5.6 CONTROL ELEMENTS
a) CIRCUIT MONITORING APPLICATIONS
Some versions of the digital input modules include an active Voltage Monitor circuit connected across Form-A contacts.
The Voltage Monitor circuit limits the trickle current through the output circuit (see Technical Specifications for Form-A).
As long as the current through the Voltage Monitor is above a threshold (see Technical Specifications for Form-A), the Flex-
Logic™ operand "Cont Op # VOn" will be set. (# represents the output contact number). If the output circuit has a high
resistance or the DC current is interrupted, the trickle current will drop below the threshold and the FlexLogic™ operand
"Cont Op # VOff" will be set. Consequently, the state of these operands can be used as indicators of the integrity of the cir-
cuits in which Form-A contacts are inserted.
b) BREAKER TRIP CIRCUIT INTEGRITY MONITORING – EXAMPLE 1
In many applications it is desired to monitor the breaker trip circuit integrity so problems can be detected before a trip oper-
ation is required. The circuit is considered to be healthy when the Voltage Monitor connected across the trip output contact
detects a low level of current, well below the operating current of the breaker trip coil. If the circuit presents a high resis-
tance, the trickle current will fall below the monitor threshold and an alarm would be declared.
In most breaker control circuits, the trip coil is connected in series with a breaker auxiliary contact which is open when the
breaker is open (see diagram below). To prevent unwanted alarms in this situation, the trip circuit monitoring logic must
include the breaker position.
Assume the output contact H1 is a trip contact. Using the contact output settings, this output will be given an ID name, e.g.
"Cont Op 1". Assume a 52a breaker auxiliary contact is connected to contact input H7a to monitor breaker status. Using the
contact input settings, this input will be given an ID name, e.g. "Cont Ip 1" and will be set "ON" when the breaker is closed.
Using Digital Element 1 to monitor the breaker trip circuit, the settings will be:
DIGITAL ELEMENT 1
I = Current Monitor
V = Voltage Monitor
Figure 5–70: TRIP CIRCUIT EXAMPLE 1
Bkr Trip Cct Out
Cont Op 1 VOff
Cont Ip 1 Off
L90 Line Differential Relay
UR Relay - Form-A
GE Power Management