Relay fixed time overcurrent protection circuit a

 Time-limited overcurrent protection , according to its action time limit characteristics, includes two types: fixed-time overcurrent protection and inverse-time overcurrent protection.
    (1) Composition of a definite time overcurrent protection circuit Definite time limit means that the action time limit of the protection circuit is fixed according to the preset action time
and has nothing to do with the size of the short-circuit current. The principle circuit of the definite time overcurrent protection device is as shown in the figure. Figure a concentrates on
the circuit principle, which is usually called the coil. All the components of the electrical appliance in this figure are grouped together, so it was also called Summary circuit
diagram. Figure h is a separately represented schematic circuit diagram, usually called an expanded diagram. All the electrical components of this circuit
are drawn separately according to the circuit to which each component belongs. From the perspective of principle analysis, the expansion circle is concise and clear, and
is most commonly used in secondary circuits (including relay protection, automatic devices, control, measurement and other circuits).
    (2) The working principle of the definite time overcurrent protection circuit. When a phase-to-phase short circuit occurs in the primary circuit, the current relay KA acts instantaneously, and the normally
open contact closes, causing the time relay KT to act. After KT passes the set time limit, its delay contact closes, causing the series-connected signal relay
(current type) KS and intermediate relay KM to act. After the KS action, its signboard falls down, and the signal loop is turned on at the same time, giving light and
sound signals. After KM acts, the trip coil YR circuit is connected to trip the circuit breaker QF and eliminate the short circuit fault.