To calculate prospective short-circuit current, this usually implies the maximum fault current that would possibly occur at a point in an electrical system, one must consider factors such as the system voltage, the impedance of the system, and the type of fault. Here’s how to do it step by step.

1. Determine the Voltage Level

The first step is knowing the voltage level of the system in which the fault might happen. For instance, if you’re working with a 480V or 13.8kV system, then this is your system voltage.

2. Find the Impedance

The impedance of the system encompasses the total impedance from the source to the fault point, and can be further broken down into:

• Source Impedance: This includes the impedance of generators, transformers, and transmission lines.
• Impedance of the Equipment: This defines impedance along the cables or transformers or along any motor between source and fault locations. The apparent impedance is called the total when all impedances are added, which can simply be stated in the following relationship.

For example, for a short-circuit calculation, the source impedance is often expressed as a percentage of the system voltage, and the impedance of the equipment can be calculated using the formula:

Z = Vsystem / Irated

Where V_system is the system voltage, and I_rated is the rated current.

3. Use the Short-Circuit Current Formula

Once you have the system voltage and impedance, you can use the following formula to calculate the prospective short-circuit current I_sc:

Isc = Vsystem / Ztotal

Where:

• I_sc is the short-circuit current (in Amps),
• V_system is the system voltage (in Volts),
• Z_total is the total impedance (in Ohms).

4. Fault Type

It can depend a bit on the fault type in its calculation as well.

• Three-Phase Fault: This is the most common and typically involves the highest fault current.
• Single-Line to Ground Fault: This involves different phase relationships and may result in a lower fault current.
• Line-to-Line Fault: Similar to a three-phase fault but involves only two phases.

For three-phase faults, the fault current is typically the largest, while line-to-line and single-line to ground faults will result in lower prospective fault currents.

5. Consider Safety Margins and Factors

Standards such as IEEE or NEC guidelines often include safety margins, factors for unbalanced faults, and other considerations in the calculation of fault currents. You may also need to consider protection devices, such as fuses or circuit breakers, which will affect the fault current in a practical system.

Example Calculation:

Assume a 480V system with a total impedance of 0.05Ω:

Isc = 480V / 0.05Ω = 9,600A

This is the prospective short-circuit current at the point of the fault.

More Examples:

Example 1: Three-Phase Fault in a 480V System

Given:

• System Voltage: 480 V (this is a typical low-voltage system).
• Source Impedance: The source is a transformer that has an impedance of 0.05Ω.
• Transmission Line Impedance: 0.01Ω.
• Fault Type: Three-phase fault.

Steps:

1. Determine the Total Impedance: The total impedance Z_total from the source to the fault point is the sum of the source impedance and the transmission line impedance.

Ztotal = Zsource + Zline = 0.05Ω + 0.01Ω = 0.06Ω

2. Use the Formula for Short-Circuit Current: Substituting the values:

Isc = 480V / 0.06Ω = 8,000A

Result: The prospective short-circuit current for a three-phase fault in this 480V system is 8,000 A.

Example 2: Short-Circuit Current for a 13.8kV System with Transformer Impedance

Given:

• System Voltage: 13.8 kV.
• Transformer Rating: 10 MVA, 13.8 kV primary, 480 V secondary.
• Transformer Impedance: 5%.
• Fault Type: Three-phase fault.

Steps:

1. Calculate the Transformer Impedance in Ohms:

Ztransformer = (5% / 100) × 19.044Ω = 0.952Ω

2. Calculate the Total Impedance: Assuming the source impedance is negligible:

Ztotal ≈ 0.952Ω

3. Use the Formula for Short-Circuit Current: Substituting the values:

Isc = 13,800V / 0.952Ω = 14,484A

Result: The prospective short-circuit current for a three-phase fault in this 13.8kV system is 14,484 A.

Example 3: Single-Line to Ground Fault Calculation

Given:

• System Voltage: 480 V.
• Impedance of the system: 0.06Ω.
• Fault Type: Single-line to ground fault.

Formula:

Isc, single-line = (Vsystem / Ztotal) × Adjustment Factor

The adjustment factor is typically around 0.577. Substituting the values:

Isc, single-line = 480V / 0.06Ω × 0.577 = 4,615A

Result: The prospective short-circuit current for a single-line to ground fault is approximately 4,615 A.

The prospective short-circuit current calculation is important when designing and planning the protection on an electrical installation because it ensures protection devices such as circuit breakers or fuses break the fault current safely without possible damage. A short-circuited PSCC reflects the maximum allowed short-circuited short circuit current generated within a three-phase system on account of line voltage and line impedance.

PSCC Calculation Situations:

• Selection of Protective Devices: Selection of suitable circuit breakers or fuses can be made if the PSCC is known. Protective devices should have an interrupting rating that exceeds the PSCC to prevent failure in fault conditions.
• System Design and Analysis: In electrical installations, calculation of PSCC during design provides an opportunity for assessing the ability of the system to handle fault currents. In this way, the ability of the components to resist short-circuit conditions can be ensured.
• Safety Compliance: In most regulatory standards, PSCC calculations are needed to check if the electrical systems are compliant with safety codes. This would prevent personnel and equipment from fault-related hazards.
• Incident Energy Assessment: Calculating PSCC is necessary in order to evaluate incident energy levels during faults. The proper PPE for the job must be determined in order to keep the workers safe.
• Upgrades and Modifications: When any upgrading or modification to electrical systems is undertaken, recalculation of PSCC reveals new configurations would maintain fault levels and thus prevent potential damage to the installed protective devices.

In summary, calculating the PSCC is a fundamental step in electrical engineering that ensures protective devices are correctly specified and that an electrical system operates safely under fault conditions.