Practical HV and LV Switching Operations and Safety Rules

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Practical HV and LV Switching Operations and Safety Rules


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 valueDepends 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 equipmentType 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


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Date post:	25-Jan-2015
Category:	Engineering
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Practical HV and LV Switching Operations and Safety Rules

Engineering