SEM-3^RD�SUB-EEM�PREPAIRED BY-SUMAN SAHU,ASST. H.O.D. ELECTRICAL ENGINEERING� CHAPTER 5:MATERIALS FOR SPECIAL PURPOSES�

CONTENTS

• Introduction
• Structural Material
• Protective Material
• Steel tapes,Wires &Strips
• Other Materials
• Bimetals
• Soldering Material
• Eyre No 7 Flux
• Fuse and Fuse Material
• Dehydrating Material

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INTRODUCTION

• Some materials used for special purposes such as fuses, solder, bimetal, storage battery plates. Those materials used for special purposes are in structural materials or protective materials.

STRUCTURAL MATERIALS :

• Cast iron, steel, timber, reinforced concrete are common materials for this purpose.
• Cast iron is used as materials for the frames of small and medium sized electrical machines. Steel is used in fabricated frames in large electrical machine, tanks in a transformers, fabrication of transmission towers.
• Timber and reinforced concrete are used for poles in OH lines.

PROTECTIVE MATERIALS :

• LEAD :
• Lead is soft, heavy and bluish grey metal. It is highly resistant to many chemical action, but it can corrode by nitric acid, acetic acid, line and rotten organic substance. The electrical conductivity is 7.8% of copper. Lead is used in storage batteries and sheathing of cables. Pure lead cable sheathing are liable to fail in service due to formation of cracks formed because of vibration.
• Lead alloys with tin and zinc and forms alloys which are used for solders and bearing metals.

STEEL TAPES, WIRES AND STRIPS :

• Steel tapes, wires and strips are used as protective materials for mining cables, underground cable, weather proof cables.

OTHER MATERIALS :

• THERMO COUPLE MATERIALS :
• When two wires of different metals are joined together an emf exist across the junction. This emf is directly proportional to the temperature of the junction. When one tries to measure this emf more junctions are to be made which will give rise to emfs. When all the junctions are at the same temperature, the resultant emf will not be zero. This resultant emf is proportional to the temperature difference of the junctions and is known as thermoelectric emf.
• Thermo couples are made of different materials such that copper / constantan, iron / constantan, platinum / platinum rhodium.
• Thermo couples can be used for the measurement of temperature.

BIMETALS :

• A bimetal is made of two metallic strips of unlike metal alloys with different coefficient of thermal expansion. At a certain temperature the strip will bend and actuate a switch or a lever of a switch. The bimetal can be heated directly or indirectly. When heated the element bends so that the metal with the greater coefficient of expansion is on the outside the are formed while that with smaller coefficient is on the inside.
• Bimetallic strips are used in electrical apparatus and such as relays and regulators.

SOLDERING MATERIALS :

• An alloy of two or more metals of low melting point used for base metals is known as soldering. The alloy used for joining the metals is known as solder. The solder is composed of 50% lead and 50% tin. Its melting point is 185oC tensile strength is 385 kg./cm2 and electrical conductivity is 10% of copper.
• For proper soldering flux is to be used. In soldering process the application of flux serves to removes oxides from the surface to be soldered. They deoxidize the metals at the time the soldering element is added. Solders are two types such as soft solders and hard solders soft solders are composed of lead and tin in various proportions. Hard solders may be any solder with a melting point above that of lead tin solders.
• The application of soft solders is in electronic devices and hard solder in power apparatus for making permanent connection.

EYRE NO.7 FLUX :

• It is an improved variety of organic flux which is used with Alca P for alumnimum cable jointing. This on decomposition at a temperature a little below the jointing temp approx 316oC removes the refractory oxide from the strands of the core and makes the surface receptable to solder.

FUSE & FUSE MATERIAL

• A fuse is a protective device, which consists of a thin wire or strip. This wire is placed with the circuit which heave to protect, so that the circuit. Current flows through it. When this current is too high the temperature of the wire or strip will increase till the wire or strip melts. So braking the circuit and interrupting the power supply.

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• FUSE MATERIAL :
• A fuse material have following properties :
• a. Low resistivity
• b. Low conductivity
• c. Low melting point
• As lead is used as fuse material because of its low melting point. But the resistivity of lead is high, thick wires are used. For rewirable fuses alloys of tin and lead are used.

DEHYDRATING MATERIAL :

• SILICA GEL :
• It is an in organic chemical, a colloidal highly absorbent silica used as a de-humidifying and dehydrating agent as a catalyst carrier. Calcium chloride and silica gel are used in dehydrating breathers to remove moisture from the air entering a transformer as it breathes. Now silica gel is used for breather of a transformer. Its main advantage is that when it becomes saturated with moisture it does not restrict breathing. Silicagel when dry is blue in colour and the colour changes to pale pink as it becomes saturated with moisture.

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