What are the hazards of electricity? (5)
- Electric shock- severe electric shock can cause involuntary muscle grip, heart fibrillation, respiratory failure and cardiac arrest
- Burns- at the point of contact, and at the point that the current flows out of the body. There may also be internal burns along the current path.
- Fire and Explosion
- Arcing
- Secondary Effects
What are the effects of the current flow on the body?(5)
- At very low current flow (less than 0.5 – 2 mA) no sensation is felt by the person receiving the shock.
- Between 2 and 10 mA current starts to flow through the body and stimulates muscles to contract. This can be felt, it causes muscles to tremble and it may hurt - but the person receiving the shock is able to control their muscles and can let go of the live object.
- Between 10 and 60 mA current starts to cause more severe muscle contractions; these may become so strong that the person cannot control their muscles and they grip on to the live object.
- At current flows above 60 mA there is the possibility of ventricular fibrillation (VF). This usually leads to cardiac arrest.
- As the current increases above 80 mA the possibility of VF becomes greater Death becomes more likely as the current increases.
What factors has influence in the severity of injury associated with receiving electric shock? (9)
- Voltage.- The higher the voltage the greater the current
- Duration.- The length of time that a person is exposed to the flow of electricity is critical.
- Frequency.- of the AC current Current path.-the route that the electricity takes as it flows through the body If it runs through the chest it is likely to affect the heart
- Resistance – as Ohm’s law shows there is a simple inverse relationship between current and resistance – the higher the resistance, the lower the current. Most of the body’s resistance to the passage of electricity is because of the skin. A person with dry skin has a resistance of about 100,000 ohms, but if their skin is wet or damaged this reduces dramatically to 1000 ohms. Any clothing that the person is wearing will also affect their resistance to the passage of electricity.
- Contact surface area – the more skin that is in contact with the live surface, the lower the resistance and the more severe the injury.
- Environment – any environmental factors that reduce resistance will cause an increase in current flow and therefore increase the severity of the shock, e.g. wet surfaces, humid air, metal surfaces, etc. Nature of the clothing and footwear – may provide some protection.
- Presence of potential secondary hazards – can result in additional injuries, e.g. if working off a ladder the person may fall.
Types of Electrical Burns
Direct- where current causes overheating as it passes through the skin and the internal tissues of the body
Indirect- dropping a spanner onto a high voltage cable can cause a short circuit, resulting in a flash of radiant heat and an explosion of molten metal.
Electricity can cause fires in several different ways: (5)
- Electrical equipment may be faulty and may overheat as a result, leading to a fire.
- The system may be overloaded; as too much current passes through, it overheats.
- Equipment may be misused, e.g. it may be connected into the mains supply by pushing bare wires into the socket rather than using the proper plug.
- A flammable atmosphere may be present which electricity ignites. This can happen in two different sets of circumstances: –– The wrong type of electrical equipment is brought into an existing flammable atmosphere (i.e. one which is already known to be flammable). –– A flammable atmosphere is accidentally created in an area where it would not be expected (e.g. due to spillage).
- Electrical equipment may produce heat or sparks as part of its normal operation. For example, a fan heater gets hot during use; if poorly positioned next to a full waste paper bin, it may start a fire.
What is the risk of static electricity?
Static electricity refers to the buildup of potential difference (voltage) between surfaces as a result of friction between them.
Little risk exists to a person from the normal static shock found in most workplaces unless there are flammable liquids or flammable atmospheres present. There is then the risk that the static shock will ignite the liquid or atmosphere, causing fire or explosion
What are the main risks associated with arcing? (4)
Arcing is where electricity jumps across an air gap. It occurs in a very limited way inside some low-voltage electrical equipment (e.g. a portable electric drill).
The main risks associated with arcing are:
- Electric shock as a result of being struck by the arc.
- Direct burns as a result of being struck by the arc.
- Indirect burns from the radiant heat given off by the arc and from the melting of any equipment struck.
- Damage to the eye as a result of the ultraviolet light (UV) that is emitted by the arc.
When we talked about secondary effects relative to electrical safety, we are talking about what?
Any sort of injury that results indirectly from receiving an electric shock is a secondary effect (e.g. falling from height)
Conditions and practices likely to lead to accidents:
- Using unsuitable equipment, e.g. the use of non-intrinsically safe equipment in a flammable atmosphere.
- Using equipment in wet, damp or humid conditions.
- Misuse, e.g. sticking wires directly into a socket rather than using a plug.
- Physical abuse, e.g. pulling the plug out by tugging at the cord; carrying the tool by the cord; allowing the cord to be pinched, trapped or crushed, driving over the cord, etc.
- Inadequate maintenance or repairs carried out by unauthorized personnel or carried out badly, e.g. a split cord taped up with insulating tape.
- Continued use of faulty, defective equipment.
- Chemical damage to the cord, e.g. by corrosive wet cement.
- Physical damage to the cord by hostile environments.
- Lack of routine inspection, testing or maintenance.
What is the risk of working near Overhead Lines
Most overhead power lines are uninsulated. Because of this, any work carried out near to these power lines has a risk of electrical arcing.
What is the electrical risk in an excavation?
Striking buried power cables is a significant risk associated with excavation work. It can lead to electric arcing, shock and burns, not to mention major business disruption to service users.
Control measures to be considered when selecting portable electrical equipment for use on construction sites: (9)
- Consider the use of battery-powered equipment.
- Consider the use of reduced and low-voltage (110V) equipment center tapped to earth.
- Provide increased protection through the use of an RCD (Residual Current Device).
- Locate cables carefully, away from hazards, e.g. vehicles that may drive over them.
- Consider the use of double insulated equipment.
- Carry out pre-use checks of the equipment for signs of damage.
- Train operators in safe use of the equipment.
- Avoid using in wet conditions (unless the equipment and supply cables are suitable for this).
- Implement a program of routine visual inspection and thorough testing of electrical equipment and cables.
Various protective systems can be used for electrical equipment such as: (6)
- Fuses – a weak link in the circuit – or miniature circuit breakers.
- Earthing – a low resistance path to earth for fault current. Isolation of supply– cutting the power.
- Double insulation – separating people from the conductors using two layers of insulation.
- Residual current devices – sensitive and fast-acting trips.
- Reduced and low voltage systems – so that less current flows during an electric shock accident.
Electrical equipment must be carefully selected to ensure that it is suitable for: (3)
The electrical system that it will become a part of. The task that it will perform. The environment in which it will be used.
Mention dangerous environment for electrical equipment (8)
- Weather – equipment and cables may need to withstand exposure to rain, snow, ice, wind, dust and lightning.
- Natural hazards, e.g. solar radiation, plants and animals (e.g. gnawing of cables by rats).
- Extremes of temperature and pressure, e.g. heat from motors.
- Dirty conditions – contamination by liquids or solids.
- Corrosive conditions – caused by chemicals.
- Liquids and vapors – immersion, splashing or spraying with water and solvent vapors, etc.
- Flammable substances, e.g. flammable gases, dusts and vapors.
Foreseeable mechanical damage must also be considered, both in terms of the environment within which the equipment is to be used and the natural operation of the equipment itself.
Advantages and Limitations of using Fuses and Miniature Circuit Breakers
A fuse is a device used to prevent current overload.
Advantages of fuses:
- Very cheap and reliable.
- Offer a good level of protection for the electrical equipment against current overload that might damage the equipment or cause overheating, fire or explosion.
Limitations of fuses:
- They primarily protect equipment and not people. It is possible to receive a severe, even fatal, electrical shock from equipment that is protected by a fuse for two reasons: –– A fuse does not stop current flow quickly enough to prevent ventricular fibrillation. –– The current flow must be above the fuse rating for the fuse to operate; this may be above the 60 mA capable of causing fatal injury.
- Very easy to bypass, e.g. by wrapping the fuse in aluminum foil. MCB does not melt in response to current overload; it simply trips out and can be reset by pressing a button.
Advantages and Limitations of Earthing
Earthing is a way of protecting equipment so that in the event of an electrical fault, current flows safely to earth rather than flowing through a person who might be touching the equipment. The earth wire of an item of electrical equipment is usually connected to the outer metal casing or chassis of the equipment.
Advantages of earthing:
It protects the person from fatal electric shock. It often provides secondary protection to the equipment because a large fault current flowing to earth will overrate the fuse or MCB.
Limitations of earthing:
A poor or broken earth connection will prevent the earth from working properly, but since the earth wire does not take part in the normal functioning of the equipment this fault can go completely undetected. It is easy to disconnect and disable
Advantages and Limitations of Isolation of Supply
Isolation is the removal of electrical power from a circuit or system. This might be achieved using a switch (isolator) or by pulling the plug out.
The advantage of isolation as a form of protection is that it is a very effective method of ensuring that people cannot be injured by electrical energy when working on an electrical system.
The limitation of isolation is that, by definition, the electrical system is dead. Certain types of testing, fault finding and electrical installation and repair work have to be carried out with the electrical system on and live.
Advantages and Limitations of Double Isolation.
The principle behind double insulation is exactly as the name suggests: there are two layers of insulation between the user and any live conductors. This eliminates the need to provide earth protection, so double-insulated equipment will have a two-core cord: live (hot) and neutral only.
The advantage of double insulation is that it relies on insulation rather than the electrical system itself for safety.
The limitation of double insulation is that the insulation must be routinely visually inspected because there is no earth protection.
Advantages and Limitations of Residual Current Devices
A residual current device is specifically designed to protect human life in the event of electric shock. It does this on the basis that it is very sensitive to small current imbalances in a circuit and is able to break the circuit very quickly.
The principle of an RCD is that it constantly compares the amount of current flowing down the live (hot) and neutral lines and trips the circuit if an imbalance is detected.
The advantage of RCDs is that they provide excellent protection for people in the event of electric shock.
The limitations of RCDs are that they:
- Do not provide over-current protection (they are not fuses; they work on a completely different principle).
- Have to be tested periodically (this is often not done).
- Can cause repeated circuit tripping if there is a fault; this can encourage people to not use them or to disable them
Advantages and Limitations of Reduced and Low Voltage Systems
The lower the voltage that an electrical system is operated at, the lower the risk of injury associated with electric shock.
The advantage of low voltage systems is that the system is inherently safer.
The limitation is that low voltage systems are inefficient at transmitting power and therefore cannot be used for many industrial applications.
In an electric work, a competent person has:
- Knowledge of electricity.
- Experience of electrical work.
- An understanding of the system to be worked on.
- An understanding of the hazards and the precautions needed.
- The ability to recognise whether it is safe for work to continue.
A SSW (Safe System of Work) to work on or near Live Electrical Systems is likely make use of the following controls (5)
- Permit-to-work system.
- Competent persons.
- Insulating PPE (such as gauntlets and boots).
- Insulated tools and equipment (such as screwdrivers).
- Designated work areas (such as “earth-free zones”).
A SSW (Safe System of Work) to Isolate is likely make use of the following controls (3)
- The breaking of the circuit.
- Physical securing of the break in the circuit.
- Some form of label (or tag).
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01 Foundations in Health and Safety8
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08 Musculoskeletal Hazards And Risk Controls.12
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Jargon Buster45
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Key Definitions8
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05 Health and Safety Management Systems 4- Measuring, Audit and Review27
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07 Safe Movement of Vehicles in the Workplace.17
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09 Work Equipment Hazards and Risk Controls35
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10 Electrical Safety29
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11 Fire Safety28
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13 Physical and Psychological Health Hazards and Risk Controls24
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02 Health and Safety Management Systems 1 - Policy15
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04 Health and Safety Management Systems 3 - Planning42
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03 Health and SafetyManagement Systems 2 - Organising57
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06 Workplace Hazards and Risk Controls46
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12 Chemical and Biological Health Hazards and Risk Controls70
