Instruction Book
Page 27
maintenance, and environmental compatibility.
Arc interruption is simple and fast. In the closed posi-
tion, current flows through the interrupter. When the
contacts are opened, the arc is drawn between the con-
tact surfaces. It is rapidly moved around the slotted con-
tact surfaces by a self-induced magnetic force which
prevents gross contact erosion as well as the formation
of hot spots on the contact surfaces. The arc burns in an
ionized metal vapor which continually leaves the contact
area and condenses on the surrounding metal shield.
At current zero, the arc is extinguished and vapor pro-
duction ceases. Very rapid dispersion, cooling, recombi-
nation, and deionization of the metal vapor plasma,
together with the fast condensation of metal vapor prod-
ucts, cause the vacuum to be quickly restored. Hence,
the opened contacts withstand the transient recovery
voltage.
5-1.2 CONTACT EROSION INDICATOR
(EXCEPT 3150A)
Figure 5-2 Behind the door configuration.
Figure 5-3 Closeup of Type W-VAC breaker vacuum
interrupters and current carrying system.
The purpose of the contact erosion indicator is to moni-
tor the erosion of the vacuum interrupter contacts, which
is very minimal over time with Eaton vacuum
interrupters utilizing copper-chrome contact material. If
contact erosion reaches 1/8 inch, the interrupter must
be replaced. A contact erosion indicator mark is located
on the moving stem of the interrupter (Figures
6-2 and
6-3
). In order to determine if the contacts have eroded
to the extent that the interrupter must be replaced, close
the breaker and observe the erosion mark placed on
each moving stem from the rear of the breaker. If the
mark on the interrupter stem is visible, the interrupter is
satisfactory. If the mark is no longer visible, the inter-
rupter assembly must be replaced.
5-1.3 CONTACT EROSION INDICATOR
(3150A ONLY)
The purpose of the contact erosion indicator on 3150
ampere circuit breakers is the same as just described in
paragraph 5-1.2 for other W-VAC breakers. The method
used to check for contact erosion on 3150 ampere
breakers is, however, different.
In order to determine if the contacts of a 3150 ampere
breaker have eroded to the extent that the interrupter
must be replaced, close the breaker and observe the
erosion mark on the right inside of the pole unit support
(Figure
6-4
). Use the erosion gauge (supplied with the
breaker) to inspect the erosion level of the interrupter. If
the pointer portion of the erosion gauge is aligned or
above the erosion mark, the interrupter assembly must
be replaced (Figure
6-5
).
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