40 Chapter 2 • Cable joints and switchgear 2.7.9 Factory sealing


Circuit-breakers are factory sealed to ensure that no one can alter the trip rating or time delay. Te seal also serves to validate the guarantee.


2.7.10 De-ion arc extinguishers Circuit-breakers are fitted with de-ion arc extinguishers (arc chutes), fig. 2.13.


Fig. 2.13 De-ion arc chutes


Teir function is to extinguish the arc formed when the circuit-breaker trips on overload or short-circuit.


Te arc chutes are designed and placed around the contacts in such a way, that the arcs formed as the contacts part, are forced outwards into the arc chutes. As the arcs moves into the chutes, they are divided into smaller segments and by so doing they are extinguished.


2.7.11 Silver tungsten contacts


Te contacts are generally made of silver tungsten as it suits the requirements for circuit-breaker contacts.


Silver is the best conductor of all conducting materials and tungsten is very hard and has an extremely high melting point. In order to combine these properties, the silver and tungsten have to be combined.


Since silver will not alloy with tungsten, a special process, called sintering, is used to combine these two materials. First the raw materials are powdered, mixed and pressed into the desired shape. Te mixture is then heat treated and the powder particles adhere (stick together).


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