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→6.4.1 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 R<sub>k</sub>, in english mostly R<sub>c</sub>.
*'''Electrical contact''' <br />is a property which is generated through the touching oftwo electrically conducting surfaces.
*'''Contact part''' <br />is a metallic component which is designed to create or interruptan electrical contact (is frequently replaced by the term “contact” if it is clearlyunderstandable that a physical piece or item is meant).
*'''Contact area''' <br />is the whole area on a contact part that may be used forcontacting.
*'''Apparent contact area A<sub>s</sub>''' <br />is the part of the contact area on contact parts thatcan make physical contact during the touching of two contacts.
*'''Load bearing contact area A<sub>t</sub>''' <br />is the part of the apparent contact area whichis affected by the contact force. It is the sum of all microscopic actual touchingpoints.
*'''Effective contact area A<sub>w</sub>''' <br />is the part of the load bearing contact area throughwhich current is flowing and therefore the sum of all current carrying touchingareas (a-spots), A<sub>w</sub>< A<sub>t</sub>< A<sub>s</sub>.
*'''Contour area A<sub>n</sub>''' <br />is the contiguous area which includes all effectivea-spots, A<sub>w</sub>< A<sub>n</sub>< A<sub>s</sub>; A<sub>n</sub>≠ A<sub>t</sub>.
*'''Contact resistance R<sub>k</sub>''' <br />is composed of the constriction resistance and the filmresistance.
*'''Constriction resistance R<sub>e</sub>''' <br />is the incremental electrical resistance generatedby the constriction of the currents paths in the touching area(a-spot).
*'''Film resistance R<sub>f</sub>''' <br />is generated by a foreign matter layer, which for ex. isformed by a reaction of the contact material surface with the surroundingatmosphere (a surface film is a substance on the contact surface withdifferent properties than those of the actual contact material).
*'''Path resistance R<sub>d</sub>''' <br />is the total electrical resistance between referencepoints (usually the device terminals) which can be freely chosen but mustbe defined. It is the sum of the conductor resistance R<sub>b</sub> and the contact resistance R<sub>k</sub>.
*'''Contact force F<sub>k</sub>''' <br />is the force that is exerted between the two contactparts in the closed position.
*'''Frictional wear''' <br />is the loss of material caused by mechanical wearbetween contact parts.
*'''Bounce''' <br />is the single or multiple interruption of conduction betweencontact parts during the make operation caused by alternating
transformation of kinetic to potential energy.
*'''Contact wear''' <br />includes all changes on a contact surface. Mechanicaland electrical wear must be distinguished.
*'''Material transfer''' <br />is the transfer of contact material from one contactpart to the other. It occurs mainly during switching of DC loads. Thedirection of the transfer depends on the load circuit properties and thecontact materials used.
*'''Arc erosion''' <br />is the loss of material into the surrounding of the contactspot which is generated by electrical arcing. It occurs during contact makeas well as break operations.
*'''Contact welding''' <br />occurs when melt-liquefied touching areas of thecontact parts come in contact with each other. The melting occurs duringhigh current carrying through these areas. During make operations thisoccurs through bounce arcs, on closed contacts a too high contactresistance or dynamic separation of the contacts due to high short circuitcurrents can cause the welding of the contacts. The welding then maycause a device failure if the device specific opening forces cannot breakthe weld connection.<br />
*'''Arc movement''' <br />happens when during the break operation a sufficiently highmagnetic field is generated which exerts a force on the electrical arc which isthen moved from the originating spot towards an arc chute (or arc splittingplates).
*'''Arc extinguishing''' <br />means the process of letting the current go to zero andtransferring the arcing gap from a conducting to the non-conducting stage.Selecting the most effective extinguishing measures depend mostly on thecurrent characteristics, the current value and the circuit voltage.
*'''Recovery''' <br />of an arc gap during contact opening is defined as the process ofthe electrically conducting plasma of an arc losing its electrical conductivity afterreaching current-zero.
*'''Symbole used'''