Prospective short circuit potential Insulation rating Busbar conductor for mechanical strength Forces on the supports Horizontal span displacement Mechanical stresses on cables & cable fittings Temperature rise Seismic restraints Civil construction requirements Fire protection Earthing:- equipotential & lightning Corrosion protection Degrees of protection :- IEC 60529 Switchgear Enclosure & Housing Sheet Steel Steel Sections AlZn Sheet Steel Rust Removal Degreasing Phosphatize Galvanize Primer coat :- Dry film 30µm Aluminum Stainless Steel Primer coat :- Dry film 30µm 1ST Top Coat :- Dry film 30µm 2ND Top Coat :- Dry film 30µm 3RD Top Coat :- Dry film 30µm Indoor Occasional Condensation Warm Dry Climate Indoor & outdoor Occasional Condensation Warm Dry Climate Indoor & outdoor Aggressive Atmosphere Warm Humid Climate The IP code is a designation code applied to indicate the degree of protection by enclosures against the access of persons to hazardous parts & against the ingress of solid foreign objects & of water & to give additional information with respect to `this kind of protection. Code letter IP 2 First digit (0-6 or “X”) Second digit (0-8 or “X”) Additional letter (Optional) Supplementary letter (optional) 3 C H Low voltage switch gear is designed for switching & protection of electrical equipment. The selection of switching devices is based on the specific task, e.g. isolation, short circuit current breaking, motor switching, protection against overcurrent & personnel hazard. Depending on the type, switching devices can be used for single or multiple switching tasks. Switching tasks can also be conducted by a combination of several units. Examples:- Circuit breakers, fuses, Disconnectors, Load break switches, Fused disconnector switches, Motor starter, Contactors, Overload relays, Residual current operated device, Miniature circuit breaker Circuit breakers must be capable of making, conducting & switching off currents under operational conditions. They must trip in accordance with defined time/ current characteristics under overload & short circuit conditions Circuit breakers without overcurrent releases are known as switch disconnectors Circuit breakers can also be equipped with residual current releases. They are mechanical switching devices with only one position of rest. Are not operated manually Are capable of connecting, conducting & disconnecting currents in the circuit under service conditions. Contactors are selected by utilization categories Are used to start motors, accelerate them to normal speed, ensure motor operation, disconnect the motor from the power supply & by means of suitable protection systems protect the motor & the corresponding circuit in the case of overload. A disconnector can only open and close a circuit if either a current of negligible quantity is switched on or off, or if there is no significant voltage difference between the two contacts of each pole. disconnector Load break switch can under normal conditions in the circuit can make, conduct and break currents. Under specified abnormal conditions such as a short circuit, conduct these short circuit currents for a specified time. load break A fuse switch disconnector is a switch disconnector in which a fuse link or a fuse holder with fuse link forms the moveable contact. The application range of fuses are identified by two letters The first letter identifies the breaking range g – general purpose fuse links These can continuously conduct currents up to their rated current and can interrupt currents from the smallest fusing to the rated breaking capacity a – back up fuse links These can continuously conduct currents up to their rated current and can interrupt only currents above a specified multiple of their rated breaking capacity The second letter identifies the application i.e. the time current characteristic G - for general application M - for the protection of motor circuits and switching devices R - for the protection of semiconductor components Tr - transformer protection B - mine substation protection D - fuse links with delay N - fuse links without delay fuse Protection functions Overload protection/automatic restarting Low load indicator Off load protection Blocking protection Phase failure monitoring Reclosing lockout Safety interlocking Thermal overload protection by thermistor Loss of supply monitoring The term earthed means “effectively connected to the general mass of earth” (ER2) The New Zealand distribution system is a multiple earthed neutral system Earthing the neutral in an MEN system limits each phase to a nominal 230V to earth The neutral is multiple earthed so that, if any live part of the supply system or consumer’s distribution system come into contact with earth, current flows back to the neutral by way of parallel paths, & the net resistance of the fault circuit is low. The lower the resistance of the fault path, the higher the fault current. The higher the fault current, the response of the tripping device in tripping out the faulty circuit will be quicker. a) b) c) d) e) The main earthing system of a MEN electrical installation shall consist of :An earth electrode; & An earthing lead; & An earth continuity busbar for the connection of earth continuity conductors within the installation; & Bonding of any available metallic water supply pipes to the earth continuity busbar; & A removable link between the earth busbar & the supply neutral The resistance of the main earthing conductor shall be not more than 0.5Ω The resistance between any point of the neutral & earth is not to exceed 10Ω Disadvantages The high fault current brings an increased risk of fire or shock if it flows through poorly made connections or joints A broken neutral is sometimes not immediately noticed All exposed metalwork associated with wiring systems must be effectively earthed The earth electrode is the physical connection between the earthing lead & the general mass of earth. Earth electrodes can be made from galvanizsed steel rod, pipe or copper rod or wire, strip or plate cad welding The connection must be clean In some area (Rotorua) the main earth connection may need some additional protection from corrosion The earthing lead must be labelled at each end Usually copper wire & forms part of a composite conductor May also use rigid metal conduit or trunking, the metallic sheathing & armour of cables or a catenary wire supporting a cable supporting a cable as protective earth conductors. Metal pipes carrying flammable or explosive gases, liquids, or dusts must not be used as a protective earthing conductor.
