A relay is an electrically operated switch that is activated by a current or signal in one circuit to open or close another circuit. Relays work by using a magnetic field created by a coil to attract a lever and change the switch contacts. Relays have applications in controlling electric motors, automatic stabilizers, and performing arithmetic operations in computers. A miniature circuit breaker is an electromagnetic device that automatically opens a circuit when the current passes a set limit, to protect from overloads or faults. Relays are used to sense faults and send a signal to circuit breakers to isolate the circuit. Contactors are high current relays used to switch loads like motors, and magnetic motor starters include contactors and overloads for protection.

1. INDUSTRIAL AUTOMATION
RELAYS AND MCB
Done by: Koribilli Mouli

2. RELAY
• An electrical device, typically incorporating an electromagnet, which is
activated by a current or signal in one circuit to open or close another
circuit.
• A relay is an electrically operated switch.

3. WORKING OF RELAY
• Current flowing through the coil of the relay creates a magnetic field which attracts
a lever and changes the switch contacts.
• The coil current can be on or off so relays have two switch positions and they are
double throw (changeover) switches.

4. • The relay’s switch connections are usually labeled COM, NC and NO:
• COM =Common, always connect to this, it is the moving part of a
switch
• NC= Normally Closed,COM is connected to this when the relay coil is
off
• NO= Normally open,COM is connected to this when the relay coil is on

5. RELAY OPERATION OFF STATE
• Drawn in natural state.
• Where the voltage present when load side is open
• In this relay’s natural state it is in the normally open state.
• When the coil is non-energized the contacts are open.

6. RELAY OPERATION ON STATE
• When there is current through the coil of wire on the control side a magnetic field is
produced.
• This magnetic field closed the contacts on the load side.
• In this relay’s natural state it is in the normally closed state.
• When the coil is energized the contacts closed.

7. APPLICATIONS OF RELAY:
• Relays are not only used in the large electrical circuits but also used in the computer
circuits in order to perform the arithmetic and mathematical operations in it.
• Used to control the electric motor switches. To turn ON an electric motor we need 230V
AC supply but in few cases/applications, there may be a situation to switch ON the
motor with a DC supply voltage. In those cases, a relay can be used.
• Automatic stabilizers are one of its applications where a relay is used. When the supply
voltage is other than the rated voltage, set of relays sense the voltage variations and
controls the load circuit with the help of circuit breakers.

8. RELAY RATINGS AND LIMITS
• Relays often have two ratings: AC and DC. These rating indicate how much power can
be switched through the relays.
• For instance, a 5 Amp relay rated at 125VAC can also switch 2.5 Amps at 250VAC.
Similarly, a 5 Amp relay rated at 24VDC can switch 2.5 Amps at 48VDC, or even 10
Amps at 12VDC.

9. MINIATURE CIRCUIT BREAKER
• A miniature circuit breaker is an electromagnetic device that carries a complete molded
insulating material. The primary function of this device is to switch the circuit. This means
to automatically open the circuit (which has been connected to it) when the current
passing through the circuit goes beyond a set value or limit. The device can be manually
switched ON or OFF just like normal switches whenever necessary.
• A miniature circuit breaker (MCB) automatically switches off the electrical circuit during an
abnormal condition of the network means in overload condition as well as faulty
condition.

10. RELAY AND MINIATURE CIRCUIT BREAKER
• The Relay is a switching device which gives a signal to the circuit breaker as
soon as the fault occurs in the power system. Circuit breaker breaks the
circuit automatically when receives the signal from the relay.
• The Relay is a switching and sensing device, but the Circuit breaker is an
isolating or disconnecting device.
• Relays operate on low power input voltage. The Circuit breaker is an
automatic on load device.
• The Relay is used to control or select one among many circuits, whereas
Circuit Breaker is one per circuit
• Relay acts an electrical amplifier for discrete signal. Circuit breaker does not
act as an amplifier.

11. CONTACTORS
Contactors are relays that switch high current loads a.k.a magnetic
starters

12. MANUAL MOTOR STARTER

13. CONTACTORS
A Contactor is a control device
that uses a small control
current to energize or de-
energize the load connected
to it.
Abouts:
• A contactor has a frame,
plunger, and a solenoid coil.
• The action of the plunger is
used to close (or open) sets of
contacts.
• A contactor does not include
overload protection.
• The closing of the contacts
allows electrical devices to be
controlled from remote
locations.

14. EXAMPLE OF A WIRED CONTACTOR

15. MAGNETIC MOTOR STARTERS
• A magnetic motor starter is an electrically-
operated switch (contactor) that includes motor
overload protection.
• Magnetic motor starters are identical to
contactors except that they have overloads
attached to them.
• The overloads have heaters or electronic
overloads (located in the power circuit) which
sense excessive current flow to the motor.
• The heaters open the NC overload contacts
(located in the control circuit) when the overload
becomes dangerous to the motor.

16. MAGNETIC STARTER

17. MAGNETIC MOTOR STARTER
L1 (1) - first line in from power source (phase 1
for 3ph / Neutral for 1ph)
L2 (3) - second line in from power source
(phase 2 for 3ph / Hot for 1ph* see below for
alternate wiring using L2 & L3)
L3 (5) - third line in from power source (phase
3 for 3ph / NC for for 1ph)
COIL
T1 (2) - first line out to motor (phase 1 for 3ph
/ Hot for 1ph)
T2 (4) - second line out to motor (phase 2 for
3ph / NC for 1ph* see below for alternate
wiring)
T3 (6) - third line out to motor (phase 3 for 3ph
/ Neutral for 1ph)

18. MOTOR STARTER CONTROL CIRCUIT
• Alternate
method of
drawing the
electrical
circuit