Low voltage blocking overcurrent protection circuit

In the protection circuit shown in the figure , in the normally open contact circuit of the overcurrent relay KA for line overcurrent protection , the normally closed contact of the low voltage relay KV is connected in series , and KV is connected to the protected line
through the voltage transformer TV.
On the bus, '
    Working principle: When the power supply system is operating normally, the bus voltage is
close to the rated voltage, so the normally closed contact of the voltage relay KV is
open. At this time, even if the current relay KA malfunctions due to line overload
(that is, the KA contact is closed), it will not cause the circuit breaker QF
to trip accidentally. Because of this, the operating current k of the overcurrent protection device equipped with low-voltage blocking does not need to be set according to the maximum load current IIAVI of the bypass line, but only needs to be set according to the calculated current I30 of the bypass line . Of course, the return current k of the protection device should also avoid I30.