Logics and circuits Questions & Answers

Logics and circuits

1. Give the definition of following.

27C Closing circuit supervisory relay
27T Tripping circuit supervisory relay
3C Interposing relay (closing coil)
3T Tripping coil
52 AC circuit breaker
88 Auxillary motor (spring charging motor)
52Y Anti-pumping relay
86.1 Lockout relay
42 Main contactor
50 Instantaneous over current relay
50N Earth fault relay
94 Trip or Trip free relay
49 Thermal overload relay
49S Stalling protection relay
27 Supervisory relay
64 Ground protection relay

2. What is the operating voltage of 3C?

48V DC.

3. DC relay coil or contactor coils must be connected to which side?

Negative side of the DC supply to avoid galvanic effect on the coil, which will corrode the coil.

4. How special current limiting resistance is connected with the seal in contact?

Special current limiting resistance is connected in series with the seal in contact.

5. How you will connect start and stop push button to control the motor from two different places?

Start push button should be connected in parallel and stop push button in series in the circuit.

6. What are the basic principles of ED?

Basic principles of ED are,

a) All the contacts of corresponding relays and contactors are shown in de-energised condition.
b) Control circuit gives us idea about ON / OFF selection of motor, fuse rating, forward reverse
control, seal in protections etc.
c) Power circuits are drawn in thick lines and control circuits are drawn in thin lines.
d) When relay or contactor energises normally open contact closes and normally closed contact
opens.
e) Auxillary contacts acts with main device such as contactors and relay.

7. What is anti pumping?

When a breaker is closed on fault condition there will be continuous tripping and closing of the
breaker because 3C is energized. Anti pumping in circuit avoids frequent tripping and closing of
circuit breaker when the breaker is closed in fault condition.

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