ABB Industrial IT enabled REX 521 Technical Reference Manual, General page 22

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Technical Reference Manual, General
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
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c
48 V dc
60 V dc
110 V dc
220 V dc
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I 20 V ac dc
s c
Ω
ext
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,
1.2 kΩ 5 W
,
5.6 kΩ 5 W
,
22 kΩ 5 W
,
33 kΩ 5 W
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