Electrical Safety

http://www.cityu.edu.hk/fmo/

1. Electrical Faults

2. Electricity Hazards

3. Lines of Defence Against Electric Shock

4. Portable Appliances

5. Electrical System Design and Installation

6. Maintenance

7. Safe Working Procedures

8. References Diagrams

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Mr. C.W. Yu

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1. Electrical Faults

1.1 Overload

load applied greater than the design value of the circuit

1.2 Short Circuit

due to cable fault or external damage to the wiring system

1.3 Earth Faults

short circuit or low impedance between phase and the protective or earth system

 

2. Electricity Hazards

2.1 Electric Shock

- is the physical stimulation that occurs when electric current passes through the body

- the effect depends on:

    a. the magnitude of the current

    b. the body parts through which the current flows

    c. duration

    d. physical condition of the person being shocked

2.1.1 Nervous System of Human Body

- controls all movements, both conscious and unconscious

-the signals are electro-chemical in nature, with levels of a few millivolts

2.1.2 Electrical Impedance of Human Body

- human body is composed largely of water, and has very low resistance

- most of the resistance to the passage of current through the human body is at the points of entry and exit through the skin

- internal impedance - depends on:

    a. the length and cross sectional area of the path

    b. conductivity of the tissues in the path

- skin impedance - depends on:

    a. surface area of contact

    b. pressure of contact

    c. degree of moisture on the skin

    d. applied voltage (at high voltage, skin breaks down)

    e. duration of current flow (the flow of current cause the victim to sweat, reducing

        the resistance very quickly after the shock commences)

2.1.3 Threshold of Perception

- is the minimum value of current which results in a sensation for person in contact with the touch voltage

- usually assumed as 0.5 mA

2.1.4 Electric Shock

- with increasing current, the sensations of tingling will result in contractions of muscles and finally lost voluntary control

- the typical 50Hz AC let-go thresholds for men is 15 mA

- current exceeding the "Let-go Current" flow through the chest, head or nerve centres controlling respiration may produce respiratory inhibition

- further increase in current will disturb the normal rhythmic expansion and contraction of the heart muscles

- very high current will cause severe burning

- the protective devices should cut off the supply to prevent shock. Socket outlet circuits should be cut off within 0.4 s and fixed equipment within 5 s

2.2 Indirect Injuries

Accidents may be caused by involuntary muscular movement. For example:

    a. falling from a ladder due to loss of balance

    b. thrown away by powerful muscular contractions

2.3 Burns due to Electric Faults and Arcing

2.3.1 Low Voltage Contact Burns

- due to heating effect of electric current passing through the body tissues

- burns destroy the protective resistance of the epidermis, thereby permitting greater currents to flow

2.3.2 High Voltage Burns

- the capacitance layer represented by the skin is punctured by the high voltage and significantly lower the body resistance

- the current increase proportionally with the touch voltage and the heating effect increases as the square of the current, result in burns along the current path and damages to internal body organs>

- severe burning effects occur when current level is 4-5 Amps

2.3.3 Radiation Burns

- result from high temperature produced by electric arc, vaporized metals, hot gases released by the arc, overheated conductors, etc.

- typical electric arc temperature is 8000 - 10000 oC (combustible flame temperature is about 1000 oC). It produces intense radiation fields which have a peak in the short wavelength (ultraviolet) region

2.4 Electrical arcing

- take place when current flows through air or insulation between conductors at different potentials

- air path becomes conducting due to ionization of the gas (air)

- molten metallic particles (about 2000 oC) will result in serious burns

- release large quantities of energy in the ultraviolet short wavelength region

2.5 Fires of electrical origin

- overheating of conductors and/or adjacent flammable materials

- ignition of flammable materials as a result of electric arcing, current leaking or scattering of hot particles due to explosion

2.5.1 Fire due to short circuit

- a short circuit current of 100 times the normal current value will increase the heat dissipation by 10,000 times (power = I2R). If the current not being cleared quickly, the temperature of the conductors will rise and set firewpe3.jpg (4140 bytes)

- the normal short circuit current will be high enough to operate the protective devices

- under high resistance short, the current may be less than the normal load and therefore too low to operate the protective devices. The fault will persist and may cause arcing and ignition of the insulation system

2.6 Explosions

- when an electric arc occurs, it superheats the air instantaneously, this causes a rapid expansion of the air with a wavefront that can reach pressures of 100 to 200 lb/ft2 which is sufficient to explode switchgear, turn sheet metals into bullets, push over concrete walls and blow molten metal at extremely high velocities

 

3. Lines of Defence Against Electric Shock

Three lines of defence in a typical installation :

    a. The current carrying live circuit is insulated ( by functional insulation)

    b. The appliance frame is either

        (i) earthed - to limit the touch voltage in the event of fault, the low impedance

                earth will normally cause protective device to operate; or

        (ii) double insulated - which provide a protective insulation as a backup in the

                event of failure of the functional insulation

    c. Resistance of footwear and floor coverings will increase the overall shock path

        resistance.

3.1 Examples of Failure of Defence Systems

    a. earth wire in an appliance cord broken or disconnected

    b. cross connected extension cords

    c. high resistance electrical system earth

    d. presence of moisture

3.2 Prevention of Electric Shock

Prevent or limit shock by:

    a. stop a dangerous potential difference from being applied across the body

    b. increase the resistance of the current path

Basically there are three catagories of protection:

    a. protection against direct contact

    b. protection against indirect contact

    c. switching off and isolation

3.3 Direct and Indirect Contact

3.3.1 Direct contact

- contact with a conductor which forms part of a circuit and would be expected to be live

3.3.2 Indirect contact with exposed conductive part

- contact with a part of the electrical installation which would not normally be expected to be live, but has become so as the result of a fault. e.g. metallic casing of the appliance

3.3.3 Indirect contact with extraneous conductive partwpe4.jpg (5678 bytes)

- contact with a conducting part which is totally unconnected with the electrical installation, but which has become live as the result of a fault. e.g. window frames, shower curtain rails, water pipes.

3.4 Protection Against Direct Contact

3.4.1 Protection by insulation of live parts

3.4.2 Protection by barriers or enclosures

- select equipment with suitable degree of protection (IP) to withstand the ingress of solid objects and liquid

3.4.3 Protection by residual current devices (RCD)

3.5 Protection Against Indirect Contact

3.5.2 Protection by Earthed Equipotential Bonding and Rapid Isolation of Supply

- The earth connect the metallic components of the equipment or system which are not normally part of the current carrying electrical conductor system. It provides a low resistance path back to the power source

- Equipotential bonding involves connecting together all non-current carrying metalwork to form a zone within which it is not possible for exposed metalwork to be at different voltage levels which could cause a shock (i.e. to create an earthed equipotential zone)

- Earthing provides two safety functions:

    a. the low resistance limit the touch voltage of bare metal which may become live

    if there is an insulation failure.

    b. it provides a reference for fault current. The low and stable earth resistance

    generate high fault current which will operate the protective device quickly.

3.5.3 Protection by Double Insulation

- Human error and mechanical failure of earthing cannot occur if there is no earth

- Remove the earth and replace by an added layer of insulation, i.e. a functional layer plus a protective layer of insulation

- Double insulated appliance has no earth connection

3.5.4 Protection by Separation (Isolation Transformer)

- Electric shock require two conditions:

    a. a current path flow back to the power source via the body; and

    b. the voltage is high enough to drive the current

- By using an isolating transformer with both primary and secondary windings well insulated, the direct current path from live conductor to the power source is cut off

3.5.5 Use of Extra-low Voltage

- Use central tap earthed step down transformer to limit the supply voltage. e.g. a 110V transformer will has 55V live to earth voltage

- Need to use double pole switches and fusing in both live poles of the supply.

- In wet environments use a maximum of 25 volts

- has limitations on appliance operation

3.5.6 Protection by Residual Current Devices (RCD)

The RCD continuously compare the current in the live with that in the neutral conductors. It detects the residual current and trip the circuit breaker if the residual current exceeds a certain value (commonly used RCD trip at 30 mA within 0.4 sec)

It does not prevent the person getting a shock, nor does it limit the magnitude of the current, it only limit the duration of the shockwpe5.jpg (3773 bytes)

There will always be some residual current in the insulation resistance and capacitance to earth. In a healthy circuit such current will be low, seldom exceeding 2 mA

Some electronic equipment may have suppression capacitors between live and earth and discharge a transient current to earth in the event of a voltage impulse being imposed on the mains voltage. This will result in unwanted tripping

3.6 Switching Off and Isolation

-A basic safety requirement for maintenance and modification to an electrical installation

Carelessness is the prime cause for accidents:

    a. Not checking that the installation is electrically dead

    b. Inadvertent energisation by other personnel

    c. Failure to discharge energy storage capacitors

    d. Inadvertently knocking a control handle to start equipment

    e. Dropping of tools onto bare, live busbars

To avoid these accidents, we need to switch off and isolate the portion of installation and equipment under work. An isolator must be:

- in an accessible position

- clearly marked the controlled circuits and systems

- not possible for unauthorized or unexpected reclosing

- be lockable

- completely separating the system or circuit from the source of supply

 

4. Portable Applianceswpe6.jpg (7773 bytes)

More than 25% of electrical accidents at work involve portable electrical appliances. The appliances should be inspected and tested periodically. The frequency depends on the type of appliance and its usage.

4.1 Inspection of Portable Appliances

Look for :

    a. damaged flexible cords with exposed live cables or failure of protective

        conductor

    b. damage to plugs

    c. damage to appliance and exposed live conductors

    d. failure of cord grip

    e. wrong connection

    f. other visible damage

    g. blocked ventilation for equipment

    h. adjacent flammable materials

    i. sign of equipment overheating

    j. ingress of foreign materials

4.2 Protection of Shock

4.2.1 By insulation

- enclose the live electrical parts

4.2.2 By earthingwpe7.jpg (3931 bytes)

- the exposed conductive parts of the appliance are connected to earth so that in the event of their becoming live due to a fault, current will flow to earth

- the impedance must be low so that a high current flows to open the protective circuit quickly

- The IEE Wiring Regulations require that in the event of an earth fault, a circuit feeding socket outlet should be disconnected by its protection within 0.4 s, as opposed to fixed appliances which may take up to 5 s to disconnect

4.3 Class of Protection

4.3.1 Class 1 protection

- appliances uses insulation plus earthing

4.3.2 Class 2 protection

- appliances uses double insulation

4.4 Safety Tests for Portable Appliances

Safety tests differ depending on the class of protection. The following basic tests should be carried out periodically:

    a. visual inspection

    b. earth continuity test

    c. high voltage insulation test

Test can be carried out by employing Portable Appliance Tester (PAT).

 

5. Electrical System Design and Installation

The designer must assess:

a. maximum demand

total current if all loads were simultaneously connected

b. diversity

-balance between financial saving and effectiveness of the installation

c. correct size and types of cables

- suitable cable size to limit temperature rise

- apply derating factors for thermal insulation, grouping an ambient temperature

d. correct rating of switches and protective devices

- cut off current before the cable temperature becomes too high under fault conditions

- capable of clearing the prospective short circuit current

e. hazardous or adverse conditions application

- outdoor, rain, high relative humidity, change in temperature

- mechanical damage

- dirts collected (affect heat dissipation)

- combustible dust (cause fire hazard)

- avoid contact between dis-similar metals which may result in electrolytic corrosion under high relative humidity conditions

- flammable materials

- use equipment with suitable index of protection (IP)

f. protection against excess current

-it takes time for a protective device to operate. e.g. a 80A cartridge fuse to BS 1361 take more than 300s to open when carrying a current of 200A, during this time the excess current will flow without hindrance from the protection system

- during the 0.4s disconnection time for socket outlets, the user of the faulty equipment is subject to full supply voltage

g. special need of users

 

6. Maintenance

The installation may deteriorate due to normal ageing, may have substandard additions installed, or may be damaged.

Preventive maintenance is required to:

    a. ensure the initial safety level of a system is maintained

    b. early detection of faults or deterioration of system performance

    c. proper keeping of test records allow comparison of results and performance

 

7. Safe Working Procedureswpe8.jpg (5875 bytes)

7.1 Working Dead

    a. switch off

    b. isolate

    c. post warning notices

        -prevent reclose

        -inform other people to stay away from the work

    d. lock off

    e. test

    f. earth

    -prevent shocks if circuits have been charged

7.2 Live Working

precautions to be observed:

    a. only involve fully trained and competent persons

    b. use suitable equipment, protective clothing and insulated tools

    c. gather information concerning the task and the system

    d. provided with insulating screens and barriers

    e. provide notices with details of emergency resuscitation

    f. the working area must be properly controlled

    g. never work alone, should be accompanied by at least one other, who will be

        able to apply rescue and resuscitation techniques or to call for help in case of

        accident happened.

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References

1. Electrical Safety: Personnel Hazards, Fire and Explosion Risks. By Trevor Blackburn & Colin Grantham.

2. IEE Wiring Regulations, 16th Edition.

3. Electrical Safety Handbook. By John Cadick.

4. Electrical Hazards and Accidents, Their Cause and Prevention. Edited by E.K. Greenwald.

5. Electrical Installations in Hazardous Locations. By Peter J. Schram and Mark W. Earley.

6. Protection Against Electric Shock. Guidance Note GS 27 from the Health and Safety Executive.

7. Practical Electrical Safety. By D.C. Winburn .

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Index of Protection (IP) Code

FIRST NUMERAL

SECOND NUMERAL

(a) Protection of persons against contact with live or moving parts inside enclosure Protection of equipment against ingress of water
(b) Protection of equipment against ingress of 'solid bodies
No./SYMBOL DEGREE OF PROTECTION No./SYMBOL   
0 (a) No Protection. 0 No Protection

(b) No Protection.

1

(a) Protection against accidental or inadvertent contact by a large surface of the body, e.g. hand, but not against deliberate access.

1

Protection against drops of water. Drops of water falling on enclosure shall have no harmful effect

(b) Protection against ingress of large solid objects less than 50mm diameter.

2

(a) Protection against contact by standard finger.

2

Drip Proof: - 
Protection against drops of liquid. Drops of falling liquid small have no harmful effect, when the enclosure is tilted at any angle up to 150 from the vertical.

(b) Protection against ingress of medium size bodies less than 12m m diameter and less than 80mm length.

3

(a) Protection against contacts by tools, wires or suchlike more than 2.5mm trick.

3

Rain Proof. - 
Water falling as rain at any angle up to 60o from vertical shall have no effect

(b) Protection against ingress of small solid bodies.

4

(a) As 3 above but against contact by tools, wires or the like, more than 10mm trick.

4

Splash Proof: - 
Water splashed from any direction shall have no harmful effect

(b) Protection against ingress of small foreign bodies.

5

(a) Complete protection against contact.

5

Jet Proof: -  
Water projected from a nozzle from any direction (under rated conditions) shall no harmful effect

(b) DUSTPROOF:- -Protection against harmful deposits of dust,dust may enter but not in amount sufficient to interfere with satisfactory operation.

6

(a) Complete protection against contact.

6

Watertight Equipment - 
Protection against conditions on ships' decks, et--. Water from heavy seas or power jets shall not enter the enclosures under prescribed conditions

(b) DUST TIGHT: - Protection against ingress of dust.

IP CODE NOTES

The degree of protection is rated in form IPXX  
 
Protection against contact or ingress of water respectively is specified by replacing far, or second X digit number tabled, e.g. IP2X defines an enclosure giving protection against finger contact but without any specific protection against, ingress of water or liquid.

7

Protection Against Immersion in Water:-  
It shall not be possible for water to enter the enclosure under stated condition of pressure and time

8

Protection Against Indefinite Immersion in Water Under Specified Pressure:-  
It shall not be possible for water to enter the enclosure.

Note: Use this table for General Guidance only - refer to BS5490 for information on degrees of protection offered by enclosures


  

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