# Definition of Terms and Symbols

### Definition of Terms and Symbols

Note: The symbols for electrical contact specific terms i.e. contact area, contact resistance, etc. have been retained from the german version of the Data Book. In related English literature some of them may vary using subscript symbols related to the language used – for example “contact resistance”: as used here from german Rk, in english mostly Rc.

Table 1: Overview of terms and formula symbols
Term Formula symbol Unit Explanation
Electrical contact A property which is generated through the touching of two electrically conducting surfaces
Contact part A metallic component which is designed to create or interrupt an electrical contact (is frequently replaced by the term “contact” if it is clearly understandable that a physical piece or item is meant)
Contact area The whole area on a contact part that may be used for contacting
Apparent contact area As [m2] The part of the contact area on contact parts that can make physical contact during the touching of two contacts
Load bearing contact area At [m2] The part of the apparent contact area which is affected by the contact force. It is the sum of all microscopic actual touching points
Effective contact area Aw [m2] The part of the load bearing contact area through which current is flowing and therefore the sum of all current carrying touching areas (a-spots), Aw < At < As
Contact resistance Rk [Ω] Composed of the constriction resistance and the film resistance
Constriction resistance Re [Ω] The incremental electrical resistance generated by the constriction of the currents paths in the touching area (a-spot)
Film resistance Rf [Ω] Generated by a foreign matter layer, which for ex. is formed by a reaction of the contact material surface with the surrounding atmosphere (a surface film is a substance on the contact surface with different properties than those of the actual contact material)
Path resistance Rd [Ω] the total electrical resistance between reference points (usually the device terminals) which can be freely chosen but must be defined. It is the sum of the conductor resistance Rb

and the contact resistance Rk

Contact Force Fk [N] The force that is exerted between the two contact parts in the closed position
Contour area An [m2] The contiguous area which includes all effective a-spots, Aw < An < As ; An ≠ At
Elastic modulus E [MPa] Material characteristic value from materials engineering that describes the proportional relationship between stress and strain during the deformation of a solid body in the case of linear-elastic behavior
Hardness (Brinell or Vickers) H The mechanical resistance that a material offers to the mechanical penetration of another body
Current I [A] The electric current strength indicates how much electric charge moves through a conductor per second
Number of effective individual contact areas N
Electrical resistance R [Ω] In electrical engineering, electrical resistance is a measure of the electrical voltage required to allow a certain electrical current to flow through an electrical conductor (component, circuit)
Path resistance Rd [Ω] The total electrical resistance between reference points (usually the device terminals) which can be freely chosen but must be defined. It is the sum of the conductor resistance Rb and the contact resistance Rk
Distance of the effective single contact areas between i and j sij [m]
Maximum contact temperature TKmax [K]
Voltage U [V] The greater the electrical voltage, the more charge can be transported and thus the electrical current strength, i.e. the amount of charge, also increases.
Contact Voltage UK [V] The electrical voltage created when two different substances come into contact. This is ultimately caused by the different electron levels in the materials
Volume V [m3] The volume is the spatial content of a geometric body
Specific resistance ρ [Ω*m] The resistivity (Rho) is defined as the resistance of a conductor of 1 m length and 1 qmm cross-section (A) at a temperature of 20 degrees Celsius
Lifting Force FA [N]
Frictional wear The loss of material caused by mechanical wear between contact parts
Bounce The single or multiple interruption of conduction between contact parts during the make operation caused by alternating transformation of kinetic to potential energy
Contact Wear Includes all changes on a contact surface. Mechanical and electrical wear must be distinguished
Material transfer The transfer of contact material from one contact part to the other. It occurs mainly during switching of DC loads. The direction of the transfer depends on the load circuit properties and the contact materials used
Arc erosion The loss of material into the surrounding of the contact spot which is generated by electrical arcing. It occurs during contact make as well as break operations
Contact welding Occurs when melt-liquefied touching areas of the contact parts come in contact with each other. The melting occurs during high current carrying through these areas. During make operations this occurs through bounce arcs, on closed contacts a too high contact resistance or dynamic separation of the contacts due to high short circuit currents can cause the welding of the contacts. The welding then may cause a device failure if the device specific opening forces cannot break the weld connection
Arc movement Happens when during the break operation a sufficiently high magnetic field is generated which exerts a force on the electrical arc which is then moved from the originating spot towards an arc chute (or arc splitting plates)
Arc extinguishing Means the process of letting the current go to zero and transferring the arcing gap from a conducting to the non-conducting stage. Selecting the most effective extinguishing measures depend mostly on the current characteristics, the current value and the circuit voltage
Recovery of an arc gap during contact opening is defined as the process of the electrically conducting plasma of an arc losing its electrical conductivity after reaching current-zero