SC Ratings
SC Ratings
(OP)
Hello,
I am pricing out some switchgear, and I am having some problems with the ratings...
One of the non-fused switches is rated as follows:
momentary: 40ka Asym (25ka Sym)
Fault Close Current:40ka Asym (25ka Sym)
Short time Current@ 2 sec:40ka Asym (25ka Sym)
Can all these ratings be the same?? and isn't momentary equal to Short time as defined by ANSI?
I thought if;
RMS Sym = 25ka then;
momentary = 1.6*25 = 40ka = short time, and;
Peak Short time withstand = 2.6*25 = 65kA
Furthermore, how do these relate to the nomenclature I have heard people use such as Isc, Ith and Ip?
Maybe a FAQ on SC ratings - IEC and ANSI would help??!
THanks
I am pricing out some switchgear, and I am having some problems with the ratings...
One of the non-fused switches is rated as follows:
momentary: 40ka Asym (25ka Sym)
Fault Close Current:40ka Asym (25ka Sym)
Short time Current@ 2 sec:40ka Asym (25ka Sym)
Can all these ratings be the same?? and isn't momentary equal to Short time as defined by ANSI?
I thought if;
RMS Sym = 25ka then;
momentary = 1.6*25 = 40ka = short time, and;
Peak Short time withstand = 2.6*25 = 65kA
Furthermore, how do these relate to the nomenclature I have heard people use such as Isc, Ith and Ip?
Maybe a FAQ on SC ratings - IEC and ANSI would help??!
THanks






RE: SC Ratings
But momentary is not the same as short time. Momentary is the maximum rms current occuring in the first half-cycle of the fault. The multiplier for asymmetrical current depends on the power factor specified in the test standard. For ANSI medium-voltage switchgear, "momentary" is the same as the close and latch rating.
Short-time rating is a withstand rating indicating maximum current the switch can withstand for the specified time without flying apart or melting down. The duration depends on the equipment standards.
RE: SC Ratings
has two basic categories of preferred ratings for medium voltage circuit breakers, namely: Rated Values and Related Required Capabilities
Rated Values:
Rated continuous current at 60Hz, rms
Rated short circuit current (at rated max kV) kA, rms
Rated Required Capabilities:
Max Symmetrical Interrupting Capability kA, rms
3-seconds short time current carrying capability kA, rms
Closing and Latching capability 1.6K times rated short circuit current kA, rms
If expressed in peak amperes, the close and latch value is equal to 2.7K times rated short circuit current.
K is the rated voltage range factor, which is a ratio of rated maximum voltage to the lower limit of the range of operating voltage in which the required symmetrical and asymmetrical current interrupting capability vary in inverse proportion to the operating voltage.
RE: SC Ratings
So how does "interrupting" current fit into the picture?
Another tender I have seen for switchgear says that the gear "shall have a Interrupting Capacity of 35kA". However, later on in the spec it says that the same gear "shall withstand a fault level 1/2 cycle, 35kA RMS symmetrical".
Are these two values the same?? From the above f I assume interupting capability = Momentary = close and latch, then this is true...
Regards,
TULUM
RE: SC Ratings
Per ANSI requirements, the close & latch rating must be at least 1.6 times the interrupting current.
It is easier for the gear to withstand the mechanical forces of fault current (the close and latch rating)than it is to actually interrupt the flow of current.
IEC terminology is a little different, but the physics doesn't change.