E l e c t r i c a l S h o c k

Presented by

Md. Noman Saber Khan

shock“High voltage is dangerous”“Voltage isn’t dangerous, current is”?

shockDepends on Current Pathway through body Nature of electrical source Duration (and voltage)

currentCurrent Level (AC 60 Hz)

Effect

1 mA Sensation that shock is occurring

5 mA Upper limit of safe or harmless range

10–20 mA Let-go threshold: flexor muscles are stronger than the extensor muscles;subject cannot shake loose from the shock source; perspiration

30–40 mA Tetany: sustained muscle contraction and cramping

50–70 mA Extreme pain, physical exhaustion, fainting, irreversible nerve damage;possibility of ventricular fibrillation (heart); respiratory arrest withpossible asphyxiation

100 mA Ventricular fibrillation (heart) and death if the current passes through the body trunk

>100 mA Fibrillation, amnesia, burns, severe electrolysis at contact sites

>5 A Little likelihood of survival

Ac/dc

http://www.allaboutcircuits.com/vol_1/chpt_3/4.html

Bodily effect DC AC 60 Hz AC 10 kHz

Slight sensation felt at hand(s)

Men = 1.0 mAWomen = 0.6 mA

0.4 mA0.3 mA

7 mA5 mA

Threshold of perception Men = 5.2 mAWomen = 3.5 mA

1.1 mA0.7 mA

12 mA 8 mA

Painful, but voluntary muscle control maintained

Men = 62 mAWomen = 41 mA

9 mA6 mA

55 mA37 mA

Painful, unable to let go of wires

Men = 76 mAWomen = 51 mA

16 mA10.5 mA

75 mA50 mA

Severe pain, difficulty breathing

Men = 90 mAWomen = 60 mA

23 mA15 mA

94 mA63 mA

Possible heart fibrillation after 3 seconds

Men = 500 mAWomen = 500 mA

100 mA100 mA

shock• “Let go” current – above this you

can’t let go (muscles contract)

• Interestingly, domestic AC happens to the most dangerous

• Also body impedance is lower

• Resistance! Your skin resistance is > 100kOhm dry, but < 1 kOhm wet!

C. F. Dalziel, Electric Shock Hazard. IEEE Spectrum (1972)

S o w h a t t o d o ?Working with 24 VDC, our biggest risk is short-circuit -> overheating -> burns or fire.• Watch out for leads• Turn things off before working on

them• Don’t wear jewelry / watches when

working with power systems

S o w h a t t o d o ? Use multimeter

correctly Watch out for correct

socket Only measure

resistance in de-energized circuit

Careful not to short out connections with the leads

http://www.allaboutcircuits.com/vol_1/chpt_3/9.html

W e a r t h e s e When soldering Working with

moving parts Using tools

O t h e r t i p s Working with the robot or robotic arm

know• Where it could move• How you will turn it off

Be aware of what’s on (and don’t forget) Don’t touch a hot iron Don’t try to catch a falling iron Keep workspace tidy Report incidents

NFPA 70E Standard for Electrical Safety Requirements for Employee WorkplacesTable 2-1.3.4 Approach Boundaries to Live Parts for Shock Protection.

(All dimensions are distance from live part to employee.)  (1) (2) (3) (4) (5)

Limited Approach Boundary1 

Restricted Approach Boundary1

Nominal System Voltage Range,

Phase to Phase 

Exposed Movable

Conductor 

Exposed Fixed Circuit Part 

Includes Inadvertent

Movement Adder 

Prohibited Approach 

Boundary1  0 to 50  Not specified  Not specified  Not specified  Not specified 

51 to 300  10 ft 0 in.  3 ft 6 in.  Avoid contact  Avoid contact 

301 to 750  10 ft 0 in.  3 ft 6 in.  1 ft 0 in.  0 ft 1 in. 

751 to 15 kV  10 ft 0 in.  5 ft 0 in.  2 ft 2 in 0 ft 7 in. 

15.1 kV to 36 kV  10 ft 0 in.  6 ft 0 in.  2 ft 7 in.  0 ft 10 in

36.1 kV to 46 kV  10 ft 0 in.  8 ft 0 in.  2 ft 9 in.  1 ft 5 in. 

46.1 kV to 72.5 kV 

10 ft 0 in.  8 ft 0 in.  3 ft 3 in.  2 ft 1 in. 

72.6 kV to 121 kV  10 ft 0 in.  8 ft 0 in.  3 ft 2 in.  2 ft 8 in. 

138 kV to 145 kV  11 ft 0 in.  10 ft 0 in.  3 ft 7 in.  3 ft 1 in. 

161 kV to 169 kV  11 ft 0 in.  11 ft 8 in 4 ft 0 in.  3 ft 6 in. 

EFCOG Electrical Safety Task Group

Working on or NearExposed Energized Parts

The Flash Protection Boundary is 4 ft. for systems 600 volts and below unless calculated otherwise under engineering supervision.

EFCOG Electrical Safety Task Group

Temperature And Heat Data• Curable burn temperature (1/10sec)

145 °F

• Cell death temperature (1/10 sec)

205 °F

• Temperature at arc terminals

35,000 °F

• Temperature of suns surface

9,000 °F

• Temperature of burning clothing 1,400

°F

• Clothing ignition temperature 700 to

1,400 °F

• Temperature of metal droplets 1,800

°F

Working on or NearExposed Energized Parts

Unqualified Persons, Safe Approach Distance:

Unqualified persons must not cross the Flash Protection Boundary unless they are wearing the appropriate PPE and under the supervision of a qualified person.

Electrical Burns vs Occupational burns OSHA - 1996

Type of Burn % of Total % of

Permanently Disabled

Scald 38 6.5 Flame 30 10.0

Tar 16 14.3 Electrical 8 86.7 Contact 4 25.0

Chemical 4 50.0

Consequences of an Arc-Flash Incident

Consequences of an Arc-Flash Incident

Review the list of items shown in slide.In this case the worker wasn’t shocked he was caught in an Arc Flash.

Electricity is a part of our daily lives, harness its power but recognize the hazards.