[0001] The present invention relates to a circuit breaker with arc restricting device, the
interrupting performance of which is enhanced. More particularly, it relates to the
relative arrangement between contactors having arc shields and arc extinguishing plates.
[0002] Heretofore, sufficient consideration has not been given to the relative arrangement
between contactors and arc extinguishing plates, especially the arrangement at the
opening of the contactors.
[0003] Figure 1, Figure 2 and Figures 3(A) and 3(B) show the opened and closed states of
contactors and the configuration of the electric arc between the contactors according
to the prior art. In Figure 1,. numeral 1 designates a movable conductor made of copper
to which a movable contact 2 is fastened. The movable conductor 1 and contact 2 constitute
a movable contactor 11. Numeral 3 designates a stationary contact which is fastened
to a stationary conductor 4 made of copper. The stationary contact 3 and conductor
4 constitute a stationary contactor 12. Shown at numeral 5 is an arc extinguishing
plate for extinguishing an electric arc 6 which occurs when contacts 2 and 3 are separated.
[0004] Figure 1 illustrates contactors 11 and 12 in a closed state, wherein electric power
is fed from a power supply side to a load side in the order of the components 4 1
3 → 2 → 1.
[0005] Now, when the movable conductor 1 causes contact 2 to open due to a trip command
which is applied to an operating mechanism portion, not shown, the electric arc 6
develops across the gap between the contacts 2 and 3 as shown, in Figure 2. Further,
as the distance between the contacts 2 and 3 increases,.the arc 6 moves outwardly
away from the contacts 2, 3 and the extremities of the arc (feet) moved onto the conductors
1, 4 as illustrated in Figures 3(A) and 3(B). The movement is due to the following
reason:
[0006] When the distance between the contacts is short as illustrated in Figure 2, the arc
6 is usually difficult to move. However, as the distance between the contacts increases
and accordingly the length of the arc 6 is increased, as shown in Figure 3(A), the
arc moves easily even for small forces imposed thereon. In the situation of Figure
3(A), the force on the arc 6 consists of an electromagnetic repulsive force due to
the current flowing through the movable conductors 1, 4 and a magnetic attractive
force of the arc extinguishing plates 5. These forces cause the arc 6 to move.
[0007] When, as shown in Figures 3 (A) and 3 (B) the arc 6 moves along the electrodes due
to the attraction force of the arc extinguishing plates 5, the positive column portion
of the arc 6 touches the arc extinguishing plates 5 and is cooled by the latter. Moreover,
the arc 6 is stretched by the arc extinguishing plates 5, whereby the cooling is highly
promoted. In this way, the extinction of the arc 6 at the current zero point is facilitated.
That is, the arc extinguishing performance is attained by utilizing the attraction
and cooling of the arc 6 by the arc extinguishing plates 5. In the prior art, circuit
breakers of this type, the feet of the arc 6 freely move on the conductors 1 and 4.
It has therefore been impossible to determine the relative arrangement between, particularly,
the arc extinguishing plates 5 and the contacts 2, 3 or the conductors 1, 4.
[0008] The present invention has for its object to enhance the performance of a circuit
breaker by surrounding the contacts thereof with arc shields which are made of a material
having a resistivity higher than that of the conductors adjoining said contacts, and
by appropriately setting the relative arrangement between the contacts and arc extinguishing
plates.
[0009] Further features and advantages of the present invention will become apparent from
the following description of preferred embodiments with respect to the prior art and
making reference to the enclosed drawings, wherein
Figure 1 is a side view for explaining the closed state of a prior-art circuit breaker;
Figure 2 is a side view for explaining the opened state of the prior-art circuit breaker;
Figures 3 (A) and 3(B) illustrate the behaviour of an electric arc, in which Figure.3(A)
is a side view and Figure 3(B) is a view seen along the direction HB - HB in Figure
3(A);
Figures 4 (A) and 4(B) illustrate a circuit breaker according to an embodiment of
the present invention, in which Figure 4(A) is a vertical sectional view and Figure
4(B) is a plan view of an arc extinguishing plate;
Figure 5 is a perspective view showing an envelope space defined by the arc extinguishing
plates;
Figure 6 is a perspective view showing the essential portions of a stationary contactor;
Figure 7 is a perspective view showing a modified embodiment of an arc shield;
Figures 8 (A) and 8(B) illustrate the function of the arc shields, in which Figure
8(A) is a vertical sectional view and Figure 8(B) is a bottom view of a movable contactor
and an arc extinguishing plate in Figure 8(A);
Figure 9 is a vertical sectional view showing another embodiment of the present invention;
and
Figures 10(A) and 10(B) show still another embodiment, in which Figure 10(A) is a
bottom view of a movable contactor and Figure 10(B) is a plan view of a stationary
contactor.
[0010] In the drawings, the same symbols indicate the same or corresponding parts.
[0011] Hereunder, embodiments of the present invention will be described with reference
to the drawings.
[0012] Referring to Figure 4(A), a pair of electric contactors 11, 12 are provided with
arc shields 7, 7 positioned on conductors 1, 4 and formed in a manner so as to conceal
the parts of these conductors 1, 4 surrounding contacts 2,3. The arc shields 7 are
made of a high resistivity material which is higher in resistivity than that of the
conductors 1, 4 which are made of copper. The arc shields 7 may comprise, for example,
an organic or inorganic electrically- insulating substance such as ceramics, nichrome,
nickel, iron, copper-nickel, copper-manganese, manganin, iron- carbon, iron-nickel,
and iron-chromium.
[0013] In a position opposite to the force end of the movable contactor 11, there are vertically
stacked a plurality of arc extinguishing plates 51, 52 and 53, which define an envelope
space K when their outer edges are joined, as illustrated in Figure 5. Symbols m,
m in Figure 4(A) indicate a pair of spaces of truncated cones whose bottom surfaces
are superposed on each other. The truncated cones have apical surfaces which are closed
curved surfaces defined by the inner edges 7a of the arc shields 7 adjoining the respective
contacts 2, 3, and conical surfaces which form angles of within 35° with respect to
normals n set on the outer surfaces of the adjoining parts of the arc shields 7,.7.
[0014] A space M is defined in such a manner that the pair of truncated cones m, m have
their bottom surfaces superposed on each other in the state in which both the contactors
11, 12 are fully open. The relationship of arrangement between both the contactors
11, 12 and the arc extinguishing plates 51, 52, 53 is so determined that the space
M contains at least a part of the envelope space K of the arc extinguishing plates
51, 52, 53. Accordingly, the space M and the envelope space K have an overlap part
D as shown in Figure 4(B). Letter S indicates the remaining section of the space M.
[0015] In the above construction, the arc shield 7 functions as follows. As illustrated
in Figure 6, the arc shield 7 limits the size of the foot of the arc 6, thereby raising
the arc density in the space surrounding the foot of the arc 6. That is, since the
arc shield 7 is made of a high resistivity material as described with reference to
Figure 4(A), the movement of the foot of the arc 6 along the conductors 1, 4, is limited.
As a result, the space proximate to the contact undergoes a pressure rise owing to
the high heat of the arc 6. Therefore, unlike the arc in the prior art wherein the
foot of the arc's movement is not inhibited, emanating metal particles a which are
emitted from the contact 3 where the foot of the arc 6 exists are confined within
a narrow range owing to the effect of the arc shield 7. The density of the metal particles
a accordingly increases to raise the arc voltage, so that the current limiting performance
is enhanced.
[0016] An experiment has revealed that the emanating directions of the metal particles are
confined within the internal space of the truncated cone m whose apical surface is
the closed curved surface defined by the inner edge 7a of the arc shield 7 and which
has the conical surface forming an angle of 35° with respect to the normal n set on
the outer surface of the arc shield 7. Even when, as shown in Figure 7, a clearance
8 where the conductor 4 is exposed is existent between the arc shield 7 and the contact
3, a closed curve formed by the inner edge 7a of the arc shield 7 defines the apical
surface of the truncated cone m similarly.
[0017] Accordingly, the arc 6 is confined within the arc space M which is formed of the
overlapping parts of the pair of truncated cones m, m set for the respective contacts
2, 3, as depicted in Figure 8(A). Here, the current of the arc 6 chiefly flows within
the arc space M in which the metal particles a are existing (refer to Figure 6).
[0018] Owing to the arc shield 7, the emanating directions of the metal particles a are
regulated. Moreover, the tendency of metal particles to flow in the regulated directions
is greater for circuit breakers having arc shields 7. Accordingly, in order to bend
the arc 6 toward the arc extinguishing plates positioned between both contacts so
as to prolong the arc, it is necessary to exert a stronger force on the are 6 than
in the prior art or to conversely exploit the intense regulation of the emanating
directions of the metal particles of the arc 6 by the arc shields.
[0019] In Figures 8(A) and 8(B), the arc space M in which the arc
6 is confined does not overlap the envelope space
K of the arc extinguishing plates 5. Accordingly, the driving effect and cooling effect
of the arc extinguishing plates 5 on the arc 6 as described before are weakened,so
that a stationary current limiting or interrupting effect cannot be demonstrated.
[0020] In the present invention, therefore, the arrangement of the contactors 11, 12 and
the arc extinguishing plates 51-53 is specified thereby to enhance the performances
of the circuit breaker. More specifically, by bringing the arc extinguishing plates
51-53 in Figure 4(A) closer to the contactors 11, 12, the arrangement is so set that
the space M contains at least a part of the envelope space K of the arc extinguishing
plates 51-53. Thus, the current limiting performance and the interrupting performance
are greatly enhanced. The overlapping parts of the arc space M and the envelope space
K are defined by the area D shown in Figure 4(B).
[0021] When the arc 6 has been developed, that part in the positive column of the arc which
has the highest temperature and accordingly includes large quantities of charges is
in the space M. Therefore, by arranging the conductors 1, 4, contacts 2, 3, arc shields
7 and arc extinguishing plates 51-53 as described above, the high-temperature and
highly- ionized positive column comes into direct contact with the arc extinguishing
plates 51-53, and the cooling capability of the arc extinguishing plates 51-53 is
utilized most efficiently. Thus, the capability of extinguishing the arc 6 is enhanced.
[0022] Figure 9 shows an example in which the opening angle of the movable conductor 1 at
the opening of the movable and stationary contactors 11, 12 is made great to enlarge
the overlapping area betwen the arc space M and the envelope space
K of the arc extinguishing plates 51-53, whereby the arc extinguishing capability is
sharply enhanced. With such relative arrangement, even when struck across the gap
between the contactors 11 and 12 having the same separation distance as in the prior
art, the arc 6 comes to have a greater effective length as indicated by a broken line
A in the figure, because the emanating directions of the arc 6 are regulated. Accordingly,
the arc voltage rises, and the current limiting performance improves. In addition,
since the arc 6 comes into direct contact with the arc extinguishing plates 51-53,
it has an intense driving force exerted thereon by the arc extinguishing plates 51-53
and its length increases more, so that the arc voltage rises still more. Further,
since the high-temperature and high-pressure part of the arc 6 is directly cooled
by the arc extinguishing plates 51-53, the interrupting performance is enhanced.
[0023] Further, in a case where as illustrated in Figures 10(A) and 10 (B), arc runways
10, 10 formed of grooves extending towards the arc extinguishing plates 5 are provided
in the arc shields 7, 7 so as to expose the conductors 1, 4, the arc 6 rapidly runs
along the arc runway 10 and comes into contact with the arc extinguishing plates 5
to be cooled thereby, for small currents wherein the feet of the arc 6 are small.
Therefore, the interrupting performance for the small currents is enhanced.
[0024] As set forth above, according to the present invention, a circuit breaker which is
excellent in the current limiting performance and the interrupting performance is
provided.