Prospective fault current (PFC) is the maximum current that could flow at a specific point in an electrical system if a short circuit or earth fault occurred, assuming the supply is not limited by protective devices. It is determined by the supply source strength (utility transformer, generator, UPS), the network impedance, and the cable length and cross-section up to the fault point.
Why Prospective Fault Current Matters in EV Charging
EV charging sites introduce new distribution boards, long feeder runs, multiple chargers, and sometimes on-site generation—each of which changes fault levels.
– Correct PFC values ensure circuit breakers, fuses, RCDs, and switchgear can safely interrupt faults
– Adequate breaking capacity prevents catastrophic device failure during a fault event
– Fault level checks reduce the risk of arc flash, equipment damage, downtime, and safety incidents
– PFC influences electrical design choices for AC chargers, DC chargers, and site main LV panels
How Prospective Fault Current Is Determined
PFC is calculated or measured at key points such as the main incomer, distribution boards, and each charger feeder.
– Utility transformer rating and impedance (or short-circuit power at the point of connection)
– Upstream network impedance (utility, service cable, main switchboard)
– Downstream impedance (feeder cables, busbars, isolators, contactors)
– Fault type and path: line-to-line, line-to-neutral, line-to-earth
– Operating configuration: parallel supplies, generator mode, UPS contribution, on-site energy systems
Prospective Short-Circuit Current vs Earth Fault Current
Different fault paths can produce different values.
– Prospective short-circuit current (PSCC) typically refers to line-to-line or line-to-neutral short circuits
– Prospective earth fault current (PEFC) refers to line-to-earth faults and depends strongly on the earthing system (TN, TT, IT) and return path impedance
– Both values may be required for selecting protection devices and verifying disconnection times
Where Prospective Fault Current Is Checked in EV Charging Projects
Fault level assessment is typically performed during design, commissioning, and major site upgrades.
– Main LV panel / building intake
– Sub-distribution boards serving EV chargers
– Feeder end points where chargers connect (especially with long cable runs)
– Sites with transformer upgrades, additional chargers, or power expansion
– Locations adding battery storage, solar PV, or backup generation that can affect fault contribution
Impact on Protection and Equipment Selection
PFC directly affects whether installed equipment is electrically safe under fault conditions.
– Breakers and fuses must have sufficient breaking capacity (interrupting rating) at the installation point
– Switchboards and enclosures must have adequate short-circuit withstand ratings
– Protection coordination requires correct PFC to ensure selectivity and avoid nuisance trips
– If PFC is too high, design may need current-limiting measures or higher-rated switchgear
– If PFC is too low, some protective devices may not trip fast enough without appropriate settings and earthing design
Common Mitigation Measures When PFC Is Too High
– Upgrade to devices with higher interrupting capacity (e.g., higher kA-rated breakers)
– Use current-limiting fuses or current-limiting breakers upstream
– Add impedance through cable sizing/length adjustments where appropriate (within voltage-drop limits)
– Reconfigure distribution to reduce the available fault level at sensitive boards
– Review transformer and supply arrangements with the utility
EV Charging Design Considerations
– High-density sites (parking hubs, depots) can increase fault levels at main boards after upgrades
– Long feeder runs can reduce PFC at charger endpoints, affecting protective device operation
– RCD type selection and coordination may depend on earthing arrangement and fault current paths
– Commissioning should verify assumptions with measurements where required by standards and local practice
Related Glossary Terms
– Fault level analysis
– Short-circuit current
– Breaking capacity
– Selectivity (protection coordination)
– Earth fault current
– Circuit breaker
– Overcurrent protection device (OCPD)
– Arc flash risk