Electricalhazards

School Science Lessons
Electrical hazards
Updated: 2007-12-14
Please send comments to: J.Elfick@uq.edu.au
See: Interesting websites
Acknowledgement

Table of contents
1.0 Electrical fires
2.0 Electrical outlets and equipment
4.0 Electrical hazards
4.1 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 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

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.

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.

4.0 Electrical hazards
4.1 Electric shock
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. Safe practices and procedures
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 ecs, 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
See the DOEM activity module. In addition, 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 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.