Technical data

1MRS751108-MUM
REX 521
31
Protection Relay
Technical Reference Manual, General
5.1.10. Trip-circuit supervision
The trip-circuit supervision consists of two functional units:
• A current limiter including the necessary hardware elements
• A software-based function block, named TCS1
The supervision of the trip circuit is based on the constant current injection principle.
By applying an external voltage over the relay’s trip contacts, a constant current is
forced to flow through the external trip circuit. If the resistance of the trip circuit
exceeds a certain limit, for instance due to a bad contact or oxidation, the supervision
function will be activated and cause a trip-circuit supervision alarm signal after an
adjustable delay time.
Under normal operating conditions, the applied external voltage is divided between
the relay’s internal circuit and the external trip circuit so that at the minimum 20 V
(15...20 V) remains over the relay’s internal circuit. Should the external circuit’s
resistance be too high or the internal circuit’s too low, for example, due to welded
relay contacts, the fault is detected.
Mathematically the operation condition can be expressed as:
V AC/DC
where:
•U
c
= operating voltage over the supervised trip circuit
•I
c
= measuring current through the trip circuit, appr. 1.5 mA (0.99...1.72 mA)
•R
ext
= external shunt resistance
•R
int
= internal shunt resistance, 1kΩ
•R
s
= trip coil resistance
The external shunt resistance is used if the trip-circuit supervision, independent of
the circuit-breaker position, is desired. If the trip-circuit supervision is required only
in closed position, the external shunt resistance may be omitted.
Should the external shunt resistance be used, it has to be calculated not to interfere
with the functionality of the supervision or the trip coil. Too high a resistance will
cause too high a voltage drop, jeopardizing the requirement of at least 20 V over the
internal circuit, while a resistance too low may enable false operations of the trip
coil.
Table 5.1.9-1 Testing of inputs and outputs
Test object Using
Test mode
required
Remarks
Digital inputs HMI Yes Physical inputs disconnected
Serial communication Yes
Output relays HMI Yes Normal operation still active
Serial communication No
IRF relay HMI Yes
Serial communication Yes
U
c
R
ext
R
int
R
s
++()I
c
20≥×–