Insulation grounding

As the anodes of the cathode-grounded tubes are cooled through the ceramic insulator, narrower insulators should lead to more heat transferable. For lower voltages, tube designs with twice the actual anode load are feasible. 

Metals are fiindamentally different from insulators as they possess no gap in the excitation spectra. Under the influence of an external field, electrons can respond by readily changing from one k state to another. The ease by which the ground-state configuration is changed accounts for the high conductivity of metals. 

Ethylene. Under the influence of pressure and a catalyst, ethylene yields a white, tough but flexible waxy sohd, known as Polythene. Polyethylene possesses excellent electric insulation properties and high water resistance it has a low specific gravity and a low softening point (about 110°). The chemical inertness oi Polythene has found application in the manufacture of many items of apparatus for the laboratory. It is a useful lubricant for ground glass connexions, particularly at relatively high temperatures. 

Fig. 7. Schematic drawing of a HaIs arc furnace. A, ground B, ignition electrode C, beU-shaped cathode D, insulator E, turbulence chamber F, ground ...

Pitch. Pitch used to be stored in solid form at the tar distillery in open bays, from which it was removed by small explosive charges. Loading of the lump pitch by mechanical shovel created a dust ha2ard both at the tar installation and at the customer s, where the lumps had to be ground before use. In the 1990s, pitch is stored in tanks heated by superheated steam or circulating hot-oil coils and transported in fiquid form in insulated rad, road tankers, or ships. When transport as a hot fiquid is not feasible, not acceptable by the customer, or for small amounts, the pitch is converted into a dust-free particulate form, ie, short rods termed pencils, pastilles, or flakes. 

The static decay test is a good measure of the static dissipative nature of an object. However, the test is controlled by the object s resistance to ground. Objects that have a high resistance, such as insulators, will not readily accept an induced charge or decay an induced charge. The static decay test is most suited for objects with resistances of 10 or less (52). 

Before conducting the insulation test one should ensure that there is no self-induced e.m.f. in the windings. It is recommended that the windings be totally discharged by grounding them through the frame of the machine before conducting the test. 

It is recommended to test eacli phase to ground separately with the other phases also grounded. This is because the insulation resistance of a complete winding to ground does not provide a check of the insulation condition between the windings. 

It is observed that the insulation resistance of one phase with the other two phases grounded is approximately twice that of the entire winding. Therefore, when the three phases are tested separately. the observed alue of the resistance of each phase is divided by two to obtain the actual insulation resistance. 

Insulation resistance of the motor windings between phases and phase to ground should be checked and made up in the event of deficiency, according to Section 9.5. 

For main insulation (ground and inter-turn insulation... 

This check may be carried out both at the works, after final assembly, and at the site after the installation before conducting routine tests or energizing the assembly. The ehecks can be carried out at a lower voltage to detect any shorting links or spanners left inadvertently on live terminals, weak insulation or insufficient clearance or creepage distances between the phases or phase to ground conduetors. In the ease of a defect the same must be rectified immediately. 

The leakage current is measured as in lEC 60298. This method is generally applicable to an HT system by applying the full rated voltage between the insulating surface, say, between a phase and the ground. The leakage current thus measured should not exceed 0.5 mA. 

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