I presumed that your statement ...IEC (61349) is a typo error.
A2.2 It is important to be aware that e.g. Icw=25kA 0.5s "gear" is [not] fit for I"k =50kA.
FYI within limit of 3s, i*i*t=k is acceptable.It can be used for lower kA with a longer duration within limit, but [not] higher kA with shorter duration without going through Icw testing.

Q3. The question remains: How to take into account thermal effect of DC component and be sure that equipment can withstand this effect as well, since only thermal current Ith from IEC60865 cares about DC component? So you can calculate it, but which characteristic of circuit breaker can you use for comparison, having in mind that Icw, not Ith is defined for circuit breakers?
A3. Reference IEC 60947-1 and 60947-2.
See IEC 60947-2 [power factor of the test circuit] and the [time constant of the test circuit]. This is similar to X/R or R/X ratio where the [thermal effect of DC component] is taken into account and more.

Che Kuan Yau (Singapore)