Contact Performance in Relays
5.6 Electrical Discharges at Contacts
There are three types of electrical discharges which affect contact behavior
in relays-the arc, the glow and the Townsend discharges.
The arc is characterized by having a high current density but a comparatively
low voltage drop across it. The consequence of this fact is that when an arc
occurs, the surfaces of the contacts are heated intensely and can experience
considerable melting of material. This action causes metallic erosion and can
provide conditions favorable for welding. On the other side, however, the
existence of the arc absorbs energy from the circuit and prevents damaging
transients from occurring during interruption. Also, the discharge tends to
clean from the contact a variety of debris that can settle or form on the
contact surfaces.
The glow discharge, on the other hand, is characterized by having a low
current density with the consequent effect of causing little heating or
melting at the surface. However, the glow discharge contains considerable
electronic energy which can catalyze substances that exist on the contact
surface or in the adjacent atmosphere to form highly resistive deposits.
Unfortunately, the thermal energy in the glow is not always sufficient to
destroy these deposits as they are formed. The consequence of this action is
that, when contacts operate predominantly in the glow discharge region,
troublesome contamination can be formed on the surface of the contact. In
addition, sometimes the action of the glow can create material transfer.
Transfer is a phenomenon that forms a metallic bridge across the gap of
miniature switches preventing the contact from opening.
The Townsend discharge like the glow discharge causes little contact heating
but is the vehicle through which contact voltage breakdown occurs.