Electricity

Introduction

Electricity is a relatively safe form of energy. It is a familiar and necessary part of everyday life, but electricity can kill or severely injure people and cause damage to property if not used sensibly. An electric shock occurs upon the contact of a (human) body part with any source of electricity that causes a sufficient current through the skin, muscles or hair. Typically, the expression is used to describe an injurious exposure to electricity- a pathophysiological effect of an electric current through the human body. Very small currents can be imperceptible. Larger current passing through the body may make it impossible for a shock victim to let go of an energized object. Still larger currents can cause fibrillation of the heart and damage to tissues. Death caused by an electric shock is called electrocution.

The risk of electric shock is greater in certain working conditions, for example wet areas. Accidents frequently involve the use of electrical appliances and tools, and unauthorized work on the electrical equipment of machinery and fixed electrical installations. The risks can be reduced by protective measures in accordance with the relevant regulations and standards.

Definitions

Source: Overview by the author

Electrical hazards

The daily use of electricity and consequent exposure to electrical hazards is almost unavoidable. We are using more and more electrical equipment in workplaces and in everyday life. There are also relatively new and affordable ways of generating electricity for example the use of photovoltaic systems that convert solar radiation into direct current electricity using semiconductors. In addition there are also new uses of electricity that need to be considered such as electric vehicles. Most electrical accidents occur because people are working on or near equipment that is:

• thought to be dead but is live;
• known to be live but those involved do not have adequate training or n appropriate equipment, or have not taken adequate precautions.

Primary (direct) hazards

The main risks associated with electrical energy are electric shock due to direct or indirect contact with live parts and electrical burns. The human body is very sensitive to current magnitude. Weak current mostly causes functional disorders, while heavy current cause tissue burn. Both effects could be fatal. Electrical burns differ from thermal or chemical burns in that they cause much more sub dermal damage. At lower currents, this damage might not be visible on the surface and therefore is more difficult to accurately diagnose but they can cause critical internal injuries. At high voltages the currents are greater. The skin resistance is high and the resistive heating causes electrical burns and can puncture the skin at the entry points. The brain and other nervous tissue lose all functional excitability when high currents pass through them. Another hazard to take into consideration is the arc. An arc can occur, for example, as a result of accidental short circuit. Even if it persists for only a very short time the heat generated can be powerful and can cause very deep and slow-healing burns. The intense ultraviolet radiation from an electrical arc can also cause damage to the eyes. The risk is grater in electrical work activities on or near live electrical conductors. It is mainly skilled persons (electricians, technicians and electrical engineers) who often carry out this work that are exposed to the risk of arc in the case of unsafe working practices ^[1]. There are a few exceptional circumstances where high voltages will not give rise to danger. High-voltage equipment should be designed and installed so that it is not necessary to work on exposed live parts. However, allowance has to be made for carrying out potential checks or tests, and also for observation from safe distances such as when phasing out. Because high voltages can arc across an air gap, it is not necessary to touch live voltage parts to suffer a shock or burns. Thermal burns are also possible. Overloaded, faulty, incorrectly maintained, or shorted electrical equipment can get very hot, and some electrical equipment gets hot in normal operation. Some equipment operates at voltages that are so low that they cannot give an electric shock but, even at these extra-low voltages, an arc can occur or thermal burns can result from overheated conductors. A good example of this is a short-circuited 12 V DC car battery which may cause a very intense arc or even explode.

Secondary (indirect) hazards

There are also secondary hazards that can also endanger those not using or working with electricity. Faulty electrical installations and electrical equipment can lead to electrical leakage currents, arcing and overheating which could cause fire or explosion by igniting flammable materials. This can cause death, injury and considerable financial loss. Even non-fatal shocks can cause severe and permanent injury. For example, the person involved could fall from a ladder, scaffolding or other work platforms or be injured by the rotating parts of equipment. Stronger electric fields are generated by circuits or equipment at high voltage. Whenever current is flowing, magnetic fields are generated around the conductor. Low-frequency electric and/or magnetic fields induce electric currents in the body, which can cause stimulation of nerves and muscles. The risk assessment should take into account the potential harmful effects of electric and magnetic fields, which depend on the type of the fields, the level of exposure and physiological characteristics of the workers.

Unwanted static electricity

Unwanted static electric charge can build up in many processes or working equipment, mainly due to friction between parts of the machinery or between the machinery and working materials or fluids used or produced by the production process. Static charge may also be created in ungrounded metal parts by induction in an electric field. When a person comes into contact with or approaches a charged part, an electric discharge current can flow through the body to the earth. The resulting physiological effects mainly depend on the amount of discharge energy and can be merely annoying or painful or can have life-threatening consequences. The effect of surprise can contribute to the risk of an accident. The discharge of static electricity can also ignite a fire or trigger an explosion or just damage electronic circuits in control systems or impede their correct functioning. This can leads to hazardous situations.

Circumstances in which accidents occur

Approximately half of electrical accidents are associated with a work activity according to the International Labour Office ^[2]. The other half tends to occur at home or during leisure activities. However; this can vary from country-to-country and year-to-year, as for example in Ireland ^[3]. The reasons for the differences are probably: different levels of development, different economic activities (this may vary within the same country from year-to-year) and different practices and regulations in the field of electrical installations and safety and health at work.

Electricity is the cause of approximately 5 % of all fatal accidents of workers and for about one percent of all non-fatal injuries ^[4], ^[5] according to statistics in Germany, GB and USA. This means that the fatality is relatively high. This is especially true for high voltage accidents ^[4]. The accidents are often caused by one or more of the following factors:

• human factors
  • use of inappropriate electrical equipment
  • being too accustomed to the danger
  • lack of awareness of the electrical danger
  • not using personal protective equipment
  • lack of competence
  • non-compliance with advice and instructions

• physical factors, which are often related to lack of maintenance or repairs
  • earth protection fault - most failures of equipment grounds occur, either at the ground contact of the receptacle or in the plug and cable leading to the line-powered equipment.
  • faulty safety equipment, for example, faulty RCD device
  • insulation fault, for example, damaged cable or damaged insulation housing

The ratio between both main groups is approximately 80/20 ^[4]. Hazards are greater in certain areas, for example ^[6]:

• in wet areas – unsuitable or damaged equipment can easily become live and can make its surroundings live, for example, the housing of many electrical equipment for dry conditions have holes and vents for cooling that provide access for spilled conductive fluids and wet parts of the housing can conduct enough electricity to cause these areas to become temporarily live

• outdoors – equipment may not only become wet but may be at greater risk of damage because of the potential exposure to shocks or vibrations during transport and to heat, oil, sharp edges, and moving parts during the use
• in cramped spaces with a lot of earthed conductive surfaces

The industrial sectors and the environments in which these listed conditions are commonly found are construction sites, the agricultural sector, metalworking industry, services performed outdoors, maintenance … Over half of the fatal incidents in GB are caused by contact with overhead power lines ^[6]. Power lines are everywhere and there is often a lack of awareness - workers and the public often do not treat power lines as a threat or danger. Contact with overhead power lines likely results in serious injury or fatality.

Accidents frequently involve the use of electrical equipment, as well as unauthorized work on the electrical equipment of machinery and fixed electrical installations. According to ISSA ^[7], the ratio of portable electrical equipment accidents to all electrical accidents is approximately 1:5. In GB nearly a quarter of all electrical accidents involve portable equipment ^[8]. Some items of equipment can also involve greater risk than others. Cables connected to portable equipment are particularly exposed to damage at the connection points since they are often moved. There is also frequent damage of the insulation over the whole exposed length of the cable due to mechanical stress and sharp objects. In these parts there is a risk of direct contact, in particular in a wet environment. The same applies for extension leads together with their plugs and sockets.

Most accidents happen at the usual consumer voltages of low-voltage distribution installations 230 V AC (against earth) and 400 V AC (between two phase conductors) ^[4]. Low voltage does not mean low hazard! Even contact with extra low voltage below the limit value of 50 V AC or 120 V DC can cause accidents. In some critical work areas, it may be necessary to limit the voltage to lower than 24 V AC or 60 V DC, for example, in conductive or wet locations with restricted movement.

Electric shock (physiological effects of electric current)

Important influential factors

IEC publication 60479-1 ^[9] provides basic guidance on the effects of shock current on human begins and livestock. There is information about body impedance and body current thresholds for various physiological effects. The harmful effect on the internal organs and their proper function mainly depends on the following factors:

• amount (magnitude) of current (which is a function of the touch voltage and total body impedance)
• length of contact time
• path of current

In practice, we mostly use a voltage sources that supplies a constant AC or DC voltage (electrical potential difference) between its terminals. It is common for DC to have an AC component (called ripple). The voltage to which the human body is subjected is called the touch voltage. The level of the electric current that could flow through the human body can be calculated using “Ohm’s Law" which defines the relationship between voltage, current and resistance (or impedance in the case of AC current). The higher touch voltage and or a lower impedance means a higher current and vice versa - higher impedance and or a lower touch voltage results in lower current. The values of body impedance depends on a number of factors, in particular, on current path, touch voltage, duration of current flow, frequency, the degree of moisture of the skin, surface area of contact, pressure exerted and temperature. In general, a value between 500 and 1000 Ohms can be expected as minimum value of body impedance under the worst case conditions (large surface, wet conditions, damaged skin, high voltage). If a person makes direct contact with a voltage source of 50 V, and the body impedance is 500 Ohms, the electric current through the person’s body would be 50 V / 500 Ohms, which would equal 100 mA. Except for cases where a person might be wet with water, the resistance of the electrical circuit with the human body would much greater than 500 Ohms, up to 100,000 Ohms. This would be especially true if the person were wearing shoes or gloves, as these articles of clothing typically involve rubber, leather, or polymeric materials that are generally highly resistive in nature. The path of the electrical current through the body affects the severity of the electric shock. Currents through the heart or nervous system are most dangerous mainly due to the risk of ventricular fibrillation. The most dangerous paths are in the following order: chest to left hand, chest to right hand, left hand to left, right or both feet, both hands to both feet. Alternating current (AC) and direct current (DC) have slightly different effects on the human body, but both are dangerous above a certain level. Under the same circumstances, alternating current is more dangerous than direct current. Accidents with direct current are much less frequent than would be expected from the number of DC applications. This is partly due to the fact that with direct current, the let-go of parts gripped is less difficult. Another reason is that for shock durations longer than the period of the cardiac cycle, the threshold of ventricular fibrillation is considerably higher than for alternating current ^[10]. The frequency of AC current is also an important factor. The risk of injury changes according to the frequency. The most dangerous range is between 15 and 100 Hz (cycles per second).

The effects of AC current in the frequency range of 15 Hz to 100 Hz

Threshold of perception and reaction

The threshold of reaction of a human body is assumed to be 0.5 mA independent of time of exposure. The threshold of perception is the minimal current that a person can detect and is under the threshold of reaction. Both thresholds vary considerably among individuals and depending on the measurement conditions (contact area, dry, wet, pressure, temperature).

Immobilization and threshold of let-go

Immobilization is the involuntary contractions of the muscles of influenced person. It may be the result of current flowing through the affected muscle or through associated nerves or the associated part of the brain. Contractions of muscles or reflex withdrawals may cause secondary physical injuries, such as those resulting from falling off a ladder. As the current increases further, the immobilization can prevent the person from voluntarily withdrawing. The let-go current is defined as the maximum current at which the subject can withdraw voluntarily. A current of about 10 mA AC, is assumed ^[1] for the threshold of let-go for adult males and 5 mA for the entire population.

Ventricular fibrillation

Ventricular fibrillation is the major cause of death due to electric shock. If the normal cardiac electrical activity is sufficiently disrupted due to the external electric current, the heart rate can rise to 300 beats/min. The heart ceases to function as a pump, blood pressure falls, there is no more oxygen supply and death occurs within minutes. Ventricular fibrillation does not stop when the current that triggered it is removed. For shock durations below 100 ms and for the hand to feet pathway, fibrillation may occur for currents above 500 mA. This threshold decreases considerably if the current flow is longer than one cardiac cycle. For shock durations of 1 s, the level is 50 mA and for durations longer than 3 s it drops to 40 mA. The threshold of ventricular fibrillation also depends on the physiological parameters of the human body.

Other effects

Electrical accidents that do not involve ventricular fibrillation can also be fatal. Strong involuntary contractions of the muscles and stimulation of the nerves can be painful and cause fatigue if there is long exposure. Currents of the same range can cause the involuntary contraction of respiratory muscles and consequently asphyxiation if the current is not interrupted.

Currents of several amperes that last for more than second cause deep–seated burns, surface burns, and other internal injuries. High voltage accidents may not result in ventricular fibrillation, other forms of cardiac arrest may occur.

Protective measures

Protection against direct and indirect contact

The IEC standard ^[11] specifies essential requirements for the protection of persons, livestock and property against direct and indirect contact. Hazardous live parts shall not be accessible and accessible conductive parts shall not be hazardous. This requirement needs to apply under normal conditions and under a single fault condition. Protection may be provided by:

• a measure that combines protection against direct and indirect contact (for example, safety class III appliances that operate on a safety extra-low voltage of 50 V AC or 120 V DC - because of their low power such appliances are not widespread in use), or
• the combination of a measure of protection against direct contact and a measure of protection against indirect contact

The measures against direct contact are:

• insulation of live parts
• barriers or enclosures
• obstacles
• placing out of reach

The measures against indirect contact are:

• automatic disconnection of supply
• class II equipment or equivalent insulation (provided by supplementary insulation or enhanced basic insulation)
• non-conducting location
• earth-free location equipotential bonding
• electrical separation

Measures of protection shall be applied to every installation, part of an installation, and equipment. The following conditions of external influences are relevant to the selection of measures of protection against electric shock:

• qualification of persons
• electrical resistance of the human body
• contact with persons with an earthing potential

The most commonly used measure of protection against indirect contact is the automatic disconnection of supply. In the event of a fault between a live part and an exposed conductive part or a protective conductor, a protective device (fuse or circuit breaker) automatically disconnects the supply to the circuit of equipment for which it provides protection. A prospective touch voltage exceeding 50 V AC or 120 V DC should not persist for a time sufficient to cause a risk of harmful psychological effect in a person in contact with simultaneously accessible conductive parts. Automatic disconnection of supply necessitates co-ordination of the type of system earthing and the characteristics of protective conductors and protective devices. Exposed-conductive-parts shall be connected to a protective conductor under the specific conditions for each type of system earthing. Class I appliances are designed to use this type of protection and are therefore equipped with a protective earth conductor. The fault protection acts in conjunction with a device in the fixed electrical installation that disconnects the power supply in the event of a fault. All components in the fixed electrical installation involved in the protection (earth connection, disconnecting device, etc.) must be in proper working order.

Additional protection – RCD

In certain circumstances all of the preceding protective measures are not sufficiently effective due to, for example:

• lack of proper maintenance
• carelessness
• normal (or abnormal) wear and tear of insulation
• accidental contact
• immersion in water
• no longer effective insulation

In order to protect users from a harmful physiological effect in such circumstances, highly sensitive fast tripping residual-current devices (RCDs) are used to disconnect the power supply automatically. These devices operate on the principle of differential current measurement. In a system supplied from an earthed source the difference between the current entering a circuit and that leaving it occurs if there is a current to earth (a leakage current), either through faulty insulation or through contact between an earthed part, such as a person, and a live conductor. RCDs with sensitivity of 30 mA of differential current are required for protection against direct contact. According to IEC 60364-4-41 ^[11], such protection must be provided for circuits supplying socket-outlets with a rated current ≤ 20 A in all locations, and for circuits supplying portable equipment with a rated current ≤ 32 A for use outdoors. This additional protection is required in certain countries for circuits supplying socket outlets rated up to 32 A, and even higher if the location is wet and/or temporary (such as construction work sites for example). There are also RCDs with intentionally slower responses and lower sensitivities, designed to protect equipment or for fire protection.

Safe work practices

Risk assessment

The health and safety risk assessment should take into account all the risks associated with electricity and should help to select appropriate measures for risk reduction. The person carrying out the assessment should have knowledge and experience of the associated risks and planning safe work procedures.

Safe and suitable equipment

The electrical installations, systems and equipment shall be designed and manufactured for safe operation. The safety features of electrical installations are also very important for the safe use of electrical equipment, therefore compliance with the IEC 60364 series of standards or any equivalent standards is required. Electrical equipment has to comply with the relevant essential health and safety requirements of the appropriate EU Directives ^[12], ^[13] and national legislation. These products should be delivered with adequate instructions on their safe use in the language of the country in which the equipment is used.

Use of RCDs and PRCDs

The use of high sensitivity RCDs is also recommended in the following cases:

• socket-outlet circuits in wet locations for all current ratings
• socket-outlet circuits in temporary installations
• circuits supplying laundry rooms and swimming pools
• supply circuits to work-sites, caravans, pleasure boats, and travelling fairs

The manufacturers or a competent person must be consulted on the selection of the appropriate type of RCD depending on the characteristics of the fault (sinusoidal alternating currents, pulsating direct currents or pure direct current). Portable residual-current devices (PRCDs to VDE 0661) ^[14] may be employed to reduce the residual risk associated with the use of portable tools on socket outlets. If there is an unknown protective measure in a fixed installation, an additional protection is required. This can be achieved using portable residual current device (often referred to as a PRCD-S) which satisfies the following requirements ^[15], ^[16]:

• rated residual current sensitivity ≤ 30 mA
• all switched pole, including protective earth (PE) conductor
• undervoltage protection
• no automatic restart after voltage comeback

And with additional functions:

• the protective device cannot be switched on if the PE conductor is interrupted or live
• if during operation, voltage occurs on the earth, or the PE conductor is interrupted, the protective device has to be switched off
• in case of external voltage on the earth, for example, by drilling, the PE conductor remain switched on

Reduced voltage

The use of reduced voltage supply equipment to supply portable tools used in areas with an increased risk of dangerous shock currents, e.g. construction sites or conducting locations with restricted movement, reduce the maximum possible shock voltage to earth (IEC 60364-7-706). The use of the appropriate type of extra low voltage, if possible, is even better. An equivalent battery-operated version of equipment, if it is available, can contribute to a significant reduction in risks.

Use in a safe manner

The electrical installations, systems and equipment shall be used in a safe manner. All electrical devices and tools are intended for use with a definite voltage and in specific environments like a dusty, humid or explosive atmosphere. Information about the environmental conditions, the device is intended for use in, have to be available on the label of the device or operating instructions. Installations and equipment must be suitable for that use, must be maintained in a condition suitable for that use, and must be used properly. Periodic checks must be made to guarantee the safe condition of the installations and equipment according to the legal provisions. New electrical installations as well as modifications and extensions of existing installations shall be inspected prior to their being brought into operation. The purpose of inspection is to verify that an electrical installation is in accordance with the safety regulations and the specified technical requirements of the relevant standards Electrical installations shall also be inspected at suitable intervals ^[17]. The purpose of periodic inspections is to discover defects that can occur after commissioning and that may impede the operation or generate hazards. An appropriate system of maintenance of equipment can include ^[6]:

• before using any electrical equipment, the user visually checks that it is safe to use, with no signs of damage or defects, and that it is correctly rated for use in the proposed location and environment
• a visual periodical inspection by someone with adequate knowledge
• where necessary, periodic testing by someone with the necessary knowledge and experience to carry out a test and interpret the results

The requirement to carry out periodic inspections and tests, as well as the frequency will depend on the outcome of a risk assessment of the hazards associated with the environment in which the equipment is used. There should be also procedures for the periodic examination and, where necessary, testing personal protective equipment and replacement as necessary. Most portable or semi-fixed equipment, certain lamps, and some types of transformer are designed to have double insulation. It is important to take particular care in the exploitation of class II equipment and to verify regularly and frequently that the class II standard is maintained (no broken outer envelope, etc.).

Work near underground cables and overhead power lines

A powerful arcing current occurs when a cable is cut by a sharp object such as the point of a tool or crushed by a heavy object or powerful machine. Its thermal effect is a major cause of serious injury. The risk of serious or fatal injury during excavation or similar work near underground power cables can be reduced by taking the following precautions ^[18]:

• mapping, recording and marking on the site of cable runs
• use of cable locating devices
• safe digging practices

People, who work on elevated platforms, scaffolding or roofs near power lines must be aware of the risk of electric shock in the case of direct contact or even in case of getting too close. Overhead power lines may also be readily accessible to people working with tall vehicles such as cranes, tipper lorries or farm machinery. While people handling metal ladders, pipes or other long articles, may also be at risk from a flashover or contact with overhead power lines. They must take into account the required safety distance.

Training

Users working on or with the electrical equipment or systems should have suitable training, skill, and knowledge for the task to prevent injury to themselves and others. They should be instructed in the method of carrying out visually checks. The relevant instructions concerning the work equipment should be available in a form and language they can understand.

Skilled personnel must be competent for the task to be undertaken. They must be equipped with and use appropriate tools, measuring and testing devices and PPE, which shall be maintained in a good condition. A specific training programme shall be set out for persons to perform live work. This programme shall comply with the special requirements for live working and shall be based on theoretical and practical exercises ^[17].

Working procedures that have to include permission for the work to be carried out are divided into three different procedures: dead working, live working and working in the vicinity of live parts. All procedures are based on the use of protective measures against electric shock and/or the effects of short-circuits and arcing. Specific and detailed instructions have to be given to the personnel carrying out the work before starting and on completion of the work.

It is also important that personnel are healthy and have no health problems that could affect safety and health at work, for example, a person with a colour vision deficiency cannot become an electrician because colour detection is crucial for identifying wires, safety signs, labels on the controls, etc.

Shock rescue procedure

The right help in the first few minutes, until the arrival of the emergency services, can be crucial for the severity of injuries or even for survival. First responders in an electrical accident must first ensure for their own safety. It is essential that all power sources are isolated before first aid is provided to injured persons. If anyone grabs the victim or pulls the person off the current with their hands, they might become part of the circuit and becomes injured as well. It is necessary to turn the power off at the mains, if possible, or remove any live part, that is still in contact with the casualty and also to isolate yourself from the ground. It is also possible to use an object of low conductivity to push away the power source. Overhead power cables are an example of a high voltage power source. High voltage has the ability to ‘jump’ or ‘arc’ distances of up to a few metres and step voltage can also be dangerous. It is necessary to remain at a safe distance until the power has been switched off by an official agency or company. Once safety is ensured, it is necessary to continue with the normal procedures applicable to first aid. It is recommended to place appropriate instructions on emergency resuscitation procedures in the event of electric shock at those locations where an increased risk of electric shock exists. Only suitably trained persons may perform effective first aid.