Overvoltage category (also called measurement category) is a safety classification that defines how well electrical equipment can withstand transient overvoltages (short, high-energy voltage spikes) that occur in power systems due to switching, lightning, faults, or inductive loads. Categories are defined in standards such as IEC 60664-1 and commonly referenced in IEC 61010 for test and measurement equipment.
In EV charging projects, CAT III and CAT IV are the most relevant because chargers, switchboards, and installation equipment often interface with building distribution and sometimes the utility supply.
Why CAT III / CAT IV Matter in EV Charging
EV charging infrastructure is connected to real-world electrical networks where transients can be severe. Correct category selection helps:
– Reduce risk of equipment failure from voltage spikes
– Improve safety for installers and maintenance technicians
– Ensure compliance for test instruments used during commissioning and troubleshooting
– Align protection design for panels, feeders, and connection points in EVSE installations
– Prevent under-rated devices from being used in high-energy locations (a major safety hazard)
CAT III vs CAT IV Explained
CAT III and CAT IV describe where equipment is used in the electrical distribution chain:
CAT III (Distribution level)
Used for equipment connected to the fixed installation within a building or facility, where fault energy and transients are significant but lower than at the supply origin. Typical locations include:
– Distribution boards and switchboards inside buildings
– Feeder circuits and fixed wiring to EV chargers
– Motor control centres, large HVAC circuits, industrial panels
– Hardwired EVSE circuits downstream of the main service disconnect
CAT IV (Service entrance / supply origin)
Used for equipment at the origin of the installation, where transients and available fault current can be highest. Typical locations include:
– Utility service entrance and meter location
– Main service disconnect / main incoming switch
– Overhead/underground supply conductors before the first protective device
– Outdoor lines feeding a site (e.g., at the point of common coupling)
How This Applies to EV Chargers and Site Work
Overvoltage category is most often encountered when selecting test equipment and specifying installation safety requirements:
– When measuring at a main incomer or near the utility connection, instruments should be CAT IV rated at the required voltage (e.g., CAT IV 600 V or CAT IV 1000 V)
– When measuring in distribution panels feeding chargers, CAT III rated instruments are typically required
– EV chargers themselves are part of a fixed installation; protection coordination (SPDs, breakers, insulation coordination) should consider the installation environment and surge exposure
Practical Selection Guidelines
– Choose CAT IV for measurements at the service entrance or upstream of the main protection device
– Choose CAT III for measurements within distribution boards and fixed wiring circuits supplying EV chargers
– Match the category with an appropriate voltage rating (e.g., CAT III 1000 V is not the same application as CAT IV 600 V)
– Prioritize reputable instruments with clear markings and certification for commissioning and maintenance work
Related Glossary Terms
Surge Protection Device (SPD)
Lightning Protection
Grid Connection
LV Panels / Switchboards
Feeder Circuit
Fault Level Analysis
Insulation Coordination
IP Ratings, IK Ratings
Commissioning Documentation
Lockout / Tagout