Relay inverse time overcurrent protection circuit a

Inverse time limit means that the action time limit of the protection circuit was originally set according to 10 times the action current, but the actual action time
changes in inverse proportion to the short-circuit current. The greater the short-circuit current, the shorter the action time.
    (1) Composition of the inverse time overcurrent protection circuit The inverse time overcurrent protection device consists of a GL type inductive current relay. Its principle
wiring circuit is shown in Figure 14-34. When a phase-to-phase short circuit occurs in a primary circuit, the current relay KA acts. After a certain delay (inverse time
characteristic), its normally open contact closes, and then its normally closed contact opens. At this time, the circuit breaker QF has its trip coil YR is "de-shunted" and trips
, eliminating the short circuit fault. When relay KA goes to shunt trip, its signal board falls down, indicating that the protection device has been activated. After the short circuit
fault is removed, the relay returns automatically, and its signal board can be manually reset using the knob on the housing.
    (2) Principle of inverse time overcurrent protection circuit The current relay KA in Figure 14-34 adds a pair of normally open contacts, which
are connected in series with the trip coil YR. The purpose is to prevent the normally closed contacts of the current relay from operating normally in the primary circuit.
Accidents in which the circuit breaker accidentally trips due to accidental disconnection due to external vibration during operation . After adding a pair of normally open contacts, even if the normally closed contacts are accidentally disconnected, the circuit breaker will not trip accidentally.
However, the action program of the two pairs of contacts of the relay must be that the normally open contact closes first and the normally closed contact opens last. That is to say, the change-over contact must be used to make first and then break
. Otherwise, if the normally closed contact is disconnected first, it will cause the secondary side of the current transformer to open with load, which is not allowed. At the same time,
the relay will return without power and will not have a protective effect. Picture a shows the connected coil, and picture b shows the unfolded coil.