An auxiliary relay can be used between the protection IED trip contact and the
circuit breaker coil. This way the breaking capacity question is solved, but the TCS
circuit 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 DC
C ext int s c
− + + × ≥(R ) /20
A070986 V2 EN (Equation 76)
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, 1 kΩ
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 causes too
high a voltage drop, jeopardizing the requirement of at least 20 V over the internal
circuit, while a resistance too low can enable false operations of the trip coil.
Table 496: 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 R
int
,R
ext
and the 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 use of the
position indication is described earlier in this chapter.
Section 6 1YHT530004D05 D
Supervision functions
576 615 series
Technical Manual
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