IEC Thermal Current and busbar withstand

[lume7006]

Hello,

IEC std 60909 for short-circuit calculations determine a thermal current Ith for 1 or 3 seconds.

I would like to get your feedback about:

If a busbar of a swicthgear was design under IEC Std, to determine if it is appropiate, is it enough to compare the calculated Ith (thermal current) vs busbar withstand current value?

I have a case in which Ith= 39 kA and manufacturer design value is 40 kA (both for 3 s), however, Ikss is 48 kA and I do not know if the design should be compared only against Ith or also Ikss (initial current, which I think will also be seen by the busbar!)

I thank your support in advance

 

[desertfox]

Hi lume7006

I am not sure what Ikss in your post is meant to be but look at this site:-

http://www.copperinfo.co.uk/busbars/pub22-copper-for-busbars/sec6.htm

The short circuit current rating of the switchgear is I believe the RMS value and not the peak value of the short circuit current.
The above link shows you how to calculate the temperature rise of the busbar for a given fault current.

desertfox

 

[k0412d]

Hi lume7006,

The design of the busbar comes, mainly, down to both mechanical and electrical.
Both of them need, among others data, short circuit current info.

Here below are the definitions of the terms you're refering from 60909 :

Ith (thermal equivalent short-circuit current)
the r.m.s. value of a current having the same thermal effect and the same duration as the actual
short-circuit current, which may contain a d.c. component and may subside in time.

Ik'' (initial symmetrical short-circuit current)
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 figures 1 and 2)

Ik (steady-state short-circuit current)
r.m.s. value of the short-circuit current which remains after the decay of the transient phenomena
(see figures 1 and 2)

ip (peak short-circuit current)
maximum possible instantaneous value of the prospective (available) short-circuit current (see figures 1 and 2)

Concerning thermal consideration
As we can see in 60909-0 Ith is the thermal equivalent current, taking into account Ik'' and decay.

If your Ith is 39kA, calculated for a short-circuit current during a 3 second window (It is Ith and not Ik)
Then from a thermal point of view the proposed design of the manufacturer (40kA - 3sec) seems to me compliant.

Concerning mechanical consideration
It is here that peak current matters most.
You can refer to desertfox link or IEC 60865 Short circuit current - Calculation of effect.

k0412d.

 

[lume7006]

Hello,

I thank you both k0412d and desertfox for your very useful inputs.

I will read on detail what you have recommended.

Best Regards