electrical safety 2011

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1.1. ELECTRICALELECTRICAL

SAFETYSAFETYPresenter: A. Zengeya, NSSAPresenter: A. Zengeya, NSSA--GweruGweru


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Electrical HazardsElectrical Hazards

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Electrical HazardsElectrical Hazards

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2.2. INTRODUCTIONINTRODUCTION Electrical safety encompasses among

other aspects; design, selection,

erection, operation, maintenance,inspection and testing of electricalequipment/installations.

The objective of this presentation is to

highlighting the basic measures thatare very vital in protecting persons andproperty against the hazards that mayarise from the use of electricity.


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3.3. VOCABULARYVOCABULARY

Electricity: A form of energy passing from onepoint to the other in the form of an electriccurrent

Voltage: Electrical pressure

Current: The quantity of electrons contained in theflow of energy

Direct Current: Current which flows in one

direction only.Alternating current: current where the

direction of flow alters at regular intervals

Conductor: Substance with low resistance to the

flow of electricity


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4.4. ELECTRICAL CIRCUITELECTRICAL CIRCUIT

A=Source of electrical current e.g. battery or

generator C=Line/phase conductors conductors

R=Appliance/Machine/Equipment

v=Voltage

i=Current


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5.5. DANGERS OFDANGERS OF

ELECTRICITYELECTRICITY Contact burnsArc burns Radiation burns Sparks and arcing Shock/Electrocution

Eye injuries caused by uv-radiationfrom arcs Fire


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Dangerous Contact

Dangerous ContactStandards and regulations distinguishtwo kinds of dangerous contact,

Direct contact

Indirect contact

and corresponding protective

measures

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Direct ContactDirect Contact

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Indirect ContactIndirect Contact

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6.6. CONCEPTS OFCONCEPTS OF

ELECTRICAL SAFETYELECTRICAL SAFETY Electrical safety is primarily concerned

with protecting persons from electric shock

and other dangers highlighted above. The principal protective measures of

achieving the aforesaid are protectionagainst direct contact and protection

against indirect contact. The working environment also has a major

impact on electrical safety.


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Protection Against ElectricProtection Against Electric

ShockShock The fundamental rule of protection is

provided by the document IEC 61140 which

covers both electrical installations andelectrical equipment.

Hazardous-live-parts shall not beaccessible and accessible conductive

parts shall not be hazardous. This requirement needs to apply under

Normal conditions & under a single faultcondition

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a)a) PROTECTION AGAINSTPROTECTION AGAINST

DIRECT CONTACTDIRECT CONTACT Proper insulation of live parts

Protection by obstacles to prevent

unintentional bodily approach/contact

Protection by barriers/enclosures providingthe relevant degree of protection

Protection by position i.e. placing out ofreach


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Protection Against DirectProtection Against Direct

Contact( Insulation)Contact( Insulation)

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Protection Against DirectProtection Against Direct

Contact (Enclosures)Contact (Enclosures)

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Additional Measures AgainstAdditional Measures Against

Direct ContactDirect Contact

In order to protect users in such

circumstances, highly sensitive fast trippingdevices, based on the detection of residualcurrents to earth (which may or may not bethrough a human being or animal) are usedto disconnect the power supplyautomatically, and with sufficient rapidity toprevent injury to, or death by electrocution,of a normally healthy human being

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Additional Measures AgainstAdditional Measures Against

Direct ContactDirect ContactAn additional measureof protection against

the hazards of directcontact is provided bythe use of residualcurrent operatingdevice, which operate

at 30 mA or less, andare referred to asRCDs of highsensitivity

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b)b) Protection AgainstProtection Against

Indirect ContactIndirect Contact Protection by earthed equipotential

bonding and automatic disconnection

(0.4s) of supply

Protection by use of double-insulated(class II) equipment.

Protection by non conducting location

Protection by earth-free localequipotential bonding.

Protection by electrical separation


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Automatic Disconnection Of AAutomatic Disconnection Of A

TT SystemTT System

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Class II EquipmentClass II Equipment

No conductive partsof a class II appliance

must be connected toa protectiveconductor:

A simple example isthat of drawing acable into a PVCconduit.

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EarthEarth--FreeFree EquipotentialEquipotential

BondingBonding

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Electrical SeparationElectrical Separation-- ShaverShaver

Socket OutletSocket Outlet

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Protection By InterpositionProtection By Interposition

Of ObstaclesOf Obstacles measure can only

be applied in a dry

location

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c)c) EarthingEarthing

Connect all extraneous conductive parts ofthe premises to the main earthing terminalof the electrical installation

Extraneous conductive parts includewater/gas/service pipes, ducting, heatingand air conditioning equipment, metallicstructural parts, etc.

Protection achieved by use of overcurrent

protective devices i.e. fuses, circuitbreakers, residual current devices.

Maintenance of earth continuity is veryvital.


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EarthingEarthing ConnectionsConnections

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SupplementarySupplementary

EquipotentialEquipotential EarthingEarthing

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Reduced VoltageReduced Voltage Most commonly used is the 110 volt centre

point earthed system. The centre point of

the secondary winding of the transformerproviding the 110 volt supply is centretapped to earth.

At no point of the 110 volt circuit can the

voltage to earth exceed55

volts. Can be provided by means of small

portable and mobile generator sets.


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Extra Low VoltageExtra Low Voltage Normally not exceeding 50 V a.c. or120 V

ripple free d.c.

SELV(50V) for swimming pools &amusement parks

FELV(=


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Unsafe Acts In The HouseUnsafe Acts In The House Replacing fuses with wire or cigarette

aluminum foils. Tieing or holding switches that keep

tripping, to maintain them in the on position. Handling/using electrical appliances while

bathing/wet. Relying on cheaper bush electricians for

repairs.

Unplugging appliances by pulling at thecable.

Supplying power to several appliances usingone plug.


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Safe Acts In The HouseSafe Acts In The House

Switch off appliances not being used and plugthe socket off

Use approved adapters.

Ensure cables are of the right size andlength. When entering a flooded room, be sure the

water is not in contact with a source ofelectricity.

Disconnect appliance cords at the walloutlets and not at the appliances.

Whenever in doubt, switch off.


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PORTABLE ELECTRICPORTABLE ELECTRIC

TOOLS/EQUIPMENTTOOLS/EQUIPMENT They should be provided with contact

voltage protection as highlighted above. Residual Current Devices (Earth Leakage

Protective Devices) can give additionalprotection where tools and equipment areused at mains voltage.

Provision of an isolator within 20 metres of

the worker using the tool. Damaged cables should be

replaced/repaired without delay. Routine inspection and maintenance.


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MAINTENACE OFMAINTENACE OF

ELECTRICAL MACHINERYELECTRICAL MACHINERY Switch off power to the machine in question. Lock off the switch wherever a locking-off

device is fitted.

Place a danger notice on the switch. Check/confirm that the switches chosen

were the right ones. Before switching on, ensure no person is in

the danger zone. Never wedge, tie or override electrical

switch gear. Report immediately any faults & provide

safety measure until fault is rectified.


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PERMIT TO WORK SYSTEMSPERMIT TO WORK SYSTEMSThis should incorporate the following clearly

defined stages;

Comprehensive assessment of the work tobe done.

Withdrawal of plant/equipment fromservice.

Physical, electrical and/or mechanicalisolation of the plant.

Completion of work and return to service.


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STATIC ELECTRICITYSTATIC ELECTRICITYGeneration

Generated when two dissimilar surfacescome together and are separated.

Insulators such as plastics are responsiblefor trapping the charges.

The electric field so created is mostly anuisance.

If sufficiently high charges develop, they cancause electric shocks or spark ignition offlammable gases.


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Static Electricity contdStatic Electricity contdSafety Measures

Earthing

Earthing of persons

Using vessels or apparatus made ofconductive materials.

Electrostatic eliminators.


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LIGH

TNINGLIGH

TNINGCreation Lightning charges are build up in

thunderstorm clouds Lightning itself is in actual fact the

electrical discharge which takes placebetween these two opposite charges.

Flashes of discharge may be within acloud, from cloud-to-cloud or from cloud-to-ground.


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Lightning contdLightning contdEffect on Humans can be

Fatal

electric shock

Burns

Tearing of the flesh due to the

explosive action of discharge.


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Lightning contdLightning contdOn hearing the rumble of thunder, persons

should seek shelter under cover listedhereunder in order of priority:-

Reinforced concrete buildings, steel framedstructures, metal-roofed buildings, buildingswith lightning conductors.

Buildings with continuously bonded metal

gutters and down-pipers. Large unprotected buildings. Cluster of trees or wood.

Large caves.


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Lightning Back CurrentLightning Back Current

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Lightning ProtectionLightning Protection

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ELECTRICAL SHOCK,ELECTRICAL SHOCK,

ELECTROCUTIONELECTROCUTION

The human body acts as a volume

conductor and hence offers littleelectrical resistance. Most of theresistance is in fact in the skin. It isquite essential to note that the let

go current of a conductor held in thehand is usually not more than 1mA.Currents above this letter are thereforequite hazardous if not deadly.


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Electrical Shock,Electrical Shock,

Electrocution contdElectrocution contd

Seriousness of Electric Shock

This is dependent on; Body Electrical Resistance Path of current flow through bodyAmount of current and duration of current

flowAmount of current and duration of current

flow


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Electrical Shock,Electrical Shock,

Electrocution contdElectrocution contd

Concepts of Electrical Shock

Asphyxia

Respiratory Arrest

Ventricular Fibrillation


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Typical Responses toTypical Responses to

Current/voltageCurrent/voltageVoltage Response Current

15 V Threshold of feeling 0.002-0.005A

20-25V Threshold of pain -

30V Muscular spasm(non-release)

0.015A

70V Minimum for death 0.1A

120V Maximum for safety 0.002A

200-240V Most serious/fatal

accidents

0.2A


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PPC/EPPC/E FOR ELECTRICALFOR ELECTRICAL

WORKWORK

This shall include among others;

Insulated tools Insulating material for covering live parts of

equipment and floors at fixed electricalmachinery/equipment

Rubber gloves and rubber boots. Non conductive head gear


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FIRSTFIRST--AID RULESAID RULES Make sure its safe to approach by switching

off current wherever possible. If switch is not readily accessible then pull

off victim using a suitable length of dry non-conducting material

Check for breathing If victim is breathing, place him/her in a

recovery position and call for medicalattention. If not then render artificial respiration


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SummarySummaryProtective measures are mostly preventive,but experience has shown that they are notinfallible due to among others;

Lack of proper maintenance Imprudence, carelessness

Normal (or abnormal) wear and tear

Accidental contact Immersion in water, etc. A situation in

which insulation is no longer effective

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Summary contdSummary contd

The fundamental principle of electrical safetyis to use as low voltages as possible

Where the use of extra low voltage is notpracticable, abundant methods of renderingthe equipment safe, do exist.

Electrical accidents are minimal but fatalities

are highly likely. If corectly used, electricity is one of the

safest forms of energy available to man.

electrical safety 2011

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