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4
4.1
4.2
4.3
76
Thermal phase overload protection (THOL)
Thermal phase overload protection (THOL)
Application
Load currents that exceed the permissible continuous value may cause damage to the conductors
and isolation due to overheating. The permissible load current will vary with the ambient tem-
perature.
The THOL thermal overcurrent function supervises the phase currents and provides a reliable
protection against damage caused by excessive currents. The temperature compensation gives a
reliable thermal protection even when the ambient temperature has large variations.
Functionality
The final temperature rise of an object relative the ambient temperature is proportional to the
square of the current. The rate of temperature rise is determined by the magnitude of the current
and the thermal time constant of the object. The same time constant determines the rate of tem-
perature decrease when the current is decreased.
The thermal overload function uses the highest phase current. The temperature change is con-
tinuously calculated and added to the figure for the temperature stored in the thermal memory.
When temperature compensation is used, the ambient temperature is added to the calculated
temperature rise. If no compensation is used, 20
perature of the object is then compared to the set values for alarm and trip.
The information on the ambient temperature is received via a transducer input with for example
0 - 10 mA or 4 - 20 mA.
The output signal THOL--TRIP has a duration of 50 ms. The output signal THOL--START re-
mains activated as long as the calculated temperature is higher than the set trip value minus a
settable temperature difference TdReset (hysteresis). The output signal THOL--ALARM has a
o
fixed hysteresis of 5
C.
Function block
THOL-
THOL
BLOCK
ALARM
TRIP
START
xx00000634.vsd
o
C is added as a fixed value. The calculated tem-
Chapter 5
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