ABB REL670 2.2 IEC - T-Feeder Protection

for 5 A circuits say bigger than 100 ohms (2500 VA). This ensures
that the current will circulate and not go through the differential
circuit at through faults.
That the settings of U>Alarm, U>Trip and SeriesResistor must be
chosen such that both U>Alarm/SeriesResistor and U>Trip/
SeriesResistor are >4% of IRated of the used current input.
Normally the settings shall also be such that U>Alarm/
SeriesResistor and U>Trip/SeriesResistor both gives a value
<4*IRated of the used current input. If not, the limitation in how
long time the actual current is allowed to persist not to overload the
current input must be considered especially during the secondary
testing.
7.1.4.3 T-feeder protection
M16850-4 v7
In many busbar arrangements such as one-and a half breaker, ring breaker, mesh
corner, there will be a T-feeder from the current transformer at the breakers up to
the current transformers in the feeder circuit (for example, in the transformer
bushings). It is often required to separate the protection zones that the feeder is
protected with one scheme while the T-zone is protected with a separate differential
protection scheme. The 1Ph high impedance differential HZPDIF function in the
IED allows this to be done efficiently, see Figure
52.
1MRK 506 369-UEN B Section 7
Differential protection
Line distance protection REL670 2.2 IEC 139
Application manual