1. Electrical Safety
Rules to provided substation safety
Enough clearance from energized parts should be provided to avoid
accidental contact with them. If that can’t be met, live parts should
be guarded or enclosed.
Rule-2- Minimum height
A minimum height from the ground to any ungrounded part of an
electrical installation should be 8’-6”, so a person staying on the
ground can’t touch a substation element or its part which may
become energized accidentally. For example, the bottom of a post
insulator supporting an energized bus does not normally have any
There should be sufficient illumination for personnel to clearly see
their surroundings and perform any work safely. Required
illumination levels are specified in NESC® .
All passageways and stairs should be wide enough for
personnel to navigate them safely, adequate railing should be
provided, and floor openings should have guard rails.
Exits should be clearly marked and evacuation routes should
be free from obstructions. Depending on the function of the
building (for example, control house), it may require several exits
to avoid personnel being trapped during equipment fault, fire,
All substation metallic structures, fences, and equipment tanks
should be connected to a station ground grid which should be
designed to ensure that step and touch potential values are lower
than the ones stipulated in the applicable standards.
3. In addition to the above the following points are also important
for safe work environment:
Make sure that people who are working with electricity are
competent to do the job. Even simple tasks such as wiring a plug
can lead to danger - ensure that people know what they are doing
before they start.
suspect or faulty equipment is taken out of use, labeled ‘DO
NOT USE’ and kept secure until examined by a competent
where possible, tools and power socket-outlets are switched off
before plugging in or unplugging;
equipment is switched off and/or unplugged before cleaning or
More complicated tasks, such as equipment repairs or alterations
to an electrical installation, should only be tackled by people
with a knowledge of the risks and the precautions needed.
4. You must not allow work on or near exposed live parts of
equipment unless it is absolutely unavoidable and suitable
precautions have been taken to prevent injury, both to the
workers and to anyone else who may be in the area.
All electric supply lines and apparatus shall be of sufficient
ratings for power, insulation and estimated fault current and of
sufficient mechanical strength, for the duty which they may be
required to perform under the environmental conditions of
installation, and shall be constructed, installed, protected, worked
and maintained in such a manner as to ensure safety of human
beings, animals and property.
Every electric supply line other than the earth or earthed neutral
conductor of any system or the earthed external conductor of a
concentric cable shall be protected by a suitable cut-out by its
Understandings the use of safety equipment required for an
electrician working on or near a low voltage installations eg.
Rubber insulating gloves, Hard hat/Helmet, Face shields, Safety
glasses and goggles, Rubber insulating blankets, Hot sticks,
Rubber mat etc
5. Completed wiring must be free from short circuits and
grounds (other than those required in the Standard).
Grounding is the method used to connect an electrical
system to the earth with a wire. Grounding adds critical
protection against electric shock and electrocution by using
a grounding rod to provide a third path for conducting
electricity in the event of a short circuit or an overload.
Grounding will help protect the person working on the
system, the system itself, and any appliances and equipment
that are connected to the system.
electrical safety equipment products including arc flash
protective clothing, insulating gloves & sleeves, insulating
blankets, switchboard matting, insulated tools, grounding
equipment, hot sticks, lockout/tag out, ground fault circuit
interrupters (GFCI's) and much more.
6. Lightening Arrester
Lightning arresters are designed to limit the line-to-ground voltage in the event
of lightning or other excessive transient voltage conditions.
Working principle : is, once the voltage surge travel throughout the
conductor then it reaches the location of the arrester where it is installed. So it
will break down the insulation of the lighting arrestor for a moment, so voltage
surge can be discharged toward the ground . Once the voltage of the system falls
under the fixed value, then the insulation will be restored among the ground and
conductor. Further, the current flow toward the ground will be stopped.
Type of arrester
A. surge arrester
B. rod gap lighting arrester
C. Sphere gap lighting arrester
D. horn gap lighting arrester
E. multi gap lighting arrester
F. electrolyte type lighting arrester
H. metal-oxide lighting arrester
7. Instrument Transformer
The term instrument transformer refers to current and
voltage transformers that are used to scale down actual
power system quantities for metering, protective
relaying, and/or system monitoring equipment.
I. Current Transformers
Current transformers or CTs are used to scale down the
high magnitude of current flowing in high-voltage
conductors to a level much easier to work with safely
type of current transformer
Wound current transformer
Toroidal current transformer
Bar type current transformer
8. Instrument Transformer…
II. Potential Transformer
Similarly, potential transformers (PTs) are used to scale down
very high voltages to levels that are safer to work with.
Type of voltage transformer
Single and three phase voltage transformer
Single and double pole three phase voltage transformer
Single phase earthed type voltage transformer
The Voltage transformer maybe:
Electromagnetic type of transformer or
Capacitive voltage transformer -100kv above
Accuracy Class and Burden
CT accuracy class is the class assigned to the ct with
the specified limits of ratio error and phase angle error
loads/impedances connected to CT secondary is called
10. Power Transformer
Transformers are essential components in electric power systems. They come
in all shapes and sizes. Power transformers are used to convert high voltage
power to low-voltage power and vice versa. Power can flow in both
directions: from the high-voltage side to the low-voltage side or from the low-
voltage side to the high-voltage side. Generation plants use large step up
transformers to raise the voltage of the generated power for efficient transport
of power over long distances. Then step-down transformers convert the power
to sub transmission.
Power transformers consist of two or more windings for each phase and these
windings are usually wound around an iron core. The iron core improves the
efficiency of the transformer by concentrating the magnetic field and reduces
transformer losses. The high-voltage and low-voltage windings have a unique
number of coil turns. The turns ratio between the coils dictates the voltage and
current relationships between the high- and low-voltage sides
Autotransformers are a specially constructed variations of regular two winding
transformers. Autotransformers share a winding. Single-phase, two-winding
autotransformers contain a primary winding and a secondary winding on a
common core. However, part of the high-voltage winding is shared with the
low-voltage winding on an autotransformer.
A disconnector is a no-load switch (not designed to break load
current or fault current) used primarily in substations and
occasionally in distribution.
Disconnector provides permanent opening of a circuit after the
tripping of a circuit breaker under fault condition. Further it provides
visual confirmation of a circuit condition (open or close) assuring
safety to substation service personnel. They also enable
sectionalizing lines during maintenance / fault isolation.
18. Circuit Breaker
Circuit breaker : a switch that automatically stop the flow of electricity to a place or
device if the current becomes too strong or another dangerous problem occurs
Operating principle-a circuit breaker essentially consists of fixed and moving
contact , make or break a circuit either manually or by remote control under
Types of circuit breaker
A. Oil type
In such circuit breakers, some insulating oil is used as an arc quenching medium
Advantages. The advantages of oil as an arc quenching medium are :
(i) It absorbs the arc energy to decompose the oil into gases which have excellent
(ii) It acts as an insulator and permits smaller clearance between live conductors and
(iii) The surrounding oil presents cooling surface in close proximity to the arc.
Disadvantages. The disadvantages of oil as an arc quenching medium are :
(i) It is inflammable and there is a risk of a fire.
(ii) It may form an explosive mixture with air
(iii) The arcing products (e.g., carbon) remain in the oil and its quality deteriorates
with successive operations. This necessitates periodic checking and replacement
19. B. FS6-sulphur hexafluoride
In such circuit breakers, sulphur hexafluoride (SF6) gas is used as the
arc quenching medium
Advantages. Due to the superior arc quenching properties of SF6 gas,
the SF6 circuit breakers have many advantages over oil or air circuit
breakers. Some of them are
(i) Due to the superior arc quenching property of SF6, such circuit
breakers have very short arcing time.
(ii) Since the dielectric strength of SF6 gas is 2 to 3 times that of air,
such breakers can interrupt much larger currents.
(iii) The SF6 circuit breaker gives noiseless operation due to its closed
gas circuit and no exhaust to atmosphere unlike the air blast circuit
(iv) The closed gas enclosure keeps the interior dry so that there is no
(v) There is no risk of fire in such breakers because SF6 gas is non-
20. (vi) There are no carbon deposits so that tracking and insulation problems
(vii) The SF6 breakers have low maintenance cost, light foundation
requirements and minimum auxiliary equipment.
(viii) Since SF6 breakers are totally enclosed and sealed from
atmosphere, they are particularly suitable where explosion hazard
exists e.g., coal mines.
(i) SF6 breakers are costly due to the high cost of SF6.
(ii) (ii) Since SF6 gas has to be reconditioned after every operation of the
breaker, additional equipment is required for this purpose
21. C. Vacuum type
In such breakers, vacuum is used as the arc quenching medium.
Vacuum circuit breakers have the following advantages
(i) They are compact, reliable and have longer life.
(ii) There are no fire hazards.
(iii) There is no generation of gas during and after operation.
(iv) They can interrupt any fault current. The outstanding feature
of a VCB is that it can break any heavy fault current perfectly
just before the contacts reach the definite open position.
(v) They require little maintenance and are quiet in operation.
(vi) They can successfully withstand lightning surges.
(vii) They have low arc energy.
(viii) They have low inertia and hence require smaller power for
22. Battery Bank- To maintain the D.C. supply continuity
during A.C. supply failure for keeping equipment in
operation for normal & abnormal conditions
Capacitor Bank- To Improve the power factor of the
system & provide compensation to reactive power absorbed
by inductive loads, reduce the over loading of the cables,
transmission lines & transformers for the same load to be
Earthing Switch- To discharge the voltage on dead lines to
Bus Section- For connecting Incoming / Outgoing Circuits
23. Parallel operation of Transformers
Parallel operation is the connection of two or more
power sources of the same size and output volt- age to
obtain a higher output cur- rent. Conditions for parallel
operations are- Convenient & Mandatory.
The convenients are:
same voltage ratio and turns ratio
same percentage impedance
same kva rating and
same position of tap changer.
The mandatory conditions are :
same phase angle shift
same phase sequence and
24. Operation sequence
The operating sequence during opening and closing of a
circuit is given as follow:
During Opening – First open circuit breaker, open isolator,
and then close earthing switch is provided.
During Closing – First open earthing switch, close isolator,
and then close the circuit breaker.
A device that detects the fault and initiates the operation of
the Circuit breaker to isolate the defective element from the
rest of the system.
The relays detect the abnormal conditions in the electrical
circuits by constantly measuring the electrical quantities
which are different under normal and faulty conditions.
Selectivity- Ability to select the faulty part and isolate that
part without disturbing the rest of the system.
Speed - Ability to disconnect the faulty part at the earliest
Senstivity- Ability of the relay to operate with low value of
Reliablity- Ability of the system to operate under pre-