Patent Application
20170103858
The invention disclosed relates to electrical switches.
Knife switches are used as disconnect switches mounted on switchboards, distribution, and control panel boards and typically are enclosed within safety switch cabinets. Knife switches are extensively used in heavy industries to handle heavy electrical loads, where visible disconnects are required. The switching of heavy currents produces arcing between the switch contacts, having the potential to cause considerable damage to the contacts and injury to operators. The contacts are typically formed of relatively soft, good conducting metals, such as copper, which have relatively low melting points and hence are very susceptible to damage by uncontrolled arcing. Past attempts to mitigate the problem of arcing-induced erosion have included providing two sets of contacts, main contacts that carry the load, and arcing contacts that open after the main contacts open and close before the main contacts close, so that the arc is drawn only between the arcing contacts and not between the main contacts. For example, U.S. Pat. No. 4,028,513 discloses a contact construction for a circuit breaker, wherein a pair of main contacts of relatively high conductivity are arranged in parallel with arcing contacts that have a steel body of relatively low conductivity. Such constructions of parallel sets of main contacts and arcing contacts are complex assemblies of parts that are expensive to manufacture and difficult to service for the replacement of eroded arcing contacts.
The invention disclosed is a knife blade switch having a simplified construction to connect or disconnect a first electrical terminal and a second electrical terminal. The knife blade switch includes copper jaws, a steel-jaw spring and a copper blade. The copper blade has a body with a first end connected to pivot and a second end (e.g., a free end) with a steel end-plate fastened to it. The copper jaws are connected to the first electrical terminal and the copper blade is connected to the second electrical terminal. The steel end-plate and the steel-jaw spring have a higher resistivity than the resistivity of the copper blade and the copper jaws. In operation, as the switch is operated from a closed position toward an open position, the copper blade is disengaged from the copper jaws while the steel end-plate at the free end of the blade remains in contact with the steel jaw-spring mounted on the copper jaws. The connection of the steel end-plate of the blade with the steel jaw-spring imposes a greater resistance path for the current flowing through the switch than the resistance path through the copper blade and the copper jaws. As a consequence, any arc formed has a diminished current when the contact separation occurs. Less arc energy occurring during separation is easier to manage. Moreover, the steel end-plate and the steel-jaw spring have a higher melting point and higher hardness than the melting point and the hardness of the copper blade and the copper jaws. By relocating the arc to the steel end-plate and the steel-jaw spring, which occurs upon separation, arc erosion is substantially eliminated for the current carrying copper blade and the copper jaws. In this manner, good contact joint integrity is maintained when the switch is fully closed.
At the start of opening an electrical switch, the area of the switch contacts that carries the electrical current diminishes, causing resistive heating and melting of the metal contact material in that area. When the contacts begin to actually separate, the electrical field strength in the small gap between the contacts is quite large and causes the air molecules to ionize, forming a plasma. The positively charged ions and negative electrons of the plasma are accelerated in the high electric field toward the respective contacts of opposite polarity and strike the metallic surfaces, causing spallation, evaporation and ionization of the metal atoms. An arc then forms between the contacts, along the conductive path created by the plasma and metal vapor. Metal atoms are eroded and ionized from the contact with the more positive potential, and are accelerated toward and deposited on the contact with the more negative potential (that temporarily exist at that particular moment in an AC cycle), resulting in arc erosion. As the switch contacts continue to separate, the electric field strength between the contacts is reduced sufficiently so that the plasma and metal vapor are no longer formed and the arc is extinguished. Arc erosion on the contacts of a switch impair good contact joint integrity when the switch is fully closed.
In accordance with an example embodiment of the invention, a knife blade switch has copper jaws and a copper blade with a steel end-plate fastened to the free end of the blade, the steel end-plate having a higher resistivity than the resistivity of the copper blade and copper jaws. As the copper blade is withdrawn from the copper jaws, the steel end-plate of the blade remains in contact with a higher resistivity steel jaw-spring mounted on and electrically connected to the copper jaws. The connection of the steel end-plate of the blade with the steel jaw-spring imposes a greater resistance path for the current flowing through the switch than the resistance path through the copper blade and copper jaws, so that an arc formed at the plate and jaw-spring has a diminished current, over what would otherwise occur with a copper blade and jaws, when the contact separation occurs. The diminished arc current reduces erosion of the copper jaws and copper blade of the switch.
An arc chute 14 is positioned at a location proximate to where the steel end-plate 6 of the blade disengages with the steel jaw-spring 12, to direct the arc and cool the hot arc gases. When the switch opens and the steel end-plate 6 moves up through the arc chute 14, the arc chute diverts the arc against an arc plate stack, to split the arc up into a number of elementary arcs, to dissipate the energy of the arc. The arc chute 14 may be fastened to the same base that supports the knife blade switch 2.
At least two properties of the material of the switch contacts affect the extent of arc erosion. First, the melting point of the contact material will affect the extent of arc erosion. A higher melting point material will reduce the extent of melting caused by the resistive heating as the switch starts to open. It will also reduce the extent of vaporization of the metal atoms when exposed to the ionized air molecules when the contacts begin to actually separate. The second property of the material is its hardness. A contact material having a higher hardness, will more readily resist the spallation and evaporation of the metal atoms when exposed to the positively charged ions and negative electrons of the plasma.
The steel end-plate 6 and the steel-jaw spring 12 may be composed of a material that has a higher melting point and higher hardness than the melting point and hardness of the copper blade 4 and the copper jaws 10. The steel end-plate 6 and the steel-jaw spring 12 may have a higher melting point material to reduce the extent of melting caused by the resistive heating as the switch starts to open. It will also reduce the extent of vaporization of the metal atoms when exposed to the ionized air molecules when the contacts begin to actually separate. The steel end-plate 6 and the steel-jaw spring 12 may be composed of a material that has a higher hardness, to more readily resist the spallation and evaporation of the metal atoms when exposed to the positively charged ions and negative electrons of the plasma.
By relocating the arc to the steel end-plate 6 and the steel-jaw spring 12, which occurs upon separation, arc erosion is substantially eliminated for the current carrying copper blade 4 and the copper jaws 10. In this manner, good contact joint integrity is maintained when the switch 2 is fully closed.
Examples of the low resistivity metal composing the blade body 4 and jaws 10 are shown as follows in Table I. The melting point and hardness of the example metals are also shown, for comparison with those for the end-plate 6 and jaw-spring 12.
Examples of the higher resistivity, higher melting point and higher hardness metal composing the end-plate 6 and jaw-spring 12 are shown in Table II:
Example embodiments of the knife blade switch 2 may be manually actuated or automatically actuated. Examples of an automatic actuation mechanism may include an electrically driven solenoid, gear motor, or linear motor that rotates the blade-body 4 about the pivot 9, to either open or close the switch. The application of such an electrically driven actuator enables a fast insertion or withdrawal of the blade end-plate 6 as it engages or disengages with the steel jaw-spring 12. A faster speed in the air, before insertion or after withdrawal, will reduce the duration of the arc in the air and thus the energy that it dissipates.
During the interval when the blade end-plate 6 is in contact with the steel jaw-spring 12, the current flowing through the switch is reduced because it must flow through a greater resistance path. The reduction in the current will diminish any arc formed when the contact separation occurs. For example, the relative position of the pivot 9 and the top of the jaws 10 shown in
Although specific example embodiments of the invention have been disclosed, persons of skill in the art will appreciate that changes may be made to the details described for the specific example embodiments, without departing from the spirit and the scope of the invention.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2014/031964 | 3/27/2014 | WO | 00 |
