How to Calculate Prospective Short Circuit Current (PSCC)?

The prospective short circuit current, PSCC, is an important electrical system parameter that defines the maximum fault current flow in case of a short circuit. It will make sure designs are proper, right equipment is selected, and safety standards are met especially on systems of high capacity. This review attempts to outline the integral principles, techniques, and considerations in the computation of PSCC.

PSCC Definition

PSCC = Isc = V / Z
  • Isc: Prospective short-circuit current (in amperes).
  • V: Pre-fault voltage at the point of calculation (in volts).
  • Z: Total impedance between the fault point and the power source (in ohms).
The PSCC value is influenced by the amount of fault current available from the source and the impedance present in the circuit constituents.

Impedance Constituents and Their Significance

The total impedance Z in a circuit has a main impact on the PSCC. It also consists of:
  • Transformer Impedance: As a percentage of the rated primary transformer. A 5% transformer impedance allows 20 times the full-load current during a short circuit.
  • Cable Impedance: Determined by the length, material, and cross-sectional area of the cable.
  • Other Equipment Impedance: Switchgear, distribution boards, etc.

Fault Types and Their Effects

The fault type affects the PSCC value:
  • Three-Phase Fault (Symmetrical): Most severe condition with the highest fault current.
  • Phase-to-Phase Fault: About 87% of the three-phase fault current.
  • Phase-to-Neutral Fault: Usually 50% of the three-phase fault current.
  • Earth Fault: Reduced additionally by the impedance of the earthing system.

Procedure to Calculate PSCC

  1. Select the Calculation Point: Determine where in the circuit PSCC is to be calculated, such as a main switchboard or a distribution board.
  2. Collect System Data: Acquire the nominal voltage at the point under study. Collect the impedance values for all the components whose currents flow toward the fault point such as the transformer, the cables as well as other equipment.
  3. Computation of Total Impedance: Add the various impedances considering phase relation if needed.
  4. Use the Formula: Isc = V / Z
  5. Compute PSCC.

Application: Factory System

Question:
A 500 kVA three-phase transformer with 5% impedance supplies 400 V three-phase system.

Calculate Full-Load Current

IFL = Transformer VA / (√3 × Voltage)
IFL = 500,000 / (√3 × 400)
IFL ≈ 722 A

Short Circuit Current Calculation:

Isc = (IFL × 100) / Transformer Impedance
Isc = (722 × 100) / 5
Isc = 14,440 A

Fault Type Adjustments:

  • Three-phase fault: 14,440 A.
  • Phase-to-phase fault: 87% × 14,440 = 12,570 A.
  • Phase-to-neutral fault: 50% × 14,440 = 7,225 A.

Practical Considerations

  • System Variations: Adjust for the impact of ambient temperature, load conditions, and aging on circuit impedance.
  • Safety Margins: Always have a safety margin so fault levels are conservatively determined.
  • Fault Type Analysis: Consider all fault types for determination of the worst case PSCC.
  • Regulatory Compliance: Calculations need to meet applicable standards like AS/NZS 3000:2018, which requires that protective devices be able to interrupt PSCC.

Regulatory Standards

AS/NZS 3000:2018 Clause 2.5.2: They must protect against overload and short circuits and deal with PSCC at their installation point. Interrupting capacity is a must of the device be more than the highest fault current available at the point of installation.

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