Trip-Circuit Supervision; Trip-Circuit Supervision (Tcs) - ABB REX 521 Technical Reference Manual, General

1MRS 751108-MUM
4.1.10.
Protection Relay
Technical Reference Manual, General

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 TCS
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, e.g. due to welded relay
contacts, the fault is detected.
Mathematically the operation condition can be expressed as:
(
U – R + R + R
c ext int
where:
• U = operating voltage over the supervised trip circuit
c
• I = measuring current through the trip circuit, appr. 1.5 mA (0.99...1.72 mA)
c
• R = external shunt resistance
ext
• R = internal shunt resistance, 1k
int
• R = trip coil resistance
s
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.
Following values are recommended for the external resistor R
Table 4.1.10-1 Values recommended for R
Operating voltage U
c
48 V dc
60 V dc
110 V dc
220 V dc
) × ≥ ⁄
I 20 V ac dc
s c
Ω
ext
Shunt resistor R
ext
,
1.2 kΩ 5 W
,
5.6 kΩ 5 W
,
22 kΩ 5 W
,
33 kΩ 5 W
REX 521
:
ext
19