Short Circuit Current Rating (Sccr According To Ul) - Siemens SINAMICS G130 Engineering Manual

Fundamental Principles and System Description
Engineering Information
1.2.3

Short Circuit Current Rating (SCCR according to UL)

In the USA a rating plate must be attached to the switchgear (referred to as Industrial Control Panel ICP) which
indicates the "short-circuit current rating" (overall panel SCCR) of the installation. Specification of the Short Circuit
Current Rating SCCR became essential when the National Electrical Code NEC 2005 came into force. The SCCR is
determined on the basis of UL508A Supplement SB4.
In order to ensure that the switchgear can withstand a short circuit in the main circuit without sustaining serious
damage, e.g. mechanical defects caused by excess current or defects resulting from overheating, the maximum
possible short-circuit current may not exceed the SCCR value of the installation.
The data of the transformer T2, which supplies the switchgear directly, generally provide an adequate basis for
making a rough estimation of the maximum possible short-circuit current at the installation site. As a general rule, the
high-voltage and medium-voltage levels have a minor influence and can therefore be ignored. Based on the rated
current I and the relative short-circuit voltage v
rated
current I is calculated according to the following equation:
k
Example:
A transformer with a voltage of 460 V 3AC on the low-voltage side and a rated power of 1 MVA has a rated current of
1255 A. The relative short-circuit voltage v
possible (continuous) short-circuit current directly at the output terminals of the transformer, i.e. on the low-voltage
busbar, is thus calculated to be I
In order to calculate the short-circuit current exactly, it is necessary to know the short-circuit power of the high-voltage
grid supply and the effective impedance of transformers T1 and T2, plus the effective impedance of the supply cable.
The maximum peak short-circuit current i
Methods for the precise calculation of short-circuit currents are given, for example, in IEC 60909-0.
Since the maximum possible short-circuit current obtained from the exact calculation method when all effective
impedance is taken into account is lower than the value estimated from the data of the supply transformer T2, the
estimation method generally yields a safer result. This applies particularly in the case of units which are not
connected directly to busbars but over long cables directly to the transformer.
In meshed systems supplied by multiple transformers connected in parallel, the process of calculating short-circuit
current I or peak short-circuit current i
k
The short-circuit current strength of the entire switchgear installation (overall panel SCCR) as specified on its rating
plate is determined by the component in the main circuit with the lowest SCCR value.
Standard SCCR values for electrical equipment are specified in UL 508A Supplement SB4.2 (September 2005).
These can be used to calculate the overall panel SCCR.
The SCCR values of the approved SINAMICS G130 converter Chassis units and the approved modular SINAMICS
S120 built-in units in Chassis format are higher than the listed standard SCCR values. These higher SCCR values
are valid only in combination with the fuses or circuit breakers stated in the catalogs and operating instructions.
Fuses or circuit breakers can be replaced by comparable types only on the condition that the peak let-through current
2
and the breaking I t value of the replacement type is not higher than those of the recommended type.
SINAMICS Engineering Manual – November 2015
38/528
Ó Siemens AG
(per unit impedance) of the transformer T2, the short-circuit
k
I = I
k rated
(per unit impedance) of the transformer is 6 % or 0.06. The maximum
k
= 1255 A / 0.06 ≈ 21 kA.
k
is reached when the short circuit occurs at the voltage zero crossing.
p
is more complex.
p
/ v .
k
× 2 I ×
2
k