Electric vehicle (EV) manufacturers integrate e-fuses, circuit breakers, and isolators into key systems to protect against fluctuating high-voltage currents. These components activate within microseconds to prevent faults, failures, and thermal runaway.
This article compares the electrical systems of EVs and internal combustion engine (ICE) vehicles, highlighting the critical role that these protective components play in ensuring EV driver safety. It also reviews the technical functions and requirements of EV e-fuses, circuit breakers, and isolators.
Comparing electrical systems: EVs versus ICE
Powered by large lithium-ion (Li-ion) battery traction packs, most primary EV systems operate at voltages between 400 and 800 V. In contrast, ICE vehicles use smaller 12 or 48-V batteries to power auxiliary components, such as lighting, infotainment systems, mirrors, and windows.
Like their ICE counterparts, EVs (Figure 1) incorporate e-fuses, circuit breakers, and isolators to protect against fluctuating currents. In EVs, these crucial protective devices ensure the safe and reliable operation of high-voltage Li-ion battery traction packs, advanced driver-assistance systems (ADAS), regenerative braking, and key powertrain components, such as electric motors.
Figure 1. The all-electric, compact Rivian SUV. (Image: Rivian)
Overcurrents and voltage spikes can cause unprotected circuits to overheat and fail, potentially leading to total system failure and electrical fires. Short circuits, damaged wiring, and loose connections further increase the risk of catastrophic failure and thermal runaway, as do cascading failures in high-power electronics.
To prevent faults, failures, and thermal runaway, e-fuses, circuit breakers, and isolators must activate within microseconds. Effective fault response minimizes risks, while mismatched coordination between protective components and mechanisms can cause nuisance tripping, contact welding, or delayed fault clearing.
Rapid response and resettable protection with EV e-fuses
Designed for direct current (dc) voltage, EV fuses are typically rated between 500 and 1000 Vdc. To resist vibration, g-forces, and humidity, some EV fuses use melamine-laminated glass fiber instead of conventional ceramic casings, ensuring rapid opening speeds and reliable handling of in-rush currents and thermal stress.
Solid-state e-fuses (Figure 2) offer significant advantages over conventional ICE or EV fuses. Unlike standard automotive fuses, which rely on thermal buildup to trigger circuit interruption, solid-state e-fuses use current measurements to detect faults proactively.
Figure 2. Microchip’s silicon carbide E-Fuse demonstrator for 400 to 800-V battery systems supports current ratings up to 30A2. SiC MOSFET-based e-fuses offer 100 to 500 times faster response than those based on conventional silicon. (Image: Microchip)
This capability allows e-fuses to react almost instantaneously to overcurrents — within one to 10 microseconds. E-fuses also provide resettable protection, eliminating time-consuming and costly replacements after fault detection. Some e-fuses incorporate advanced features such as temperature protection, programmable trip currents, and diagnostic capabilities for real-time system monitoring accessible through Local Interconnect Network (LIN) interfaces.
E-fuses typically operate in temperature ranges from -40°F (-40°C) to 185°F (85°C), with specialized versions handling up to 257° F (125° C). These fuses comply with industry standards, including ISO 8820-8 (up to 450mV) and UL 248-20 (up to 1500 Vdc).
Advanced circuit breakers for dc fast charging
EV circuit breakers (Figure 3) function as automatic switches, using electromagnetic mechanisms to disconnect power during overcurrent events caused by short circuits, overloads, or collisions. They handle currents up to 25,000 amps at 900 volts, with trip times depending on the fault level: up to 7 milliseconds for 4 kA, 6 milliseconds for 4-10 kA, and within 4 milliseconds for 10-25 kA.
Figure 3. A cross-section view of Eaton’s Breaktor complete circuit protection solution. (Image: Eaton)
EV circuit breakers offer resettable protection, enabling reactivation after safety checks without requiring replacement. They’re available in various configurations, such as multi-pole setups, adjustable overcurrent triggers, and external triggering options.
Some advanced models enhance safety and reliability with features such as permanent magnets and splitter plates to manage arcs, current sensors, and driver electronics that oversee actuation and de-energize the electromagnetic coil in the event of faults. Notably, many advanced EV circuit breakers can support fast dc charging up to 350 kW, safeguarding the battery and power electronics from overcurrent and voltage spikes.
Safer energy transfer with isolators
High-voltage isolators (Figure 4) regulate energy transfer in EVs, ensuring electrical separation between high-voltage and low-voltage systems. These isolators also suppress electromagnetic interference (EMI) and harmonic distortion, further bolstering safety by mitigating power spikes that could lead to thermal runaway or component failure.
Figure 4. A schematic representation of an isolated electrical system, showing circuit A and circuit B separated by an isolation barrier to prevent current flow, and highlighting dual power supply inputs and outputs. (Image: Power System Design)
Isolation barriers ensure compliance with minimum resistance requirements — 100 ohms per volt for dc circuits and 500 ohms per volt for ac circuits. Key components include ground fault circuit interrupters (GFCIs), which detect current imbalances as small as 0.005A, and manual service disconnects (MSDs) for emergency power cutoff.
Battery management system (BMS) isolators separate high-voltage batteries from low-voltage electronics. They’re used in switch-mode power converters, such as dc/dc converters, which regulate voltage and current for critical systems like propulsion, ADAS, and regenerative braking.
These converters step down high-voltage dc from the main battery to safely power components such as climate control, lighting, and infotainment while also supporting key safety functions, including automated braking and driver monitoring. Additionally, isolators help maintain signal integrity in the controller area network (CAN).
High-voltage interlock systems (HVILs), which verify that connectors and covers are properly engaged before power flow activation, rely on isolators to enforce circuit safety. Constructed from materials like polyimide or ceramic, isolators act as gatekeepers to prevent damage and electric shock by ensuring full connector engagement before activation. This precaution is crucial during maintenance and post-accident assessments when high-voltage components may be exposed and unstable.
Conclusion
Powered by large Li-ion battery traction packs, most primary EV systems operate at voltages between 400 and 800 V. EV manufacturers incorporate e-fuses, circuit breakers, and isolators to protect against fluctuating high-voltage currents that could lead to electrical fires, component damage, or total system failure.
Related EV Engineering content
- What Role Do Isolators Play in Electric Vehicles?
- How Do 12V and HV Systems and Grounding Work in an EV?
- How to Ensure EV Battery Safety with Advanced Temperature Monitoring
- What is Thermal Runaway and Why Does It Matter for EVs?
- Myths vs. Facts: How EVs Operate in the Rain and Snow
References
- How Eaton Breaktor Circuit Protection Works, Eaton
- Safeguarding Electronics: The Vital Role of E-Fuse Technology, LemonFlip Solutions
- SiC E-Fuse Board for Electric Vehicles, Power Electronic News
- Ensuring Safety in Electric Vehicles: High Voltage Systems and Safety Measures, Vasudha Foundation
- Driving Ahead for Performance and Sustainability, Mersen
- Fuses and Circuit Protection, e-Mobility Engineering
- Electronic Fuses and the Evolution of Overcurrent Protection, Charged EVs Magazine
- Understanding Electric Vehicle Fuse Applications, Swe-Check
Filed Under: FAQs, Power Electronics