Thermal protection estimates temperature from current and cooling conditions. It is fundamentally different from instantaneous overcurrent because the protected object stores heat and cools over time.
Learning objectives
Configure thermal capacity, heating and cooling constants, alarm/trip levels and restart inhibition using equipment data.
Core engineering principles
The model integrates losses over time
Heating is commonly related to current squared, while cooling follows one or more time constants. Load history therefore changes available thermal margin.
Cold and hot limits differ
A motor starting from cold can tolerate more heating than one recently stopped. Restart logic should retain thermal memory across power interruptions where required.
Unbalance adds rotor heating
Negative-sequence current can create significant rotor loss even when average phase current appears acceptable. Motor models may combine 49 and 46 effects.
Cooling state changes the model
Forced ventilation, pump status, transformer fans and ambient temperature affect permissible loading. Binary inputs or temperature sensors can adapt the model.
Direct temperature and replica models are complementary
RTDs measure local temperatures with delay and limited coverage; a current-based model estimates internal heating. Combining both improves protection.
Engineering application method
- Step 1: Obtain equipment thermal limits and time constants.
- Step 2: Define cold/hot current capability and service factor.
- Step 3: Configure heating, cooling, alarm, trip and restart thresholds.
- Step 4: Add cooling-state and temperature inputs where available.
- Step 5: Test thermal memory, reset and restart inhibition.
Practical example
A motor stopped after a heavy start may have normal current at zero but insufficient thermal capacity for an immediate restart. A retained 49 model correctly blocks the second start.
Common mistakes
- Using 49 as a simple delayed 51 element.
- Resetting thermal memory on relay reboot.
- Ignoring start heating and negative sequence.
- Copying motor constants to transformers.
- Relying on one RTD location.
Design and review checklist
- Are time constants manufacturer-based?
- Is hot restart handled?
- Does loss of cooling change limits?
- Are alarm and trip capacity levels defined?
- Is thermal memory retained correctly?
Standards basis and official sources
- IEC 60255-1:2022 — Common requirements for measuring relays and protection equipment.
- IEEE C37.2-2022 — Device function numbers, acronyms and contact designations.
Engineering note: Verify the contracted standard edition, amendments, manufacturer evidence and project-specific studies before applying these principles to a supplied assembly.