Understanding short-circuit analysis

Short-circuit analysis calculates fault currents when an electrical fault (short circuit) occurs in your system. These calculations determine the interrupting capacity required for protection devices and verify equipment ratings.

What short-circuit calculates

Fault current types:

Initial symmetrical short-circuit current (Ik'')
Peak short-circuit current (Ip) - maximum instantaneous value
Breaking current (Ib) - RMS value at circuit breaker opening
Thermal equivalent short-circuit current (Ith) - for thermal calculations

For each bus:

Three-phase fault current
Phase-to-phase fault current
Single-phase-to-ground fault current
Fault MVA at each point

Equipment verification:

Breaker interrupting capacity requirements
Bus bracing requirements
Cable short-circuit withstand
Protective device coordination

Fault current types

Initial short-circuit current (Ik''):

First half-cycle RMS current
Subtransient component
Highest magnitude
Used for peak current calculation

Peak current (Ip):

Maximum instantaneous value
Includes DC offset component
Determines mechanical forces on busbars
Formula: Ip = κ × √2 × Ik'' (κ depends on X/R ratio)

Breaking current (Ib):

Current when breaker contacts separate
Typically 3-5 cycles after fault
Lower than Ik'' due to decay
Determines breaker rating

Thermal current (Ith):

Equivalent steady-state current for heating
Used for cable short-circuit withstand
Calculated from fault duration and current magnitude

Why short-circuit analysis matters

Protection device selection:

Breakers must have interrupting capacity ≥ fault current
Undersized breakers cannot clear faults safely
May explode or fail to interrupt

Equipment rating verification:

Buses, switchgear must withstand fault forces
Cables must handle thermal stress during fault
Transformers must withstand through-faults

Coordination:

Upstream devices must remain closed while downstream operates
Requires fault current at each point
Time-current curves based on fault current

Safety:

Arc flash energy depends on fault current
Higher fault current = higher incident energy
Personnel protection requirements

Pre-fault voltage

Every short-circuit run starts each bus from one pre-fault voltage basis. Choose it in Diagram settings, Short Circuit tab.

Fixed, % of nominal: every bus starts at that percent of its nominal voltage, and the utility

impedance is its voltage squared over its short-circuit rating. This is the ANSI-style basis ETAP and most North American studies use. New projects start at 100%.

IEC 60909 voltage factor (c max 1.10): every bus starts at 1.10 per unit, and transformer

impedances carry the IEC correction factor. Projects created before this setting existed keep this basis until you change it.

The two bases agree at a bus fed directly by the utility. Downstream of a transformer the IEC basis reads higher: a 480 V bus behind a 1500 kVA, 5.75% transformer reads about 8% higher than at a fixed 100%.

Both bases leave out loads, cable capacitance and transformer magnetizing current, and use transformers at their nominal ratio. Changing the setting marks completed short-circuit and arc flash runs stale, and each run records the basis it used. The arc flash study has its own pre-fault voltage setting, on the Arc Flash tab.

Prerequisites for analysis

Required components:

At least one source (utility feed or generator)
Complete network topology
All component impedances specified
Voltage ratings for all buses

Source impedance:

Utility short-circuit MVA and X/R ratio
Generator subtransient reactance (X''d) and stator resistance Ra

Enter manufacturer Ra under the generator's Study tab in percent on its rated voltage and kVA base. For example, enter 1 for 1% (0.01 pu). Blank Ra retains the existing zero-resistance assumption; an entered 0 records explicit zero. Short-circuit and arc flash both use this resistance. Zero resistance does not guarantee the highest incident energy.

Transformer impedance percentage

Missing data causes:

Calculation failure
Incorrect results
"Insufficient data" errors

Fault types - Three-phase, phase-to-phase, single-phase faults
Running analysis - How to execute short-circuit calculation
Protection devices - Interrupting capacity requirements