Increased contact resistance of switching contacts during current carrying

The contact resistance of contacts switching under load does not just vary from operation to operation but also changes during current carrying. Measurements revealed strongly scattering contact voltage drops up to the softening voltage when AC carrying, which may yield high overtemperatures. Especially at Ag/SnO2 those high values, which may lower and again deteriorate during current carrying, appeared frequently. Investigations focused on conditions and parameters (e.g. contact material, atmosphere) which result in this increased scattering contact resistance behaviour. Furthermore potential remedies to avoid high resistance values were evaluated. The studies have shown that the current waveform has a crucial impact on the contact resistance behaviour: High contact resistance existing at AC immediately lowers when carrying DC.