2 CNS/GSS/2008/M3 2 Basic materials SHEET STEEL : The Sheet Steel used are of Cold Rolled base metal, commercial quality type. 1.2mm, 1.5mm, 2.0mm and 3.0mm thick sheets are used in various enclosures. Two types of sheet steel are used viz., Hot dip galvanised and Electro galvanised. Hot dip galvanized sheets are produced on continuous galvanizing lines which guarantee smooth and uniform quality, according to JIS G-3302 or ASTM A-526. The Electro galvanized sheets have coating thickness of 2.5 microns per side, according to JIS G-3313 or ASTM A-591 Definition of steel Steel is a material composed mainly of iron, with a carbon content lower than 2% and which contains other elements.
3 CNS/GSS/2008/M3 3 Basic materials Classification of steel Steels are classed according to their: - chemical composition in: - unalloyed steels, - stainless steels, - other alloyed steels. - quality class (except for stainless steels) in: - quality steels, - special steels. Stainless steels Stainless steels are subdivided according to the following criteria: - their nickel content: - nickel < 2.5%, - nickel > 2.5%, - their principal feature: - corrosion resistance, - high temperature oxidation resistance, - creep resistance.
4 CNS/GSS/2008/M3 4 Basic materials METAL TREATMENT : Degreasing is carried-out on the sheet steel items by soaking in hot chemical solution, at a temperature of 65 to 85 C, for a duration of 6 to 8 minutes. This removes linolin-based oils, grease etc. from the items. Then the sheet steel is rinsed twice with flowing water at ambient temperature, for about two minutes, to remove alkali solution deposits on the surface. After rinsing, Phosphate refiner is used, at ambient temperature for about two minutes. This avoids formation of coarse phosphate coating and ensures a fine-grained crystal coating with excellent adhesion characteristics. Next, it undergoes zinc phosphating at a temperature of 54 C, for 5 – 10 minutes, to have a crystalline coating of Zinc phosphate. Again, it is rinsed with water at ambient temperature and applied with chromate solution for preserving the zinc phosphate on to the sheet, thereby enhancing the anti-corrosion properties. Finally, the sheet steel is applied with paint using Electro-static powder coating. The powder is of polyester type, finishing to Light Grey Color RAL 7032, as a standard. The overall thickness of paint shall be maintained at 70 to 80 microns.
5 CNS/GSS/2008/M3 5 Basic materials Reaction of Phosphating: Fe + H3PO4 --- Fe(HPO4)2 + H2 > Metal + Acid Phosphoric --- Primary metal Phosphate (O) Iron Phosphate -- FePO4 (sludge) + H2O 3Zn(H2PO4)2 -- Zn3(PO4)2+4H2O Zinc Primary Phosphate -- Hopite 2Zn(H2PO4)2+Fe – Zn2Fe(PO4)2+H2O Phosphophyllite After Phosphating -- Water rinsing is a must. Cannot be allowed a gap of more than 2-3hrs. Before Painting; Degree=60min. Phosphating = 2.5 – 4gm/m2
6 CNS/GSS/2008/M3 6 Basic materials BUSBARS Busbars are of High Conductivity Hard Drawn Electrolytic Grade Copper. The copper used for electrical purposes shall be of highest purity. In general, its total content of impurities, including oxygen, shall be less than 0.1 per cent. Copper of this type is known as “High Conductivity” Copper. As per IEC, the following are the characteristics of Annealed copper: At a temperature of 20 C, the resistance of a wire of uniform section is Ohm/Mt./mm2. At a temperature of 20 C, the density is 8.89 grammes per cubic centimetre. At a temperature of 20 C, the ‘constant mass’ temperature coefficient of resistance is per C. At a temperature of 20 C, the resistance of a wire of uniform section one Metre in length and weighing one gramme is ohm.
7 CNS/GSS/2008/M3 7 Basic materials For busbar purposes, hard drawn or medium hard drawn copper conductors are preferable to annealed conductors on account of their greater stiffness, strength, hardness and better surface finish. The temperature at which copper starts to anneal is influenced by the presence of impurities and the extent to which the metal has been cold worked and usually a temperature of about 200 C is required. The current carrying capacity of a busbar is usually determined by the maximum temperature at which the bar is permitted to operate. In British Standard 159, for Busbars and Busbar connections, it is stated that the temperature rise of busbars and Busbar connections, when carrying rated normal current at rated frequency shall not exceed 50 C. This temperature rise is based on an ambient temperature having a peak value not exceeding 40 C and an average value not exceeding 35 C, measured over a 24 hour period.
8 CNS/GSS/2008/M3 8 Basic materials In practice, these limitations on temperature rise may be relaxed for copper busbars if suitable insulation materials are used. A nominal rise of 60 C or more above an ambient of 40 C is allowed by IEC 439 (BS 5486 Part 1) provided that suitable precautions are taken viz., the rise is limited by the mechanical strength of the busbar material, the effect on adjacent equipment, the permissible temperature rise of insulating materials in contact with the bars, and the effect on apparatus connected to the busbars. The amount of heat generated in a copper conductor is proportional to its resistance and to the square of the current flowing. Under short-circuit conditions, the heat capacity of the bar itself play an important part in regulating the temperature, but in normal circumstances, the temperature of the conductor attains a steady value when the rate of heat generation is equal to the rate of dissipation.
9 CNS/GSS/2008/M3 9 Basic materials The cooling effect of natural convection currents is greatest in the case of a Rectangular bar when the bar is mounted on edge. By increasing the thickness of a bar, for example from 4 in x ¼ in to 4 in x ½ in, its resistance is reduced to half, but its current carrying capacity is only increased by about 45 percent since the surface area is increased marginally. The dissipation of heat by radiation is increased in case of busbars having dull black surface. Hence, a coat of paint, therefore, may be expected to increase the heat dissipation from a busbar and thereby improve its current rating, by 20 to 25 percent. When a number of conductors are used in parallel, the total current capacity is less than the rating for a single bar times the number of bars used. The following factors may be used to obtain the total rating (DC) for conductors in parallel. No. of laminationsMultiplying factor
1010 CNS/GSS/2008/M3 10 Basic materials TINNING : The tinning of the surfaces is normally un-necessary, although advantages can be gained in certain circumstances. For best results, the surfaces are to be tinned where the busbars are subjected to corrosive atmospheres. BOLT SIZES AND RECOMMENDED TIGHTENING TORQUE: Used for Bolt SizeBusbar Sizes, mm2Torque, Nm M615 x 5, 15 x M825 x 5, 30 x 10, x 5, 40 x 10 M10 40 x 5, 40 x 10, x 5, 50 x 10, 60 x 10 M12 80 x 10, 100 x x 10
1 CNS/GSS/2008/M3 11 Basic materials SLEEVES The Busbars are sleeved using Heat shrinkable PVC sleeves. The properties of sleeve is as follows: Thickness : mm Di-electric withstand:more than 7 kV Material: Poly Vinyl Chloride blended with suitable additives and pigments. Shrinkage:25 to 30% along width Max. 1% along length Shrink temperature:140 – 175 C Application:The busbars are inserted into selected sleeve and Hot air is blown over the sleeve uniformly along the length of the busbar using Air Gun Usually Black colored sleeves are used, as a standard. However, Red Yellow and Blue colored sleeves are also used on specific requirement.
1212 CNS/GSS/2008/M3 12 Basic materials SELECTION OF SLEEVES : Busbar size, mm2Sleeve Flap size, mm x x x x x x x x x x x
1313 CNS/GSS/2008/M3 13 Basic materials METALLIC COATINGS – ELECTROPLATED ZINC COATINGS on IRON or STEEL Electroplating is achieved using three principal methods: -Cyanide alkaline bath -non-cyanide alkaline bath -Acid bath DESCRIPTION OF COATINGS: Designation – Using four symbols : Zn X1 X2 / Fe, Where Zn – is the metal constituting the coating X1 – minimal coating thickness X2 – Finish symbol – A:Clear ; B-bleached ; N-Black; C-indescent; D- Opaque Fe – Support metal symbol Thickness of Coating: The degree of protection afforded by Zinc coating is directly proportional to its thickness. However, the maximum thickness is 25microns. Quality of Coating: Coatings must be free of all defects such as spots, scoring, marks,etc
1414 CNS/GSS/2008/M3 14 Basic materials Hot Dip Aluminium Zinc (AZ) coated steel strips, sheets & blanks Aluminium-zinc alloy coated products are mainly used in the building sector (cladding and coverings). However this type of coating can be used to in products: covering parts, structural parts (frames, supports, etc.) inside devices. The high aluminum content of this coating means that the sheet withstands temperatures exceeding those of galvanised steel (> 300°C). This type of coating yields a corrosion resistance 2 to 6 times greater than that of galvanised steel for the same thickness. It also has a better heat reflecting capacity than galvanised sheet (virtually twice as high), thus enabling improved heat discharge. However the aluminium content means that contact with certain products (copper, lead, unprotected steel, etc.) must be avoided. The coating deposited on the surface of the steel is an alloy containing 55% of aluminium, 1.6% of silicon and the top-up in zinc.
1515 CNS/GSS/2008/M3 15 Basic materials Just as for galvanised products, zinc provides steel with a sacrificial protection: if the coating is damaged in a corrosive environment (e.g. during transport, storage) zinc is attacked first (appearance of white rust), thus protecting the iron. The aluminium increases corrosion resistance by the formation on the surface of an insoluble alumina layer.
1616 CNS/GSS/2008/M3 16 Basic materials "Belleville" conical spring washers Description These washers are described by their outer diameter and thickness, completed by the surface treatment. Materials These spring washers are manufactured from steel strips in carbon steel grades with a carbon content of > 0.60 Mechanical properties These are obtained after quenching and tempering: maximum hardness 48 HRC at all points of the washers. The treatment can be followed by a surface hardening by shot blasting.
1717 CNS/GSS/2008/M3 17 Basic materials ABS Acrylonitrile Butadiene Styrene It is a amorphous Thermoplastic Prorperties:ElectricalComparative tracking index600V EnvironmentMoisture absorption0.2% TechnologySelf threadno unscrewing AppearanceAesthetic appearancevery good
1818 CNS/GSS/2008/M3 18 Basic materials What Is SF6? Sulphur hexafluoride. SF6 is a gas that is used in electrical power equipment. It is colourless, odourless, non-flammable and chemically stable. This means that at room temperature it does not react with any other substance. Pure SF6 is not poisonous. The gas is not dangerous to inhale, provided the oxygen content is high enough. In principle you can inhale a mixture of 20% oxygen and 80% SF6 without danger. SF6 is about 6 times heavier than air. That means that it may collect in cable ducts or at the bottom of tanks. Where and How Is SF6 Used? SF6 is used as an insulating gas in substations, as an insulating and cooling medium in transformers and as an insulating and arc quenching medium in switchgear for high and medium voltage applications. These are all closed systems which are extremely safe and unlikely to leak. In electrical power systems, high and medium voltage switchgear is required to cut off the power in case of a fault, in order to protect people and equipment. When power is switched, an electric arc strikes between the circuit-breaker contacts. Breakers filled with SF6 are electrically insulating and effectively control arcing.
1919 CNS/GSS/2008/M3 19 Basic materials What Is the Benefit of SF6 ? There are two reasons for using SF6 in electrical equipment: SF6 provides extremely good electrical insulation and very effectively quenches electric arcs. These properties of SF6 make it possible to build electrical equipment and apparatus that are compact, use a small amount of material, are safe and will last a long time. At normal atmospheric pressure, SF6 has a dielectric withstand capability that is 2.5 times better than air. Usually the gas is used at 3-5 times atmospheric pressure and then the dielectric properties are ten times better than for air. SF6 insulates so well because it is strongly electronegative. SF6 effectively controls circuit-breaker arcs because it has excellent cooling properties at temperatures ( K) at which the arcs extinguish (the gas uses energy when it dissociates and therefore produces a cooling effect).