A magnetic trip is a short-circuit protection function in a circuit breaker that trips (opens) the circuit instantly when current rises suddenly to a very high level, typically due to a short circuit or severe fault. It works using an electromagnetic mechanism (a coil/solenoid) that reacts in milliseconds, limiting damage to cables, devices, and downstream equipment.
What Is Magnetic Trip?
Magnetic trip is the “instant” trip part of many breakers, especially MCBs (miniature circuit breakers) and MCCBs (molded case circuit breakers). When fault current exceeds a defined threshold, the breaker trips without waiting for heat buildup, unlike a thermal trip, which reacts more slowly to overload current.
Magnetic trip is commonly described by a breaker’s trip curve (for example, B-curve, C-curve, D-curve), which defines how quickly the breaker trips at certain multiples of its rated current.
Why Magnetic Trip Matters in EV Charging
EV charging installations include long cable runs, high continuous loads, and sensitive electronics in EV chargers, distribution boards, and site power infrastructure. A fast short-circuit response is critical to:
– Reduce arc flash and fault energy at the point of failure
– Protect EVSE components such as contactors, power supplies, and RCD/RCBO devices
– Prevent cable insulation damage and overheating from extreme fault currents
– Improve overall site safety and reduce downtime after faults
For AC charging sites, correctly selected magnetic trips support reliable protection coordination between the charger feeder breaker and upstream protection in the building or at the utility connection.
How Magnetic Trip Works
– A sudden high current creates a strong magnetic field in the breaker’s coil
– The magnetic force pulls a plunger/armature that releases the latch mechanism
– The breaker contacts open immediately, interrupting fault current
– The breaker remains open until manually reset (or remotely reset in some systems)
Magnetic trip responds to rapid current spikes, which is why it is the primary protection against short circuits rather than long-duration overloads.
Magnetic Trip vs Thermal Trip
Magnetic trip (instant protection)
– Reacts in milliseconds
– Designed for short circuits and severe faults
– Triggered by current rising above a high threshold
Thermal trip (time-delayed protection)
– Reacts in seconds to minutes
– Designed for overload conditions (too much current over time)
– Based on heating of a bimetal element
Many breakers used in EV charger feeders are thermal-magnetic devices combining both behaviors.
Trip Curves and Typical Settings
Trip curves define the magnetic trip threshold relative to the breaker’s rated current (In). Common MCB curves include:
– B-curve: trips instantly at ~3–5 × In (more sensitive, often residential/light commercial)
– C-curve: trips instantly at ~5–10 × In (common in commercial circuits and motor-like inrush)
– D-curve: trips instantly at ~10–20 × In (high inrush loads; requires careful short-circuit design)
For EV charging, curve choice depends on charger characteristics (inrush behavior), cable impedance, available fault current, and required selectivity with upstream protection.
EV Charging Design Considerations
– Verify prospective short-circuit current (Ik) at the installation point and confirm breaker breaking capacity (Icn/Icu)
– Ensure magnetic trip is not so high that a short circuit at the charger end may fail to trip quickly due to long cable impedance
– Coordinate breaker curve with RCD/RCBO selection (Type A, Type B, Type F) where required by local standards
– Consider selectivity so downstream breakers trip first, avoiding unnecessary site-wide outages
– In three-phase AC charging (e.g., 11 kW or 22 kW), confirm protection for all phases and neutral where applicable
Common Issues When Magnetic Trip Is Misapplied
– Nuisance tripping caused by inappropriate curve selection (too sensitive for inrush conditions)
– Delayed or non-trip on end-of-line faults if magnetic threshold is too high relative to available fault current
– Poor coordination causing upstream breakers to trip before the EV charger feeder breaker
– Mismatch between breaker breaking capacity and site fault level, risking unsafe interruption
Related Glossary Terms
Circuit breaker
MCB (Miniature circuit breaker)
MCCB (Molded case circuit breaker)
Thermal trip
Trip curve (B, C, D)
Short-circuit current (Ik)
Breaking capacity (Icn / Icu)
Selectivity
RCD / RCBO
EVSE (Electric Vehicle Supply Equipment)