Overload protection (L)
The ID letter for overload protection is L (stands for "Long-time delay").
The trip unit is inverse-time delayed and exhibits the following characteristics depending on the trip unit type:
Bimetal characteristic with thermal-magnetic trip units
I2t characteristic with electronic trip units
The letters Ir refer to the current setting value, and tr to the associated time delay.
The time delay with the device-specific reference point defines the current-dependent curve shape of the setpoint characteristic. With 3VA molded case circuit breakers, this trip unit reference point corresponds to 6 times the current setting value Ir. The following mathematical equation determines the remaining curve of the setpoint characteristic:
I2t = constant
The electronic trip units feature a thermal image function that can be deactivated on some electronic trip units.
Thermal image activated (ON)
If the thermal image is activated, the previous thermal history is taken into account. After tripping, the thermal memories of the phases are set to a value corresponding to 85 % of the thermal equivalent of the warmest phase. This setting allows reclosure of the contacts of the molded case circuit breaker. This is followed by a cool-down period according to an exponential function with a time constant Tau (τ).
Thermal image deactivated (OFF)
The thermal image for ETU 5-series and 8-series can be deactivated to allow use of the TD500 test device for characteristic curve tests. The thermal image must remain activated in normal operation. When the image is deactivated, the thermal prehistory of the circuit breaker, the cables/busbars, other components of the lower-level power distribution system or an end consumer is ignored.
It is the responsibility of the commissioning engineer/operator to provide additional thermal overload protection for the lower-level power distribution system while the thermal image is deactivated.