( )
1 1
1
t
n n final n
e
t
D
-
- -
æ ö
Q = Q + Q - Q × -
ç ÷
è ø
EQUATION1168 V1 EN (Equation 93)
where:
Q
n
is the calculated present temperature,
Q
n-1
is the calculated temperature at the previous time step,
Q
final
is the calculated final temperature with the actual current,
Dt is the time step between calculation of the actual temperature and
t
is the set thermal time constant for the protected device (line or cable)
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 taken from a separate sensor or can be given a constant
value. The calculated component temperature is available as a real figure signal, TEMP.
When the component temperature reaches the set alarm level AlarmTemp the output
signal ALARM is set. When the component temperature reaches the set trip level
TripTemp the output signal TRIP is set.
There is also a calculation of the present time to operate with the present current. This
calculation is only performed if the final temperature is calculated to be above the
operation temperature:
ln
final operate
operate
final n
t
t
æ ö
Q - Q
= - ×
ç ÷
ç ÷
Q - Q
è ø
EQUATION1169 V1 EN (Equation 94)
The calculated time to trip is available as a real figure signal, TTRIP.
After a trip, caused by the thermal overload protection, there can be a lockout to
reconnect the tripped circuit. The output lockout signal LOCKOUT is activated when
the device temperature is above the set lockout release temperature setting ReclTemp.
The time to lockout release is calculated that is, a calculation of the cooling time to a
set value. The thermal content of the function can be reset with input RESET.
1MRK505222-UUS C Section 7
Current protection
451
Technical reference manual
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