To determine the ratings and settings of fuses,
breakers, relay, etc.
To isolate the fault or overloads.
An electrical device, typically incorporating
an electromagnet, which is activated by a
current or signal in one circuit to open or
close another circuit.
How to work a relay………
Relays are switches that open and close
circuits electromechanically or electronically.
Relays control one electrical circuit by
opening and closing contacts in another
circuit.As relay diagrams show, when a relay
contact is normally open (NO), there is an
open contact and the relay is not energized.
When a relay contact is Normally Closed
(NC), there is a closed contact and the relay is
Basic Parts of Relay………
Frame: Heavy-duty frame that contains and
supports the parts of the relay.
Coil: Wire is wound around a metal core.The coil
of wire causes an electromagnetic field.
Armature: A relays moving part.The armature
opens and closes the contacts. An attached
spring returns the armature to its original
Contacts:The conducting part of the switch that
makes (closes) or breaks (opens) a circuit.
Basic design or operation
A simple electromagnetic relay consists of a
coil of wire wrapped around a soft iron core, an
iron yoke which provides a low reluctance path
for magnetic flux, a movable iron armature, and
one or more sets of contacts. The armature is
hinged to the yoke and mechanically linked to
one or more sets of moving contacts. It is held
in place by a spring so that when the relay is de-
energized there is an air gap in the magnetic
circuit. In this condition, one of the two sets of
contacts in the relay is closed, and the other set
Protection vs. Coordination
Coordination is not an exact
Comparison between protection
Limit the extend and
duration of service
Selective fault isolation
Provide alternate circuits
Prevent injury to
Minimize damage to
Quickly isolate the
affected portion of the
Minimize the magnitude
of available short-circuit
Fundamental requirements Of a
Selectivity: maximum continuity of service
with minimum system disconnection.
Speed : minimum fault duration and
consequent equipment damage.
Sensitivity: operate with low value of
Reliability: assurance that the protection will
Simplicity: minimum protective equipment
and associated circuit to achieve the
Economics: maximum protection at
minimum total cost
RELAY PROTECTION COORDINATION……
Relay Protection Coordination means that
downstream devices (breakers/fuses) should
activate before upstream devices.This
minimizes the portion of the system affected
by a fault or other disturbance. At the
substation level, feeder breakers should trip
before the main breaker. Likewise,
downstream panel breakers should trip
before the substation feeder supplying the
Classification of Relays
Classification can be done by different ways, such as
by function, input, performance characteristics an
operating be divided into five types:
Protective relays and fuses operate on the intolerable
power system conditions.They are applied to all
parts of the power system; i.e. generates, buses,
distribution lines and feeds,etc
Regulating relays are associated with tap changer of
TFs, on governor of generating equipment to
control the voltages level with varying load (used
during normal conditions).
Relays of this type are used in energizing or
restoring lines to service after an outage and in
interconnecting pre-energizing parts of the
An auxiliary relay is a relay that
assists another relay or device in performing an
There are two categories:
contact multiplication and circuit isolation
Other relay classification
Protective relays classified by input
are known as current, voltage, power,
frequency and temperature relays.
Those classified by operating
principles are electromechanical, solid
Detecting and isolating faults on transmission
and distribution lines by opening and closing
Isolating the controlling circuit from the
controlled circuit when the two are at
In logic functions.
Time delay functions.
Used to control high voltage circuits with the
help of low voltage signals.
Advantages of Relays
Used as a kind of safety switch to allow a circuit with
a small current through to switch on a circuit that
will have a larger current flow through it.
Minimize the amount of damage to the system
during a fault.
Small control signal controls a larger load current or
Decreased electrical noise when switching.
Totally silent operation.
Output resistance remains constant regardless of
amount of use.
High burden level instrument transformers
are required (CTs and PTs of high burden is
required for operating the electromagnetic
relays compared to static relays)
Requires periodic maintenance and testing
unlike static relays
Relay operation can be affected due to
ageing of the components and dust, pollution
resulting in spurious trips
Operation speed for an electromagnetic
relays is limited by the mechanical inertia of