I have a question regarding switch usage in a high voltage, very low current circuit (micro amps). Basically, we're taking the 120VAC out of the wall, converting it to DC (output can be either 5, 12, 15 or 24VDC) where it's going to a switch that is going to be the input to an SEL relay. Currently, it uses 120VDC as the input. What I want to know is can a push button with this high of a rating be used for a low voltage application? There are microamps going through so the power dissipated is <1W. Everything I've been taught tells me a push button will work regardless of the voltage running through it, as long as it's pushed (and the button switch isn't melted).
You have to observe the minimum current and voltage for the switch.
Typically mechanical contacts aren't gold-plated so a thin oxide layer will build up with time and it needs some amount of energy to "burn" that thin layer. See https://en.wikipedia.org/wiki/Wetting_current
A capacitator to generate some inrush current may help but this depends on your setup. If this is a home project and reliability isn't essential just use the switch...
Generally, people think: "if it can do more, it can do less", but this may not be the case with a switch in a humid environment, because of the oxydation phenomena.
It is like an insulating film between 2 electrodes. You have to observe the required minimum voltage & current as indicated by the manufacturer.
International standards such as IEC 60947-5-4 (2002) define the reference voltages for tests as +5 VDC or +24 VDC, where the current is chosen among the following values: 1 mA, 5 mA, 10 mA, 100 mA; 10 mA is the preferred value. Note that when tests are performed, there is a delay of 10 ms before actually checking if the contact is closed.
For additional information on the subject, you can also check the MAX13362, manufactured by Maxim, a 24-Channel Automotive Switch Monitor with selectable wetting current (0 mA, 5mA, 10 mA, 15 mA). If minimum voltage & current are not both present, even in normal conditions (T and humidity) problems arise after a few years.