case study - hong kong polytechnic...

EE1D01 Electrical Science for Everyone

1

Case Study

Dr. X was conducting an experiment related to plant growth. He needed to expose the plants to light for specific time-periods each day. Accordingly, he had a portable fluorescent lighting rack constructed and mounted on a wooden frame. He also obtained an electric timer to automatically turn the lights on and off. The light fixture was plugged into the timer, which was plugged into the wall outlet. However, since the timer only accepted a two prong plug, an adapter was used to allow the three prong plug of the lighting rack to be used with the two prong outlet of the timer.


2

The Incident

Dr. X adjusted the timer so that the lights would be on and plugged the timer into a standard 3-prong wall outlet. Then another person in the lab noticed Dr. X grasping the lighting rack and appearing to be rigid. A third person grasped Dr. X by the shoulders and pulled him away from the lighting rack. Dr. X was transported to a hospital where he was pronounced dead.


3

The Investigation

The investigation revealed the following details:

A potential difference of 397 volts was measured between the metal fluorescent light fixture and the adjacent stainless steelsink. (There should not have been any potential difference).

Both the fixture and the wall outlet were found to be wired correctly.

The light fixture was rated for 800 mA, but the lamps used were 1500 mA.

The transformer in the ballast was found to have short circuitedto the case.


4

What Probably Happened

Use of bulbs drawing more current than approved for the fixture caused overheating of the ballast resulting in melting of insulation around the transformer coil. This allowed an energized transformer wire to touch the metal cover of the ballast which in turn energized the metal fluorescent fixture. The lights probably continued to function since they were wired correctly.

When Dr. X grasped the fixture, some part of him probably brushed against the nearby metal sink. This completed a circuit to ground through Dr. X, electrocuting him.


5

Preventing this Fatality

If overrated lamps had not been used, the ballast would probably not have overheated and failed.If the ground wire connection between the light fixture and the plug not been interrupted by the "cheater" adapter and the two-prong timer, the fixture would have safely shorted to ground tripping the circuit breaker when the equipment was plugged in before Dr. X touched it.Had the standard outlet near the sink been Ground Fault Circuit Interrupter (GFCI) or RCD protected (as it should have been because it was near a sink), the GFCI would have tripped at the first flow of current through Dr. X immediately cutting off power to the fluorescent fixture and saving his life.

As with many very serious accidents, a number of factors were involved,any one of which could have prevented this fatality.


6

Recommendations

1) Follow manufacturer's recommendations for use of electricalequipment. Do not use electrical equipment for a task it is not designedfor.

2) Generally, equipment used in research should have a 3 prong plug or be double insulated. Equipment with neither of these features are less safe but may meet electrical codes. A 3 prong plug must always go into a 3 prong outlet. Do not use a "cheater" plug or pull out the 3rd prong.

3) Do not use multiple cube taps in a standard outlet. If you must plugmore than two pieces of low demand equipment into a standard outlet,use a fused power strip that will trip if too much power is used.

4) Make sure that any outlet near a sink or other water source is GroundFault Circuit Interrupter (GFCI) protected. If you have a GFCI,periodically test it by plugging something into it and pushing the “test”button. The equipment should turn off and then turn it back on.


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5) If you see a person being electrocuted, DO NOT TOUCH HIM! Theelectricity can go though you too. If possible, remove the power (pullplug or trip circuit breaker), or use a non-conductive item (e.g. woodenbroom handle) to pry him away from the contact.

6) Above all, do not disable any electrical safety feature expecting thatanother safety feature will protect you. In the incident described above,if proper lamps were used, the ballast could still have been defectivefrom other causes. Also, GFCIs can be defective. That is why theyshould be periodically tested.

7) Check that power cords are in good condition.8) Do not use extension cords as a substitute for permanent wiring.

Recommendations


8

Lightning

Lightning kills nearly 100 people each year in the United States. It injures hundreds more. It may travel as far as 40 miles. Each second of the day it falls somewhere on earth.

Lightning will strike. However, people may be protected from most of lightning’s effects by using common sense during storms.


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LightningLightning is an unbelievably huge discharge

Clouds get charged through air friction

1 kilometer strike means 3 billion volts!

Main path forms temporary “wire”along which charge equalizes

often bounces a few times before equal

Thunder is bang produced by the extreme pressure variations induced by the formation and collapse of the plasma conduit

www.stormchasing.nl/lightning.html


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ThunderWe see the light emitted from the air molecules as they become excited from the electrical discharge.The thunder is the sound that accompanies the lightning. It results from the fast expansion of the gas that is heated to high temperatures from the electrical discharge.Sound propagates much slower than light

V(sound) = 330 m/sV(light in vacuum) = 300,000,000 m/s

That is why we hear the thunder long after the lightningHow far is the lightning? Count the seconds between the flash and the thunder. Each second is worth 330m~1000ft.What if there is no thunder? Sometimes the sound wave is reflected or absorbed while traveling through the air and may not reach usSonic boom: just a jet going supersonic.


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Charging a Thundercloud

Raindrops, snow crystals and hail stones collide inside the cloud. During these collisions they may exchange electrons and ions. The exact mechanism is not well understood, but the bottom line is:

Larger particles become negatively charged. Smaller particles become positively charged.

Larger particles settle down to the bottom of the cloud. Smaller particles are lifted to the top of the cloud by strong updrafts.Note precipitation area: void of large particles positively charged.


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Type of Discharges

Cloud-to-ground (20%, 1-200kA, 10GJ)90% of the time: negative cloud to positive ground10% of the time: positive cloud to negative ground

Cloud-to-cloud: between oppositely charged regions of different clouds.Cloud-to-atmosphere:Base-to-top: discharge

within the same cloud


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14


15


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Different Types of Lightning

Heat lightning

Forked lightning

Ribbon lightning

Bead lightning

Different Types of Lightning

Ball

Cloud-to-ground

Sheet

Cloud discharge


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Cloud-to-Ground strokes are the most

dangerous.


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2424

Basic lightning physics+ + +

+ ++

+ ++

++ +

+ + ++++

- -- - -

- -- -

----

Charge builds up inside thundercloud, reaches breakdown threshold (~300 kV/m)

t=0


2525

+ + ++ +

++ ++

++ +

+ + ++++

- -- - -

- -- -

----

Preliminary breakdown within cloud

t=1 ms

Basic lightning physics


2626

+ + ++ +

++ ++

++ +

+ + ++++

- -- - -

- -- -

----

Propagating “Stepped leader” forms ~30 μs after electric field exceeds breakdown fields of air

t~1.1 ms

v~200 m/ms



2727

+ + ++ +

++ ++

++ +

+ + ++++

- -- - -

- -- -

----

“Stepped leader”propagates down one “step” at a time, pauses (10s of μs), might split into pieces

t~20 ms

~50mI~1kA



2828

+ + ++ +

++ ++

++ +

+ + ++++

- -- - -

- -- -

----

Ground electrified as stepped leader approaches

+ ++

++

+ +

+

t~25 ms



2929

+ + ++ +

++ ++

++ +

+ + ++++

- -- - -

- -- -

----

Ionized (~10,000 K) plasma streamers rise from ground (often from high points like treetops, poles, etc) to meet leader

+ ++

++

+ +

+

t~30 ms



3030

+ + ++ +

++ ++

++ +

+ + ++++

- -- - -

- -- -

----

Leaders and some streamers connect

+ ++

++

+ +

+

t~35 ms

Attachment process



3131

+ + ++ +

++ ++

++ +

+ + ++++

- -- - -

- -- -

----

Channel formed: high current flows at bottom of channel, beginning “return stroke”

+ ++

++

+ +

+

t~35 ms

I~10s of kA



3232

+ + ++ +

++ ++

++ +

+ + ++++

- -- - -

- -- -

----

Return stroke propagates upward, comparable to speed of light30,000 K temperatures

+ ++

++

+ +

+

t~35 ms

v~0.4c



3333

+ + ++ +

++ ++

++ +

+ + ++++

- -- - -

- -- -

----

Return stroke reaches cloudOccurs much faster than stepped leader

+ ++

++

+ +

+

t~35 ms



3434

+ + ++ +

+ ++

++ +

+ + +++

- - -- -

---

Return stroke weakens as charge is depletedTotal charge transfer typically in 1s of C

+ +

++

+

t~36 ms



3535

+ + ++ +

+ ++

++ +

+ + +++

- - -- -

---

Often several return strokes ~50 ms apartTypically weaker than first return stroke

+ +

++

+

t~90 ms

I~10 kA



3636

+ + ++ +

+ ++

++ +

+ + +++

--

---

Weaker “continuing currents” may continue for 100ms or more, transferring more charge. Sprites may be triggered at 10s of C transferred

+ +

t~200 ms

I~100 A



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38

Typical Lightning Stroke Impulsive Transients


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40


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Struck flag pole and dispersed


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Positive Lightning


43

Effects of Lightning

There are two main effects of lightning strikes.Direct strikes can cause damage to buildings equipment and property, injury or death to people and animals.Because of the high levels of electrical current discharged during strikes in addition to the above electrical surges can cause damage to electrical equipment.


44

Lighting injures humans by three general mechanisms:

Electrical effects- through direct injuries

Heat

Concussive forces

May also directly cause a house or forest fireCauses 75,000 forest fires/year and responsible for 40% of all forms of fire

Effects of Lightning


45

Direct Injuries from Lighting

Direct Strike - most likely to injure those in open pace, unable to find adequate shelterSplash injuries - lightning first comes into contact with a solid object then “splashes” into a victim in close proximity

Current seeks the path of least resistance and may jump from object to person or person to person

Contact injury - occurs when a victim is in direct contact with an object struckStep Voltage (ground current - stride voltage) - current of ground strike travels outward radially (like a pebble in water)

Humans offer less resistance than ground (being primarily composed of salt water) and potential difference exists between even a normal human stride

Blunt Trauma - occurs as consequence of proximity to lightning’s concussive wave, may throw patient 10 yards away.


46


Lightning Step Voltage200,000 Volts

0 Volts far away

Current flow

thru earthgenerates

voltage

8,000 voltsacross feet(Typical)

Step Voltage


48

Lightning RodsPerform two functions

provide safe conduit for lightning away from house

diffuse situation via “coronal discharge”

Charges are attracted to tip of rod, and“electric field” is highly concentratedthere.

Charges “leak” away, diffusing chargein what is sometimes called “St. Elmo’sFire”, or “coronal discharge”


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50

Cone of ProtectionLightning rod protects areas within its cone

60 degree cone is 99% effective

45 degree cone is 99.9% effective

Less current flow from sharp pointed tipMore current flow from blunt or ball tip


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Unsafe Ground System

Multiple unconnected Grounds Problem

Lightning currents flowing in each ground system not equal

Dangerous voltages will develop between equipments due to differentground system impedances

Extreme shock hazard.


52

Safer Ground System

Multiple, Connected Grounds much Safer

Connecting all grounds together createsan EQUIPOTENTIAL environment

Voltage drop between ground systemsideally ZERO if wire has zero resistance

Ground rise will be same everywhereand differential voltages will be minimal

Multiple ground points leads to lowering resistance to ground thus lowering of Ground Rise overall


53

Bonding

Bonding is also for Lightning Protection

Purpose – keep all metal surfaces at zero potentialTo prevent electrical shockTo prevent stray current corrosionTo prevent induced potential from lightning strike

Bonding conductorCross section of #6 AWGStrap not less than #20 gauge (0.032 inch thick)Normally #6 bare copper wire


54

Typical Lightning Protection System


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Lightning Protection System- Early Streamer System (AS 1768)- Air Terminal- Co-axial Cable Down Conductor- Stroke Counter- Lightning Earthing Termination

Earthing System to BS 7430- Main Earthing- Supplementary Bonding- Equipotential Bonding

Lightning Protection and Earthing Systems


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Radio and Radio Equipment(1.2%)

Farm andHeavy Equipment

(5.0%)

Telephone(4.0%)

Golf(6.5%)

Lightning Casualties In U.S. (1959-2001)

Open Fields(45.0%)

Under Trees(23.0%)

Water(13.6%)

Golf and Trees(1.7%)

When thunderstorms nearby, avoid these activities like your life depends on it -- It Does!

When thunderstorms nearby, avoid these activities like your life depends on it -- It Does!

Lightning Facts


57

If outdoors:Seek shelter inside, preferably inside a building with a lightning rodIf not, stay inside the carDo not stay under trees.If hiking above the tree line, descend immediately.It is NOT safe to be in a tent, small picnic shelters, near heavy machinery.

If indoors:Stay away from water, plumbing, doors and windowsDo not use land line telephones.Turn off, unplug, and stay away from appliances, computers, power tools, TVs. Consider purchasing a heavy duty surge protectorBring pets inside - especially dogs chained to trees

Lightning Safety


58

Spread out and away from others - Don’t stand in a crowd of people.Move away from metal objects including bikes, fencing and machinery.Get out of and move away from pools, lakes, and other water bodies.Seek areas of land that are lower than the surrounding landscape.If a tingling sensation is felt, or the hair stands on end, lightning may be about to strike. Immediately crouch low to the ground on the balls of the feet and cover the ears. Do not lie down, orplace your hands on the ground.Return to activities no sooner than 30 minutes after the last sound of thunder or flash of lightning.

Lightning Safety


59

Safe Locations #1

No place is absolutely safe from lightning. Ideally, evacuation to a lightning certified or lightning protected building is the best when available; however, this will probably not be the case.

THEN

Large enclosed structures (substantially constructed buildings).


60

Safe Locations #2

Fully enclosed metalvehicles – car, bus, etc.

Close windowsKeep hands on lap

It’s not the rubber tires that make a vehicle safe – it’s the metal enclosure.

Not Canvas Type (soft) TopsSo, what does this say aboutUsing 2 ½ and 5 ton vehicles?


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Faraday Cage


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Inside your vehicle is

actually a safe place. It’s a kind

of Faraday Cage.

Don’t touch the metal parts.


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Safe Locations #3

Remaining OutdoorsStay away from rivers, lakes, or other bodies of water. Be aware of the potential for flooding in low-lying areas.Stay away from natural lightning rods/tall structures such as: towers, tall trees, telephone poles/lines, tents with metal supports, etc.Take shelter under a small tree among several large ones if possible. Stay at least six feet away from the tree trunk to minimize a side strike and step voltage. NEVER stand under an isolated tree.


64

Lightning Safety Position (LSP)

Assume LSP.Crouch with feet as close

together as possible. Have heels touch.

Place hands over ears.

-REMEMBER-DO NOT LIE FLAT ON THE GROUND


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Lightning Safety Position


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Seek Proper Shelter

• Buildings MuchBetter Than Vehicles

• Large, Fully Enclosed,Substantially Built

• Vehicles OfferSome Safety

• No Place OutsideIs Safe Near AThunderstorm

• Near ≡ 6 Miles

Lightning Safe Locations 10

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1

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Rel

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Pro

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Lightning Certified Facility

Outdoors

Lightning Protected Building

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