Technical data

Vacuum interrupter principles
With Siemens type 38-3AH3 vacuum
circuit breakers, the chopping currents
are held to 5 A or less. This is low enough
to prevent the build-up of unduly high
voltages, which may occur on switching
of inductive circuits. The chrome-copper
contact material keeps overvoltages to a
minimum, so special surge protection is
not required in most applications.
When the contacts open, the current to
be interrupted initiates a metal vapor arc
discharge, and current continues flowing
through this plasma until the next current
zero.
The arc is extinguished near the current
zero, and the conductive metal vapor
recondenses on the contact surfaces and
the arc chamber wall or arc shield within
a matter of microseconds. As a result, the
dielectric strength of the break recovers
very rapidly and contact erosion is almost
negligible.
The arc drawn in the vacuum interrupter
is not cooled. The metal vapor plasma is
highly conductive and the resulting arc
voltage is only 20 to 200 volts. This low
arc voltage, combined with very short
arcing times, produces only a very small
arc energy in the vacuum interrupter,
accounting for the long electrical life
expectancy of the Siemens vacuum
interrupter.
Axial magnetic-field design is employed.
In this configuration, the current flow
creates a magnetic field along the
longitudinal axis of the vacuum
interrupter. This magnetic field prevents
constriction of the arc, and this forces the
arc to remain in diffuse mode. Since the
arc remains in diffuse mode, localized
overheating is avoided and contact
erosion is held to low levels.
Vacuum circuit breakers
21
Figure 26: Siemens vacuum interrupter family