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3.4 Contacts
A pair of contacts comprises a moving contact and a
fixed contact. The instants of opening and closing
impose the most severe duty. Contact materials must
be selected with consideration to three major criteria:
1. Minimum contact resistance
2. Maximum resistance to wear
3. Maximum resistance to welding
Silver or silver-alloy contacts are low in resistance,
but wear rather easily. Tungsten, or majority-tungsten
alloys are strong against wear due to arcing, but rather
high in contact resistance. Where feasible, 60%+ sil-
ver alloy (with tungsten carbide) is used for contacts
primarily intended for current carrying, and 60%+ tung-
sten alloy (with silver) is used for contacts primarily
intended for arc interruption. Large-capacity MCCBs
employ this arrangement, having multicontact pairs,
with the current-carrying and arc-interruption duties
separated.
3.5 Arc-Extinguishing Device
Arcing, an inevitable aspect of current interruption,
must be extinguished rapidly and effectively, in nor-
mal switching as well as protective tripping, to mini-
mize deterioration of contacts and adjacent insulat-
ing materials. In Mitsubishi MCCBs a simple, reliable,
and highly effective “de-ion arc extinguisher,” consist-
ing of shaped magnetic plates (grids) spaced apart in
an insulating supporting frame, is used (Fig. 3.7). The
arc (ionized-path current) induces a flux in the grids
that attracts the arc, which tends to “lie down” on the
grids, breaking up into a series of smaller arcs, and
also being cooled by the grid heat conduction. The
arc (being effectively longer) thus requires far more
voltage to sustain it, and (being cooler) tends to lose
ionization and extinguish. If these two effects do not
extinguish the arc, as in a very large fault, the elevated
temperature of the insulating frame will cause gas-
sing-out of the frame material, to de-ionize the arc.
Ac arcs are generally faster extinguishing due to the
zero-voltage point at each half cycle.
3.6 Molded Case
The integral molded cases used in Mitsubishi MCCBs
are constructed of the polyester resin containing glass
fibers, the phenolic resin or glass reinforced nylon.
They are designed to be suitably arc-, heat- and gas-
resistant, and to provide the necessary insulating
spacings and barriers, as well as the physical strength
required for the purpose.
3.7 Terminals
These are constructed to assure electrical efficiency
and reliability, with minimized possibility of localized
heating. A wide variety of types are available for ease
of mounting and connection. Compression-bonded
types and bar types are most commonly used.
3.8 Trip Button
This is a pushbutton for external, mechanical tripping
of the MCCB locally, without operating the external-
accessory shunt trip or undervoltage trip, etc. It en-
ables easy checking of breaker resetting, control-cir-
cuit devices associated with alarm contacts, etc., and
resetting by external handle.
Supporting
frame
Grids
Fig. 3.7 The De-Ion Arc Extinguisher
Grid
Arc
Attraction force
Induced flux
Fig. 3.8 The Induced-Flux Effect
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