Distribution Switchgear
Covering general principles of the Switchgear function, this invaluable reference discusses topics such as interruption techniques, fault level calculations, switching transients and electrical insulation.
TABLE OF CONTENTS Distribution Switchgear
Chapter 8: Operating Mechanisms
It is the function of a circuit breaker operating mechanism to transmit stored energy via a mechanical drive to the moving contacts, so as to cause them to close and open, when commanded, within defined operating times and speeds. What is more, it should operate without hesitation even after prolonged periods of inactivity. Operating mechanisms will incorporate drives to ancillary devices such as auxiliary switches for remote control and indication, motor drives for spring charging, position indicators and local manual trip and close facilities. In many cases, an operations counter will also be required.
8.1 Materials
Special attention has to be paid to the material specification and design of shaft and latch bearings in order to ensure operation without hesitation even after very prolonged periods of inactivity. Under these circumstances, the moving contacts should still part within about 25 ms when the protection system issues an opening command. Trip and close latches are intentionally designed to be relatively lightly loaded in order to minimise trip and closing coil burdens. To achieve this, they have a high mechanical advantage. However, this brings with it a high velocity ratio with resultant high-impact speeds of the latch faces. It is, therefore, common practice to use case hardened steel for the latch faces to prevent deformation which would lead to changes in tripping load and, therefore, operating time.
The intended service environment of the switchgear will have a marked influence on the choice of materials and protective finishes. All components within operating mechanisms, such...
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