Fault level analysis is an electrical engineering study that calculates the prospective short-circuit (fault) current at different points in an electrical system. It is used to verify that switchboards, protective devices, cables, and EV charging equipment can safely withstand and interrupt fault currents, and that protection can be coordinated correctly.
What Is Fault Level Analysis?
Fault level analysis (also called short-circuit analysis) determines how much current would flow during fault conditions such as:
– Phase-to-phase short circuit
– Phase-to-earth fault
– Three-phase bolted fault (often the highest fault current case)
– Faults at different locations along feeders and distribution boards
The output is typically expressed as:
– Fault current (kA) at each busbar or distribution node
– Fault contribution from transformers, generators, and other sources
– Let-through energy (I²t) and peak current estimates for equipment stress checks
Why Fault Level Analysis Matters for EV Charging
EV charging installations add new circuits and significant electrical load, often requiring new switchboards and feeders.
– Verifies switchgear breaking capacity (kA rating) is sufficient
– Confirms busbars, cables, and enclosures can withstand mechanical and thermal stress during faults
– Supports correct selection of protective devices (breakers, fuses, RCD/RCBO strategy)
– Improves safety and reduces risk of catastrophic equipment failure
– Enables protection coordination so a downstream fault does not trip the whole site
– Supports compliance documentation and commissioning quality for large projects
Where Fault Level Analysis Is Used
– New fleet depots with multiple chargers and large feeders
– Public charging hubs with high connection capacity (LV or MV supplies)
– Retrofit sites where existing boards may have limited fault withstand ratings
– Sites with onsite generation or storage (PV, BESS, generators) that change fault contributions
– Projects requiring utility approvals, design sign-off, or tender documentation
What Inputs Are Needed
Accurate inputs are essential for meaningful results.
– Utility supply parameters at point of connection (fault level, network impedance)
– Transformer details: rating, vector group, impedance (%Z), tap position
– Cable types, lengths, and installation method (impedance and temperature assumptions)
– Switchboard and busbar ratings, breaker types, and settings
– Earthing system type and earth fault loop impedance assumptions
– Contributions from onsite generation, inverters, or rotating machines (if applicable)
– Single-line diagram and system topology (what feeds what)
Typical Outputs and Checks
A fault level analysis usually supports several design decisions.
– Fault current at main incomer, switchboard busbars, and charger feeders
– Required breaking capacity for breakers and fuses at each level
– Peak current and I²t checks for busbars and cable thermal withstand
– Identification of locations where fault level is too high for existing equipment
– Recommendations: upgrade switchgear, add current-limiting devices, adjust topology
– Input to a protection coordination (discrimination/selectivity) study
How Fault Level Results Affect EV Charging Design
– High fault levels may require higher-rated switchboards or breakers (higher kA ratings)
– Current-limiting solutions (fuses/breakers or dedicated fault current limiters) can reduce stress
– Feeder sizing and protective device selection may change based on withstand requirements
– Multi-board architectures (sub-distribution) can improve selectivity and manage fault levels
– Integration of PV/BESS may require reassessing fault contributions and protection settings
Common Pitfalls
– Using outdated utility fault level data or assuming generic values
– Ignoring transformer tap position changes and future network reinforcement
– Missing contributions from onsite generation or storage systems
– Not validating cable lengths and impedances (as-built differs from design)
– Treating the analysis as a one-time step; changes in topology require recalculation
– Failing to link fault level analysis to protective device coordination and commissioning tests
Limitations to Consider
– Results are only as accurate as the input data and modeling assumptions
– Inverter-based resources can have complex, limited fault contributions that require correct modeling
– Fault level can change over time as the utility network is upgraded or reconfigured
– Site extensions (more chargers, new transformers) can increase fault levels and invalidate earlier studies
– Analysis does not replace safe installation and commissioning; it supports correct equipment selection
Related Glossary Terms
Short-Circuit Current
Fault Current
Fault Current Limiter
Protection Coordination
Distribution Board (DB)
Switchboard
Electrical Commissioning
Earthing System