ABSTRACT
Relevant IET parts, chapters and sections: Part 4, Chapters 41, 42, 43, 44; Part 5, Chapter 53
Important terms/topics covered in this chapter:
▪ Arm’s reach A zone of accessibility to touch, extending from any point on a surface where persons usually stand or move about, to the limits which a person can reach with his hand in any direction without assistance
▪ Barrier A part providing a defined degree of protection against contact with live parts, from any usual direction
▪ Basic protection Protection against electric shock under fault-free conditions
▪ Circuit protective conductor A protective conductor connecting exposed conductive parts of equipment to the main earthing terminal
▪ Class II equipment Equipment in which protection against electric shock does not rely on basic insulation only, but in which additional safety precautions such as supplementary insulation are provided. There is no provision for the connection of exposed metalwork of the equipment to a protective conductor, and no reliance upon precautions to be taken in the fixed wiring of the installation
▪ Design current The magnitude of the current intended to be carried by a circuit in normal service
▪ Enclosure A part providing an appropriate degree of protection of equipment against certain external influences and a defined degree of protection against contact with live parts from any direction
▪ Exposed conductive part A conductive part of equipment which can be touched and which is not a live part but which may become live under fault conditions
▪ External influence Any influence external to an electrical installation which affects the design and safe operation of that installation
▪ Extraneous conductive part A conductive part liable to introduce a potential, generally earth potential, and not forming part of the electrical installation
▪ Fault current A current resulting from a fault ▪ Fault protection Protection against electric shock under single fault
conditions ▪ Insulation Suitable non-conductive material enclosing, surrounding
or supporting a conductor ▪ Isolation Cutting off an electrical installation, a circuit or an item of
equipment from every source of electrical energy ▪ Live part A conductor or conductive part intended to be energized in
normal use, including a neutral conductor but, by convention, not a PEN conductor
▪ Obstacle A part preventing unintentional contact with live parts but not preventing deliberate contact
▪ Overcurrent A current exceeding the rated value. For conductors the rated value is the current-carrying capacity
▪ Overload An overcurrent occurring in a circuit which is electrically sound
▪ Residual current device (RCD) An electromechanical switching device or association of devices intended to cause the opening of the contacts when the residual current attains a given value under specified conditions
▪ Short-circuit current An overcurrent resulting from a fault of negligible impedance between live conductors having a difference of potential under normal operating conditions
▪ Skilled person A person with technical knowledge or sufficient experience to enable him to avoid the dangers which electricity may create
By the end of this chapter the reader should:
▪ be aware of the influences from which systems must be protected, ▪ be able to select equipment etc. with respect to IP and IK codes, ▪ know and understand the measures that can be taken to protect
against shock, ▪ understand the requirements for protection against overcurrent, ▪ be able to select protective devices suitable for various applications, ▪ understand the relationship between ‘let-through’ energy and the
thermal capacity of cables, ▪ understand the importance of discrimination,
▪ be aware of the need to protect against the effects of overvoltage and undervoltage.
