5.9 - electrical safety
TRANSCRIPT
Syllabus points there is an inherent danger involved with the use of electricity that
can be reduced by using various safety devices, including fuses, residual current devices (RCD), circuit breakers, earth wires and double insulation
Learning GoalsIdentify the causes of electric shockIdentify hazardous situations which could lead to an
electric shockDescribe safety precautions you could take to avoid
electric shocks in your everyday lifeRecall that current and time of contact determines
the amount of damage done to a personIdentify the colour codes on wires in plugs (e.g.
Green/yellow = earth)Identify and name safety devices used to prevent
electric shocks (e.g. fuse)Explain how a variety of electrical safety devices
work (e.g. fuse, RCD)
Hazards Two types of hazard associated with electricity:
Thermal hazard - excessive electric power causes undesired thermal effects, such as starting a fire in the wall of a house
Shock hazard - electric current passes through a person. Shocks range in severity from painful to lethal
Short circuit
Low resistance path between positive and negative terminals of a voltage source
Resistance is small so power dissipated is large, can cause things to melt, catch on fire
This wire provides a low resistance path between terminals. The current does not flow through the lamps in this case.
Shock hazard Which of these can injure or kill a person: current or
voltage?
How is a person harmed?
Shock Hazard Harm is done by current (flow of charge) through the body
Voltage is related to possible flow of current, energy available for the flow
Amount of current depends on the body’s resistance
Dry skin = high resistance
Wet skin = low resistance
Lower the resistance the higher the current, can be lethal
Shock hazards Most electrical shock fatalities occur because a current put the heart
into fibrillation (irregular heart beat)
Effects of electrical shock depend on:
Amount of current
Path taken by the current
Duration of the shock
Frequency of the current
Effect of Current on the body
Current (mA) Effect1 Threshold of sensation
5 Maximum harmless current
10 - 20 Muscles contract (squeeze)
50 Pain
100 – 300+ Fibrillation possible (often fatal)
300 Burns (depending on concentration of current)
6000 (6 A) Sustained ventricular contraction and respiratory paralysis, stops when shock ends. Used to defibrillate the heart.
Lichtenberg Figure People can survive being
struck by lightning
these marks are formed when capillaries beneath the skin burst due to the electrical discharge and they usually appear "within hours" of the strike though they tend to disappear within a few days.
From: http://geardiary.com/2011/06/17/meet-winston-kemp-lightning-strike-survivor-and-lichtenberg-figure-owner/
Preventing electrical shocks Unplug appliances completely when working on them/cleaning
them
Keep appliances dry, use dry hands (e.g. hairdryer)
If you suspect something is ‘live’ only touch it with the back of your hand. Then if your muscles contract you will pull away from the object instead of grabbing it
Use a qualified professional (electrician) to carry out repairs, maintenance
Fuse/circuit breaker If the current or resistance is too large wires
can overheat
Fuses/circuit breakers limit excessive currents (8-16 A)
Opens the circuit automatically when current is above safe limit
Works for small voltages/currents (e.g. in a household)
Doesn’t work for large voltages/currents as spark can jump through air
A fuse will not save someone from electrocution if they touch a live wire as 0.1A can kill
Wire in middle breaks when overheated. This creates a gap in the circuit.
House wiring In Australia the voltage supplied by mains
electricity is 240 V In your house there are three wires involved
in supplying power to appliances These are colour coded:
Active = brown Neutral = blue Earth = green/yellow
Active wire – carries current from the power station
Neutral wire – closes the circuit so current can flow
Earth wire – safety device, connected to conductive parts of appliance
Residual Current Device (RCD)
RCD’s have a switch which is intended to cut the power when the Active and Neutral currents are different and/or current flows in the Earth.
Safety switches can ‘trip’ with as little difference in currents between active and neutral as 30 mA.
This is below the ventricular fibrillation level for adults - the level at which the normal function of your heart is disturbed.
RCD switches that trip on 10 mA prevent muscular cramping. These are recommended for areas where young children play or are cared for.
ResourcesAV
Electrical Safety (23:00)
One Flash and Your Ash (15:00)
Further Reading
Build.com.au - How electrics are installed
UWA Electrical Engineering - Safety Devices Explained
WA Government - RCD Laws
Electrical Safety First - RCD’s Explained