Woodward
Manual MRI3-ITER GB
18
DOK-TD-MRI3 ITERE Rev.A
The temperature equivalent
n-1
has not been calculated after start of the protection program (con-
nection of auxiliary voltage), therefore, the temperature state of the electrical item to be protected is
expected to be cold.
If, however, the electrical equipment has been initially loaded, it will take approximately 3 x
warming up
, (at constant load) until the thermal equivalent equals the real state.
Different time constants:
After switching off the electrical equipment (In = 0) temperature will decrease and tend towards n
= 0 (environmental temperature).
e.g.:
cooling down
= 2
warming up
Depending on the comparison between the last measured current and the actual current the
cooling down or warming up constant will be applied for calculation.
In
In-1
warming up
In < In-1
cooling down
If the relay is supplied by the mains to be protected, i.e. if the relay is not supplied to auxiliary vol-
tage after tripping of the mains c.b., the following has to be considered:
After return of the supply voltage, it is assumed that the electric item is in cold condition. In this
case this assumption does not comply with the reality. Therefore an external supply voltage should
be applied.
The tripping criterion for the alarm or tripping element of the thermal characteristic is:
Trip
> k
2
·
100%
The effective values of the measured phase currents are detected by calculating the root out of the
integral of a period’s momentary current squares. Calculation of the thermal equivalent is always
based on the highest of the three phase currents.
