The device current in thermal and magnetic circuit breakers passes through both a detection mechanism and a set (or sets) of electrical contacts. The contacts are generally spring loaded and latch restrained. When triggered by the overcurrent detection mechanism, the latch will release a movable contact arm. The arm then withdraws from the fixed contact at a rate determined by spring loading and electromagnetic forces due to the contact current.
When the contacts are closed, or “latched”, current flows between the contacts only at very small physical contact points, or asperities, due to surface roughness on the bulk contact faces. The actual area of electrical contact is only a small fraction, less than 1%, of the apparent area of the bulk contact surface (see Figure 5.1).
Current flowing in the contact bulk regions is constricted at these contact points, much like fluid flowing through a pipe with an insert containing very small holes. The extra electrical resistance due to this current restriction is referred to as the spreading or constrictive resistance of the contact. It can be shown [5.1] that the constriction resistance on each side of an individual contact “spot” is given by
where ϱr is the bulk resistivity of the contact material, and a is the effective radius of the asperity or actual contact spot area. If the contacts are constructed of two different materials, with respective bulk resistivities ϱr1 and ϱr2, the total series spreading resistance due to current constriction in both contacts is 
Normally, contacts are fabricated with identical materials and, normally, actual contact is made at N number of spots on the contact surfaces. The net constriction resistance for the contacts is then the parallel combination of all the individual contact values, or 
The effective radius of each contact spot, ai, is dependent on the preparation of the bulk contact surface, the normal forces applied to the contacts, the “hardness” of the contact material (i.e will each contact asperity be under elastic or plastic deformation?), and the temperature at the contact interface.
In addition to constrictive resistance at contact asperities, there may be a resistance due to a thin film or layer of material oxide between contacting asperities. Electrons either tunnel quantum mechanically through this thin film, or break through the film by a process Holm refers to as “fritting” [5.1]. The film resistance is between the constriction resistances of individual asperities, so the net “contact” resistance would be a modification of Equation (5.1):
where Rfi is the film resistance at asperity i.
In practice there is no attempt to determine contributions to Rcontact due to individual contact spots. The net excess resistance of the contact system, beyond the bulk resistances of the two contacting bodies, is simply referred to as the contact resistance. The voltage drop across this resistance is commonly referred to as the contact drop. In most cases this contact drop does not exceed .1-.2 volts. Contact drops tend to saturate at these levels since, as the magnitude of the current rises, the asperity interface temperature rises softening the asperity material. The softer material spreads out and increases the actual asperity contact area, thus lowering the contact resistance.
When two bulk metallic contacts which are carrying an electrical current separate, the last point or points of physical and electrical contact will be at one or more (if more than one, a small number) constriction asperity spots. The current density at these points will be very large, easily enough to melt the asperity material and form molten bridges between the two contacts. These bridges are then heated and stretched to the point that they vaporize. The process initiates the arc between the two contacts. If the contacts are not metallic, such as carbon, the asperity points do not melt, but rather arc immediately upon physical separation.