Electromagnetic relays are classified based on technology and function. An attracted armature relay uses electromagnetic force and is fast acting, making it suitable for protection applications. Relays respond to electrical quantities like voltage, current, frequency, and phase angle. A polarized relay only operates based on the direction of current or voltage. Burden refers to the power consumed by a relay. Static relays offer advantages over electromagnetic relays like lower power consumption and compact size.

1. UNIT-2
ELECTROMAGNETIC RELAY
Two Marks
1. How the relays are basically classified?
i Based on Technologies,
 Electromagnetic relay
 Static relay
 Microprocessor based relay
ii Based on function
 Overcurrent-relay
 Under/over voltage relay
 Impedance relay
 Over/under frequency relay
 Directional relay
2. Mention the features and the applications of attracted armature type Electromagnetic relay?
 Protection of ac and dc equipments
 Used as measuring relays or auxiliary relays
 Used as differential relays
Features:
 It can be operate both on ac and dc, as torque is proportional to I^2.
 The relay is fast due to small length of travel and light moving parts.
3. What are the actuating quantities in respect of relays?
 Voltage
 Current
 Frequency
 Phase angle
4. What is meant by polarized relay?
Operation on the relay based on the direction of current or voltage then it is said to be
polarized and the relay is polarized relay.
5. Define ‘burden’ of a relay?
It is the power consumed by the relay circuitry at the rated current.
6. What are the various types of electromagnetic relays?
 Attracted armature type
 Balanced beam type
 Induction disc type
 Induction cup type
 Moving coil type
 Moving iron type
7. What is meant by directional relay?
A relay which respond to the flow of power in a definite direction with reference to the location
of CT’s and PT’s.
8. What is meant by over current protection and list the applications of over current relays?
This relay when the magnitude of the current exceeds the pick-up level of this relay. The
applications are motor protection, transformer protection, transmission line protection, industrial
utility equipments like furnaces etc.

2. 9. What is differential relay and list the applications of differential relay?
A relay depends to the vector difference between two or more similar electrical quantities is
differential relay.
The applications are:
 Protection of generator, generator-transformer unit.
 Protection of transformers
 Protection of transmission line by pilot wire differential protection
 Protection of large motors
 Protection of bus bar
10. What are the difficulties of differential protection?
 Difference in pilot wire lengths
 Saturation of CT magnetic circuits during short-circuits conditions.
 Magnetizing current in-rush in transformers which switching in.
 Tap changing operation in the transformers.
11. What is a distance relay and mention its kinds?
Distance relay is double coil and double acting quantity relay with one coil energized
by voltage and the other coil by current. It is the one whose operation is based on measurement
of impedance, reactance, or admittance of line between the location of relay and the fault point.
Its kinds are
 Impedance relay
 Reactance relay
 Admittance relay
12. What is R-X diagram?
It is a representative diagram of impedance/reactance/admittance relay characteristics
on R-X planes with R on the X axis and X on the R axis.
13. What is the use of negative sequence relay?
The negative sequence relay is used to protect electrical machines against overheating
due to unbalanced currents n the stator.
14. What is a comparator? What are all its types?
Comparator is a part of static relay, which receives two or more inputs to be compared
and gives output based on their comparison.
There are
 Amplitude comparator
 Phase comparator
 Hybrid comparator
15. List out the advantages of static relay?
 Low power consumption as low as 1 milli watt for EM relay it is 2 watts.
 Static relay is compact
 Static relays can THINK and ACT
 Defective module can be replaced quickly
16. What are the limitations of a static relay?
Static relays are temperature dependent, as characteristics of a transformer, diode
change with temperature variation. To avoid this temperatures compensation is provided by
means of thermistor circuits.

3. Static relay can is sensitive to voltage transient or voltage spikes, caused by
operation of breaker and isolator in the primary circuit of CT’s and PT’s.
17. What is static relay?
A static relay prefer to a relay in which the measurementor comparison of electrical
quantities is done in a static network which designed to give an output signal , when a threshold
condition is passed ,which operates a tripping device.
18. Define power swing.
During switching of lines or under wrong synchronization surges of real and reactive
power flowing in the transmission line cause severe oscillations in the voltage and current
vectors. It is represented by a curve originating in load region and travelling towards relay
characteristics.
19. What is mho relay?
The relay measuring a component of admittance Y angle theta. The other names of mho
relays are admittance relay and angle admittance relay.
20. Why an offset mho is used with a main MHO relay?
In order to operate for power swings, the main MHO relay is provided with offset unit,
which will block the main relay during power swings.
21. What are the characteristics of a reactance relay?
The relay has characteristics such that all the impedance radius vectors whose outer ends
lie on the straight line having a constant X components, where X is the reactance of protected
lines between the relay location and the fault point.
22. What are the factors on which the earth fault protection depends on?
The earth fault protection depends up on the type of neutral earthing and size of the
alternator. In case of low rating alternator, the resistance and for large alternators, kept connected
between the neutral and earth are the deciding factors.
23. Why a shading ring is provided in an induction disc relay?
The shading ring is provided to produce flux in the shaded portions of the magnet which is
displaced in phase and space from flux in the remaining portion of the same magnet.