Θ
Θ
Θ
Θ
∆
n
n
final
n
t
e
=
+
−
(
)
⋅ −
−
−
−
1
1
1
τ
(Equation 12)
Θ
n
calculated present temperature
Θ
n-1
calculated temperature at previous time step
Θ
final
calculated final temperature with actual current
Δt
time step between calculation of actual temperature
t
thermal time constant for the protected device (line or cable), set
Time con-
stant
The actual temperature of the protected component (line or cable) is calculated by
adding the ambient temperature to the calculated temperature, as shown above.
The ambient temperature can be given a constant value or it can be measured. The
calculated component temperature can be monitored as it is exported from the
function as a real figure.
When the component temperature reaches the set alarm level
Alarm value, the
output signal
ALARM
is set. When the component temperature reaches the set trip
level
Maximum temperature, the
OPERATE
output is activated. The
OPERATE
signal
pulse length is fixed to 100 ms.
There is also a calculation of the present time to operation with the present current.
This calculation is only performed if the final temperature is calculated to be above
the operation temperature:
t
operate
final
operate
final
n
= − ⋅
−
−
τ
ln
Θ
Θ
Θ
Θ
(Equation 13)
Caused by the thermal overload protection function, there can be a lockout to
reconnect the tripped circuit after operating. The lockout output
BLK_CLOSE
is
activated at the same time when the
OPERATE
output is activated and is not reset
until the device temperature has cooled down below the set value of the
Reclose
temperature setting. The Maximum temperature value must be set at least two
degrees above the set value of
Reclose temperature.
The time to lockout release is calculated, that is, the calculation of the cooling time
to a set value. The calculated temperature can be reset to its initial value (the
Initial
temperature setting) via a control parameter that is located under the clear menu.
This is useful during testing when secondary injected current has given a calculated
false temperature level.
t
lockout
release
final
lockout
release
final
n
_
_
ln
= − ⋅
−
−
τ
Θ
Θ
Θ
Θ
(Equation 14)
Here the final temperature is equal to the set or measured ambient temperature.
1MRS757644 H
Protection functions
620 series
Technical Manual
339
