School Science Lessons
Electrical hazards
2012-05-10 SPwp
Please send comments to: J.Elfick@uq.edu.au
Acknowledgement
Table of contents
1.0.0 Electrical hazards
1.0 Electrical fires
2.0 Electrical outlets and equipment
4.0 Electrical hazards, Electric shock
4.1.1 Effects of electric current passing through the human body
4.1.2 "Let-go" current
4.1.3 Causes of electric current passing through the human body
4.2.0 Safe practices and procedures
4.2.1 Power points and switches
4.2.2 Mains-operated equipment
4.2.3 Electrical leads
4.2.4 Electrical inspections
4.2.5 Reporting an accident
4.2.6 Isolating transformers [deleted]
4.2.7 Precautions to take when using an isolating transformer [deleted]
4.2.8 Core balance earth leakage devices
4.2.9 Precautions to take when using core balance earth leakage devices
4.2.10 Management of electric shock victims
1.0 Electrical fires
1. The electrical wiring in science laboratories and other activity areas should be adequately protected
against overloading by circuit breakers. Repairs should be done only by a qualified electrician.
2. Wiring should not be overloaded by using appliances or equipment whose current rating is greater than
that of the power outlet. General purpose outlets (GPOs) were usually rated at 10 amps. Some GPOs
may be rated at 15 amps.
3. The use of double adapters should be avoided. Portable power boards (multiple point power boards)
especially those with circuit breakers, should be used.
4. All power leads must be in good condition, of the appropriate current rating for the appliance(s) being
used, and of the minimum length necessary for practical use.
2.0 Electrical outlets and equipment
Electrical outlets
1. Permanent general purpose electrical outlets must be in a safe and operational condition.
2. New installations must be fitted with automatic blind outlets. For further information regarding electrical
safety refer to Section Commercial electrical equipment
3. The Regulatory Unit of the Department of Minerals and Energy (DME) (formerly the Queensland
Electricity Commission [QEC] now called the Queensland Electricity Industry [QEI] has set down
stringent guidelines for equipment operating on 240 volts (V).
4. Teachers are reminded that home-made 240V equipment would be unlikely to comply with legal
specifications.
5. All 240V electrical equipment should be serviced and checked by a qualified electrician.
6. Equipment suppliers must supply schools with equipment that complies with QEI specifications,
including appropriate earthing and use of approved insulating materials.
4.1.1 Effects of electric current passing through the human body
1. Breathing can stop
2. The heart can stop
3. Burns to the skin and internal organs
4. Muscle spasm
5. Clinical shock (a reaction of the body's nervous system to severe injury)
6. Falls may cause injuries
The danger to persons caused by an electric current passing through the human body depends upon the
value of the current. However, it is difficult to calculate what this current will be under any given set of
circumstances. The relationship of current to voltage is not linear because body resistance varies with the
touch voltage and with the path taken by the current.
4.1.2 "Let-go" current (Sometimes called the "cannot let-go" current)
This is defined as the maximum current a person can tolerate when holding an electrode and still let go of
this electrode using muscles directly stimulated by this current. Experimental evidence places the value of
this current at about 10 milliamperes. Higher currents would cause muscle freezing, preventing the person
from releasing his/her hand from the conductor.
4.1.3 Causes of electric current passing through the human body
An electric shock could be received from the following:
1. wall or bench outlets (power points),
2. switches,
3. mains operated equipment,
4. leads connecting equipment to power points,
5. extension leads.
In Queensland, the electricity reticulation system is MEN (Multiple Earth Neutral) i.e. the neutral wire is
earthed at the powerhouse, at the point of supply to each consumer and at other points along the line
between powerhouse and consumer.
If a person comes into contact with a live conductor connected to the mains, e.g. the active pin in a power
point or switch or an exposed active wire in a lead, for a touch voltage of 240 volts AC, the body
resistance between its extremities is of the order of 1000 ohms. Under these conditions the current flowing
through the person would be 240 milliamperes. This current could produce serious results.
If the earth lead is faulty or broken (i.e. high resistance) the greater part of the earth leakage current will
flow through the person. The fuse will not blow because the current is insufficiently high and the effect on
the person could be fatal.
4.2.1 Power points and switches
1. Dry hands thoroughly before operating any switch.
2. Do not allow the cover plates of power points or switches to be removed. If, for any reason, the cover
plate has been removed or damaged, do not allow the point or switch to be used until it has been
repaired by a qualified electrician.
3. Switches must be in the "off" position before inserting plugs or removing plugs from power points.
4. Grasp the plug, not the lead, when removing plugs from power points. Hold the plug firmly and pull
away.
5. Do not allow students to tamper with power points or switches, e.g. by pushing metal objects into
power points or
poking at pilot lights with metal objects.
6. If a power point or switch is faulty in any way, do not attempt to repair it yourself. Do not allow
students to use it until it is repaired.
7. Cover power points with plug protectors when not in use.
8. Hand-held equipment (e.g. electric drills) unless double insulated, must be operated through an
isolating transformer or a core balance earth leakage device. These are designed to protect the operator
from injury from faulty earth connections.
4.2.2 Mains-operated equipment
Mains operated equipment has to meet the stringent requirements laid down by the Queensland
Electricity Industry (QEI), e.g. that it be effectively earthed (unless double insulated) and that the lead from
the equipment to the power point be effectively anchored within the equipment so that it cannot be pulled
loose.
1. If the equipment installed by the Department of Public Works and Housing, e.g. hot water systems,
stills, fans, refrigerators, reticulated d.c. supplies fails to function, teachers should not attempt to repair it.
This is not only dangerous but illegal. Faults should be reported to the nearest depot of the Department of
Public Works and Housing or the principal may arrange for a local electrician to effect repairs.
2. Avoid using 240 volt equipment near sinks or water outlets or close to gas outlets where accidental
sparking may ignite gas leaks.
3. Disconnect portable appliances from the power outlet when not in use.
4. Before attempting to clean any mains operated equipment (portable or fixed) ensure that it is switched
off and disconnected from the power point.
5. Hand held equipment, e.g. electric drills, unless double insulated, must be operated through an isolating
transformer or a core balance earth leakage device. These are designed to protect the operator from
injury in the event of faulty earth connections.
4.2.3 Electrical leads
Most mains operated equipment is connected to power points through leads which contain three wires.
These are colour coded as follows:
Lead Current Australian standard International standard
Live Red Brown
Neutral Black Blue
Earth Green Green/Yellow
Both these standards are presently acceptable to the Queensland Electricity Industry. The earth lead
(green or green/yellow) is meant to earth the frame or metal case of the equipment to prevent shock to the
user should a short circuit occur between the electrically "live" parts of the equipment and the frame or
case. The other two wires in the lead (red and black or brown and blue) carry the electric current
operating the equipment. The following safe practices should be followed:
1. Inspect all leads and fittings regularly for frayed or cracked insulation. If a lead is considered defective,
it should be
checked by a qualified electrician.
2. Never use a lead if the plug which fits into the mains operated equipment or the three pin plug which fits
into the power point are damaged or loose.
3. Never twist or knot a lead or bend it sharply.
4. Never tack leads to walls.
5. Store leads when not in use in such a way that the insulation will not be damaged.
6. Avoid using extension leads, where possible. If necessary, use commercially available leads. Home-made
leads must not be used.
7. Always switch off the equipment before attempting to disconnect extension leads.
8. Use the shortest possible continuous lead.
9. Use a power board when several appliances need to be plugged in. [Do NOT use double adapters!] 4.2.4 Electrical inspections
Checks are usually arranged through the principal. All movable electrical equipment which has a lead
fitted or which requires a lead to be attached, and all extension leads, are to be checked at prescribed
intervals and tagged by a qualified electrical worker in accordance with Australian Standard AS
3760-1990 "In-service inspection and testing of electrical equipment". It should be noted that damage may
be caused to specialized electronic equipment such as photocopiers, computers, typewriters and facsimile
machines if they are not tested correctly. The following table outlines the testing requirements:
Environment where equipment is used Intervals between inspection and test Intervals between inspection and test Intervals between inspection and test Intervals between inspection and test
.
 Class of equipment Class of equipment Additional tests for portable safety
switches		 Additional tests for portable safety
switches
.
 Class I (Earthed) TV, Stereo
Class II (double Insulated) hair dryer, electric drill, shaver		 Interval for push-button test by user Interval for performance test
Manual arts, cleaning room, home economics, tuck shop, ag. science
 6 months 12 months Daily 12 months
Science dept.
staff rooms
 12 months 12 months Daily 5 years
Records are required for all items of electrical equipment, including the dates of each test, and the name
and licence number of the qualified electrical worker, to be kept and to be made readily available to a
Division of Accident Prevention inspector upon request.
4.2.5 Reporting an accident
Any accident involving mains power should be reported to the local electrical authority, which will
conduct an investigation and report to the QEI. Such action may prevent further accidents from similar causes.
4.2.8 Core balance earth leakage devices
The devices work on the principle of current balance. In any electrical equipment which is in good
condition, the currents in the active and neutral conductors are equal and opposite. A current transformer
placed around these two conductors will have no secondary voltage induced or current flowing. If the
equipment is earthed and the insulation resistance between equipment and the earth is low, some of the
load current will flow to earth instead of returning through the neutral (earth leakage current). Since only
the active and neutral conductors pass through the current transformer, the currents in these two
conductors are no longer balanced and do not cancel. This out-of-balance earth leakage current induces a
voltage in the secondary of the current transformer which is used to operate a relay and trip a circuit
breaker.
If, for any reason, an earth leakage current flows through the operator, as shown in the diagram, the
imbalance between the currents in the active and neutral leads causes a current to flow in the circuit of the
secondary of the current transformer which operates a relay and a circuit breaker, thus switching off the
mains. Notice that the mains system is MEN (Multiple Earth Neutral) as stated previously.
This protective system does not prevent current passing through the operator, but design considerations
can keep this
current low in value and flowing only for a limited time, i.e. until the circuit breaker operates.
Portable devices of this kind operate on an earth current of 20 milliamperes and tripping times as low as
30 milliseconds.
The result of this combination of current and time is that the operator will be aware of the shock but will
experience no dangerous effects.
All these devices have re-set buttons which must be operated after the circuit breaker has been tripped.
Test buttons are also provided to enable the operator to ascertain whether the device is working
according to the design specifications.
Most high schools have a device of this kind protecting the electric circuits in the laboratories and
demonstration rooms.
4.2.9 Precautions to take when using core balance earth leakage devices
If the core balance earth leakage device trips during the operation of some equipment, a qualified
electrician should be
called to examine the equipment before it is used again.
Operate the test button once a week to ensure that the device is operating satisfactorily.
4.2.10 Management of electric shock victims
The rescuer must:
1. Turn off the electricity supply if possible.
OR
1a. Disconnect the victim from the electricity supply by the use of a dry non-conducting material e.g. dry
clothing, a wooden stick.
2. Avoid any direct contact with the skin of the patient or any conducting material touching until
disconnected.
3. Commence resuscitation. Use cardiopulmonary resuscitation if the patient's heart has stopped beating.
4. Seek medical help.
Acknowledgement
The contents of this item have been adapted from "Aspects of Science Management: A Reference
Manual for Schools", Department of Education, Queensland, Australia. This manual have been adapted
from many sources, including the previous Primary Science Safety Manual and Queensland Safety
Handbook for Schools, Science, handbooks of other state education Departments, and material from
government agencies. These sources are gratefully acknowledged. All efforts were made to check that the
information was accurate and current at the time of publication. New legislation and regulations may be
enacted from time to time. New information regarding equipment and chemicals may become available.
Emergency procedures may be revised. It remains the principal's responsibility to check for the latest
information regarding legislation, regulations, policy and procedures that might affect a science activity.
These guidelines were prepared by the Studies Directorate. Representatives from many sectors of the
Education Department, tertiary institutions and professional associations have contributed to the development of this
document. Questions about this reference manual should be directed to the Studies Directorate,
Department of Education, PO Box 33, Brisbane Albert Street, Q 4002, Australia.