What is meant by the “normally open” switching function with pressure switches?

Pressure switches (for example, a “normally open contact”), are often used for the monitoring and control of safety functions in order to signal any over or underpressure, and thus to prevent any damage to the machinery or the system. Switches with the “normally open” switching function are the most frequently used switches, since, with the triggering of the switching function, they complete a circuit and thus can directly activate the follow-on process.
“Normally open contacts” or switches with the “normally open” switching function are well-known in daily life – such as with light switches. A light switch is generally a “normally open contact” which, on actuation, closes a circuit and thus allows the current to flow to the series-connected light bulb (see Fig.). A “normally open contact” changes its switching status from “open” to “closed” when it exceeds the set switch point.
Greatness of pressure switches with “normally open” switching function
The “normally open” switching function is widely used in industrial applications, since it enables direct control of a safety or control function. When a pressure switch with “normally open” function detects that a critical pressure value has been reached, i.e. the limit value has been exceeded, then the switch completes a circuit. If one has (for example) a safety system, such as an emergency stop system or an alarm klaxon, connected in series with the switch, then this will then be energised and, with this, activated directly – effectively “switched on”. Thus a pressure switch with a “normally open” function fulfils direct control functions when the pressure exceeds or falls below the critical value.
Mechanical pressure switches are often used for the direct control of larger loads, such as motors or pumps since, depending on their design, they often feature a sufficient switching current. Electronic pressure switches, and also mechanical pressure switches with gold contacts, are nowadays usually monitored by a PLC or central control system. The PLC drives the safety function with a separate circuit. Primarily, pressure switches are still used today as a safety component where, although the system pressure is already being monitored continuously, a critical pressure (and therefore a hazard to people and machinery) should be avoided.
Apart from “normally open”, manufacturers of pressure switches offer a wide range of switching functions for all applications, such as “normally closed” or “change-over”. “Normally open” and “normally closed” cover well over 95 % of all pressure switch applications, whereas “change-over” is primarily used for specific applications.
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