- Electrical Contact Material Definition: Electrical contact material is defined as a substance chosen for its ability to conduct electricity efficiently and withstand various operational stresses in electrical contacts.
- Contact Force Impact: The force applied to contacts affects their resistance and required contact area, crucial for efficient performance.
- Voltage and Current Handling: Materials must be chosen based on the voltage and current they need to manage, with specific considerations for DC applications.
- Oxidation Issues: Oxidation during operation can create non-conductive layers on contacts, increasing resistance and affecting performance.
- Thermal and Electrical Conductivity: High electrical and thermal conductivity are essential for materials used in electrical contacts to reduce losses and dissipate heat.
Materials used for electrical contacts are chosen so a machine contact can carry current, survive make and break, and keep a low voltage drop. That choice depends on contact force, voltage, current, resistance and oxidation. Those factors are listed below:
Contact Force
Contact resistance falls as the closing force rises, because more of the surface actually touches. Curved faces need more force than flat ones for the same metallic area. That force can range from a fraction of a gram to a kilogram. The contact metal has to stand that load.
Voltage and Current
The performance of contacts is related the voltage and current, the contacts must make and break during their operation.
The contacts used in DC supply are subjected to material transfer from the face of one contact to the face of its making contact. Which results in mound on one contact and hole or cater on the face of its making contact. The direction of transfer of material whether form positive to negative or vice versa depends on the polarity of ions formed by material.
Contact Resistance
The main function of electrical contacts is to carry electric current, so they must have low contact resistance to avoid voltage drops, especially in low voltage applications. Contact resistance includes both the material resistance and the interface resistance between contacts. The material’s resistance is low compared to the interface resistance. Flat surfaces have small projection points that limit contact area.
These small projection points restrict the area of interference surfaces touch other. Due which the effective area at interference through which the current passes is very small as compare to gross area of interference surfaces. Due to which the resistance at contact interference is very high. To reduce this contact resistance, have to make the interference surfaces as smooth as possible to increase the area of interference surfaces touches other.
Contact resistance can increase due to contamination from chemicals formed by oxidation of the contact material. Oxidation is a major issue, as arcing during operation heats and erodes the contacts, forming chemicals like oxides and carbonates. These non-conductive films raise contact resistance.
Corrosion Resistance
At the time of selection of material for electrical contacts, we must keep in mind that the material must possess high corrosion resistance at operating temperature and must be free from oxidation during arcing. Otherwise, oxides, carbonates, chlorides, sulphate and sulphide etc. formed during oxidation can make anon-conducting layer of thin film at the surfaces of contacts, which results in increased contact resistance.
Stickiness or Weldability
High current circuits can cause arcing, leading to very high temperatures. This heat can make contacts stick or weld together. Therefore, materials for electrical contacts must withstand high temperatures and avoid welding.
Arc Extinguishing Property
During the operation of electrical contacts, the arc is developed. For successful operation of electrical contacts, this arc must be extinguished as earlier as possible. Hence, while selecting the material for electrical contacts, we must keep it in mind that the material must possess such property that it must help in arc extinguishing.
High Electrical Conductivity
The total resistance of electrical contacts must be as minimum as possible to reduce the switch losses. Hence, material being selected for electrical contacts, must possess high electrical conductivity.
High Thermal Conductivity
During the operation of electrical contacts, which are used at high current rating circuits, the arcing develops the heat at very high temperature. To avoid the concentration of this heat at contact surface, this heat must be conducted be contacts and dissipated in atmosphere. Hence, material being used for electrical contacts must possess high thermal conductivity.
