Section 4: Description And Operation; Vacuum Interrupter; Vacuum Interrupter Assembly; Contact Erosion Indicator - Eaton VR+ Series Instruction Book

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AMH-4.76-VR
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VR-Series Replacement Circuit Breaker
SECTION 4: DESCRIPTION AND
OPERATION
VR-Series + vacuum replacement circuit breakers are designed to be
used with existing installations of equivalent air-magnetic metal-clad
switchgear circuit breakers. The front mounted spring type stored
energy mechanism facilitates inspection and provides improved
access to components for servicing. The long life characteristics of
the vacuum interrupters and proven high reliability of spring-type
stored energy mechanisms assure long, trouble-free service with
minimum maintenance.

4.1 VACUUM INTERRUPTER

Vacuum interrupters offer the advantages of enclosed arc
interruption, small size and weight, longer life, reduced
maintenance, minimal mechanical shock, and elimination of contact
degradation caused by environmental contamination.
In the closed position, current flows through the interrupter
moving and fixed stems and the faces of the main contacts. As the
contacts part, an arc is drawn between the contact surfaces. The
arc is rapidly moved away from the main contacts to the slotted
contact surfaces by self-induced magnetic effects. This minimizes
contact erosion and hot spots on the contact surfaces. The arc
flows in an ionized metal vapor and if the vapor leaves the contact
area, it would condense into the metal shield which surrounds the
contacts.
At current zero, the arc extinguishes and vapor production ceases.
Very rapid dispersion, cooling, recombination, and deionization
of the metal vapor plasma and fast condensation of metal vapor
causes the vacuum to be quickly restored and prevents the
transient recovery voltage from causing a restrike across the gap of
the open contacts.

4.1.1 VACUUM INTERRUPTER ASSEMBLY

Each vacuum interrupter assembly (also referred to as pole unit) is
assembled at the factory as a unit to assure correct dimensional
relationships between working components. Three interrupter
assemblies are used per circuit breaker. Each vacuum interrupter
assembly consists of a molded insulator frame and includes the
vacuum interrupter, its lead assembly, bell crank, operating drive/pull
rod, stand-off insulator, and contact load spring. The pole units are
fastened to the circuit breaker's stored energy mechanism frame.
A silver-plated copper laminated shunt transfers current from the
moving interrupter stem to the primary bushings via a Holm-free,
non-sliding conical current transfer. A silver-plated copper casting is
attached to the stationary stem, completing the primary circuit to
the disconnect assemblies. The operating rod, loading spring, and
bell crank transfer the mechanical motion from the circuit breaker's
stored energy mechanism to the moving stem of the vacuum
interrupter.

4.1.2 CONTACT EROSION INDICATOR

The purpose of the contact erosion indicator is to monitor the
erosion of the vacuum interrupter contacts, which is very minimal
over time with Eaton vacuum interrupters utilizing copper-chrome
contact material. The VR-Series + vacuum interrupter assembly
incorporates both the original vacuum interrupter erosion indicator
and the contact-spring wipe into one all-encompassing indicator. The
adequacy of the remaining contact erosion and wipe can easily be
determined by observing the moving end of the vacuum interrupter
assembly on a closed circuit breaker. The procedure to determine
the adequacy of the "T" cutout on the vacuum interrupter assembly
is depicted in Figures 6.1 and 6.2. If the wipe is inadequate (no part
of the "T" cutout is visible) then the vacuum interrupter assembly
must be replaced. Field adjustment is not possible.
12 Instruction Book
IB182923EN September 2018 www.eaton.com
Figurt 4.1. Vacuum Inttrrupttr Asstmbly
Figurt 4.2. Vacuum Inttrrupttr Asstmbly (All Thrtt Polt Units)