Short-circuit Apparent Power of the System

In this article, I have presented the effects of utility fault level on low voltage equipment. Assuming an infinite bus as the utility fault level will have a significant cost implications particularly on the power transformer.

To resolve this issue, IEEE (C57.12.00 - 2015) and IEC (60076-5:2006) have presented typical values of short-circuit apparent power of the system to be used in design if no data is available. Please note that IEEE values are higher than the IEC values despite IEC presenting current North American practice, which should have covered IEEE. However, the IEEE standard is more current than its IEC counterpart.

Which value to use? It will all be up to the Electrical Engineer to decide which value of short-circuit apparent power will be used for the analysis.

IEEE Std C57.12.00-2015

Table 14 on short-circuit apparent power of the system to be used unless otherwise specified has been completely revised.

In establishing the fault values shown in Table 14, consideration was given to currently available maximum circuit breaker ratings and existing fault levels determined from user surveys. It is recognized that some systems will present fault levels that exceed these levels. If additional margin is desired, users may specify pre-fault voltage conditions (e.g., 1.05 p.u.) or an infinite bus. For low impedance transformers, specifying an infinite bus may increase the cost of the transformer significantly.

Table 14 - Short-circuit apparent power of the system to be used unless otherwise specified
Maximum system voltage (ANSI C84.1) (kV rms) Nominal system voltage (kV rms) System fault capacity (based on maximum system voltage)

(kA rms) (MVA)
Below 48.3 Below 46 63 -
48.3 46 63 5,270
72.5 69 63 7,910
121.0 115 80 16,770
145.0 138 80 20,090
169.0 161 80 23,420
242.0 230 80 33,530
362.0 345 80 50,160
550.0 500 80 76,210
765.0 735 80 106,000
800.0 765 80 110,850

IEC 60076-5:2006 The short-circuit apparent power of the system at the transformer location should be specified by the purchaser in his enquiry in order to obtain the value of the symmetrical short-circuit current to be used for the design and tests.

If the short-circuit apparent power of the system is not specified, the values given in Table 2 shall be used.

Table 2 - Short-circuit apparent power of the system
Highest voltage for equipment, Um kV Short-circuit apparent power
Current European practice Current North American practice
7.2, 12, 17.5 and 24 500 500
36 1,000 1,500
52 and 72.5 3,000 5,000
100 and 123 6,000 15,000
145 and 170 10,000 15,000
245 20,000 25,000
300 30,000 30,000
362 35,000 35,000
420 40,000 40,000
525 60,000 60,000
765 83,500 83,500
NOTE If not specified, a value between 1 and 3 should be considered for the ratio of zero-sequence to positive-sequence impedance of the system.

A Filipino Engineer, Registered Professional Engineer of Queensland (RPEQ) - Australia and Professional Electrical Engineer (PEE) - Philippines with extensive experience in concept select, front-end engineering, HV & LV detail design, construction and commissioning of Hazardous and Non-Hazardous Area electrical installations in water and waste water pipeline and pumping facilities, offshore platforms, hydrocarbon process plants and pipelines including related facilities. Hazardous area classification and design certification (UEENEEM015B, UEENEEM016B, UEENEEM017B).