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The contacts are the current-carrying part of the contactor. This includes power contacts, auxiliary contacts, and contact springs. Contact material is chosen for high electrical conductivity, mechanical strength, and stability under arcing and oxidation. Commonly used metals include alloys of tungsten, molybdenum, copper, and others.
When the contacts touch, the switch is closed; when the contacts are separated, the switch is open. The gap must be an insulating medium, such as air, vacuum, oil, SF 6. Contacts may be operated by humans in push-buttons and switches, by mechanical pressure in sensors or machine cams, and electromechanically in relays.
Every time the contacts of an electromechanical switch, relay or contactor are opened or closed, there is a certain amount of contact wear. If the contact is cycling without electricity (dry), the impact of the contact electrodes a slightly deformed by the resulting cold forging. [1]
The mechanical motion of the reeds is below the fatigue limit of the materials, so the reeds do not break due to fatigue. Wear and life are almost entirely dependent on the electrical load's effect on the contacts along with the properties of the specific reed switch used. Contact surface wear occurs only when the switch contacts open or close.
This action causes the material in the contacts to degrade and coordination, resulting in device failure. This contact degradation drastically limits the overall life of a relay to a range of about 10,000 to 100,000 operations, a level far below the mechanical life of the device, which can be in excess of 20 million operations. [36]
Breaker arm with contact points at the left. The pivot is on the right and the cam follower is in the middle of the breaker arm. A contact breaker (or "points") is a type of electrical switch, found in the ignition systems of spark-ignition internal combustion engines. The switch is automatically operated by a cam driven by the engine.
The formation of oxide layers at contact surface, as well as surface roughness and contact pressure, determine the contact resistance, and wetting current of a mechanical switch. Sometimes the contacts are plated with noble metals , for their excellent conductivity and resistance to corrosion.
In a uniselector, the stepping switch steps only along or around one axis, although several sets of contacts are often operated simultaneously. In other types, such as the Strowger switch, invented by Almon Brown Strowger in 1888, mechanical switching occurs in two directions, across a grid of contacts.
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