Contact connections are included into all electrical circuits and are very responsible elements. Trouble-free operation of an electrical wiring and electric equipment depends on a condition of electric contact connections. At the same time the size of transient contact resistance is important.
Definition
In an electrical circuit, in the place of contact of two or more conductors, the electric transitional contact, or conducting connection on which current flows from one part in another is created. At simple imposing the contacted surface of the connected conductors does not give good contact. Actual contact area several times is most less than contact surface, what can see confirmation by means of a microscope.
In view of small contact area the contact connection gives very noticeable resistance when passing current from one surface in another and is called transient contact resistance. Transient resistance of contact a priori is more, than resistance of the continuous conductor of the same form and the sizes.
The factors influencing the size of transient resistance
Resistance of a zone of contact does not depend on the size of contact surfaces and is defined by the pressure force or force of contact pressing. Contact pressing is called the effort with which one contacting surface affects another. In general, the total area of contact will depend on the size of force of pressing and durability of material of contact. The number of contacts always grows in contact when pressing. With small pressure there is a plastic deformation of contact, at the same time tops of ledges are rumpled and then, at increase in pressure, all new and new points come to contact. As a result, pressure has to be quite big to provide small transient resistance, but also should not generate the plastic deformations in metal of contact leading to its destruction. Transient resistance considerably depends on the oxidation level of contact surfaces of the connected conductors. Irrespective of material of the conductor, the film of oxide creates bigger electrical resistance.
The intensity of oxidation of conductors depends on temperature of contact and the it quicker, the more transient resistance.
Aluminum conductors are very strongly subject to oxidation. For example, their oxide film which is formed on air possesses with a specific resistance of 1012 om*sm. Since property of contact connection can change. Only the new, well processed and smoothed out transitional contact can have the smallest probable transient contact resistance with a sufficient pressure. When forming contact connections apply different ways of fastening of conductors. For example, soldering, welding, pressure testing, mechanical connection with the help of bolts and also reduction in contact with the help of elastic pressing of springs. Actually at any way of connection of wires it is possible to achieve permanently small transient contact resistance. It is important to connect, at the same time, wires strictly on technology and to use for every way of connection of wires of the necessary tool and materials.
Contact connection of electrochemical incompatible conductors is contact of two oxides which will have high value of transient resistance.
For decrease in transient contact resistance consider all above-mentioned factors influencing its size and carry out compliance of types of connecting contacts to materials of conductors and to conditions of their operation.