Prospective Fault Current
Prospective fault current (PFC) is the maximum overcurrent that could flow at a given point in an electrical system if a short circuit or earth fault of negligible impedance occurred, such as a bolted fault. It is determined by the supply voltage and total loop impedance back to the source, and is used to verify that protective devices and equipment can safely withstand and interrupt worst-case fault energy.
On a manufacturing shop floor, prospective fault current is critical for selecting circuit breakers and fuses with adequate interrupting ratings, ensuring switchboards and motor control centers (MCCs) have sufficient short-circuit withstand ratings, and calculating arc-flash incident energy for PPE selection. It is measured or calculated at main service entrances, sub-distribution panels, and large equipment terminals using fault loop testers or impedance calculations. Higher PFC generally increases potential incident energy unless mitigated by fast protective clearing times. Regular re-evaluation is needed when system changes occur, such as transformer upgrades or new generator additions.
- Protective device interrupting capacity less than PFC at its location, risking catastrophic failure during a fault.
- Panel or equipment short-circuit rating exceeded by actual PFC, leading to enclosure rupture or arc-flash escalation.
- Inadequate documentation and testing of PFC at relevant points, omitting line-earth readings or failing to update records after system changes.
How is prospective fault current different from actual fault current?
Prospective fault current assumes a fault of negligible impedance (bolted fault), representing the worst-case theoretical current. Actual fault current is often lower due to added impedance from arcing or partial contact. Standards use PFC because protective devices must safely interrupt this worst-case scenario.
Why must both prospective short-circuit current and prospective earth fault current be considered?
Regulations treat PFC as including both values. The greater of the two must be compared to protective device breaking capacity and recorded. In some systems, line-earth fault current can be similar or higher than line-neutral, depending on earthing arrangements and loop impedances.
How is PFC used to select a circuit breaker in a plant MCC?
Determine PFC at the MCC bus by calculation or measurement. Identify the maximum value (line-neutral vs. line-earth). Select breakers with a short-circuit interrupting rating (kA) greater than or equal to that PFC. Confirm the MCC assembly short-circuit rating also meets or exceeds that PFC.