Definition of Terms and Symbols

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_k, in english mostly R_c.

• Electrical contact is a property which is generated through the touching of

two electrically conducting surfaces.

• Contact part is 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 is the whole area on a contact part that may be used for

contacting.

• Apparent contact area A_s is the part of the contact area on contact parts that

can make physical contact during the touching of two contacts.

• Load bearing contact area A_t is 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 A_w is 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), A_w< A_t< A_s.

• Contour area A_n is the contiguous area which includes all effective

a-spots, A_w< A_n< A_s; A_n≠ A_t.

• Contact resistance R_k is composed of the constriction resistance and the film

resistance.

• Constriction resistance R_e is the incremental electrical resistance generated

by the constriction of the currents paths in the touching area (a-spot).

• Film resistance R_f is 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 R_d is 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 R_b and the contact resistance R_k.

• Contact force F_k is the force that is exerted between the two contact

parts in the closed position.

• Frictional wear is the loss of material caused by mechanical wear

between contact parts.

• Bounce is 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 is 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 is 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.

• Symbole used

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