1. electrical design factors
2. mechanical design factors
3. environmental design factors
4. economic factors.

1. welding or sticking of contacts
2. excessive resistance
3. contact transfer
4. contact erosion.

Simply put, the function of an electrical contact is to make and break an electrical circuit. In this broad spectrum, circuits may be handling currents ranging from microamps to hundreds of amperes. There is no universal contact construction or all-purpose contact material. Over the years, a great number of contact materials and alloys have been formulated to better meet the electrical and performance requirements of specific applications.

Before discussing the many possible combinations of precious and base metal alloys, consider the factors that influence contact performance and life.

Four Factors Which Affect Contact Life

1. Welding or sticking may be caused by:
   1. mechanical hang up - most often the result of excessive material transfer and subsequent interlock caused by wipe action.
   2. butt welding - the result of heat caused by high resistance at the interface of the contacts.
   3. arc welding - caused by closing two molten contacts which have been heated by an arc discharge.

2. Excessive resistance is the sum of the resistance of:
   1. contact material resistance - this can be defined as the specific resistance of a given material.
   2. constriction resistance - this is the resistance at the interface where the contacts touch. It is caused by crowding current through a very small area and is a significant amount of the total contact resistance.
   3. film resistance - this is the result of resistance caused by non-conducting or semiconducting particles between the contacts. These films or particles can be chemical and/or mechanical. Chemical films are caused by oxidation, corrosion or surface reaction to foreign materials. Mechanical films are the result of dust, oil and other foreign particles. Contact failure may be said to occur when the closed resistance is too high or the open resistance is too low.

3. Contact transfer is the term employed when the contact material moves from one contact to another. In A.C. circuits this does not have a general direction unless there is an imbalance of contact temperatures. In this case, the material moves to the cooler contact. Contacts working in synchronism with line frequency will transfer, since they always break at the same point on the waveform. This, in effect, is the same as breaking a D.C. circuit. In a D.C. circuit, material may transfer in one of the following ways:
   1. to the negative contacts, commonly called bridge transfer. Bridge transfer is caused by operating below minimum arc voltage or current values for a given material.
   2. to the positive contact, usually termed arc transfer. Arc transfer is caused by operating above minimum arc voltage and current values.

4. Contact erosion can be either electrical or mechanical. Mechanical wear in straight butt type operations is of little concern, but in wiping or rotary action it is a major problem. Mechanical design factors are covered more fully in Contacts Selection.

   The primary source of contact erosion is the arc. The energy in the arc heats the contact surface to the boiling point and causes material loss by vaporization. This loss of material directly relates to contact life and can have an effect on the dielectric strength of the surrounding mechanisms. The characteristics of arc erosion vary between different materials, but the rate of arc erosion for a given material is proportional to the circuit current and the frequency and number of operations.