Siemens 7SC80 Manual page 452

Mounting and Commissioning
3.1 Mounting and Connections
Trip Circuit Supervision 74TC
Please note that two binary inputs or one binary input and one bypass resistor R must be connected in series.
The pick-up threshold of the binary inputs must therefore stay substantially below half the rated control DC
voltage.
If two binary inputs are used for the trip circuit supervision, these binary inputs must be volt-free i.o.w. not be
commoned with each other or with another binary input.
If one binary input is used, a bypass resistor R must be used (see following figure). The resistor R is inserted
into the circuit of the 52b circuit breaker auxiliary contact to facilitate the detection of a malfunction also
when the 52a circuit breaker auxiliary contact is open and the trip contact has dropped out. The value of this
resistor must be such that in the circuit breaker open condition (therefore 52a is open and 52b is closed), the
circuit breaker trip coil (52TC) is no longer energized and binary input (BI1) is still energized if the command
relay contact is open.
[prinzip-ausloesekreisueberwachung-1-binein-150502-kn, 1, en_US]
Figure 3-2
This results in an upper limit for the resistance dimension, R
value of the arithmetic mean R should be selected:
[formel-mittelwert-r-260602-kn, 1, en_US]
In order that the minimum voltage for controlling the binary input is ensured, R
[formel-rmax-260602-kn, 1, en_US]
So the circuit breaker trip coil does not remain energized in the above case, R
452
Trip circuit supervision with one binary input
, and a lower limit R , from which the optimal
max min
is derived as:
max
is derived as:
min
SIPROTEC Compact, 7SC80, Manual
E50417-G1140-C486-A8, Edition 07.2017