Short-circuit level (also called fault level) describes the magnitude of prospective short-circuit current available at a specific point in an electrical system. It is used to size and coordinate electrical equipment and protection. Depending on context, it may be expressed as:
– Short-circuit current (kA)
– Short-circuit apparent power (MVA) at a given voltage (often used by utilities)
In EV charging sites, short-circuit level is a core design parameter for switchboards, feeders, and charger protection devices.
Why Short-Circuit Level Matters in EV Charging Infrastructure
Short-circuit level impacts safety, equipment selection, and system reliability.
– Determines required breaking capacity of breakers, fuses, and switchgear (Icu/Ics)
– Influences selectivity (discrimination) between downstream and upstream protection
– Affects cable thermal and mechanical withstand during faults (I²t stress)
– Impacts arc fault energy and safety planning in electrical rooms
– Must be verified when adding chargers or upgrading distribution, as fault levels can change
If short-circuit level is underestimated, protective devices may not interrupt faults safely.
How Short-Circuit Level Is Determined
Short-circuit level depends on the impedance between the source and the fault point.
– Utility transformer rating and impedance (%Z)
– Supply voltage and network strength at the point of connection
– Length and cross-section of cables (and busbar impedance)
– Impedance of switchgear, distribution boards, and protective devices
– Earthing arrangement and fault loop path (especially for earth faults)
– Contribution from on-site generation (PV inverters, batteries, generators), if applicable
It is typically calculated for multiple points: main intake, sub-panels, and individual charger feeders.
Short-Circuit Level vs Short-Circuit Current
These terms are closely related.
– Short-circuit current is the fault current magnitude (kA) at a point
– Short-circuit level often refers to the same concept, but can also mean fault power (MVA)
– Utilities may provide fault level in MVA, which can be converted to current at the site voltage
Practical Implications for Charger Installations
– Choose protective devices with adequate kA rating at the installation point
– Confirm panelboards and busbars have sufficient short-circuit withstand ratings
– Coordinate protection to ensure a fault on one charger trips only its circuit protection
– Validate fault loop impedance and disconnection times, especially when RCDs are used
– Reassess fault levels when expanding capacity or modifying the upstream supply
Key Benefits of Proper Fault Level Design
– Safe interruption of short circuits without equipment failure
– Higher site uptime through better protection coordination
– Reduced risk of fire, equipment damage, and hazardous fault energy
– Cleaner commissioning and easier compliance documentation
– More predictable expansion planning for additional chargers
Limitations to Consider
– Fault levels vary across a site and can change if the utility network configuration changes
– Accurate calculations require correct upstream data and real cable routing information
– Some on-site generation changes fault contributions in non-obvious ways
– Achieving full selectivity at high fault levels can increase equipment cost
Related Glossary Terms
Short-circuit current
Fault level analysis
Prospective fault current
Breaking capacity (Icu/Ics)
Overcurrent protection device (OCPD)
Selectivity (discrimination)
Feeder circuit
Main LV panels
RCD
Grid connection capacity