Practical HV and LV Switching Operations and Safety Rules

Practical HV and LV Switching Operations and Safety
Rules
Technology www.idc-online.com/slideshare Technology TTrraaiinniinngg tthhaatt Wwoorrkkss

Topics
• Electric shock and its causes
• Direct and indirect contact
• Touch and Step potential
• Role of electrical insulation in safety
• Avoiding electrical shock
• Earthing systems and safety implications
• Earthing of outdoor installations

Electrical hazards
• Invisible nature and potent power
• Electricity – Good slave but a bad master
• Result in disabilities, loss of precious life, damage to
equipment and property, huge financial losses, loss of
reputation
• Main causes:
– Misuse of electricity
– Carelessness
– Disregard to safety precautions while working with
electricity

Electric shock and associated effects
• Internal organ damage by passage of electricity
through body
• Burns on skin at point of contact
• Injuries by electric shock combined with fall
• Temperature hazards due to high temperatures
of electrical equipment
• Loss of consciousness
• Burns and injuries due to Arc flash, Arc blast

Electric shock
• Passage of electric current through body results in
shock
• Electric shock, a result of following conditions:
– Exposure to live parts (Direct contact)
– Exposure to parts that accidentally become live
(Indirect contact)
– Potential difference between different points of
earth under certain conditions

Resistance of human Body to Electric current

Effects of D.C. Current
Slight sensation Men = 1.0 mA Women = 0.6 mA
Threshold of perception Men = 5.2 mA Women = 3.5 mA
Painful, but Men = 62 mA Women = 41 mA
voluntary muscle
control maintained
Painful, unable Men = 76 mA Women = 51 mA
let go of wires
Severe pain, Men = 90 mA Women = 60 mA
difficulty breathing
Possible heart Men = 500 mA Women = 500 mA
fibrillation
after 3 seconds

Main factors determining seriousness of shock
• Path of current flow through body
• Magnitude of current
• Duration of current flow
• Body’s electrical resistance
• Human body presents resistance to flow of electric current - However
is not a constant value
Depends on factors such as:
• Body weight
• Manner in which contact occurs
• Parts of body that are in contact with earth and resistance values between
contact points

Direct and Indirect contact
• Electric shock hazard can arise due to ‘Direct contact’ or
‘Indirect contact’
• Direct Contact:
– Contact of person/livestock with live electric parts
– Condition when human body comes into contact with part that
is normally live
– Current through body governed by voltage at point of contact,
voltage across body and earth, and resistance of human body
– Voltage to which human body is subjected, a main factor
influencing current through body

Direct contact
Direct contact hazard can be minimized by:
• Using appropriate insulation for live parts
• Providing barriers for exposed live
conductors
• Use of residual current devices

Indirect contact
• Contact of person/livestock, with exposed conductive parts
that have become live under fault conditions
• Potential applied on human body in situations other than
‘direct’ contact
• Usually happens when:
– Human body is in contact with an external conductive part and a system fault
involving live conductor and external conductive part
– Potential difference between two points on earth arising out of system earth
fault gets applied across two feet (with distance being about 1 metre) of a
standing person
• Separated extra low voltage systems (SELV) provide safety
against direct, indirect contact hazards

Step potential
• Voltage difference between a person's feet
• Caused by voltage gradient in soil at the point
where a fault enters earth
• Potential gradient steepest near fault location and
thereafter reduces gradually

Touch potential
• Represents same basic hazard as Step potential, except
potential exists between person's hand and his/her feet
• Eg. Person standing on earth touches structure that is
conducting fault current into earth
• Safe limit of touch potential usually much lower than
that of step potential

Touch Voltage
Voltage that appears between any
point of contact with uninsulated
metal work located within 2.5
metres from earth surface and any
point on earth surface within
horizontal distance of 1.25 metres
from vertical projection of point
of contact with uninsulated metal
work

Step voltage, Touch voltage and
Transferred voltage

Safety against Step, Touch potentials
• Limiting step, touch potentials to safe values in
substation, vital to personnel safety
• Step, Touch potentials greatly reduced by
equipotential wire mesh, safety mat
• Mesh
– Installed in immediate vicinity of any equipment a worker
might touch
– Connected to main earth grid
• Mat minimises touch, step voltages experienced by
operating personnel

Transferred Earth Potential
• Pipe-work, rails, cables with metallic sheaths connect services
inside substation with external installations
• During earth fault, entire earth grid, earthed parts in substation
experience potential rise above remote earth
• Dangerous potential differences introduced between metal
parts connected with substation earth grid and services not
connected to grid
• If metal parts of services are connected to earth grid, potential
rise of earth grid gets transferred to remote points that are at
true earth potential - Pose danger to personnel of remote
installations

Role of electrical insulation in safety
• Insulating materials in some form or other used in all Electrical
equipment
• Insulation materials can be:
– Solid
– Liquid (eg. dielectric oils used in transformers)
– Gas (eg. SF6 used in HV switchgear and circuit breakers)
• Insulation helps:
– To prevent short circuit between live conductors and between live
conductor and enclosures of equipment
– To prevent live conductors coming in contact with human body
• Solid insulating materials play crucial role in safety

Properties of insulation materials
Properties that decide suitability of insulation material for
an application:
• Voltage withstand rating:
− Expressed usually as kV/mm - Electrical stress beyond
limit may result in breakdown of insulation material
• Operating temperature limit:
− At temperatures higher than operating limit of insulator,
insulation properties of insulator may deteriorate and
cause it to fail

Hazards posed by electrical equipment
Type of equipment Hazards
Generation equipment Electric shock, arc flash, mechanical hazards
Transformers Electric shock, arc flash, fire hazard
Overhead Transmission/distribution lines Electric shock, arc flash, fall from heights
Cables Electric shock, arc flash, fire hazard
Bus ducts Electric shock, arc flash, thermal hazard
Distribution equipment Electric shock, arc flash, thermal hazard, fire hazard
Motive equipment Electric shock, arc flash, thermal hazard,
mechanical hazards
Heating equipment Electric shock, arc flash, thermal hazard
Lighting equipment Electric shock, arc flash, thermal hazard, fall from
heights
Uninterrupted power supplies with battery Electric shock, arc flash, hazards from corrosive
liquids and explosive gases

Causes of electrical accidents
• Failure to isolate live parts/inadequate or insecure
isolation of live parts
• Poor maintenance and faulty equipment
• Insufficient information about system being worked on
• Carelessness, lack of safety procedures, failure of
adherence to procedures including failure to prove DEAD.

Live work
• Mainly performed to minimise disruption of supply to
consumers
• Restrict work on live equipment to specific situations
– On-line washing of insulators in HV outdoor substations
– LV maintenance work such as lamp/fuse changes
– Testing work using alternate power supplies
• Should be carried out only where specifically permitted by
applicable rules and legislation
• Take appropriate precautions against direct contact
• Properly shroud, install temporary barriers for adjacent
exposed live parts
• Use insulated tools

Documentation and work instructions
• Clear set of accurate documentation on installation -
Foremost factor in ensuring safety
• Should contain schematic diagrams, floor plans, wiring
diagrams, cable schedules, literature of all equipment
forming part of installation
• Clearly enunciated operating, safety instructions must
be available for each operational task carried out by
operators
• Particular attention needed by operating personnel to
procedures on isolation, securing isolation and
earthing of equipment

Operator training
• No amount of documentation is a substitute for a
knowledgeable operator
• All operators must be trained in:
– Equipment and installations intended to be operated by
them
– Safety procedures, equipment operating principles. Should
acquire familiarity with all available documentation
– Emergency actions expected of them in the event of
accidents

Utilizing interlocks
• Well designed equipment likely to have safety
interlocks
• Mechanical or key interlocks supplemented by
electrical interlocks
• NEVER override safety interlocks
• If at all an interlock must be defeated (due to equipment
malfunction), should be done after complete
verification, with proper authorisation. Must never be
done in a hurry

Earthing power supply systems and
safety implications
Earthing in electrical systems:
• Provides electrical supply system with electrical
reference to earth mass
• Two types:
• System earthing or earthing
• Protective earthing

Impedance earthing using neutral
reactor
• Inductor (also called earthing reactor) used to
connect system neutral to earth
• Limits earth fault current
• Value of earthing reactor chosen to restrict
earth fault current to value between 25% and
60% of three phase fault current

Resonant earthing using tuned reactor
• To avoid very high earth fault currents (common on single
circuit o/h lines in remote locations)
• Variant of reactor earthing. Value of earthing reactor chosen
so that earth fault current through reactor equals current
flowing through system capacitances under fault-condition
• Common in systems of 15 kV (primary distribution) range
with mainly overhead lines
• Not used in industrial systems where reactor tuning can be
disturbed due to system configuration changes by frequent
switching (on/off) of cable feeders

Impedance earthing through neutral resistance
• Most common type of earthing method adopted in Medium
voltage circuits
• System earthed by resistor connected between neutral point
and earth
• Advantages:
– Reduced damage to active magnetic components (reduced fault
current)
– Minimized fault energy – Minimal arc flash effects, increased safety of
personnel near fault point
– Avoiding transient over voltages, resulting secondary failures
– Reduced momentary voltage dips
– Obtaining sufficient fault current flow to permit easy detection,
isolation of faults

Protective earthing
• Connecting enclosure to earth - Enclosure’s potential firmly
‘clamped’ to that of earth
• In event of accidental connection of live parts to conducting
metallic enclosure, person coming in contact with enclosure
does not experience dangerous high voltages
• Provides low impedance path for accumulated static charges,
surges caused by atmospheric or electrical phenomenon to
earth
• Earthing of shields, screens of signal wires ensures noise
control by minimizing electromagnetic interference

System earthing and equipment earthing

Role of equipment earthing
(protective earthing) in human safety
• Connecting conductive metallic enclosures of
equipment (not normally live) to earthing
system of substation, other consumer facility
• For effective earthing, current should flow
through equipment enclosure to earth return
path without enclosure voltage exceeding
value of safe touch potential

Role of equipment earthing
(protective earthing) in human safety

Earth fault loop impedance in LV systems
• Should not be high since:
– It will restrict earth fault current to values not detectable
easily
– Affect performance of fuses/ other over current protective
devices
– If fault loop impedance too high, earth fault current will be
insufficient to operate protective devices (over-current
release, fuses)
– Low impedance earth return path to source necessary for
adequate fault current flow to operate protective devices
• Earthing conductor fulfills function of low impedance
connection

Earthing path through earth connection

Earthing outdoor substation equipment

Earthing pole mounted transformer

Earthing outdoor air break isolator

DO YOU WANT TO KNOW MORE?
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please visit:
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Practical HV and LV Switching Operations and Safety Rules

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