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5.3.3 Breaking principle
In a vacuum interrupter, the electric arc begins at the instant
the contacts separate, persists until zero current is reached
and can be influenced by the magnetic field.
Diffuse or contracted vacuum arc
Individual points of fusion form on the surface of the cathode
following separation of the contacts. This leads to formation
of metallic vapours that support the arc itself.
The diffuse arc is characterized by expansion over the surface
of the contact and by evenly distributed thermal stress.
The electric arc is always the diffuse type at the rated current
value of the interrupter. There is very little erosion of the
contact and a very high number of interruptions.
As the value of the interrupted current increases (beyond the
rated value), the electric arc tends to change from diffuse to
contracted owing to the Hall effect.
Starting from the anode, the arc contracts and tends to
concentrate as the current increases. There is a temperature
increase in the affected area, and the contact is therefore
subjected to thermal stress.
To prevent the contacts from overheating and becoming
eroded, the arc is kept rotating. By rotating, the arc resembles
a moving conductor through which the current passes.
Radial magnetic field contact
arrangement with a rotating
vacuum arc.
System voltage
Development of current and voltage trends during a single phase vacuum interruption process.
Spiral geometry of ABB interrupter contacts
The special geometry of the spiral contacts generates a radial
magnetic field in all areas of the arc column, concentrated
over the contact circumferences.
An electromagnetic force is self-generated and this acts
tangentially, causing rapid arc rotation around the contact
axis.
This means the arc is forced to rotate and to involve a wider
surface than that of a fixed contracted arc.
Apart from minimising thermal stress on the contacts, all this
makes contact erosion negligible and, above all, allows
the interruption process to be controlled even with very high
short-circuits.
ABB vacuum interrupters are zero-current interrupters and are
free of any re-striking.
Rapid reduction in the current charge and rapid condensation
of the metal vapours simultaneously with the zero current,
allows maximum dielectric strength to be restored between
the interrupter contacts within a few microseconds.
Diffuse arc.
Schematic diagram of the transition from a diffuse arc to a contracted arc
in a vacuum interrupter.
Current of
short-circuit
Arc voltage
Contact
separation
Contraction
Contraction over anode
over anode.
and cathode.
Short-circuit current
interruption
Time
Recovery voltage
(system frequency)
Transient recovery voltage (TRV)
(high frequency)
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