In a marginal situation as discribed would it matter to switch life if it was used on-off or momentary? Is one mode harder on the switch than the other?
I borrowed this from wikipedia, the same principles apply here.
A switch is most vulnerable when the contacts are opening or closing. As the switch is closed, the resistance of the switch changes from nearly infinite to nearly zero. At infinite resistance, no current flows and no power is dissipated. At zero resistance, there is no voltage drop and no power is dissipated. When the switch changes state however, there is a brief instant of partial contact when resistance is neither zero nor infinite and power is dissipated. During that transition the contacts heat up. If the heating is excessive, the contacts can be damaged or even weld themselves closed.
In addition, during the transition when the contact is broken there is an additional issue that if an inductive load is being switched, the stored energy in the inductor is dissipated as an arc within the switch, prolonging the transition and worsening the heating effect on the contacts. Thus switches are commonly rated by the current they are designed to break, as this is the most stringent constraint.
The arc that results when the switch operates corrodes the switch contacts, in time leading to erosion of the contact surface and fouling of the contact area by corrosion byproducts. A switch therefore has a finite life, again often being rated at a given number of cycles of disconnection at a specified current. Operation outside its design envelope will shorten the switch life very drastically.
To combat contact corrosion a switch is usually designed to have a wipe action such that the contact corrosion is cleaned off the area of the contact that forms the low resistance path when the switch is closed. It's also designed so that the initial point of contact, and thus the majority of the contact corrosion, occurs at a sacrificial part of the contact, rather than the face that is in contact when the switch is fully closed. Depending on the switch rating and price, the contact area of the switch is often a sophisticated construction of brass contact, silver contact button, and plated finish to minimize the amount of contact corrosion and thus extend the life of the switch.
