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power2020 (Electrical) (OP)
4 Dec 07 14:33
Dear Folks,
How to choose short-time withstand current and duration for MV and LV switchgear and also pl. specify IEC/IEEE standard.
Advanced thanks..
dpc (Electrical)
4 Dec 07 16:57
One of the ANSI C37 standards.  I don't know about IEC.

In ANSI world, you don't get many options for duration.  Just need to make sure the 1/2 cycle and 5 cycle fault currents are less than the published ratings for the breakers you select.

The trick is in how the fault current is calculated.  It is not a simple Ohms law calculation for ANSI-rated breakers.  Guidelines are in ANSI C37.  

You can find applicable standards and the standard ratings on GE, Cutler-Hammer or Square D websites.  

power2020 (Electrical) (OP)
5 Dec 07 1:22
Thank you dpc for your response.
I have read in one of the academic books that short-time withstand current duration shall be 1-sec if fault current / rated current is more than 40 and
3-sec if fault current/rated current is less than 40.
But it is not given clearly that which fault current - 1/2 cycle or 1.5 -4 cycle. And also I could not find in any IEC/IEEE standards.
And I enquired one of the famous switchgear manufacturer regarding short-time with stand current(Icw), they said it will be derived from 1/2 cycle and 5-cycle fault currents (from symm rms and Asymm peak currents).
dpc (Electrical)
5 Dec 07 11:52
In addition to the 1/2 cycle (close & latch) rating and the rated short circuit current, ANSI-rated breakers also have a maximum interrupting current rating and a 3-second short-time current carrying capability.  I believe this is covered in ANSI C37.06, but I'm going from memory.  

From practical standpoint, the main concerns are usually the 1/2 cycle and interrupting ratings.  
alehman (Electrical)
7 Dec 07 21:35
ANSI C37.20.1 is the standard for "Low Voltage Metal Enclosed Power Circuit Breaker Switchgear". It specifies short-time withstand values for two 30-cycle intervals separated by 15 seconds of no current. IEC 60439-1 specifies 1 second duration.

from:
IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 40, NO. 6, NOVEMBER/DECEMBER 2004, "Comparison of ANSI/IEEE and IEC Requirements for Low-Voltage Switchgear"
EddyWirbelstrom (Electrical)
8 Dec 07 7:16
In IEC and AS world :-
For HV switchgear :-
IEC 60694 ( AS2650 )Clause 4.5
Rated Short-Time Withstand Current ( Ik )
The rms value of the current which the switchgear and control gear can carry in the closed position during a specififed short-time under prescribed conditions of use and behaviour.
The rated short-time withstand current is equal to the rated short-circuit breaking current.
Duration 1 sec unless otherwise mentioned.

LV Switchgear :-
AS 3439.1 : 2002 Section 7.5
See Table 4 for the relationship between peak withstand current and short-time withstand current.
power2020 (Electrical) (OP)
8 Dec 07 8:56
Thanks alot for your replies.
I have seen in many locations using 3-sec duration for short-time withstand current which is not mentioned in IEC 60694.
Mr.Mnewman- As I dont have AS standards, pl. attach table-4, in AS 3439.1:2002.
Advanced thanks...
EddyWirbelstrom (Electrical)
8 Dec 07 16:51
Hello Power2020
1 sec and 3 sec are the most common duration times specifed for short-circuit withstand capability.
I do not know how to attach documents in Eng-Tips so I will reproduce Table 4 from the AS 3439.1 :2002 for LV switchgear

rms value of short-circuit current = Ik
powerfactor = pf
n = ratio of 'peak short-circuit current' to 'short-time withstand current'

Ik => 5kA,   pf = 0.7,   n = 1.5
5 < Ik =< 10kA   pf = 0.5,  n = 1.7
10 < Ik =< 20kA   pf = 0.3,  n = 2.0
20 < Ik =< 50kA   pf = 0.25,  n = 2.1
50 < Ik   pf = 0.2,  n = 2.2

'Peak short-circuit current' is the peak (crest) of the current waveform half a cycle after fault inception.

Ik = i peak / ( sqrt(2) * ( 1.02 + 0.09 *e^-3R/X ) )

Note for HV switchgear the peak to short-time withstand current ratio is 2.5

If someone can tell me how to attach documents I can send more information.





EddyWirbelstrom (Electrical)
8 Dec 07 16:57
I should have mentioned that the values of peak to short-time withstand current ratios are those which the switchgear must withstand.
The actual calculated prospective ( duty ) values must not exceed these values.
power2020 (Electrical) (OP)
8 Dec 07 23:59
Hi mnewman,
Thanks alot for your info.
It is clear to calculate short-circuit with-stand current. How do we decide duration of short-circuit withstand time?
Regarding attachment: We type our information in Step-1 message, similarly there is step 3 for attachments. After clicking "..or upload your file to ENGINEERING.Com", separate window will open. Again select browse for required location of the file and then select upload  file.
File will be uploaded along with your information.
EddyWirbelstrom (Electrical)
9 Dec 07 5:13
Protection device operating time plus breaker interrupt time determine the required duration of the short-circuit withstand time.   Recommended values when specifying switchgear are 0.5s, 1s, 2s, and 3s.
Short-circuit clearing times are generally less than one second.
Thanks for instructions on attaching files.
power2020 (Electrical) (OP)
9 Dec 07 23:51
Dear All,
Many thanks for your great response.

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