A thermal trip is a protective action where an electrical protection device interrupts a circuit because heat has exceeded a safe limit. It is typically triggered by sustained overcurrent (which causes heating) or by an internal temperature rise detected in a protective component. In EV charging installations, thermal trips most commonly occur in circuit breakers, motor protection devices, or thermal overload relays feeding chargers or related equipment.
A thermal trip is different from a magnetic trip, which reacts quickly to short-circuit currents.
Why Thermal Trips Matter in EV Charging
Thermal trips can stop charging unexpectedly and are a common cause of downtime at busy sites. They matter because they:
– Protect cables, terminals, breakers, and distribution equipment from overheating
– Indicate that a circuit is operating above its continuous design limits
– Can reveal poor design assumptions (diversity, simultaneous charging, ambient temperature)
– Can point to installation quality issues (loose terminals, undersized conductors)
– Affect customer experience and charger uptime, especially in fleet depots and public hubs
Repeated thermal trips usually mean the circuit needs investigation, not just a reset.
How Thermal Trip Protection Works
Thermal protection is designed to respond to heating over time:
– Current flowing through a conductor or breaker generates heat (I²R losses)
– A thermal element (often a bimetal strip or thermal sensor) warms up as current persists
– If the temperature reaches a calibrated threshold, the device trips and opens the circuit
– The trip time depends on the level of overcurrent and the device’s time-current curve
This “time delay” characteristic helps tolerate short peaks while protecting against sustained overload.
Common Causes of Thermal Trips in Charging Sites
Thermal trips in EV charging installations often occur due to:
– Too many chargers drawing high current simultaneously (limited diversity assumptions)
– Incorrect breaker sizing for continuous EV loads (EV loads can be long-duration)
– High ambient temperature in cabinets, plant rooms, or outdoor enclosures
– Poor ventilation or overcrowded sub-distribution boards (SDBs)
– Loose terminals or poor crimping causing localized heating
– Cable routing issues (bundling, insulation, long runs) increasing temperature rise
– Incorrect settings on adjustable breakers or protective devices
– Aging components or repeated high-load operation near limits
Thermal Trip vs Temperature Derating
These protections operate at different levels:
– Temperature derating reduces charger output to avoid overheating while staying online
– A thermal trip disconnects the circuit when limits are exceeded, stopping charging entirely
Well-designed systems aim to avoid thermal trips by managing load and thermal conditions before protective cut-off is reached.
Operational Impacts
Thermal trips can cause:
– Charger outages and increased support tickets
– Repeat call-outs if the root cause is not addressed
– Reduced site throughput and longer queues
– Potential safety risk if overheating is occurring at terminations or cables
Monitoring trip events and correlating them with load profiles helps prevent recurrence.
Prevention and Mitigation Strategies
– Verify circuit design for continuous charging loads (cable sizing, breaker ratings, diversity)
– Apply load management to enforce a maximum site demand limit
– Improve cabinet ventilation and spacing in distribution equipment
– Use thermal scanning and inspections to catch hot terminations early
– Ensure correct torqueing and quality of terminations during installation
– Review breaker settings and coordination to match the installation design
– Plan expansion carefully to avoid overloading existing feeders and SDBs
Related Glossary Terms
Magnetic Trip
Overcurrent Protection Device (OCPD)
Sub-distribution Boards (SDBs)
Load Management
Maximum Site Demand Limit
Temperature Derating
Charger Uptime
Power Quality