Electrical contact

Contacts may be operated by humans in push-buttons and switches, by mechanical pressure in sensors or machine cams, and electromechanically in relays.

[7] In AC systems, where the current passes through zero twice for each cycle, all but the most energetic arcs are extinguished at the zero crossing.

[8] Ragnar Holm contributed greatly to electrical contact theory and application.

If the small a-spot is treated as a circular area and the resistivity of the metal is homogeneous, then the current and voltage in the metal conductor has spherical symmetry and a simple calculation can relate the size of the a-spot to the resistance of the electrical contact interface.

, ballistic conduction of electrons occurs, resulting in a phenomenon known also as Sharvin resistance.

The contacts are open when the energizing force (magnet or relay solenoid) is not present.

SPDT toggle switches with a center off position are common, but relays with this configuration are relatively rare.

These are commonly found in contactors and in toggle switches designed to handle high power inductive loads.

As with forms X and Y, both current paths involve two contacts in series, mechanically linked and operated by a single actuator.

[12] Where a switch contains both normally open (NO) and normally closed (NC) contacts, the order in which they make and break may be significant.

In most cases, the rule is break-before-make or B-B-M; that is, the NO and NC contacts are never simultaneously closed during the transition between states.

The less common configuration, when the NO and NC contacts are simultaneously closed during the transition, is make-before-break or M-B-B.