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PRESENTATION ON
RESTRICTED NEUTRAL SYSTEM OF POWER
SUPPLY FOR HV/EHV INSTALLATIONS.
BY
AJAY SINGHAJAY SINGH
DY. DIRECTOR OF MINES SAFETY(ELECTRICAL),
DIRECTORATE GENERAL OF MINES SAFETY, MINISTRY
OF LABOUR AND EMPLOYMENT, GOVT OF INDIA.
REG 100: PROTECTIVE EQUIPMENT.-
(1) appropriate equipment shall be suitably placed in
the mines for automatically disconnecting supply to
any part of the system, where a fault, including an
earth fault, occurs and fault current shall not be more
than 750 milliampere in installations of voltage
exceeding 250 V and up to 1100 V for below groundexceeding 250 V and up to 1100 V for below ground
mines and oil fields and 50 ampere in installations of
voltage exceeding 1100 V and up to 11 kV in open cast
mines and the magnitude of the earth fault current
shall be limited to these specified values by
employing suitably designed, restricted neutral
system of power supply.
WHAT IS RESTRICTED NEUTRAL SYSTEM
IEEE Standard 142-1991, Recommended Practice for
Grounding of Industrial and Commercial Power
Systems (Green Book), defines a high resistance
grounded system as follows:
A grounded system with a purposely inserted
resistance that limits ground-fault current which can
flow for an extended period without exacerbating
damage. This level of current is commonly thought to
be 10A or less.
ADVANTAGES OF RESTRICTED NEUTRAL SYSTEM
• Reduced magnitude of transient over-voltages
• Improved functioning of protective
relays,
• Greater safety for personnel
• The limited fault current and fast response time • The limited fault current and fast response time
prevents overheating and mechanical stress on
conductors.
• Reducing equipment damage.
The reason for limiting the current by resistance
grounding may be one or more of the following, as
indicated in IEEE Std. 142-1991, IEEE Recommended
Practice for Grounding of Industrial and Commercial
Power Systems.
(1) To reduce burning and melting effects in faulted
electric equipment, such as switchgear, transformers, electric equipment, such as switchgear, transformers,
cables, and rotating machines.
(2) To reduce mechanical stresses in circuits and
apparatus carrying fault currents.
(3) To reduce electric-shock hazards to personnel
caused by stray ground-fault currents in the ground
return path.
(4) To reduce arc blast or flash hazard to personnel
who may have accidentally caused (or who happen
to be in close proximity) to the ground fault.
(5) To reduce the momentary line-voltage dip
occasioned by the occurrence and clearing of a
ground fault.
(6) To secure control of transient over voltages(6) To secure control of transient over voltages
while at the same time avoiding the shutdown of a
faulty circuit on the occurrence of the first ground
fault.
Body resistance= 1 ohm
SHOCK VOLTAGE = 317V
Flow of current through body=317ADeath almost certain.
Body resistance= 1 ohmBody resistance= 1 ohm
Extra resistance introduced in the path
of current = 423Ω
SHOCK VOLTAGE = 0.75V
Flow of current through body=0.750ASURE SHOT CHANCES OF SURVIVAL.
Electrocution
UNGROUNDED POWER SYSTEM
the “ungrounded system” is, in reality, a “capacitively
grounded system” by virtue of the distributed
capacitance
Ungrounded power system
MAIN DISADVANTAGE OF
UNGROUNDED SYSTEM
Ungrounded System
A voltage 1.73 times the normal voltage is present on all
insulation on the ungrounded Phase, causes failures in older
motors and transformers, due to insulation breakdown
As per IEEE 242-1986 7.2.5 the main
disadvantages of Ungrounded systems are
transient over-voltages, locating the first fault and
burn downs from a second ground fault.
Over-voltages caused by intermittent faults, can Over-voltages caused by intermittent faults, can
be eliminated by grounding the system neutral
through an impedance, which is generally a
resistance, which limits the ground current to a
value equal to or greater than the capacitive
charging current of the system.
If this ground fault is intermittent or allowed to
continue, the system could be subjected to possible
severe over-voltages to ground (transient voltage),
which can be as high as six or eight times phase
voltage. This can puncture insulation and result in
additional ground faults.
A second ground fault occurring before the firstA second ground fault occurring before the first
fault is cleared will result in a phase to- ground-to-
phase fault, usually arcing, with a current
magnitude large enough to do damage, but
sometimes too small to activate over-current
devices in time to prevent or minimize damage.
The voltage across a typical capacitor can’t change
instantaneously, hence while energisation of
capacitor, the voltage across capacitor will be zero
which is followed by a voltage recovery(overshoot)
in the form of oscillating transient voltage
superimposed on Fundamental waveform.
Grounded power system
Restricted neutral power system
Resistance-Grounded System
PREACAUTIONS TO BE FOLLOWED WHILE ADOPTING
RESTRICTED NEUTRAL SYSTEM
The resistor must be sized in a manner as to
ensure that the ground fault current limit is greater
than the system's total capacitance-to-ground
charging current. If not, then transient over-charging current. If not, then transient over-
voltages can occur.
Sympathetic tripping can occur on an unfaulted
feeder if the operating value of the feeder’s
ground-fault relay is less than the feeder’s charging
current.
TRIPPING RATIO AND NGR SELECTION
Tripping ratio is the ratio of prospective ground-
fault current to the operating value of the ground
fault protection.
An adequate tripping ratio ensures that
sufficient ground-fault current is available forsufficient ground-fault current is available for
detection when a ground fault occurs.
Tripping ratio of at least 7 is necessary to detect
a two phase- to-ground fault.
METHOD TO MEASURE THE CHARGING CURRENT OF
A SYSTEM
SYSTEM
VOLTAGE
(V)
CHARGING
CURRENT
(3IC0)
AMPS/1000
KVA OF
REMARKS
TYPICAL CHARGING CURRENTS
KVA OF
SYSTEM
CAPACITY
480 0.1- 2
600 0.1- 2
2400 2- 5
4160 2- 5
SYSTEM
VOLTAGE
(V)
PHASE TO
PHASE
ESTIMATED LET
THROUGH
CURRENT
REMARKS
APPROXIMATE VALUES OF CHARGING CURRENT
PHASE
600 1A/2 MVA
2400 1A/1.5 MVA
4160 1A/1 MVA
NUISANCE/SYMPATHETIC TRIPPING
• FOR EARTH FAULT IN THE OUTGOING SIDE OF 1,
THE BREAKERS BELONGING TO FEEDERS 2, 3, AND 5
ALSO TRIPS. SYMPATHETIC TRIPPING.
• THE SETTING OF EARTH FAULT RELAYS SHALL BE SET
AOVE THE CHARGING CURRENT SO AS TO PREVENT
NUISANCE TRIPPING.
METHODS OF NGR MONITORING
harmonic voltages: proper filtering circuits.
Charged clouds.
Inrush currents.
Presence of high resistance earth fault, making
FACTORS WHICH MAY CAUSE MAL-FUNCTIONING
OF MONITORS
Presence of high resistance earth fault, making
the circuit complete, fooling monitor to feel that
NGR is in order.
Presence of artificial neutral points: primary of
P.T. etc.
COMMON MISTAKES WHILE ADOPTING
RESTRICTED NEUTRAL SYSTEM OF POWER SUPPLY
BYPASSING OF NGR DUE TO POOR WORKMANSHIP
PARTIAL BYPASSING OF NGR DUE TO PRESENCE OF ARTIFICIAL
NEUTRAL POINT
POOR INSULATION RESISTANCE.
SETTING OF EARTH FAULT RELAY IN EXCESS OF
RESTRICTED EARTH FAULT CURRENT.
PRESENCE OF PERMANENT EARTH FAULT OF
HIGH IMPEDANCE. HIGH IMPEDANCE.
REG44: USE OF ELECTRICITY AT VOLTAGE EXCEEDING
650 VOLTS: -
(vii) (b) provisions shall be made for suitable oil soak
pit and where use of more than 9000 litres of oil in
any one oil tank, receptacle or chamber is involved,
provision shall be made for the draining away or
removal of any oil which may leak or escape from theremoval of any oil which may leak or escape from the
tank, receptacle or chamber containing the same, and
special precautions shall be taken to prevent the
spread of any fire resulting from the ignition of the oil
from any cause and adequate provision shall be made
for extinguishing any fire which may occur;
(ix) he shall ensure that the transformers of 10 MVA
and above rating or in case of oil filled transformers
with oil-capacity of more than 2000 ltrs are provided
with fire fighting system as per IS - 3034: 1993 or
with Nitrogen Injection Fire Protection system;
INTRODUCTION: Power transformer may be
considered as a metal tank filled with oil i.e.
flammable liquid.
The inside active part of transformer can be
considered as “insulation materials under high
dielectric stress” & “conducting parts i.e. copper etc.
carrying very high current”.
Owing to this any internal or external fault on aOwing to this any internal or external fault on a
transformer may trigger a fire & the volume of oil in
the transformer is sufficient to cause a major fire
accident/explosion in spite of the safety devices like
pressure relief valve, Buccholz relay etc.
REASONS OF FIRE:
•Internal arcing fault due to insulation failure.
•Internal Fault generated during external Short circuit
fault.
•Internal Fault generated during lightning/switching
impulse stroke.
•Improper preventive maintenance of transformer for•Improper preventive maintenance of transformer for
e.g. oil quality etc.
•Improper operation of transformer i.e. overloading,
over voltage etc.
•Bypassing the safety & protection devices.
•Inherent design/manufacturing defect.
•Ageing of transformer
COMMAND FROM
VARIOUS RELAYS,
LIKE,
DIFFERENTIAL,
RAPID PRESSURE
RISE, BUCHOLZ,
PRESSURE RELIEF
VALVE
COMMAND
FOR DRAINING
OF OIL
INJECTION
OF
NITROGEN
FUNCTIOING OF AUTO FIRE SYSTEM
VALVE
As soon as the fire is detected, immediately signal is sent to the
quick drain valve & within fractions of a second (0.2secs) drain
valve is opened to release the pressure & partially drain the oil.
The conservator oil is isolated from main transformer by closing
of automatic shutter valve so that the conservator oil does not
replenish the tank. Within 20 seconds the partial oil draining is
complete.
COURTESY: M/S CTR
The nitrogen injection valve opens thereby injecting dry Nitrogen
into the transformer from bottom. Explosive gases released from
decomposition of oil/insulation are evacuated. Because of thermal
stirring, the surface temperature is dropped below the flash point &
fire is extinguished in approximately 5 minutes. The Nitrogen
injection is continued for approximately 45 minutes to sufficiently
cool the transformer & to prevent any recombustion.
1. BUCCHOLZ RELAY1. BUCCHOLZ RELAY
2. TEMPERATURE DETECTORS
3. SHUTTER VALVE
4. FIRE PROTECTION SYSTEM
5. TRANSFORMER QUICK DRAIN VALVE
6. NITROGEN CYLINDER
7. NITROGEN PYROTECHNIC VALVE
8. PRESSURE REDUCER
USE OF NON INFLAMMABLE LIQUID IN TRANSFORMER
• LOOKING INTO THE FLAMMABLE NATURE OF
TRANSFORMER OIL, NON-INFLAMMABLE LIQUID I.E.
SILICONE OIL IS ALSO BEING USED FOR TRANSFORMERS.
• SILICONE OIL IS NON-HYDROCARBON, NON FIRE-
PROPAGATING OIL & IS USED IN ELECTROSTATICPROPAGATING OIL & IS USED IN ELECTROSTATIC
PRECIPITATOR TRANSFORMER INSTALLED OVER ROOFTOP
OF THERMAL POWER STATIONS .
• IN EVENT OF FIRE THE INSULATION OF TRANSFORMER
BURNS BUT QUENCHES DUE TO NON-HYDROCARBON
NATURE OF THIS OIL. THE FLASH POINT & FIRE POINT OF
THIS OIL IS VERY HIGH.
DGA : By Vacuum Gas Extraction Apparatus and Gas
Chronographs first gasses are extracted from oil by stirring it
under vacuum. These extracted gasses are then introduced in
gas Chronographs for measurement of each component.
Generally it is found that hydrogen and methane are
produced in large quantity if internal temperature of power
transformer rises up to 150oC to 300oC due to abnormal
thermal stresses. If temperature goes above 300oC, thermal stresses. If temperature goes above 300 C,
ethylene(C2H4) are produced in large quantity. At the
temperature is higher than 700oC large amount of
hydrogen(H2) and ethylene(C2H4) are produced.
Ethylene(C2H4) is indication of very high temperature hot spot
inside electrical transformer. If during DGA test of
transformer oil, CO and CO2 are found in large quantity it is
predicted that there is decomposition of proper insulation.
ELECTRICAL ACCIDENTS
REG 19: HANDLING OF ELECTRIC SUPPLY LINES AND
APPARATUS: -
(1) Before any conductor or apparatus is handled,
adequate precautions shall be taken, by earthing or
other suitable means, to discharge electrically such
conductor or apparatus, and any adjacent conductorconductor or apparatus, and any adjacent conductor
or apparatus if there is danger there from, and to
prevent any conductor or apparatus from being
accidentally or inadvertently electrically charged
when persons are working thereon.
(2) Every person who is working on an electric supply
line or apparatus or both shall be provided with tools
and devices such as gloves, rubber shoes, safety belts,
ladders, earthing devices, helmets, line testers,
hand lines and the like for protecting him from
mechanical and electrical injury and such tools andmechanical and electrical injury and such tools and
devices shall always be maintained in sound and
efficient working condition.
NO USE OF SAFETY GADGETS
•LINE TESTER.
•SAFETY ROPE.
•HALMET.
•SAFETY GLOVES.
NO PROVISION OF INTERLOCKING
• SHUTDOWN OF BREAKER 1 WAS
TAKEN WHEREAS 2 WAS STILL ENERGISED.
• NO PROVISION OF INTERLOCKING
BETWEEN 1 & 2.
•LINE TESTER.
•SAFETY ROPE.•SAFETY ROPE.
•HALMET.
•SAFETY GLOVES.
REG 74: PROTECTION AGAINST LIGHTNING.-
(1) every overhead line, sub-station or generating
station which is exposed to lightning shall be
equipped with efficient means for diverting to
earth any electrical surges due to lightning which
may result into injuries.
(2) The earthing lead for any lightning arrestor shall (2) The earthing lead for any lightning arrestor shall
not pass through any iron or steel pipe, but shall
be taken as directly as possible from the lightning
arrestor "without touching any metal part to a
separate-vertical ground electrode
Remedial Approach
Conventional air terminal (Franklin Rod)
Faraday Cage:
metallic material completely surrounding the protected
structure and resulting in its electrostatic shielding.
conductors are spaced in a criss-crossed fashion across the
roof structure and sides.roof structure and sides.
IONIZING AIR TERMINAL:
streamer emission system employs either a terminal of
specific shape (Sphere as in the case of Dynasphere) or
enhanced ionizing radioactive air terminal for the
generation of ions. Air terminal is connected to a special
down conductor attached to an earthing system.
G
LIGHTNING STRIKE TO LAUNCH COMPLEX 39, KENNEDY SPACE CENTER,
FLORIDA. THE AIR TERMINAL PROTECTING THE SHUTTLE ON THE PAD
INTERCEPTED THE STRIKE
FARADAY CAGE
LASER BEAM:
The laser beam would produce multi-photon ionization.
The laser beam could thus intercept a leader as it
developed towards the earth, and act as a conductor from
the cloud to the ground and then be terminated to a down
conductor and the earth mass.
Preventive Approach
preventing build-up of charge
in the area to be protected.
The system shall be able to
reduce the potential between
the protected area and the
charged clouds, so that thecharged clouds, so that the
potential difference is not
high enough to enable the
generation of a leader to the
earth within the protected
area.
ESTIMATION OF EXPOSURE RISK
Expected number of lightning flashes per square kilometre
per year.
For either polarity, however, the current flow is
unidirectional with a rise time of less than 10 μs for the
negative flash (but considerably longer for positive flash)
and then decays to a low value, for a simple single stroke,
in 100 μs or less.in 100 μs or less.
Assuming the charge in the cell to be 100 C and the radius of an equivalent spherical cell to
be 1 km. The capacitance of the cell is, therefore, about 10-7 F and from Q == CV the
potential is estimated to be 109 V. It is reasonable, therefore, to assume that the cloud
potential is more than 100 MV. This potential is high enough to ensure that the potentials
sustained by whatever is struck will be controlled by the product of current and
impedance, because this product will never be high enough in comparison with the cloud
potential to modify the current magnitude.
The lightning stroke starts by the step by step descent from the cloud of a leader stroke
stepping some tens of metres at a time. When the last step brings the tip of the leader
sufficiently close to earth an upward streamer leaves the earth to join the tip of downward
leader.
The initiation of this upward streamer depends on a critical field being exceeded at the
earth emission point and so is a function of the charge deposited by the down-coming
leader and any enhancement of the field caused by the geometry of the earth. The
length of the upward streamer will be greater for greater charges and hence high
current flashes will start preferentially from high structures for which the field
enhancement is high.
Electrical Effect
As the current is discharged through the resistance of the earth electrode of the lightning
protective system, it produces a resistive voltage drop which may momentarily raise the
potential of the protective system to a high value relative to true earth. It may also produce
around the earth electrodes a high potential gradient dangerous to persons and animals.
Side Flashing,
The point of strike on the protective system may be raised to a high potential with respect to
adjacent metal. There is. therefore, a risk of flash. over from the protective system to any
other metal on or in the structure. If such flashover occurs, part of the lightning current is
discharged through internal installations, such as pipes and wiring, and so this flashover
constitutes a risk to the occupants and fabric of the structure.
ThermalThermal
As far as it affects lightning protection, the effects of a lightning discharge is confined to the
temperature rise of the conductor through which the current passes. Although the current is
high, its duration is short, and the thermal effect on the protective system is usually
negligible. In general, the cross-sectional area of a lightning conductor is chosen primarily to
satisfy the requirements of mechanical strength, which means that it is large enough to keep
the rise in temperature to 1°C . For example, with a copper conductor of 50 mm2 cross section
a severe stroke of 100 kA with a duration of 100 μs dissipates less than 400 J per metre of
conductor resulting in a temperature rise of about 1°C. The substitution of steel for copper
results in a rise of less than 10°C.
Mechanical Effects
Where a high current is discharged along parallel conductors at close proximity, or along a
single conductor with sharp bends, considerable mechanical forces are produced. Secure
mechanical fittings are, therefore, essential.
A different mechanical effect exerted by a lightning flash is due to the sudden rise in air
temperature to 30 000 K and the resulting explosive expansion of the adjacent air in the
channel along which the charge is propagated. This is because, when the conductivity of
the metal is replaced by that of an arc path, the energy increases about one hundredfold. A
peak power of about 100 MW/m can be attained in the return stroke and the shock wave
close to this stroke readily dislodges tiles from a roof .
BASIC CONSIDERATIONS FOR PROTECTION
• Decide whether or not the structure needs protection
Need for Protection
Structures with inherent explosive risks; for example, explosives factories, stores and
dumps and fuel tanks; usually need the highest possible class of lightning protective
system.
- where large numbers of people congregate;
- where essential public services are concerned;
- where the area is one in which lightning strokes are prevalent- where the area is one in which lightning strokes are prevalent
- where there are very tall or isolated structures;
- where there are structures of historic or cultural importance,
assessment can be made taking account of the exposure risk ( that is the risk of
the structure being struck) and the following factors:
• Use to which the structure is put,
• Nature of its construction,
• Value of its contents or consequential effects,
• The location of the structure, and
• The height of the structure ( in the case of composite structures the overall height).
Estimation of Exposur Risk
The probability of a. structure or building being struck by lightning in anyone year is the The probability of a. structure or building being struck by lightning in anyone year is the
product of the 'lightning flash density' and the 'effective collection area' of the structure.
The lightning flash density, Ng is the number of ( flashes to ground ) per km2 per year.
The effective collection area of a structure is the area on the plan of the structure extended in
all directions to take account of its height. The edge of the effective collection area is displaced
from the edge of the structure by an amount equal to the height of the structure at that point,
Hence. for a simple rectangular building of length L, width W and height H metres, the collection
area has length (L + 2H) metres and width ( W + 2H) metres with four rounded corners formed
by quarter circles of radius H metres.
This gives a collection area, Ao (in m2) of:
Ao = ( L X W) + 2 ( L X H) + 2 ( W X H) + пH2
The probable number of strikes ( risk) to the structure per year is,
P = Ao X Ng X 10-6P = Ao X Ng X 10-6
Ng: is lightning flash density , the number of ( flashes to ground )
per km2 per year,
Ao (in m2): collection area
Suggested Acceptable Risk
For the purposes of this Code, the acceptable risk figure has been taken as
10-5, that is, 1 in 1,00,000 per year.
Overall Assessment of Risk
Having established the value of P, the probable number of
strikes to the structure per year the next step is to apply the
'weighting, factors',
This is done by multiplying P by the appropriate factors to
see whether the result, the overall weighting factors,
exceeds the acceptable risk of P = 10-5 per year.
Interpretation of Overall Risk FactorInterpretation of Overall Risk Factor
If the result obtained is considerably less than 10-5 ( 1 in
100,000 ) then, in the absence of other overriding
considerations, protection does not appear necessary; if
the result is greater than 10-5, say for example 10-4( 1 in 10
000 ) then sound reasons would be needed to support a
decision not to give protection.
Zone or Protection
CONE CONCEPT: volume within which a lightning conductor gives protection against a direct
lightning stroke by directing the stroke to itself. For a vertical conductor rising from ground
level, the zone has been defined as a cone with its apex at the tip of the conductor its base
on the ground. For a horizontal conductor the zone has been defined as the volume
generated by a cone with its apex on the horizontal conductor moving from end to end.
Protection angle
AIR TERMINATION AND ZONE OF PROTECTION FOR SIMPLE STRUCTURE WITH EXPLOSIVE OR
HIGHLY IMFLAMMABLE CONTENT
Where a structure is simply a continuous metal frame, it requires no air termination or down
conductor. It is sufficient to ensure that the conducting path is electrically and mechanically
continuous and that the requirements of the code in respect of the connection to the general
mass of the earth are met.
Fences Surrounding Structures Containing Flammable Liquids or Gas
Earthing of All Metal Fences
Where fences which surround hazardous locations, are of the all-metal type, no particular
problems arise, and they can be earthed at intervals not exceeding 75 m.
The temperatures of lightning are 15,000-60,000°FThe temperatures of lightning are 15,000-60,000°F
Lightning strokes can heat the air through which they travel to an amazing 30,000 degrees
Celsius (54,000F).
This extreme heating causes the air to explosively expand, creating a shockwave that travels
outwards through the air as the booming sound that we call "thunder."
Lightning forms through the electrification of clouds, and the most popular theory is that this
comes about through graupel and hail falling through the region of the cloud containing
supercooled water droplets and ice crystals.
As liquid droplets collide with hailstones and freeze on contact, the latent heat
released keeps the surface of the hailstones warmer than the surfaces of the
surrounding ice crystals. When the hailstones collide with the colder ice crystals, there
is a net transfer of positive ions from the warmer hailstones to the colder ice crystals.
The hailstones therefore acquire a negative charge, and the ice crystals become
positively charged. The lighter ice crystals get carried upwards into the cold part of the
thunderstorm by strong updrafts, causing the upper part of the cloud to become
positively charged. The heavier hailstones fall toward the bottom of the cloud, causing
the middle part of the cloud to become negatively charged. The lower part of the cloud
is generally of negative and mixed charge, with some positive charged areas where
precipitation is leaving the cloud.
C: A second surge of negative
charge descends along the
ionized path of the previous
stroke creating the Dart
Leader.
D: A second return stroke follows
the first.
The whole process repeats until the cloud is discharged.
This all happen in 10 microseconds. Your eyes can’t see
any of this details.
Other types of cloud-to-ground lightning strokes 1
• Between anvil and ground
• Positive charges travel downward to
the ground
• 8% of all cloud-to-ground lightning
stroke are of this type.
• More dangerous
• Exits from the side of a
thundercloud and comes to the
ground away from the thundercloud
• The ground strike can be over 10 km
from the cloud boundary, arriving at
the ground in an area where the sky
is blue.
Lightning Safety• Signs of lightning stroke: Sizzling sound or
hair standing up indicates a strong field
• 5 s gap between flash and thunder implies a
1 mi range
• 30-30 rule (conservative)
– 30s lag 6 mi range
– Wait 30 min
• Take refuge in a car or a building with • Take refuge in a car or a building with
plumbing and wiring
• Not under trees
• Assume a low crouch with only feet in
contact with the ground
• Avoid holding elongated metal objects, like
rifles or golf clubs; avoid open water outdoors
• In june 1998, 13 people were badly injured during a rock
concert in Baltimore.
This stadium was provided with LP.
• In Oct 1998, republic of congo, 11 team members were
killed by lightning.
• Empire building was struck 25 times in one year by
lightning.
• A direct lightning on your car will flow through metal• A direct lightning on your car will flow through metal
frame and usually may flat one or more tires and
concurrently may result in damage to the electrical
system but no injury to the occupant.
But the strike may ignite the fuel hence we should get out
of the car as soon as possible.
Because opposite charges attract each other, the negatively charged bottom of the cloud
causes positive charges to build in the ground directly beneath it. These positive charges
will follow the cloud wherever it goes. The largest buildup of positive charges will occur in
protruding objects such as houses, trees, and poles, which reach up toward the sky. The
difference in charges causes an electric potential to build between the cloud and the
ground, but in dry air, a flow current does not occur because air is a good electrical
insulator. However, as the electrical potential gradient gradually builds, it will eventually
become large enough (on the order of a million volts per meter) to overcome the
insulating properties of the air, causing a current to flow, and lightning occurs. Cloud-to-
ground lightning begins within a cloud when a sufficiently strong localized electric
potential gradient causes a discharge of electrons to surge towards the cloud base and
then towards the ground. This discharge proceeds in a series of steps, each about 50-then towards the ground. This discharge proceeds in a series of steps, each about 50-
100m long, with pauses of about 50 millionths of a second between each step. This is
called a "stepped leader," and is usually quite faint, sometimes too faint to see with the
naked eye.
As the tip of the stepped leader nears the ground, the electric potential gradient
increases sharply, and a current of positive charge starts upward from the ground to
meet it. Once they meet, a surge of electrons flows to the ground, and then a much more
intense "return stroke" races upward towards the cloud along the same path traced by
the stepped leader. This return stroke is what causes the bright flashes that we see as
lightning, and the transfer from ground to cloud is so fast that it is imperceptible to the
human eye, and thus we see it as just a continuous flash of light.
The charges on the ground are influenced by the charge build up in the clouds. Normally,
the ground has a slight negative charge however, when a thunderstorm is directly
overhead, the large negative charge in the middle of the storm cloud repels negative
charges on the ground underneath the storm. This causes the ground and any objects (or
people) on the ground directly underneath the storm to become positively charged
Lightning Formation
The sky is filled with electric charge. In a calm sky, the positive (+) and negative (-) charges are
evenly spaced throughout the atmosphere. Therefore, a calm sky has a neutral charge.
Inside a thunderstorm, the electric charge is spread out differently. A thunderstorm is made up of
ice crystals and hailstones. The ice crystals have a positive charge, and the hailstones have a
negative charge. An updraft pushes the ice crystals to the top of the thunderstorm cloud. At the
same time, the hailstones are pushed to the bottom of the thunderstorm by its downdraft. These same time, the hailstones are pushed to the bottom of the thunderstorm by its downdraft. These
processes separate the positive and negative charges of the cloud into two levels: the positive
charge at the top and the negative charge at the bottom.
During a thunderstorm, the Earth's surface has a positive charge. Because opposites attract, the
negative charge at the bottom of the thunder cloud wants to link up with the positive charge of
the Earth's surface.
Once the negative charge at the bottom of the cloud gets large enough, a flow of negative charge
rushes toward the Earth. This is known as a stepped leader. The positive charges of the Earth are
attracted to this stepped leader, so a flow of positive charge moves into the air. When the
stepped leader and the positive charge from the earth meet, a strong electric current carries
positive charge up into the cloud. This electric current is known as the return stroke and humans
can see it as lightning.
ANY QUESTIONS PLEASE
