iosh msqp electricity

IOSH MANAGING SAFELY Preventing Accidents & Incidents in the Oil & Gas Sector

in partnership with

TOBY CLARK BSc FIOSH AIEMA MCot Cert Ed

IOSH MANAGING SAFELY

MODULE 2

Electrical Safety

Electricity

To enable electricity to flow there needs to be a complete circuit between two terminals normally made by using wires known as conductors. Voltage pushes the electricity through the wires.

Current is the flow of electrons through a conductor. The faster the current, the more power is generated

Resistance and Impedance stop the current from flowing too quickly

Ohm’s Law: Voltage (V) = current (I) multiplied by the circuit impedance (Z)

OHM’S LAW

VOLTS ( V )

AMPS ( I ) RESISTANCE ( R )

V = I x R

I = V / R

R = I / V

Basic Electrical Circuitry

• Circuits

• Earthing

• Direct current

• Alternating current

DC supply from battery

Resistance or load

Dangers of Electricity

Effects on the bodyELECTRIC SHOCK resulting from the flow of

electricity through nerves, muscles and organs causing:

• Muscular contractions• Respiratory failure• Fibrillation of the heart• Cardiac arrest• Internal burns

Electricity at Work Regs. 1989

Defines causes of injury as:1 Shock2 Burns3 Arcs4 Fires of electrical origin5 Explosions of electrical originDoes not include consequential injury e.g. falls resulting

from shock

Common voltages

• 12V, 24V, 36V battery powered equipment

• 110V (55V centre tapped) site equipment

• 230V single phase mains supply

• 415V 3-phase supply – greater power

• 700 – 1000V third rail supply

• 25KV overhead catenary railway supply

• 11kV – 400kV electricity Grid

Pole-mounted transformer

Grid substation

Transformer – Doha City

Incorrectly sized enclosure!

Padlocked!

Safe by position


The severity of electric shock is proportional to current flow and depends on:

conductivity (opposite of resistance) • voltage • duration • state of the point of contact with the body,

particularly moisture content– Also: whether direct or alternating current (and it’s

frequency)


• Direct shock – contact with a charged or energised conductor that is intended to be so charged or energised, i.e. the installation is in its normal condition

• Indirect shock – contact with a conductor that is normally at a safe potential but has become dangerously live through a fault condition.

Fault current to earth via body

Flow across chest involves breathing muscles and cardiac arrest

Earth-fault loop

Earth return pathway

Class 1 metal bodied equipment


Effects of current flowing in the human bodyCurrent Length of time Likely effects0.1(mA) not critical Threshold of feeling.

Undetected by person1-15(mA) not critical Threshold of cramp.

Can’t let go15-30(mA) minutes cramplike pulling

together of arms, breathing difficult.Limit of tolerance

30-50(mA) seconds to Strong cramplike effects, loss of minutes consciousness due to restricted

breathing. Cardiac fibrillation


Current Length of time Likely effects

50-500(mA) <one heart No fibrillation. Strong

period (750mS) shock effects

>one heart period Fibrillation. Loss of

consciousness. Burn

marks

Over 500 <one heart period Fibrillation. Loss of

consciousness. Burn

marks


Secondary Effects of Electric Shock

• Involuntary muscular reaction may cause strained muscles/torn ligaments

• Falls

• If working at height, fall can be fatal


Electrical Burns

Burns are likely to be most severe at locations along the path of the current where the resistance is greatest. Burns usually occur on the surface of the skin at points of contact i.e where the current enters and exits the body, but high currents can create internal burns which cause damage to red blood cells and muscle tissue. Deep seated and slow to heal

Entry wound

Exit wounds

First Aid

• Quick medical attention

• Disconnect current

• Rescuer must not make contact with anything that is live

• Artificial respiration until qualified medical help arrives

• Personnel trained in resuscitation

Fires

Fires of electrical origin can be caused in several ways:

• Leakage of current due to poor or inadequate insulation, poor connections

• Overheating of electrical equipment• Incorrect fuse rating• Overheating of flammable materials too close to

electrical equipment• Mechanical damage eg adaptors damaging sockets

Heating Effect of an Electric Current

H = I2 x R

Heating effect in Watts

Square of current flow

Resistance

Note also: Watts = Volts x Amps (W = V x I)

High resistance due to:

a) generally under-rated equipment

b) Local hotspots – dirty connections, frayed, cut or stretched wires etc.

Switchrooms should never be used as stores. Particularly

not for flammables!

Explosion

• Ignition of a flammable substance such as gas, dust, liquid of vapour by an electric arc, spark or hot surface

• Explosion of electrical equipment due to excessive currents or prolonged internal arcing faults

Inductive Loop Heating

Uncoiled electrical cables generate heat within themselves and may melt down under load.

Cable reels should always be fully run out

Cables to appliances, especially cookers and washing machines should be uncoiled and run tidily

Electric Arcs

• An electric arc is a high temperature electrical discharge between two electrodes in close proximity. The energy released is in the form of heat and light. This extremely high temperature can result in the conductor melting. If this phenomenon is controlled it has great benefits for industry i.e. electric arc welding. However, uncontrolled it can lead to molten metal being discharged or the light (UV radiation) produced can cause burns and ‘arc eye’

We don’t know which picture is before and which is after!

Safe operation near overhead power lines

Safe by position?

‘Dial before you Dig’ – cable plan available free of charge from

electricity Board

Gas plan contact Transco

Cable detecting equipment – CAT Scanner

sometimes used with a portable signal generator – ‘Cat & Jenny’

Portable appliance labelling system to conform with electricity at Work Regs. 1989

Bar code system identifies appliance

Selection of suitable equipment

Evaluation of suitability of construction of electrical systems should consider:-

• The manufacturer’s recommendations• The likely load and fault conditions• The probable use of the system• The need for suitable electrical protection

devices• The environmental conditions

Dodgy plug connection

Domestic standard equipment

240V equipment

Jet washing – safe system of work?

Protective Systems

• Fuse – weak link (low resistance) in a circuit which by design overheats and melts if the current exceeds the safe limit thereby cutting off the power supply. Protects the equipment

• Fuse rating is calculated by: Amps = Power Volts• Too slow to protect people• Easy to replace with wrong rating• Needs a tool to replace• Easy to override

Cartridge fuses for 3 amp plugs

Sealed safety plug

Fuse rating should be matched to the

appliance

Blown fuseIntact fuse

Protective Systems

• Circuit breaker – mechanical device in the form of a switch which automatically opens if the circuit is overloaded. Protects the equipment

Miniature Circuit Breaker (MCB)

Advantages over a fuse:

1 More reliable

2 Resettable

Protective Systems

• Reduced Voltages used in harsh environments such as construction sites 110 volt centre-tapped transformer is recommended

• The maximum shock voltage to earth is 55 volts• Lower voltage systems called Safety Extra Low

Voltage (SELV) are those in which the voltage does not exceed 50 volts - used in swimming pools/vehicle washing areas

Transformer from 240V to 110V (Centre tapped at 55V). Note robust water and dirt resistant connections

Protective systems

Battery Powered

• The common method is to use a rechargeable battery to power the equipment. It eliminates the need for a cable to feed power to the equipment and gives a greater flexibility of use for the user

Cordless drills operating at 24V or 36V are powerful and safe

Charger unit

Battery pack

Why are these situations especially dangerous?

Residual current device protecting high risk equipment

RCD is a sensitive trip device that limits shock current to 30mA for 30mS

Class 1 (metal bodied) equipment

Relies on integrity of earth connection for safety

Class 2 all insulated, double insulated equipment

Relies on insulation of the casing for safety.

No earth wire is needed

Protective Systems

Double Insulation

• Equipment which is double insulated will carry this symbol:

Equipotential bonding of water pipes

All conductive surfaces e.g. stainless steel sinktops are bonded together and connected as a ring main to the earthing point of the transformer – so that they can never become electrically live

Protective Systems

Insulation/Enclosure of Live Parts (IP Rating)

Enclosure suitable for the environmentNot allow ingress of water, swarf or dustEquipment is classified by the degree of

protection – the greater the numeric value, the greater the degree of protection

Static Electricity

• Energy needed to ignite petrol vapour = 0.02mJ

• Energy in a clothing-generated static spark = 0.2mJ

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