stopped. This can be used to account for reduced cooling capacity of the motor when it is not running. A
Cooling multiplier of 1 means the cooling is independent of whether the motor is running or not – e.g. a
water-cooled motor. A setting of 2.5 may be appropriate for a fan-cooled motor, however for the best
protection consult the motor manufacturer. Values are provided from 1.0 to 5.0.
The thermal model continues to simulate the motor’s thermal behaviour even if the power is removed
from the relay. When power is restored the thermal memory would be at the same level had there been
no loss of power.
The “Thermal Accumulator” shows the state of the thermal model: 0% = Cold, 100% = Trip. When a trip
occurs it cannot be reset until the current is below the pick-up level.
When the Protection Element is not in the ‘running’ state, if the thermal accumulator is not below the
selected start block level, a ‘start block’ (trip) will occur. This will automatically clear once the thermal
accumulator drops to, or is below the selected level. This functionality is used to ensure adequate
thermal capacity is available before another start is allowed. If necessary it will force sufficient cooling
time to allow the restoration of sufficient thermal reserve. If for example the application required
capability to permit a locked rotor start lasting up to 60% of the allowable ‘cold’ locked rotor start time
(before tripping on Motor Overload), the ‘Start Block Level’ parameter would be set to 40%.
In order to allow an emergency restart on a hot motor, a reset of the thermal memory is possible – but
the risk of permanent motor damage must be carefully considered before taking this step.
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