Page 177 - Electromechanical Devices and Components Illustrated Sourcebook

Chapter 8 Circuit Protection 139

Long Throw Interrupters Fixed Contact Moving Contact

Figure 8-13 shows a fast acting, long throw interrupter. The
unit has a moving contact that is spring loaded for fast open-
Open
ing. When the trigger is pressed, the moving contact is pulled
into the open position. The distance between the contacts is
too wide to allow the arc to sustain. The speed at which the
contact retracts limits the duration of the arc and mitigates the
Vacuum Bellows
damage that it causes. Long throw interrupters are only prac- Metal Vapor Shield
tical to a certain upper limit. As the voltages become higher, Contacts Flange
the size and speed of these units becomes a factor. Ceramic Insulator

Closed Terminal

Vacuum
Vessel Copper
Terminal O-Ring
Flexible Cable
Figure 8-14 Vacuum Interrupter
Latch
Retract Spring
Terminal Trigger
from the contacts. Figure 8-15 shows a typical pneumatic
Bronze Bushing
suppression interrupter. The contacts are built into a pair of
Insulating Column
Moving Contact bolted clam shells. A diaphragm is sandwiched between the
Moving Rod
flanges. The center of the diaphragm is fixed to the moving
Preload Spring
Fixed Contact contact. When the contacts are opened, the cavity behind the
Contacts diaphragm is pressurized and a jet of gas is forced through the
Terminal center orifice. The gas jet blows the arc away from the con-
Open Closed
tacts and it is extinguished. To improve the performance of
Figure 8-13 Fast Acting, Long Throw Interrupter
these interrupters, the air can be replaced with a gas having a
higher dielectric, such as sulfurhexifloride.

Vacuum Interrupters

To reduce size, interrupters are often constructed so that the Pressurized Cavity
Orifice
contacts can operate in a vacuum. The lack of ionizing gas Gas Flow Gas Port
surrounding the contacts provides considerably improved arc
control. Vacuum interrupters are not typically preferred
because maintaining a suitable vacuum over extended periods
Fixed Contact
of time can be problematic. If the vacuum leaks down, the
interrupter could open and maintain a continuous arc,
Moving Contact
severely damaging the interrupter and providing almost no
protection. A variation of the vacuum interrupter uses a high
Open
dielectric oil in place of the vacuum. The oil provides better
insulating qualities and draws the heat from the arc with great Outer Diaphragm Flange
efficiency. These interrupters have better long-term reliability High Dielectric Gas
and do not require the rigorous monitoring of a vacuum unit. Diaphragm Insulator Flange
Figure 8-14 shows a sectional view of a typical vacuum inter- Fixed Clam Shell
Insulator
rupter. Diaphragm Flange
Terminal Mount Terminal
Pneumatic Interrupters Contacts
Terminal
Pneumatic, or air blast, interrupters are the most common Terminal Clam Shell
High Dielectric Gas
high-power units used. These units can be seen at many of the
high-power switching stations that dot the country. These Closed
units use a blast or jet of compressed gas to blow the arc away Figure 8-15 Pneumatic Suppression Interrupter