A fault current limiter (FCL) is a device or system designed to reduce the magnitude of fault current (short-circuit current) during an electrical fault. By limiting peak current and the rate of rise, it helps protect equipment, improve system selectivity, and reduce mechanical and thermal stress on cables, switchgear, and power electronics. In EV charging infrastructure, fault current limiting can be relevant in switchboards, distribution networks, and high-power installations where prospective short-circuit levels are high.
What Is a Fault Current Limiter?
A fault current limiter introduces additional impedance or a fast-acting limiting mechanism only when a fault occurs (or it behaves in a way that naturally limits fault current), so that:
– Fault current stays within the withstand ratings of equipment
– Protective devices can operate safely and predictably
– Downstream systems see less destructive peak energy (I²t)
Fault current limiting can be implemented as a dedicated FCL device or as an inherent characteristic of certain equipment (for example, current-limiting protective devices).
Why Fault Current Limiters Matter in EV Charging
EV charging sites often add significant electrical infrastructure to existing buildings and grids. In some locations, the prospective short-circuit current can be high enough to create challenges:
– Switchboards and protection devices may need higher breaking capacity ratings
– Cable and busbar thermal/mechanical stress increases during faults
– Coordination/selectivity between upstream and downstream protective devices becomes harder
– High fault currents can increase damage severity and downtime after an incident
Using fault current limiting can help:
– Reduce required breaking capacities in parts of the system (site-dependent)
– Improve protection coordination and reduce nuisance outages
– Protect sensitive power electronics and reduce collateral damage risk
How Fault Current Limiting Works
Fault current is limited by reducing available fault energy through fast impedance changes or current-limiting behavior.
– Impedance insertion: adds effective resistance/inductance during fault conditions
– Fast interruption/limiting: limits peak current before it reaches its maximum
– Saturation effects: some magnetic devices change behavior under high current to limit the rise
– Current-limiting fuses/breakers: protective devices that restrict peak current and let-through energy (I²t) during a fault
The exact mechanism depends on the technology and application.
Common Types of Fault Current Limiting Solutions
– Current-limiting fuses: very fast limitation of peak current and let-through energy
– Current-limiting circuit breakers: designed to reduce peak fault current and I²t
– Series reactors: add inductive impedance (often continuous, not only during faults)
– Dedicated FCL devices: specialty equipment used where short-circuit levels are exceptionally high
– Transformer impedance selection: choosing transformer parameters to manage prospective short-circuit current (system-level approach)
Where Fault Current Limiters Are Used in Charging Projects
– Main LV switchboards feeding large EV charging clusters
– Sites with short-circuit levels above the withstand rating of existing distribution equipment
– Depot or hub installations with many chargers and large feeders
– Retrofit projects where upgrading all switchgear to higher breaking capacity is costly or impractical
– Locations where improved selectivity is needed to avoid whole-site outages
Key Considerations When Specifying an FCL
– Prospective short-circuit current (Ik) at the installation point and at downstream nodes
– Required breaking capacity (kA) of protective devices and switchgear
– Target coordination/selectivity between upstream and downstream protection
– Impact on voltage drop, power quality, and thermal performance in normal operation
– Maintenance, replacement strategy, and spare parts availability
– Compliance with local electrical installation rules and utility requirements
– Documentation: short-circuit studies, protection coordination study, and as-built settings
Benefits of Fault Current Limiting
– Reduced equipment stress and damage severity during faults
– Better protection coordination and potentially fewer site-wide outages
– Potentially lower cost than replacing upstream switchgear with very high breaking capacity equipment (case-dependent)
– Improved operational resilience for high-utilization charging sites
Limitations to Consider
– Not always required; many sites can meet fault-level requirements through correct switchgear selection and coordination
– Some approaches (like series reactors) can introduce continuous losses and voltage drop
– Dedicated FCL devices add cost and complexity and may require specialist design input
– Limiting fault current must not compromise protective device operation or discrimination
– The correct solution depends on a proper short-circuit and protection coordination study
Related Glossary Terms
Fault Current
Short-Circuit Current
Overcurrent Protection
Protection Coordination
Distribution Board (DB)
Switchboard
Earthing System
Electrical Commissioning