Current switches exist to serve some purposes. Each purpose assists an industrial situation in maintaining the status quo. You also need to know which type of switch to use and what type of current. Not knowing these things will result in some major electrical damage to your plant's equipment. Let's start with familiarizing everyone with the three key purposes of AC current switches.
Checking for Electrical Current
The old-fashioned (and considerably less intelligent) way of checking a line for current is to touch it. Electrocution is not exactly the way anyone wants to find out that they have a live wire, but it worked. Now, you just use an AC current switch to test for electrical current. Most of the current used in the U.S. is alternating current, so you are safe to use AC current switches. However, if you have a foreign machine that clearly states in the owners manual, is labeled on the machine, or has a warning sign to the same effect that it operates only on AC current, make sure you have on hand and use an AC switch with this machine!
Monitoring the Flow of Current
Once you have established that you have a live line of electrical juice flowing into and out of a machine, you want to be sure that the electricity keeps flowing. During brown-outs and blackouts, certain machines in the plant need to keep running. The switches will tell you if the power is still fueling the machines. If not, wait about sixty seconds for the plant's generators to come online, and then check these machines again. It is especially important to monitor the electrical flow in machines that keep things cold, freeze things, and/or provide a power source for welding, sealing, driving fasteners, etc..
Signaling with an Alarm
Most importantly, AC switches typically come with an alarm system, of sorts. If something is not right with the electrical flow, the switch sends an alarm signal to the employees responsible for monitoring the equipment. This helps repair technicians get on the problem and fix it immediately.
The alarm continues to signal until the problem is fully fixed. Then it stops. Alarm signals that continue beyond the repairs signify a bigger problem with the switch itself, in which case the technicians will have to replace the AC current switch. If there is still an unidentified problem within the system after a switch has been replaced, then the new switch will signal a problem, too.