Shock indirect contact

Two types of contact will result in a person receiving an electric shock. Direct contact with live parts involves touching a terminal or line conductor that is actually live. The regulations call this basic protection, indirect contact results from contact with an exposed conductive part such as the metal structure of a piece of equipment that has become iive as a resuit of a fauit. The regulations call this fault protection. 

If, for any reason, there was a breakdown of insulation in a part of an electric circuit or in any apparatus such as, say, a hand-held metal-cased electric drill, it is conceivable that current would flow external to this supply circuit, if a path were available. For example, the metalwork of the drill may be in contact with a live internal conductor at the point of insulation breakdown. Or, take the example of someone working at a switch or socket outlet from which the cover had been removed before the electricity supply had been isolated. In both circumstances the person concern could come into contact with a live part, either metalwork of the drill made live because of the internal fault condition or a live terminal exposed by removal of the cover which, if the conditions were right, would allow an electrical current to flow through the body to earth. The former is an example of electrical shock received by indirect contact whilst the latter is an example of electrical shock from direct contact. If the total resistance of the earth fault path were of a sufficiently low value, the current could kill or maim. 

Accidents involving an electric shock are usually subdivided into two categories - direct contact and indirect contact shocks. The standards that will be considered later use this distinction. A direct contact shock occurs when conductors that are meant to be live, such as bare wires or terminals, are touched. An indirect contact shock is usually associated with touching an exposed conductive part that has become live under fault conditions an example of an exposed conductive part would be the metal casing of a washing machine. 

The majority of direct and indirect contact electric shock and burn accidents occur at 230 V on distribution systems or on connected equipment. There are many instances in which high voltage overhead lines are touched, so this is a form of direct contact however, they usually result in predominantly burn injuries rather than electric shock. 

Under the fault condition shown, the casing of the washing machine will no longer be earthed and it will become live at the full mains potential of about 230 V. When the person depicted simultaneously touches the live casing of the heater and an earthed object, such as the water pipe shown, a hand-to-hand mains voltage shock is suffered. The flow of current in this case of the indirect contact scenario is depicted in Fig. 2.8 and the equivalent circuit is shown in Fig. 2.9. 

The nature of the fault described means that the touch voltage is in the order of 230 V. However, many indirect contact shock accidents occur at less than mains voltage. This can be quite fortunate for the injured person because the shock current will be lower, thereby reducing the adverse effects and improving their chances of being able to let go of the conductors and survive the incident. 

The essence of the concept is the prevention of direct and indirect shocks by contact between a hve part or a conductive part made live by a fault and earth. To this end, insulating walls and floors are used with the minimum amount of touchable conductive and extraneous conductive parts. Any that are used and that could become live under fault conditions have to be so spaced as to prevent anyone touching two of them at the same time. The area has to be earth-free, so no protective conductors are employed and conductive and extraneous conductive parts are not earthed. Extraneous conductive parts, such as metal pipes which are in the location and outside it, need protection by insulation, barriers or placing out of reach so that in the event of a fault they cannot transmit a potential, including an earth potential, in either direction. 

Earthed equipotential bonding and automatic disconnection of supply, known as EEBADS, is the most common technique employed for protection against indirect contact electric shock. Essentially, for earth-referenced supplies, the technique requires the exposed conductive parts of Class I apparatus and equipment to be earthed by means of the protective conductor, with the protective conductor connected back to the main earthing terminal of the installation. 

The intention of Regulation 7 is to prevent electric shock and burn injuries from direct and indirect contact, and fire and explosion consequent on short circuits or leakage currents between circuit conductors or between circuit and other conductors. 

As indirect contact shocks are the most common form of electric shock incident, five methods of avoidance are dealt with extensively in section 413. 

Regulation 551-04-04 addresses protection against indirect contact for static inverters, typically used for uninterruptable power supplies in installations where continuity of supply is crucial. Where the disconnection times of section 413-02 cannot be achieved, supplementary bonding must be used to minimise the risk of a shock between exposed metalwork. A warning is provided in Regulation 551-04-05 about the possible deleterious effects on the operation of protective devices, such as circuit breakers, of direct current generated by the static inverter or filters. 

From an historical perspective, in order to avoid indirect contact electric shock, the now-superseded 1966 edition of BS 638 Specification for arc welding plant, equipment and accessories, recommended that the power source metalwork and the work piece should be earthed. This was a safeguard against an interwinding fault in a transformer source causing the... 

Protective measures against direct and indirect contact (electric shock) are required depending on the battery nominal voltage and the chosen ground system of the electric network (Table 6.4). In the case of a system short circuit an effective protection can be achieved by incorporating a system with protective conductor and associated protective devices. In battery installations mainly an IT network or TN network is used. 

Electrical protective devices are used to protect machines and equipment from damage due to values of voltage or current exceeding design levels. They are also used to protect individuals from the risk of injury, by isolating electrical faults in part of the system. Electrical hazards associated with workshop machinery and tools may be caused by overload current, short-circuit current, or electric shocks to individuals arising from direct or indirect contact with live conductors. Overload current is an excessive current... 

Boyle et al (Ref 5, p 855) stated that direct measurements of particle velocity, density and pressure are not feasible at present and the indirect methods must be used. They describe a method using determination of the shock velocities in an explosive (such as Comp B) and in a Plexiglas plate placed in contact with the explosive [See under Detonation (and Explosion), Pressure of ]... 

OSHA has requirements for safe work practices at 1910.333. By not complying with these work practices employees performing work near or on equipment or circuits (which are or may be energized) could be exposed to electric shock or other injuries resrdting from either direct, or indirect, electrical contacts. Your company-specific safety-related work practices must be consistent with the nature and extent of the electrical hazards. 

In the following text, a direct shock is from contact with a live part which is intentionally live. An indirect shock is from contact with an exposed conductive part or an extraneous conductive part made live from a fault. 

The more usual description for section 606 is confined conductive locations. These are spaces where freedom of movement is restricted and the body is likely to be in contact with exposed and extraneous conductive parts. This section covers work inside boilers, metal ventilation ducts and tanks, for example, where extensive contact with the metalwork increases the indirect shock hazard. The risk is enhanced if these interiors are wet or so hot that the operator s clothes are soaked with perspiration. Incidentally, although not... 

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