Siemens SIPROTEC 7SD610 Manual page 272

Mounting and Commissioning
3.1 Mounting and Connections
Trip Circuit Supervision
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 be substantially below half the rated control DC volt-
age.
If two binary inputs are used for the trip circuit supervision, these binary inputs must be isolated, i.o.w. not be
communed with each other or with another binary input.
If one binary input is used, a bypass resistor R must be inserted (see following figure). The resistor R is con-
nected in series with the second circuit breaker auxiliary contact (Aux2) to allow the detection of a trip circuit
failure even when circuit breaker auxiliary contact (Aux1) is open and the command relay has dropped out. The
value of this resistor must be such that in the circuit breaker open condition (Aux1 is open and Aux2 is closed)
the circuit breaker trip coil (TC) is no longer picked up and binary input (BI1) is still picked up if the command
relay contact is open.
Figure 3-2
TR
CB
TC
Aux1
Aux2
U-CTR
U-BI
R
UR
This results in an upper limit for the resistance dimension, R
value of the arithmetic mean R should be selected:
In order that the minimum voltage for controlling the binary input is ensured, R
272
Principle of the trip circuit supervision with one binary input
Trip relay contact
Circuit breaker
Circuit breaker trip coil
Circuit breaker auxiliary contact (NO contact)
Circuit breaker auxiliary contact (NC contact)
Control voltage for trip circuit
Input voltage of binary input
Bypass resistor
Voltage across the bypass resistor
, and a lower limit R , from which the optimal
max min
is derived as:
max
SIPROTEC, 7SD610, Manual
C53000-G1176-C145-6, Release date 02.2011