In this element we will look at the hazards which can arise from the use of electrical equipment in the workplace. We all use electrical equipment on a day-to-day basis but rarely consider the hazards. Electricity is a dangerous but very useful source of power. Unfortunately, electrical faults are one of the main causes of fires and many people each year suffer injuries from electric shocks and burns due to faulty equipment and wiring, or from the use of inappropriate electrical equipment in a particular environment. We should ensure that each piece of equipment we use is safe every time we use it.

We start with a brief outline of the basic principles of electricity and the terminology associated with it. We have to understand this in order to appreciate how risks arise, the harm they can do and how some of the protective measures work. We go on to look at the hazards posed by electricity, particularly electric shocks and burns, before examining the measures which can be taken to control the risks.

The element is designed to meet the following aims and learning outcomes as specified by NEBOSH for this part of the syllabus for the International General Certificate.

Overall Aims

On completion of this element, you will have knowledge and understanding of:

The hazards and risks associated with the use of electrical equipment and systems operating at mains voltages.
The measures that should be taken to minimise the risks.

Specific Intended Learning Outcomes

When you have worked through this element, you will be able to:

Identify the hazards and evaluate the consequential risks from the use of electricity in the workplace.
Advise on the control measures which should be taken when working with electrical systems o r using electrical equipment.

Principles Of Electricity

Electricity is a form of energy associated with the flow of charged particles or electrons from one point to another through a conductor (usually a metal wire). There must be an unbroken path between the two points through which the particles can travel, the complete path being known as the circuit. A switch can be used to complete or break the circuit, thus controlling whether the electricity flows through it or not.

This basic principle is simple, but to appreciate the hazards posed by electricity when it is used to power tools and machinery, and the methods by which these hazards may be controlled, we have to understand the operation of circuits in a little more detail.

Voltage, Current and Resistance

A basic electrical circuit can be made by connecting one end of a conductor to a source of electricity and the other to another point, thus allowing power to flow from one “terminal” to the other. The source can be a battery or a socket in your living room or at the workplace. Both of these incorporate two terminals, the positive and negative terminals on a battery or the live and neutral connections (the two at the bottom) in a three pin socket.

Voltage

Power flows along a conductor where there is a difference in electrical “pressure” between the two terminals. This pressure difference provides the push to move the electrons and is measured in volts (symbol “V”). The higher the difference in pressure (the voltage), the harder the push.

The correct name for this electrical pressure is “electrical potential”, so voltage is the measure of potential difference between the two terminals.

To help understand this, it may be useful to compare an electrical circuit to a piped water system. For water to flow along the pipe there must be a supply of water being pumped into the system. The point at which the supply is connected will have a higher pressure than elsewhere in the system and the water will then flow along the pipe towards the point of lower pressure.

Current

This is the flow or speed of power along the conductor. It is measured in amperes (usually shortened to “amps”, with the symbol “I”).

The current (or the amperage) in a circuit is determined by the voltage. Thus, the higher the voltage, the higher the amperage.

To continue the comparison with the water system, the flow through the pipe can be increased by making the diameter of the pipe larger. Similarly, in an electrical circuit the current flow can be increased by using thicker conductors. Thus the mains cables which carry the electricity supply from a substation to houses and business premises are much thicker than the mains wiring inside a building, and that in turn is thicker than the wiring from an appliance to the plug.

Resistance

If a circuit was made consisting only of the source and conductor connected between the two terminals, the flow of power would continue to be pushed through and speed up. This can be dangerous, so to ensure that this does not happen there has to be something which slows the rate of flow, usually by removing power from the circuit.

This process is known as resistance and is measured in ohms (symbol “R” or “”). Any item of equipment which is connected to the circuit and uses some of the power flowing through it is a resistor; for example, light bulbs, heaters, power tools, etc. They take the electrons and convert them into another form of energy, such as heat or motion. Other resistances come from the conductor itself where the type of material forming the conductor may slow the current (when it is known as “impedance”).

We can show this by comparing the process with a piped water system. If there is nothing to slow the water down it will pick up speed as it flows through the system. However, if part of the flow is diverted to power a water wheel, that will act as a resistance to the flow. Similarly, rough surfaces, obstructions and bends in the pipe will also impede it.

Relationship Between Voltage, Current and Resistance

We can consider the way in which the three elements of a circuit are interrelated by referring to the following simple circuit diagram below.

Simple Electrical Circuit

As the voltage pushes the current through the circuit, the electrons are used in one form or another by the resistances connected. Thus the voltage applied to a circuit must be just sufficient to push the current all the way through the circuit and all its components. This can be expressed in a simple equation:

V = I  R

This is known as “Ohms Law” and is important in that it provides a means of working out any of the values if we know the other two. Thus:

I =

and

R =

The power in a circuit is a product of the voltage and the current and is measured in watts (“W”). Thus:

W = V  I

For example, if the current taken by an electrical heater is 5 amps when plugged into a 240 volt supply, then its power load would be 240  5 = 1,200 watts.

These equations can be used to calculate the current which flows through any circuit, given the resistance or power loading connected to a known voltage. They can be used to determine fuse ratings for particular loads or the current flowing through a person who may inadvertently touch a live supply (as we shall consider later).

Basic Circuitry

An electrical circuit consists of a number of components – a source of electrical current, a conductor connected to the source and another terminal, and equipment connected to the conductor and powered by the current (the load).

The conductor should be a material which allows the current to flow with as little impedance as possible and copper wire is usually used for this purpose. The wire will be enclosed in an insulator, a sheath of material which does not conduct electricity such as rubber or plastic. This has two effects:

It prevents electricity leaking out of the conductor as it passes along it.
It prevents any other conductor touching the enclosed wire and creating a short circuit, as we explain below.

Short Circuits

Once a voltage is applied to the conductor in a perfect circuit current will flow from the terminal with the higher electrical potential to the terminal with the lower potential. Normally this will be from the source terminal (the live terminal) to the return (neutral) terminal.

Electricity will flow to the point of lowest potential in a circuit along any path it can find. If it can find an alternative path to a terminal with a larger potential difference than the neutral terminal it will take it. Thus, if another conductor touches the circuit and provides such a path, at least some of the current will flow along that conductor. This is known as a short circuit.

The lowest potential (zero on the voltage scale) is the potential of the mass of the earth. So any conductor which connects the circuit to earth, deliberately or accidentally, will cause a short circuit. This is a severe danger because the new path may have less resistance to the current, allowing the flow to increase.

In the UK the public electricity supply is based on a 240 volt circuit. This means that at an electrical switch board or fuse box the live terminal has a potential of 240 volts and the neutral terminal is at a potential of zero volts. However, at any point along the conductor connecting them, for example between the live and neutral terminals in a plug, the potential difference at the neutral terminal will not be truly zer0 because of impedance. Any other conductor which connects to earth will immediately cause a short circuit.

The short circuit current which would result if a particular point in a circuit was connected directly to earth is called the fault current and the new circuit created is called the earth fault loop.

Earthing

Providing a connection to earth can be used to safeguard electrical systems as well as representing a hazard. Earthed conductors in a circuit provide a safe path for any fault current to be dissipated to earth. They will be connected to any exposed metal parts of a component connected to an electrical circuit which should not normally carry a current. If a fault develops and they do become live, then the earth conductor will carry the fault current away.

Direct and Alternating Currents

A current which flows in one direction only is known as direct current (dc). A battery gives a direct current.

If the direction of flow alters at regular intervals, it is an alternating current (ac). The public electricity supply varies in different regions of the world. In the United States the voltage is around 110 volts; in the UK it is 240 volts ac, with the alternating cycle being 50 Hz (cycles per second). Shock injury from dc is generally a lot less severe than from ac.

The flow of ac is cyclic. For instance, the 240 volt public supply peaks at 338 volts. This cyclic variation is not suited to some installations, such as driving electric motors. To overcome this problem a three-phase system is used in which three electricity supplies are fed into the circuit, each of which is out of phase with the others but which in combination produce a steady current. The additional hazards with three-phase systems are that three live terminals are present and an electric shock at 415 volts can result between them.

Hazards of Electricity

Now we have looked at basic electrical processes, we can consider the problems which can occur in using electrical equipment. In essence they come from situations where the current passing through a circuit escapes from its intended path along the conductors in the circuit and finds another path along a different conductor.

Electric Shock

An electric shock is received when a person makes a contact with a live conductor and the current passes through his/her body. The person’s body acts as the conductor for the current, interrupting the circuit and providing an alternative path for it to flow along. Where the person is in simultaneous contact with an earthed object – for example, standing on the ground or also touching an object which is in direct contact with the ground – this results in a fault current with the person (and the other object he/she is in contact with) completing the earth fault loop.

Once an electric current has passed the barrier of the skin, which has a relatively high resistance, the body itself offers little resistance and the current may take one of numerous pathways through it. The effects will depend mainly on the amount of current which passes through and the duration of the connection.

Effects on the Body

An electric shock results in a convulsive response by the nervous system to the passage of electricity through that part of the body. This causes the muscles to contract, often violently. A well known feature is that if you grasp a live wire in the palm of your hand, the shock will cause the hand to close around the wire, gripping it more firmly and being unable to release it.

Depending on the path that the current takes through the body, the muscular contractions caused can have different effects. The most serious involve the current passing through the heart where it may cause fibrillation – interference with the timing of the beats of the heart – and/or cardiac arrest. Interference with the muscles controlling breathing may also be serious and cause respiratory failure. Both of these may be fatal.

Even if the current does not pass through vital life organs, the effects can have serious consequences. The involuntary muscular reaction caused may result in arms, legs, torso or neck convulsing. This is likely to be sufficient to knock the victim off balance and cause a fall, and where this is from a height or in dangerous surroundings, the fall may result in more serious injuries than the shock itself. The force with which the muscles contract may well be sufficient to overstrain and damage the muscles themselves or other moving parts of the body and to cause fractures of bones.

The passage of electricity can also cause burns, both inside and on the surface of the body. We shall consider this later.

Factors Influencing Severity

The severity of the shock and type of injury caused will depend upon the amount and nature of current passing through the body and the parts of the body through which the current passes.

Amount and nature of the current

The amount of electrical current passing through the body is a function of the voltage applied, the resistance of the path through which it travels and the duration of the contact:

The voltage of the circuit formed by the contact which the body makes with the electricity supply will determine the force with which the electricity passes through the body. No voltage can be considered safe in all circumstances, although low voltages may reduce the risk. We must always assume that a public electricity supply of 240 volts is potentially fatal.
The electrical resistance of the body varies depending upon the skin condition (moist or dry) and the path travelled through the body between the entry and exit points. Generally, skin resistance is quite high, although it may vary from less than 1,000 ohms for wet skin to over 500,000 ohms for dry skin. Perspiration brought on by fear and shock may increase moisture on the skin and further lower resistance where there is continuing contact with an electrical source. Internally, the resistance is much less; for example, between the ears the internal resistance (less the skin resistance) is only 100 ohms, while from hand to foot it is closer to 500 ohms.
Many shocks are of very short duration with the person remaining in contact with the live source for less than a second. Depending on the current voltage received this may be long enough to receive a fatal shock. Longer contact will clearly be more damaging.

We can determine the amount of current received from a shock by applying the equations of Ohm’s Law noted earlier. If we assume a voltage of 240V from the public electricity supply and a low body resistance of 1,500 ohms, the current received would be:

I =

= 0.16 amps

Current at this level is generally measured in milliamps (one thousandth of an amp), so this level of current is expressed as 160 mA. The following table shows the effects of various levels of current for a duration of one second.

Current (mA)	Effects
0.5 - 2	Threshold of perception
2 - 10	Painful sensation
10 - 25	Inability to let go
25 - 80	Breathing laboured, danger of asphyxiation, loss of consciousness from heart or respiratory failure
100 - 200	Ventricular fibrillation – fatal

Above 200 milliamps, the muscular contractions are so severe that the heart is forcibly clamped during the shock. This clamping protects the heart from going into ventricular fibrillation, and the victim’s chances for survival are good provided that resuscitation is performed immediately. However, this does not imply that such a severe shock will not be extremely serious. Severe burns occur over 200 mA.

The current passing through the body can be either direct or alternating current and the effect is proportional to both its magnitude and the type. Direct current is less damaging.

Current path

The key points are the entry and exit points for the current, the part of the body which comes into contact with the live source and the part from which the current leaves the body. This may be where there is contact with the ground or another conductor which will take the current. The most common points of contact are hands or arms and for departure, the feet or the other hand or arm.

The path that the current takes between the entry and exit points will generally be that of the least resistance. Paths through the heart or through the respiratory organs are the most dangerous.

The effect on the body will vary with the general condition of the person; so, for example, a weak heart will be less able to withstand the effects of the shock.

First Aid Treatment

Electric shock victims should receive quick medical treatment. However, it is important not to touch a person who remains in contact with an electrical source as you will also receive a shock.

The first action should be to break any continuing contact between the victim and the current. This should be by switching off the supply or disconnecting the equipment. If this is not possible, the victim should be separated from the current using a non-conductive material; for example, a wooden broom handle, piece of cardboard or dry clothing, etc. to push or drag the person away.
Artificial respiration should start immediately and continue until the victim recovers or qualified medical aid intervenes. In situations where there is a high risk of electric shock accidents, first aid personnel trained in resuscitation methods should be readily available.
Emergency medical attention should always be sought as there is a risk of internal injury which may not be visible.

Electrical Burns

Burns resulting from electricity may be divided into two groups:

Those which result from direct contact with a live conductor.
Those which arise indirectly from situations where there is no contact with a live conductor, – similar to being struck by lightning.

Direct Burns

The passage of an electric current along any conductor is accompanied by the release of heat. Thus, when a current passes through the body, it is likely to cause burns, even though the duration of exposure may be very short.

The heat given off by a current is directly related to the amount of the current and the resistance encountered. So the higher the current, the greater the heat and the more likely it is that burns will result. Similarly the higher the resistance, the greater the heat and the more likely it is that burns will result. As the skin is the site of highest resistance in the body, it is on the surface of the skin at the point of contact with a live conductor that the most severe burning will occur. However, high currents can create internal burns all along the path travelled through the body, causing damage to red blood cells and muscle tissue. Such burns are often deep-seated and slow to heal.

Indirect Burns

The most common cause of electrical burns, excluding situations where a person touches a live conductor, is arcing. Arcing can occur when one conductor with a very high potential is brought into close contact with another earthed conductor. The voltage may be large enough for the natural insulation of the air between them to break down and a spark to jump the gap. This ionises the air, considerably lowering its resistance, which in turn allows the current to increase so that an electric arc is set up between the conductors. Very large currents can flow through the arc in a very short time, possibly less than one second, in a similar way to lightning.

Arcing will only occur where the live conductor is uninsulated, where the insulation is insufficient to prevent the force generated by the high voltage or where a fault has developed exposing the live conductor. The other earthed conductor may be a metal object on the ground or a person.

If the arc is connected to a person, which may happen for example when he/she gets too close to an overhead high voltage power line, the victim may be subject to both a flame burn from the arc and electric shock from the current which passes through the body. The burns are often worsened by clothing catching fire.

Even where the arc does not actually touch a person there is a danger of burns. Arcing generates ultraviolet radiation which can burn the skin and the retina of the eye (causing arc eye or eye flash). Additional burns may result from radiated heat, where the body absorbs the heat energy at a distance, resulting in burns located deep in the body.

Common Causes of Electrical Fires

Fires require three elements in order to start: a source of heat, combustible material such as fuel, and oxygen. Electricity may provide the source of heat in two main ways:

Arcing – the generation of electrical sparks or arcs between an uninsulated or poorly insulated conductor and another earthed conductor. This may be on a very small scale – for example, within a plug – but so long as there is some combustible material to burn it may be sufficient to start a fire. Damage to the insulation enclosing a live wire, broken parts or incorrect wiring of a connection may be the cause.
Overheating of conductors – This may be due to poor or inadequate insulation allowing the natural heat created by the flow of electricity to escape, to overloading the conductor with too high a current for the capacity of the wire, or to excessive resistance within the conductor (for example, passing a current through a flexible cable wound onto a drum, where the bends in the cable increase resistance).

If the temperature generated in either of these circumstances is sufficient to ignite any combustible material, either in contact with the source of the heat or nearby, fire may be started. There are special dangers where flammable vapours may be present in the atmosphere; for example, in a paint spray booth.

Fires of electrical origin can be very dangerous since it is not possible to use water to extinguish them; water is a good conductor of electricity and its use would create a live electrical hazard.

Portable Electrical Equipment

Nearly a quarter of all reportable electrical accidents involve portable equipment. Most of these accidents result in electric shock, with the next most common result being fires.

Many accidents are caused by faulty leads to appliances, although faults in the equipment itself are also a major cause.

The conditions and practices likely to lead to these problems are:

Using equipment in inappropriate conditions – most particularly where cables are liable to be damaged in use and/or where there is water present. For example, using an electric powered pressure water cleaner outside, where the trailing cable may be damaged by vehicles and other equipment, and live wires exposed in a wet environment. In offices, the leads from floor cleaners or kettles are often exposed to damage where they trail across corridors.
Using damaged equipment – either from a failure to carry out routine maintenance checks and repairs or continuing to use equipment after it has been damaged. This is a particular problem on construction sites where the harsh operating conditions mean there is a high probability of mechanical damage and often many people use the same piece of equipment with few if any checks. Damaged or missing insulation or insulation failure can easily expose people to the risk of electric shock.
Having incorrect wiring and connections, usually as a result of poor maintenance and repairs.
Servicing equipment without disconnecting supply.

Secondary Hazards

In addition to the risk of personal injury from shocks, burns and fire as a direct result of electrical problems, any failure in the electrical circuit will have an impact on all machines and other systems on that circuit. The interruption in the power supply, for however short a time, may cause inadvertent mechanical movement of plant or machinery which may pose a risk in itself. There is also the risk of a failure of electrical protection devices such as fire alarms and smoke detectors or machine guards.

Remember too that secondary hazards may arise from electric shocks if they cause a fall from a height or against dangerous objects.

Revision Question 1

What is the voltage of a circuit?
What determines the current in a circuit?
What is the difference between resistance and impedance?
What is a short circuit?
What does arcing do?
What is the main effect of electric shock on the body?
If a person receives a shock for one second which passes through the body along a path with an impedance of 10,000 ohms, what would be the current received and what effect might it have on the person if the voltage of the circuit touched was:

(i) 240 volts.

(ii) 110 volts.

(iii) 50 volts?

What is the first step in treating a victim of electric shock?
What is arcing and what risks does it pose?
Why are cable drum extension leads dangerous?

The suggested answers are given at the end of the element.

Control Measures

The golden rules for handling faulty electrical equipment are to ensure that it has been disconnected from the electricity supply and that it cannot be reconnected, whether accidentally or intentionally, and to check that the circuit is dead. Safe handling of electrical equipment in normal conditions depends on:

Proper selection of suitable equipment.
The use of appropriate protective devices.
Effective inspection and maintenance routines, undertaken by competent people.

Selection and Suitability of Equipment

The selection of suitable work equipment for particular tasks and processes makes it possible to reduce or eliminate many risks to the health and safety of people in the workplace. This applies both to the normal use of the equipment as well as to other operations such as maintenance.

There are basically three points to the safety of work equipment:

Its initial design and quality.
The purpose for which it will be used.
The place where it will be used.

Integrity and Use

When evaluating the suitability of the construction of electrical systems, several factors should be considered:

The manufacturer’s recommendations.
The likely load and fault conditions.
The probable use of the system(s).
The need for suitable electrical protection devices, such as overload protection.
The environmental conditions which may affect the mechanical strength and protection required.

No electrical equipment should be put into use where its electrical strength and capability may be exceeded in such a way as may give rise to danger. In other words equipment should not be subject to electrical stresses with which it would be unable to cope. Equipment should be able to withstand normal, overload and fault currents. It should be used within the manufacturer’s rating and in accordance with any instructions supplied with the equipment.

The conditions which a piece of equipment will withstand can be found from electrical specifications and tests undertaken by the manufacturer and accredited testing organisations, based on international and national standards.

Hazardous Environments

If it is reasonably foreseeable that electrical equipment may be exposed to adverse or hazardous environments, then it should be constructed and protected to prevent danger arising from the exposure. The protection necessary will vary depending on the type of hazard and the degree of risk.

It is necessary to select the correct type of equipment for the environment after considering the present and future conditions the equipment is likely to be exposed to. Hazardous environments include the following:

Weather

Equipment and cables must be able to withstand exposure to weather (rain, snow, ice, wind and dust). There is a particular risk of corrosion of exposed parts. Precautions include containment of equipment in suitably weather-proofed enclosures. Additional protection may be necessary to protect equipment from lightning.

Natural hazards

These include solar radiation (sunlight) as well as animals and plants which may affect cables; for example, there may be a need to protect against gnawing of cables by rats. Siting is also very important here.

Extremes of temperature and pressure

The temperature of equipment may be raised by heat generated in the equipment itself or by an external source. It may also be caused by a build-up of debris and dust. Suitable protection includes containment of equipment in a suitably designed container to protect against extremes of temperature and pressure. Also, means of dispersing excess heat can be incorporated into the design of equipment; for example, fans built into motors. In the case of debris, accumulations should be removed or preferably prevented from occurring. Accumulations of waste should be removed or preferably prevented from occurring.

Dirty conditions

This includes contamination from both liquids or solids. Precautions include containment in a construction to resist the entry of dirt and dust. In less important cases, regular inspection and cleaning as part of a maintenance programme would be acceptable.

Corrosive conditions

Substances may be corrosive alone or in combination with moisture. Protection may have to be via total enclosure in corrosion resistant housing, that is not ventilated to the atmosphere.

Liquids and vapours

Electrical equipment must be protected against immersion, splashing or spraying with water and solvent vapours, as well as against condensation. Precautions include housing equipment in waterproof casing and enclosing in airtight containers.

Flammable substances

The presence of flammable materials, including flammable dusts and vapours, presents a danger in the use of electrical equipment. A dust cloud may pose an explosion hazard, while combustible dusts which settle on electrical equipment can create fire hazards. The selection, construction or installation of the equipment should be such as to guard against the possibility of ignition. If equipment is used in potentially explosive atmospheres, it should be constructed so that it is not liable to cause ignition of the atmosphere.

Mechanical Damage

The susceptibility of the equipment to mechanical damage must be assessed, both in terms of the environment within which it is to be used and the natural operation of the equipment itself. Damage may arise from impact, stress, wear and tear, vibration, hydraulic and pneumatic pressure.

Abrasion may be caused by mechanical movement or the movement of people and can cause extensive damage to equipment, particularly portable equipment and flexible cables. In the case of cables, protection against abrasion includes enclosing them within a protective cover such as flexible armouring, protective braiding or superior forms of sheathing, burying them below ground or placing them at a height.

Movement at the point of entry of a flexible cable into a rigid joint, such as a plug or cable connector, causes much damage. Where it is impossible to avoid rigid connections to flexible cables, a pliable supporting sleeve should be used. Cable joints are also subject to stress and individual wires from which the conductors are made can be pulled loose from their terminals and make accidental contact. Cable clamps within the connectors which take the stress should be used. An additional precaution is the correct use of cord grips located in plugs and connectors which also have moulded-in plastic channels or barriers which can prevent accidental contact.

Protective Systems

Protective devices incorporated into electrical circuits or the equipment itself act to cut off the electricity supply in the event of a fault and/or to reduce the current delivered to a person in the form of an electric shock.

Fuses and Circuit Breakers

Both these devices act to break a circuit in the event of an overload of power:

A fuse forms a weak link in a circuit by overheating and melting by design if the current exceeds the safe limit.
A circuit breaker is a mechanical device in the form of a switch which automatically opens if the circuit is overloaded.

Both protective devices should be chosen so that their rating is above the operating current required by the equipment, but less than the current rating of the cable in the circuit. They should be fitted to the conductor as close as possible to the live terminal.

Primarily designed to protect the equipment/circuit/system, these features may also offer secondary protection to a person.

Earthing

By earthing the exposed metal parts which should not normally carry a current, any fault current is provided with a low impedance path to earth should it become live. If all exposed metalwork is properly bonded to earth, it cannot be made live by a fault and the risk of shock is eliminated. The design and quality of the earth conductor is vital because if it fails, the protection is removed.

Earthing measures should be connected so that the fault current will operate protective devices (fuses, residual current devices) and cut off the supply by breaking the circuit.

In certain cases, such as wet environments, additional protection is necessary due to the hazard posed by the close proximity of water, electricity and metal objects; for example, in a central heating system driven by an electric pump. All external metalwork should be connected by a common bonding conductor which ensures that all the metalwork is at the same potential. This measure is called equipotential bonding. A current will not flow between two points at the same potential, so if any of the metal fittings become live, any of the other metal fittings may be touched simultaneously without the risk of electric shock. A common connection to earth is usually made.

Isolation

There is a difference between isolation and switching off:

Switching off refers to depriving the equipment of electric power whilst still leaving it connected.
Isolation refers to physically separating it from any source of electric power, with the additional step being taken of ensuring that it cannot be accidentally re-energised.

Isolation should establish an effective barrier between the equipment and the supply and ensure that no unauthorised person is able to remove the barrier. In particular, it should:

Establish an air gap between the contacts in the switch or some other barrier which would prevent the flow of current under all conditions of use.
Include a device such as a padlock or lock which will prevent the removal of the barrier by unauthorised persons.
Be accessible, easy to operate and clearly labelled.

Reduced/Low Voltage Systems

Where environmental conditions are harsh, as on construction sites or in areas which are wet, and there is a high risk of electric shocks, the use of reduced or low voltages is advisable to reduce the effect of any shock.

For hand-held portable tools and the smaller transportable units, the 110 volt centre-tapped (CTE) system is recommended, using a transformer to reduce the voltage from the public supply. The system relies on the mid-point of the transformer to be earthed (centre-tapped). The maximum shock voltage to earth is then half the supply voltage, that is 55 volts in the event of direct contact. As most shocks occur between a live part and earth this is a major step in the reduction of the shock effect. The full 110 volt supply is available to power the equipment.

Lower voltage systems which are called “safety extra low voltage” or SELV are those in which the voltage does not exceed 50 volts ac between conductors in a circuit which is isolated from the supply mains and from earth by means of a safety isolating transformer. These systems represent even less of a hazard and should be used in other environments such as vehicle washing areas and in the vicinity of swimming pools. They are also recommended for hand lamps, soldering irons and other small hand tools where the risk of shock is high.

Residual Current Devices

Sensitive earth leakage protective devices provide another means of circuit interruption in the event of an earth fault and are also intended to provide protection from indirect shock.

Residual current devices (RCDs) or sensitive current-operated earth leakage circuit breakers (ELCBs) detect when a current flows to earth by comparing the currents flowing in the live and neutral conductors. They are sensitive enough to detect a leakage current too small to operate a fuse, but which may nevertheless be large enough to deliver an electric shock or to start a fire. In that event they interrupt the supply by means of automatic circuit breakers. The sensitivity of the device to the level of leakage can be adjusted so that any shocks experienced are not lethal.

Every RCD has a test button which should be checked regularly to ensure correct operation.

It is important to note that ELCBs only operate when a fault to earth occurs. They do not provide overload protection. They reduce the effects of a shock, not the chances of getting a shock.

Double Insulation

If equipment has a metal enclosure, precautions must be taken to prevent the metalwork from becoming live. This can be achieved by “double-insulation” in which the live parts of the equipment are covered by two layers of insulating material.

Each layer is capable of insulating the live parts alone, but together they ensure that the occurrence of insulation failure and its associated danger is extremely improbable. This method is also suitable for portable equipment which often suffers particularly rough use, but regular maintenance is essential as the insulation only remains effective while it is intact. In addition to maintenance, the insulation must be soundly constructed and the equipment properly used. It reduces the chances of getting a shock.

The principle of double insulation also applies to the use of insulating mats for operators to stand on and use of insulated tools and personal protective equipment, such as rubber footwear, heat resistant face shields, clothing and insulating gloves.

Inspections and Maintenance Strategies

Proper maintenance is at the heart of ensuring the safety of electrical equipment. It is a general term which in practice includes visual inspection, testing, repair and replacement, as both part of the routine of using the equipment and as a specific activity in its own right.

There should be an appropriate system for this which is designed to ensure all aspects of maintenance are carried out. The basic requirements are:

Identification of the equipment which has to be maintained and where/how it is to be used.
Discouragement of “unauthorised” equipment in the workplace.
Carrying out simple user checks for signs of damage – for example, casing, plug pins and cable sheath.
Formal visual inspections carried out routinely by a competent person.
Periodic testing of equipment by a competent person.
Systems for the reporting and replacement of defective equipment.
Recording of all maintenance and test results along with the inventory of equipment in use.

User Checks

The person utilising the electrical equipment should be encouraged to look at it critically and, after a minimum basic training, check visually for signs that the equipment is not in sound condition; for example:

Damage to the cable sheath (apart from light scuffing)
Damage to the plug – for example, the casing is cracked or the pins are bent.
Inadequate joints, including taped joints in the cable.
The outer sheath of the cable is not effectively secured where it enters the plug or the equipment – obvious evidence would be if the coloured insulation of the internal cable cores were showing.
The equipment has been subjected to conditions for which it is not suitable – for example, it is wet or excessively contaminated.
Damage to the external casing of the equipment or there are some loose parts or screws.

These checks also apply to extension leads and associated plugs and sockets. Checks should be undertaken by the user before and during use.

Any faults should be reported to management and the equipment taken out of use immediately. Management should take effective steps to ensure that the equipment is not used again until repaired by a person competent to carry out the task; the defective equipment could be labelled as “faulty” and its associated plug removed.

Formal Inspection and Tests

It is common practice to have formal inspections of electrical equipment and installations. There are two types of formal inspection:

Routine visual inspections, carried out by a competent person to control immediate risks and monitor the user checks. These will be visual checks similar to those discussed above, but undertaken in a more formal and systematic manner.
Periodic detailed inspections, including testing the equipment, again carried out by a competent person. Such inspections and tests may also be required in particular situations where there is reason to think that the equipment may have a fault.

Any equipment found to be faulty should be taken out of service and not used again until properly repaired.

In common with all tasks, the inspection and maintenance of electrical appliances and systems should be carried out by a “competent person”, who is

‘… a person possessing such knowledge or experience, or who is under such degree of supervision as may be appropriate having regard to the nature of the work’.

Because of the extreme danger of coming into contact with live electrical conductors, any person involved with the maintenance, repair and inspection of electrical equipment must be familiar with the requirements of the particular task undertaken, or be supervised by somebody who is. Therefore, the person should, as part of their competency, have either carried out these tasks before or been specifically trained in them.

The competent person can normally be a member of staff who has sufficient information and knowledge, following appropriate training on what to look for and what is acceptable, and who has been given the task of carrying out the inspection. To avoid danger, competent persons should know when the limit of their knowledge and experience has been reached (this is one of the definitions of a competent person).

Combined inspection and testing should be carried out by someone with a higher level of competence than that required for visual inspection alone, because the results of the tests may call for interpretation and appropriate electrical knowledge will be essential. However, the same worker can often carry out both types of inspection.

Visual inspections

Regular visual inspections are generally the most important part of a maintenance regime and most potentially dangerous faults can be detected in this way.

Simple written guidance relating to the visual inspection can be produced, summarising what to look for, procedures to follow when faults are found and when unauthorised equipment is found in use. It can help whoever is carrying out the formal visual inspection and also users.

The formal visual inspection should not include taking the equipment apart. This should be confined, where necessary, to the combined inspection and testing discussed below. However, additional checks could include removing the plug cover and checking that a fuse is being used (for example, it is a fuse, not a piece of wire, a nail, etc.), the cord grip is effective, the cable terminations are secure and correct, including an earth where appropriate, and there is no sign of internal damage, overheating or presence of liquid or foreign matter. Checks may also be made to ensure that there is no evidence of overheating (burn marks or staining) and that the correct rating for the fuse and the correct cable rating is being used (to prevent overloading).

The inspections should be carried out at regular intervals. The period between inspections can vary considerably depending on the type of equipment, the conditions of use and the environment. For example, equipment used on a construction site or in a heavy steel fabrication workshop will need much more frequent inspection than such equipment as floor cleaners in an office. In all cases, however, the period between inspections should be reviewed in the light of experience.

Combined inspection and tests

The checks and inspections outlined above will, if carried out properly, reveal most (but not all) potentially dangerous faults. However, some deterioration of the cable, its terminals and the equipment itself can be expected after significant use; for example, a broken earth wire within a flexible cable, deterioration of insulation quality or contamination of internal and external surfaces. Additionally, equipment may be misused or abused to an extent which may give rise to danger.

Inspection and testing are the only reliable ways of detecting such faults and should be carried out on a regular basis to back up the inspection regime. The regularity will depend on the type of equipment, the manner and frequency of use and the environment. In addition, other occasions when testing is likely to be justified are:

Whenever there is reason to suppose the equipment may be defective, but this cannot be confirmed by visual inspection alone.
After any repair, modification or similar work.

Testing is often carried out at two levels, demanding different levels of competence from the person carrying out the task:

Simple “pass/fail” types of test where no interpretation of readings is necessary. Providing the appropriate test procedures are rigorously followed and acceptance criteria are clearly defined, the routine can be straightforward.
The use of more advanced test instruments which give readings that require interpretation. This requires technical knowledge or experience and specific electrical skills.

The inspection carried out in conjunction with the testing should usually include checks for:

Correct fusing.
Effective termination of cables and cores.
Suitability of the equipment for its environment.

Frequency of Inspection and Testing

If there are no specific legal requirements in your country, deciding on the frequency of maintenance is a matter of judgement for those responsible for the equipment and should be based on an assessment of risk factors, which include:

Type of equipment and whether or not it is hand-held.
Manufacturer’s recommendations.
Initial integrity and soundness of the equipment.
Age of the equipment.
Working environment in which the equipment is used (such as whether it is wet or dusty) or the likelihood of mechanical damage.
Frequency of use and the duty cycle of the equipment.
Foreseeable abuse of the equipment.
Effects of any modifications or repairs to the equipment.
Analysis of previous records of maintenance, including both formal inspection and combined inspection and testing.

Records of Inspection and Testing

It is useful to record maintenance, including test results. A suitable log is a management tool for monitoring and reviewing the effectiveness of the maintenance scheme and indeed to demonstrate that a scheme exists. It can also be used as an inventory of portable electrical equipment and a check on the use of unauthorised equipment (for example, domestic kettles or electric heaters brought to work by workers).

The log can include faults found during inspection and may give an indication of the types of equipment or environment which are subject to a higher than average level of wear or damage. This will help monitor whether suitable equipment has been selected. Entries in a test log can also highlight any adverse trends in test readings which may affect the safety of the equipment, thus enabling remedial action to be taken. Care should be taken in interpreting trends where a subsequent test may be carried out with a different instrument from that used for an earlier test, since differences in the results may be due to differences in the test instruments rather than indicating deterioration in the equipment being tested.

Records do not necessarily have to be on a paper system since test instruments are available which store the data electronically for downloading directly onto a computer database.

It is useful to label equipment to indicate that it has been tested satisfactorily and has been passed as safe, and when the date for the next test is due. Otherwise individual items may be missed on following checks.

Portable Appliance Testing

It is important that portable electrical appliances are maintained in exactly the same way as fixed equipment. By their very nature, electric drills, saws, trimmers, etc. are out in use much of the time and there must be an effective system for keeping track of each item to ensure that the maintenance schedule is followed and use can be monitored.

All pieces of equipment should be identified by a serial number and recorded in a register which specifies when each item should be recalled for inspection. A nominated person should be appointed to ensure that recall and inspection do take place.

As mentioned earlier for fixed equipment, the frequency of inspection will depend on the risk assessment. It should be determined by the type of equipment and its use, the manufacturer’s recommendations and the experience of the user. The equipment should either be marked to indicate to the user when the inspection is due or should be clearly indicated on the checking-out sheet used to obtain an appliance from store.

The inspection and any subsequent tests and repairs should be carried out by a competent person experienced in this type of work. A record of inspection should be made and kept for the life of the equipment. Inspection and testing should cover the same points as described above in general, although particular attention should be paid to leads and plugs which are more vulnerable on portable equipment.

Revision Question 2

What five factors should be used to assess the suitability of the construction of an electrical system?
What protection is offered by the cord grip in a plug?
What is the difference between a fuse and a circuit breaker?
What is the purpose of a fan in an item of electrical equipment?
What is equipotential bonding?
What is the difference between switching off and isolation?
What protection is offered by a reduced voltage transformer in a circuit?
State the main features of a proper system of maintenance.
What checks should be carried out before an item of electrical equipment is used?
What is the safest method of powering electric hand tools which are being used outdoors?

The suggested answers are given at the end of the element.

Summary

The basis of an electrical system is a circuit in which a conductor links two terminals, one providing the electricity supply (the live terminal) which then flows along the conductor to the other (neutral) terminal. The force which makes electricity flow is the difference in electrical potential between the terminals and is measured in volts. The current which flows is measured in amperes (or amps). Resistance, measured in ohms, is provided by components connected to the circuit which transform the electricity into other forms of energy, and the impedance offered by the conductor itself.

The relationship between voltage, current and resistance is expressed by Ohms Law and this can be used to calculate the value of any one of the three elements where the other two are known.

Short circuits are formed by another conductor connecting to the circuit and providing an alternative path along which the current can flow, usually to earth. The conductor carrying the short circuit may be an object or a person.

The main hazards of electricity are electric shock, electric burns and fire. There may be secondary hazards caused by the effects of electric shock (such as falls) or by an interruption of the power supply to machinery or protection devices/systems.

Electric shock causes a convulsive response by the nervous system, resulting in the muscles contracting, often violently. The severity of the effect is determined by the amount and nature of the current, the duration of the contact with the live conductor and the path the current takes through the body. Most shocks are potentially fatal where they affect the heart or respiratory organs.

Electrical burns may be the direct result of contact with a live conductor (in which case they may be both external and internal) or be the result of arcing.

Arcing is also a significant cause of electrical fires. The other main electrical cause of fire is overheating of a conductor, either through overloading or excessive resistances.

Control measures to reduce the risk from electrical hazards are based on the proper selection of suitable equipment for the task in the particular operating conditions, the incorporation of specific protective devices and the adoption of systematic maintenance procedures.

Protective devices act mainly to cut off the supply in the event of a fault, or to reduce the current and the effect of any electric shock received. The main measures are:

Fuses and circuit breakers.
Earthing.
Isolation.
Reduced/low-voltage systems.
Residual current devices.
Double insulation.

Maintenance strategies are based on regular user checks, routine visual inspections and periodic detailed inspection and testing of all items of equipment. Records should be kept of all formal visual inspections and tests.

Portable electrical equipment presents particular hazards, especially in relation to cables connecting to the mains supply and its use in adverse conditions, such as wet environments or where it may be subject to mechanical damage. It is very important to follow strict maintenance strategies to ensure their continued safety.

(i)	I = = = 24 mA.	This will cause strong muscle contraction and possibly some breathing difficulties.
(ii)	I = = = 11 mA.	This will be painful and there will be some muscle contraction.
(iii)	I = = = 5 mA.	This will be barely perceptible, perhaps some mild tingling will be felt.

Wednesday, 1 October 2014

Element 10 | Electrical Hazards and Control

INTRODUCTION

Principles Of Electricity

Voltage, Current and Resistance

Voltage

Current

Resistance

Relationship Between Voltage, Current and Resistance

Basic Circuitry

Short Circuits

Earthing

Direct and Alternating Currents

Hazards of Electricity

Electric Shock

Effects on the Body

Factors Influencing Severity

First Aid Treatment

Electrical Burns

Direct Burns

Indirect Burns

Common Causes of Electrical Fires

Portable Electrical Equipment

Secondary Hazards

Revision Question 1

Control Measures

Selection and Suitability of Equipment

Integrity and Use

Hazardous Environments

Mechanical Damage

Protective Systems

Isolation

Reduced/Low Voltage Systems

Residual Current Devices

Double Insulation

Inspections and Maintenance Strategies

User Checks

Formal Inspection and Tests

Frequency of Inspection and Testing

Records of Inspection and Testing

Portable Appliance Testing

Revision Question 2

Summary

Suggested Answers to Revision Questions