ABB REF615 - Page 197

in the protection IED monitors the healthy auxiliary relay coil, not the circuit breaker
coil. The separate trip circuit supervision relay is applicable for this to supervise the
trip coil of the circuit breaker.
Dimensioning of the external resistor
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:
U R R I V AC DCc ext s c− + + ≥(R ) /int x 20
A070986 V1 EN (Equation 11)
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
If the external shunt resistance is 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 78: Values recommended for the external resistor R
ext
Operating voltage U
c
Shunt resistor R
ext
48 V DC 1.2 kΩ, 5 W
60 V DC 5.6 kΩ, 5 W
110 V DC 22 kΩ, 5 W
220 V DC 33 kΩ, 5 W
Due to the requirement that the voltage over the TCS contact must be 20V or higher,
the correct operation is not guaranteed with auxiliary operating voltages lower than
48V DC because of the voltage drop in the R
int
, R
ext
and operating coil or even voltage
drop of the feeding auxiliary voltage system which can cause too low voltage values
over the TCS contact. In this case, erroneous alarming can occur.
At lower (<48V DC) auxiliary circuit operating voltages, it is recommended to use
the circuit breaker position to block unintentional operation of TCS. The usage of the
position indication is described earlier in this chapter
1MRS756378 D Section 7
Supervision functions
REF615 191
Application Manual