Four Factors Governing Contact Selection

1. Electrical design factors - the electrical characteristics of the circuit in which the contact must perform are of prime importance in contact selection. Following are four important electrical considerations:
   1. electrical current - as the major component of electrical energy, current affects all factors involved in contact performance. A high percentage of the energy dissipated at the contact is in the form of heat generated by constrictive resistance. Normally, contact temperature rises during operation until it reaches a point of equilibrium between radiation and conduction losses and heat input of resistance and the arc energy. Arc erosion, welding and sticking all occur in direct proportion to the contact current.
   2. voltage -circuit operating voltages are important in contact selection. All materials possess a characteristic arcing voltage in the 10-20 volt range. If the maximum voltage is below this range, arcing will not occur. Above this voltage, arcing and subsequent material loss must be expected.

   3. load - the type of load will have a direct bearing on contact performance. Here are the common types of electrical loads:

      resistance - a straight resistive load is very predictive and behaves consistently.

      inductive - An inductive load complicates the problem on contact break as it releases a considerable amount of energy.

      capacitive - A capacitive load creates a problem because of a high current inrush.

      motor - A motor load, while basically inductive, has a heavy staring current inrush.

      lamp - A lamp load, with a high current inrush, requires contacts of sufficient ability to handle the electrical load.

   4. contact protection - arc suppressers such as condensers, capacitors and blowout coils can reduce arcing at the contact points, thus offering longer life. Care must be exercised in selecting the type of suppresser. The best protective circuit is most often determined by actual test.

2. Mechanical design factors - The following mechanical factors also affect contact selection:
   1. force - The mechanical forces that close a pair of contacts play a vital role in the selection of contact material. The force employed on the make should be the maximum the material will withstand without excessive wear or deformation. To achieve stable conditions, the minimum contact force is:

      METAL	GRAM PRESSURE
      Gold	1-5
      Platinum	10-50
      Gold Plate	5-50
      High Percentage Silver Alloys	20-250
      Tungsten	100
      Base metals	1000

   2. frequency of operation - the number of make and break operations in a given time will greatly affect the life of a contact. An accepted classification of frequency is as follows:
      low frequency - up to 1/min.
      intermediate frequency - 1/min. to 10/sec.
      high frequency - more than 10/sec.
      Problems encountered in low frequency operations are oxidation or film formation during idle time. In high frequency operations, the design must allow for high temperatures and heat dissipation.

   3. speed of operation - the speed of closing or opening a pair of contacts is important in both A.C. and D.C. operations. In D.C. operations, the snap action is most advantageous in both make and break. In A.C. applications, a fast make and a slow break result in the least arc damage.

   4. bounce or chatter - closing and opening contacts more than once per designed cycle will reduce contact life. In these cases, it normally requires some change in the spring material or contact mass to obtain a firm make and break.

   5. gap - the contact gap or space between the contacts in their open position must be sufficient to prevent continued arcing. This is particularly true in inductive loads with high counter EMF.

   6. wipe (over-travel) - depending on the application, a certain amount of wipe can be advantageous or undesirable. Wipe or sliding will break down some films and can increase the contact area. In D.C. circuits this can be a detriment due to material transfer and the resulting possibility of mechanical hang-up between the peak and crater.

3. Environmental design factors - as stated in the factors which affect contact life, film resistance caused by the environment can have an adverse affect on the operation and life of a contact. These environmental factors include the following:
   1. gases and fumes - sulfur fumes are primarily responsible for the tarnishing (silver sulfide) formation on contacts. This film will increase resistance and has been known to cause open circuits. Hydrocarbon fumes will have an adverse effect on the platinum alloys. Another source of fumes is the organic material used in the device (plastics), which on outgasing, can produce harmful films. To combat these problems, devices can employ gold flashing or plating.

   2. foreign materials - such contaminants as dirt, dust, lint, grit and loose metallic particles, as well as lubricant silicone (which breaks down to silicon dioxide and silica) can greatly increase arcing between contacts or cause excessive temperature rise through their inadvertent intrusion. Should enough particles lodge in the contact area, an open circuit will result. This can be alleviated somewhat by mounting the contacts vertically or designing a point or projection in the face.

   3. temperature and humidity - contact materials employed in high temperature application must be able to withstand abnormal oxidation and chemical reactions. High humidity will promote or increase corrosion and oxidation.

4. Economic factors - Because contact materials are precious or rare metals, they are expensive. Depending on the material used and the size of the contact, cost can vary from less than a cent to several dollars each. On all devices, the prime requisite is that the contacts operate dependably for their designed life. In some sophisticated devices, this cost may be immaterial. In other applications, it may represent a major portion of the total cost.