64
1MRS755225
Transformer Terminal
Technical Reference Manual, General
RET 54_
5.1.11.
Trip circuit supervision
The trip circuit supervision inputs TCS1 and TCS2 in the RET 54_ transformer
terminal consist of two functional units:
• a constant-current generator including the necessary hardware elements
• a software-based functional unit for signalling
The functional units are based on the CMTCS1 and CMTCS2 function blocks
included in the condition monitoring category.
The supervision of the trip circuit is based on the constant-current injection
principle. If the resistance of the trip circuit exceeds a certain limit, for instance due
to bad contact or oxidation, or if the contact has welded, the voltage over the
supervised contact falls below 20 V AC/DC (15...20V) and the supervision function
of the trip circuit is activated. If the fault persists, the trip circuit supervision alarm
signal ALARM is obtained once the preset delay time of the function block
CMTCS_ elapses.
The input/output circuits are galvanically isolated from each other. The constant-
current generator forces a 1.5 mA measuring current through the circuit-breaker trip
circuit. The constant current generator is connected over the trip contact of the
transformer terminal circuit. The current generator for the TCS1 is connected to the
terminals X4.1/12-13 and the current generator for the TCS2 to the terminals X4.1/
17-18 of the RET 54_ transformer terminal.
Under no-fault conditions, the voltage over the contact of the constant current
generator must be above 20 V AC/DC.
The following instructions must be read very carefully.
Mathematically, operating condition can be expressed as:
where
• U
c
= operating voltage over the supervised trip circuit
• I
c
= measuring current through the trip circuit, approximately 1.5 mA
(0.99... 1.72 mA)
• R
ext
= external shunt resistor value
• R
int
= internal shunt resistor value, 1 k
• R
s
= trip coil resistance value
The resistor R
ext
must be so calculated that the trip circuit supervision current
through the resistor is low enough not to influence the trip coil of the circuit breaker.
On the other hand, the voltage drop over the resistor R
ext
must be low enough not to
jeopardize the operating condition presented in the formula above.
!
U
c
R
ext
R
int
R
s
+
+
I
c
20
VAC
/DC
–
