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Element 11: Electricity
11.1 Hazards and risks
Causes of Electrical Accidents
Current, not voltage, is responsible for electrical accidents. The voltage will induce a current to flow along a ground-created route. Any obstruction to the passage of current, referred to as “resistance,” leads in the loss of energy, typically in the form of heat. This current or the resulting heat is the fundamental cause of all electrical accidents..
Direct contact – Contact with a conductor that is part of a circuit and would be anticipated to be live results in an electric shock. An example would be if someone removed the plate from a switch and then touched the live wires within. It is feasible that a residual current device (RCD) with an operational current of 30 mA or less could provide protection in this circumstance.
Indirect contact– It is possible to acquire an electric shock from a conductor that is not connected to the electrical installation but has become live due to a failure. Such a component is known as an unnecessary conductive component. This condition is hazardous because to the presence of a phase-to-earth fault. This renders the case of the equipment live, such that contact with it and a good ground incorporates the human body into the shock circuit. The failure would generate a “dead short” if the protective system had zero resistance, and the fuse or circuit breaker would then open the circuit.
Another classification of the causes are;
Electrical Shock
Electric shock is the sensation and muscular spasm caused when electric current passes through the body.
There are 4 factors determining the severity of the shock.
(i) The resistance of the body against the stream flow. (Or the amount of circulating shock current)
(ii) Current follows a path through the body..
(iii) The duration of the current’s passage through the body.
(iv) Supply Frequency
Body resistance (or the quantity of current)
The human body is mostly water, and it doesn’t have much resistance. The skin, on the other hand, has a very high resistance. The value depends on the type of skin, whether or not it has been burned, and whether or not it has water on it. So, most of the resistance to the flow of electricity through the human body is at the places where the skin opens and closes. A person with hard, dry skin will have a much higher resistance to shock current than someone with soft, wet skin. If the skin has been burned, carbon particles that conduct electricity will make the resistance very low. When skin is wet, the resistance of the body could be as low as 300 ohms. Also, if the skin is broken at the point of contact, the resistance of the skin could drop to almost nothing. When working with voltages less than 240 volts, skin resistance is important. If someone gets shocked by more than 240 volts, the voltage arc will burn through the skin and cause deep, third-degree burns where it enters the body.
Path of the current flow
Current can travel through a person’s body in two dangerous ways: from hand to hand and from the left hand to either foot. The second path is the most dangerous because the current will flow through the heart and other vital organs. A path made through the head also makes it more likely that someone will die.
Duration of the current flow
Fibrillation is when the heart is shocked and starts to beat in a useless way. The longer someone is shocked, the more likely it is that their heart will start to beat irregularly. Most electric shock deaths are caused by fibrillation. If the current in milliamperes is less than 116/t, where “t” is the length of the shock in seconds, it is unlikely that the person is an adult. Less electricity is needed to make the heart fibrillate the longer the shock lasts. Some examples of shock current levels and times that could cause fibrillation are 21 milliamps for 30 seconds, 44 milliamps for 7 seconds, or 67 milliamps for 3 seconds. But a 500 mA shock current that lasts less than 20 ms may not cause any long-term damage.
Supply Frequency
Because direct current (DC) moves through a conductor in a continuous way, it can easily cause muscle tetanus (contraction). Alternating current (AC) switches between moving in one direction and moving in the opposite direction. This gives a muscle a chance to relax between changes. So, DC is more dangerous than AC because of the risk of getting “frozen on the circuit.” But AC’s alternating nature is more likely to cause the pacemaker neurons in the heart to go into a state called fibrillation, while DC tends to make the heart stop beating. Once the shock current is turned off, a heart that is “frozen” has a better chance of beating normally again than a heart that is “fibrillating.” The effect of fibrillation is strongest when the frequency of the power supply is between 9 and 12 Hz. High Frequency (MHz) currents are safe and may not even cause a shock because of the “skin effect.”
Electric burn
Burns from electricity are not the same as burns from fire. They happen when an electric current passes through body tissue and heats it up. Because of this, they are usually caused by an electric shock and usually happen in or on the skin at the point of contact with the electric system. Electric burns are often very painful, take a long time to heal, and leave permanent scars.
Fires of electrical origin
Electricity can cause fire in a number of ways, including:
Electric arcing
When current travels through air or insulation between two conductors at different potentials, an electric arc occurs. Injuries from arcs may be the direct result of burning from the arc, in which case it is not uncommon for molten metallic conductor particles to penetrate the burn and increase its severity. Arc burns are typically extremely severe and frequently lethal.
Explosion caused by the use of electricity
Explosions of all types often involve the rapid release of a tremendous amount of energy. When electrical equipment such as motors, equipment, and cables are subjected to far higher amounts of current than they are designed to carry, they may explode violently.
Working Near Live Overhead Power Lines
Contact with overhead electricity lines is one of the leading causes of fatalities involving mobile plant and equipment. Any voltage that permits sufficient current to travel through the heart is potentially harmful or lethal if it creates contact with live overhead power lines. Contact with live electricity can potentially result in severe burns due to the release of electrical energy. Additional concerns include fires and explosions that could disable the involved equipment. To receive a lethal electric shock from a high voltage overhead power line, direct contact is not required. Being too close can be fatal.
People can be killed and injured by electric shock, electrical arcs (which cause explosions), and fires when underground cables are destroyed. This frequently leads to serious burns on the hands, face, and body, even when protective equipment is worn.
Contact with underground power cables during excavation work
Damage can be caused when a cable is:
Cables that have been previously damaged but left unreported and unrepaired can cause incidents.
Work on mains electricity supplies.
The voltage of the mains supply may vary between workplaces. In certain locations, the voltage could be 230V or 440V. In certain locations, such as power plants and the transmission sector, the supply voltage may be measured in the thousands or millions. Working on these mains is fraught with danger, as a shock from any of these voltages can be lethal.
Secondary Hazards
11.2 Control measures
Ensure people working on or with your electrical equipment or systems are Page No. 204-207
‘Competent’ for the task
Competent refers to the possession of adequate training, skill, and knowledge for the task in order to prevent injury to oneself and others. Assure the safety of the electrical installation
Make sure that:
Provide safe and suitable equipment
Reduce the voltage
Provide a safety device
An RCD (residual current device) can provide additional safety if equipment operating at 230 volts or more is utilized. An RCD is a device that detects certain, but not all, electrical system defects and quickly disconnects the power supply.
The ideal location for an RCD is the main switchboard or the socket outlet, as this provides permanent protection for the supply lines. If this is not practicable, a plug containing an RCD or an RCD adapter that plugs into an outlet can also give further safety.
People-protecting RCDs have a maximum rated tripping current (sensitivity) of 30 milliamps (mA). Remember:
– You should use this regularly.
Carry out preventative maintenance
An appropriate system of maintenance is strongly recommended. This can include:
Work safely
Ensure that individuals working with electricity are qualified for the job. Even basic actions, such as wiring a plug, can be hazardous; therefore, individuals should be well-informed before beginning.
Check that:
Repairing equipment or making changes to an electrical installation are more complicated jobs that should only be done by people who know the risks and precautions they need to take.
You can’t let people work on or near exposed, live parts of equipment unless it’s absolutely necessary and the right precautions have been taken to keep workers and anyone else in the area from getting hurt.
Underground power cables
When digging in the street, sidewalk, or near a building, you should always expect to find cables. To avoid danger, make sure your service plans are up-to-date, use tools to avoid cables, and dig carefully.
Service plans should be available from regional electricity companies, local governments, highway authorities, etc. The HSE has a publication called Avoiding danger from underground services that gives more information (HSG47).
Overhead power lines
Overhead lines are the cause of more than half of the electrical accidents that kill people every year.
When working near power lines, the owners may be able to turn them off if given enough notice. If this can’t be done, talk to the owners about how close you can work without getting hurt.
Don’t forget that power lines can still spark even if plants and equipment don’t touch them. The HSE has a publication called Avoiding danger from overhead power lines that gives more information (GS6)
Not every electrical item needs a PAT and those that do may not need to be tested every year
Most of the electrical risks can be controlled by using a simple, cheap system to look for visible signs of damage or faults.
There is no law that says equipment that has been inspected or tested has to be labeled or that you have to keep track of what has been done.
Even though it’s not required by law, keeping records and labels can be a good way to check how well the maintenance scheme is working and see if it needs to be changed.
The booklets listed below and the frequently asked questions page at www.hse.gov.uk/electricity have information on how to test portable appliances and how often to do so.
A qualified person should check and test fixed installations (such as the wiring and equipment between the supply meter and the point of use, such as plug sockets) on a regular basis.
References
Element 11: Electricity
11.1 Hazards and risks
Causes of Electrical Accidents
Current, not voltage, is responsible for electrical accidents. The voltage will induce a current to flow along a ground-created route. Any obstruction to the passage of current, referred to as “resistance,” leads in the loss of energy, typically in the form of heat. This current or the resulting heat is the fundamental cause of all electrical accidents..
Direct contact – Contact with a conductor that is part of a circuit and would be anticipated to be live results in an electric shock. An example would be if someone removed the plate from a switch and then touched the live wires within. It is feasible that a residual current device (RCD) with an operational current of 30 mA or less could provide protection in this circumstance.
Indirect contact– It is possible to acquire an electric shock from a conductor that is not connected to the electrical installation but has become live due to a failure. Such a component is known as an unnecessary conductive component. This condition is hazardous because to the presence of a phase-to-earth fault. This renders the case of the equipment live, such that contact with it and a good ground incorporates the human body into the shock circuit. The failure would generate a “dead short” if the protective system had zero resistance, and the fuse or circuit breaker would then open the circuit.
Another classification of the causes are;
Electrical Shock
Electric shock is the sensation and muscular spasm caused when electric current passes through the body.
There are 4 factors determining the severity of the shock.
(i) The resistance of the body against the stream flow. (Or the amount of circulating shock current)
(ii) Current follows a path through the body..
(iii) The duration of the current’s passage through the body.
(iv) Supply Frequency
Body resistance (or the quantity of current)
The human body is mostly water, and it doesn’t have much resistance. The skin, on the other hand, has a very high resistance. The value depends on the type of skin, whether or not it has been burned, and whether or not it has water on it. So, most of the resistance to the flow of electricity through the human body is at the places where the skin opens and closes. A person with hard, dry skin will have a much higher resistance to shock current than someone with soft, wet skin. If the skin has been burned, carbon particles that conduct electricity will make the resistance very low. When skin is wet, the resistance of the body could be as low as 300 ohms. Also, if the skin is broken at the point of contact, the resistance of the skin could drop to almost nothing. When working with voltages less than 240 volts, skin resistance is important. If someone gets shocked by more than 240 volts, the voltage arc will burn through the skin and cause deep, third-degree burns where it enters the body.
Path of the current flow
Current can travel through a person’s body in two dangerous ways: from hand to hand and from the left hand to either foot. The second path is the most dangerous because the current will flow through the heart and other vital organs. A path made through the head also makes it more likely that someone will die.
Duration of the current flow
Fibrillation is when the heart is shocked and starts to beat in a useless way. The longer someone is shocked, the more likely it is that their heart will start to beat irregularly. Most electric shock deaths are caused by fibrillation. If the current in milliamperes is less than 116/t, where “t” is the length of the shock in seconds, it is unlikely that the person is an adult. Less electricity is needed to make the heart fibrillate the longer the shock lasts. Some examples of shock current levels and times that could cause fibrillation are 21 milliamps for 30 seconds, 44 milliamps for 7 seconds, or 67 milliamps for 3 seconds. But a 500 mA shock current that lasts less than 20 ms may not cause any long-term damage.
Supply Frequency
Because direct current (DC) moves through a conductor in a continuous way, it can easily cause muscle tetanus (contraction). Alternating current (AC) switches between moving in one direction and moving in the opposite direction. This gives a muscle a chance to relax between changes. So, DC is more dangerous than AC because of the risk of getting “frozen on the circuit.” But AC’s alternating nature is more likely to cause the pacemaker neurons in the heart to go into a state called fibrillation, while DC tends to make the heart stop beating. Once the shock current is turned off, a heart that is “frozen” has a better chance of beating normally again than a heart that is “fibrillating.” The effect of fibrillation is strongest when the frequency of the power supply is between 9 and 12 Hz. High Frequency (MHz) currents are safe and may not even cause a shock because of the “skin effect.”
Electric burn
Burns from electricity are not the same as burns from fire. They happen when an electric current passes through body tissue and heats it up. Because of this, they are usually caused by an electric shock and usually happen in or on the skin at the point of contact with the electric system. Electric burns are often very painful, take a long time to heal, and leave permanent scars.
Fires of electrical origin
Electricity can cause fire in a number of ways, including:
Electric arcing
When current travels through air or insulation between two conductors at different potentials, an electric arc occurs. Injuries from arcs may be the direct result of burning from the arc, in which case it is not uncommon for molten metallic conductor particles to penetrate the burn and increase its severity. Arc burns are typically extremely severe and frequently lethal.
Explosion caused by the use of electricity
Explosions of all types often involve the rapid release of a tremendous amount of energy. When electrical equipment such as motors, equipment, and cables are subjected to far higher amounts of current than they are designed to carry, they may explode violently.
Working Near Live Overhead Power Lines
Contact with overhead electricity lines is one of the leading causes of fatalities involving mobile plant and equipment. Any voltage that permits sufficient current to travel through the heart is potentially harmful or lethal if it creates contact with live overhead power lines. Contact with live electricity can potentially result in severe burns due to the release of electrical energy. Additional concerns include fires and explosions that could disable the involved equipment. To receive a lethal electric shock from a high voltage overhead power line, direct contact is not required. Being too close can be fatal.
People can be killed and injured by electric shock, electrical arcs (which cause explosions), and fires when underground cables are destroyed. This frequently leads to serious burns on the hands, face, and body, even when protective equipment is worn.
Contact with underground power cables during excavation work
Damage can be caused when a cable is:
Cables that have been previously damaged but left unreported and unrepaired can cause incidents.
Work on mains electricity supplies.
The voltage of the mains supply may vary between workplaces. In certain locations, the voltage could be 230V or 440V. In certain locations, such as power plants and the transmission sector, the supply voltage may be measured in the thousands or millions. Working on these mains is fraught with danger, as a shock from any of these voltages can be lethal.
Secondary Hazards
11.2 Control measures
Ensure people working on or with your electrical equipment or systems are Page No. 204-207
‘Competent’ for the task
Competent refers to the possession of adequate training, skill, and knowledge for the task in order to prevent injury to oneself and others. Assure the safety of the electrical installation
Make sure that:
Provide safe and suitable equipment
Reduce the voltage
Provide a safety device
An RCD (residual current device) can provide additional safety if equipment operating at 230 volts or more is utilized. An RCD is a device that detects certain, but not all, electrical system defects and quickly disconnects the power supply.
The ideal location for an RCD is the main switchboard or the socket outlet, as this provides permanent protection for the supply lines. If this is not practicable, a plug containing an RCD or an RCD adapter that plugs into an outlet can also give further safety.
People-protecting RCDs have a maximum rated tripping current (sensitivity) of 30 milliamps (mA). Remember:
– You should use this regularly.
Carry out preventative maintenance
An appropriate system of maintenance is strongly recommended. This can include:
Work safely
Ensure that individuals working with electricity are qualified for the job. Even basic actions, such as wiring a plug, can be hazardous; therefore, individuals should be well-informed before beginning.
Check that:
Repairing equipment or making changes to an electrical installation are more complicated jobs that should only be done by people who know the risks and precautions they need to take.
You can’t let people work on or near exposed, live parts of equipment unless it’s absolutely necessary and the right precautions have been taken to keep workers and anyone else in the area from getting hurt.
Underground power cables
When digging in the street, sidewalk, or near a building, you should always expect to find cables. To avoid danger, make sure your service plans are up-to-date, use tools to avoid cables, and dig carefully.
Service plans should be available from regional electricity companies, local governments, highway authorities, etc. The HSE has a publication called Avoiding danger from underground services that gives more information (HSG47).
Overhead power lines
Overhead lines are the cause of more than half of the electrical accidents that kill people every year.
When working near power lines, the owners may be able to turn them off if given enough notice. If this can’t be done, talk to the owners about how close you can work without getting hurt.
Don’t forget that power lines can still spark even if plants and equipment don’t touch them. The HSE has a publication called Avoiding danger from overhead power lines that gives more information (GS6)
Not every electrical item needs a PAT and those that do may not need to be tested every year
Most of the electrical risks can be controlled by using a simple, cheap system to look for visible signs of damage or faults.
There is no law that says equipment that has been inspected or tested has to be labeled or that you have to keep track of what has been done.
Even though it’s not required by law, keeping records and labels can be a good way to check how well the maintenance scheme is working and see if it needs to be changed.
The booklets listed below and the frequently asked questions page at www.hse.gov.uk/electricity have information on how to test portable appliances and how often to do so.
A qualified person should check and test fixed installations (such as the wiring and equipment between the supply meter and the point of use, such as plug sockets) on a regular basis.
References
