Parameters influencing the contact compatibility of organic vapours in telecommunication and control switches

The formation of carbon on contact surfaces of commercial switching devices for low power applications by thermal decomposition of organic vapours emanating from various organic materials may cause an undesirable increase of the contact resistance in the range of several Ohms. A specially developed test system and procedure is used to determine the contact compatibility of individual organic vapours: the contacts of a model switch are switching a well defined circuit in an atmosphere consisting of synthetic air and an increasing admixture of one organic vapour. The so called critical concentration is attained when well defined criteria of carbon contamination of the contacts become detectable. Previous investigations revealed that the contact compatibility of an organic vapour in combination with a certain contact material depends mainly on its chemical structure. The influence of the parameters oxygen concentration and humidity of the switching atmosphere, contact bouncing, and contact temperature on contact compatibility of individual organic vapours has been investigated systematically. The variation of these parameters showed the often rather strong influence on the effect and yielded more insight into the mechanism of contact carbonization.