In my last article (The Diabolical & the X-Ray), I got the chance to talk about remote-controlled switches and diabolical devices. This time I want to talk about selecting a mechanical switch. I will limit this to basic snap-action switches and their specifications.
Switches with a small form factor, typically 0.5 x 0.5 inches, can usually handle up to 3 amps. The smaller the switch size, the shorter the travel is. That means lower voltage rating (contact spacing when open) and the smaller the amount of current that can be handled (contact size). Another specification many of us are concerned with the operating force of the switch. To maintain a good contact, strong springs are needed.
If you are looking for light-weight switch action then you will be limited in both size and current. In snap-action switches, the useful current capacity range is from a few milliamps up to perhaps 30 amps. Switches are also rated with different specs for DC voltage and AC voltage. There should be details for the normally open (NO) position, typically called insulation resistance. You can see 100 MΩ or higher value. This rating is at a stated voltage higher than normal operating conditions.
Reliability is very key in mechanical switches. It not unusual to have a mechanical life of more than one million operations. The electric specification can be equal to the mechanical life. Typically though it is lower — sometimes only one-tenth the mechanical life. I have seen some limited-production switches with a mechanical life of 25 million cycles. Price and delivery could set any project back, especially if your stipulations are higher than the design requirements. But in some cases, there may be just pennies in the difference between a basic switch and a more reliable version.
Another detail you should be concerned with is environmental qualifications. You should understand what conditions the switch will be used in and the conditions you might encounter during maintenance, storage, or shipping. Water is not the only item that can get into a switch. Contaminants such as oils and greases can stop a switch from functioning properly.
Another specification is outflow (although you'll see other terms that mean the same thing). This refers to contaminants coming from the switch and getting into a medical or similar delicate environment. The potential of heating in the switch elements could make the outflow of switch lubricant an issue in some instances. Contaminant leakages in and out are usually the same specification.
I will wrap up with a few last details. Actuators can range from being small pin plungers, to hinge-style levers, to panel-mounted roller plungers. The total travel is the total movement of switch without damage occurring (operating travel plus pre-travel). Pre-travel is the distance of the plunger button (actuator) movement from rest position to the point of operation. Pre-travel is often specified as a maximum amount.
The maximum operating frequency is the fastest rate at which the switch can be operated. There is typically a difference between mechanical and electrical speeds. Mechanical action is faster. Approvals for the switch are definitely a factor especially in interchangeability with other switches. UL94B flammability rating, IP50 terminal type, and IP67 lead wiring and splash rating. Check your requirement regarding UL, CSA, VDE, RoHS, and other foreign designators.