Principles of electrical safety:Precautions against electric shock and contact burn injuries.

Precautions against electric shock and contact burn injuries

General principles

Shock and contact burn injuries can be prevented first by ensuring that the electrical system is designed, installed and maintained in accordance with sound engineering principles and in compliance with accepted and published standards, and secondly by ensuring that any work on the system is carried out in a safe manner. This section provides some detail on the hardware design principles, safe systems of work being described at the end. Details can be found in reference 16A.

Prevention of direct contact injuries

Direct contact shock and burn injuries are commonly prevented by ensuring that conductors energized at dangerous voltages cannot be touched. There are, however, some instances where conductors have to be uninsulated and available to touch, such as when live testing work is being done or because they need to be exposed for functional reasons, examples being power pick-up rails on fairground rides and overhead travelling cranes.

The most commonly used preventive techniques are listed. The list is not exhaustive and there are other less commonly used and specialized techniques available.

Insulation and enclosures

Direct contact injuries are most commonly prevented by covering the conductors with suitably rated insulating material, as in the case of cables, or by placing them inside an enclosure that is constructed so as to prevent access to the live parts.

Insulation must be suitably rated so that it can withstand the applied voltage and must be selected or protected to withstand external influences, such as impact and abra- sion, high and low temperatures, sunlight, water and corrosive liquids. Information on cable insulation is provided in Chapter 9 and general information on the properties of insulating materials and dielectrics is provided in Chapter 3.

Enclosures must be constructed of materials that will withstand the environment in which they are installed. They must also prevent access to the internal live parts and prevent the ingress of liquids and dusts that may be present in the operating environment and which may create a hazard if allowed to get into the enclosure.

IEC 60529 (see Table 16.2) defines an Ingress Protection (IP) code that is used to describe how well an enclosure prevents the ingress of moisture and dust. The code uses two figures; a third can be added to cover mechanical impact protection but it is rarely used. The first figure shows the degree of protection against ingress by solid objects and the second figure shows the degree of protection against liquid ingress. As an example, IP20 describes an enclosure that will prevent finger access but will not prevent the ingress of moisture – a domestic socket outlet would fall into this category. An enclosure with an IP rating of IP55, such as a halogen lamp for general illumina- tion, would be suitable for external use. The highest rated enclosures, at IP68, will prevent the ingress of dust and can be immersed in water.

Enclosures are often also classified according to the criteria set out in IEC 60536- 2 (see Table 16.2). There are two main classifications. Class I equipment has a metal casing that needs to be earthed for safety reasons; metal-clad switchgear is an example. Class II equipment has internal electrical conductive parts on which the basic insula- tion is supplemented by additional insulation to provide an additional insulating barrier; this type of equipment, of which the common plastic-encased power drill is an example, does not require any metal parts of the casing to be earthed on the grounds that it would only become live if both layers of insulation were to fail, an event that is very unlikely to occur.

Another important standard for enclosures is IEC 60439-1 (see Table 16.2). Among other things, this standard describes ‘forms’ of enclosures according to the protection against contact with live parts belonging to adjacent functional units inside the enclosure, the limitation of the probability of initiating arc faults, and the protection against the passage of solid foreign bodies from one internal unit to an adjacent unit.

There are many other standards covering the design and configuration of enclosures.

Safe by position

Uninsulated conductors energized at dangerous voltages can, in principle, be made safe by being placed out of reach. For example, high-voltage overhead lines with uninsulated conductors are raised to heights above ground level at which they are safe from being inadvertently touched or approached so closely by a conducting object (particularly an object with a sharp pointed end) that an arc can develop from the line to the object. Safety is not assured, however, as exemplified by the common occur- rence of direct contact with the conductors by the likes of construction and agriculture vehicles, fishing rods and kites.

Reduced voltage

The direct contact hazard can be minimized by reducing the shock voltage. In the UK, the most common reduced voltage system operates at 110 V three-phase or single- phase. In the former, the star point of the supply generator or transformer is earthed and, in the latter, the centre point of the output winding is earthed, reducing the shock voltage between a phase conductor and earth to 55 V. Reduced voltage is most frequently used to supply Class I and Class II portable tools in work locations, such as construction sites.

Extra-low voltage

The two common types of extra-low voltage systems are Safety Extra-Low Voltage (SELV) and Protective Extra-Low Voltage (PELV).

The SELV systems operate at voltages no greater than 25 V ac or 60 V ripple-free dc between conductors or between any conductor and earth. These are considered to be safe voltages for most applications, although lower voltages may be needed for work in wet conditions and confined spaces. In an SELV system, any exposed conductive parts should not be connected to, or be in contact with, the protective conductor of another system, nor with extraneous metal which could be energized by another system. A step-down transformer, to provide the safe low voltage, should be a safety trans- former to EN 60742 (see Table 16.2).

PELV differs from SELV only by having its circuits earthed at one point only. The conductors are usually protected either by barriers or enclosures to at least IP2X or with insulation capable of withstanding 500 V dc for 60 s. Where the voltage does not exceed 25 V ac or 60 V ripple-free dc in a dry location within the equipotential zone, these additional enclosure precautions are not required, but otherwise the limits are 6 V ac or 15 V ripple-free dc.

Limitation of energy

Limiting the amount of energy that a system can deliver into the body is a method of protection against direct-contact injuries. The electric stock-control fence is one exam- ple. In these systems, the fence wires are energized with pulses, with the peak voltage of each pulse being in the order of 5–10 kV, with a pulse duration in the order of 1 ms and pulse repetition frequencies of about 1 Hz. The amount of energy delivered into a 500 Ω load is limited to 5 J per pulse, where 500 Ω represents the typical lowest value of human body resistance at the fence operating voltages. Five joules is esti- mated to be below the energy needed to cause cardiac fibrillation effects in the large majority of the population, but it is high enough to cause sufficient pain to any animal that may touch the fence wire to deter them from touching it again.

Electrical separation

Here the source of supply is usually a safety isolating transformer or its equivalent. The secondary windings are not earthed or otherwise referenced, which explains why such systems are commonly known as isolated or unreferenced supplies. The principle is illustrated in Fig. 16.2. A person simultaneously touching one pole of the unrefer- enced supply and earth will not experience an electric shock because there is no complete circuit back to the point of supply, although small amounts of reactive current may flow due to capacitive coupling.

Earth leakage protection

Circuit breakers and fuses protect against excess current arising from overload conditions and faults. The most common type of fault is an earth fault, but the current

Principles of electrical safety-0207

flowing due to these faults may be too low to operate the overcurrent protection devices. In addition, the overcurrent protective devices will not operate in the event of some- body making direct contact with a live conductor because the current which flows through the body to earth will be too low to operate the devices, although it will often be high enough to cause fatal electric shocks. These two problems can be obviated by the use of earth leakage protection devices that, in low-voltage systems, will be RCDs as described in Chapter 7. Earth leakage protection on higher voltage systems is provided using protection devices of the type described in Chapter 8, although these do not provide protection against direct conduct with live HV conductors.

RCDs should not be relied upon as the sole means of protecting against injury from direct contact. This is because RCDs have failure modes that can lead to unrevealed dangerous fault conditions and because, for an RCD to operate in the event of direct contact, current of at least 30 mA must flow. This amount of current is large enough to cause muscular contraction whereas in most cases it will almost certainly prevent electrical injury effects, such as ventricular fibrillation, it may not prevent injury arising from the muscular contraction, such as falling off a ladder, or being thrown against a wall.

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