By Earl Haines
NKK Switches
Scottsdale, Ariz.
Edited by Miles Budimir
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M-Series and S-Series toggle switches from NKK Switches are rated for 6 A at 125 Vac and 50 A at 125 Vac, respectively. |
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These 6 A fixed contacts were damaged by 38 A of inrush current. In general, silver contacts are better able to withstand the arcing from high inrush current. |
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The main disadvantage of silver contacts is tarnishing. The dome contact on the left shows considerable tarnishing, as opposed to the clean contact on the right. |
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The electrical resistivity of a material tells how well the material conducts electrical current. Conductivity is the reciprocal of resistivity. Good conductors of electricity have low resistance or high conductivity. |
Switch contacts are made from a variety of metals, but gold and silver are the most common. Each has advantages and disadvantages. The big plus for gold is that it doesn't corrode or tarnish. However, its higher cost is a disadvantage, and it's less conductive than silver.
Another problem with gold contacts is the potential for oxidation from organic compounds, simple versions of which are present in the atmosphere. In vapor form, they can be absorbed by the gold contact surface. Sliding or fretting between contacts causes a chemical reaction between the gold, which acts as a catalyst, and organic compounds. This reaction produces long-chain compounds or polymers.
One way to get around these drawbacks is to place a thin gold layer over a copper contact. Doing so demands barrier plating. Due to the cost of gold, the plating is usually thin, on the order of 1 to 2 microns. Without a barrier between the two materials, copper can migrate to the gold surface, where the exposed copper begins to oxidize forming a copper-oxide layer that acts as an insulator. The barrier layer is usually nickel.
Silver contacts, on the other hand, need no such barrier because the silver plating is much thicker, 2 to 5 microns. A barrier also isn't necessary in gold over silver contacts because the silver is thick enough to act as a barrier.
Some applications such as high-reliability edge-connectors require 30 microns of gold plating by specification. The thick plating is needed because of the high-force, high-wear nature of insertions and disconnections. However, a 30-micron gold plating isn't reasonable for switch contacts. First, a plating of this thickness would eliminate the spring characteristics of the contacts, and there just isn't enough room for such a thick plating. Moreover, a switch with 30 microns of gold on the contacts would be more expensive.
Soft mechanical wear is another disadvantage of using gold. It's much softer than silver or silver alloys and gold wears much easier by sliding friction.
In addition, silver contacts are better in applications where arcing and welding are likely. This is because in low-current situations, typically less than 50 A, the circuit resistance or reactance is usually high, so the relatively high contact resistance of gold is not significant to the total circuit resistance.On the other hand, in high current situations (typically over 50 A) using silver contacts, the circuit resistance or reactance is usually low. Therefore, the contact resistance should be as low as possible otherwise it would become significant to the total circuit resistance. The I2R heating of the contacts would result in premature contact failure.
Silver, however, does have a notable disadvantage: Tarnish. Contacts get tarnished in the presence of hydrogen sulfide and water vapor. Sources of hydrogen sulfide include decaying organic material, some packaging material, and industrial pollution. Tarnish is usually not a problem, however, where arcing, mechanical forces, and contact movement are present.
During long static periods tarnish on silver contact surfaces continues to accumulate. However, gold-over-silver plating produces no tarnish and the contact will close reliably even after long periods of inactivity. If a switch must close only a few times in its life, as in an emergency shut-off, it should have gold over silver contacts.
Arc oxidation produces another problem for silver contacts. An arc is a plasma of negative ions, similar to lightning. The temperature within the plasma arc can easily reach 10,000°F or higher. At these temperatures, everything oxidizes including the silver, along with any particulate contamination caught in the plasma. Evaporated or vaporized silver may condense on contacts and the surrounding area in the form of a metallic powder. The area around the contact erosion will be covered with a black ash, arc oxidation that comes mostly from the silver and any lubricant present.
In applications where power switching is infrequent (months or years between operation), gold over silver contacts provide high reliability. This is because silver tarnishes in the presence of hydrogen sulfides and other industrial pollutants. The tarnish raises the contact resistance as it forms a thin insulative layer on the surface of the contacts. Eventually, the contact may not close, depending on arc energy available and spring force.
Silver contacts fare better in moderate wear. For example, PCB edge connectors have high-contact spring force, insertion force, and wear. Contact wear in rotary switches might be considered moderate. Wear is low in butt contacts where there is little relative movement between the contacts and where most of the wear comes in the form of fretting (small sliding movements). Wear is also low in cross-bar contacts, the most ideal design.
Furthermore, snap-action contacts keep operators from "teasing" contacts. Snap-action contacts close or open after reaching a predetermined plunger force. They open and close much quicker than other designs, thereby shortening the arc period. In fulcrum or see-saw type contacts, an operator can tease the toggle in such a way as to stop the contacts in mid-travel. If an arc is created during teasing, the arc will be maintained causing more damage to the switch contacts.
Think cross bars for reliability The STC mechanism blends three basic technologies: slide action, crossbar, and duality. The sliding contacts produce a self-wiping motion that cleans the switch contacts with each actuation. By creating pressure, the crossbar stabilizes the contact and ensures a crisp feel. Finally, twin or dual crossbars pinch the stationary contact and provide a double safety mechanism. If for any reason one side should fail to make contact, the other side completes the circuit. |