A typical application is to maintain pressure in the compressor tank. Symbol C from Figure 1 is an NO switch with the switch arm above the terminal. If the pressure drops below a safe level, the switch will open and stop the motor. A typical application is the monitoring of oil pressure. This would be applied as a low-pressure switch. When the pressure drops, the switch will change to the open position. Symbol B from Figure 1 is an NC switch with the switch arm below the terminal. If the pressure gets too high, the switch will open and shuts off the pump that is adding pressure to the system. This would typically be applied as a high-pressure switch. A pressure rise will cause the switch to open. For each of the four symbols, a use case is given. This leads to multiple possible combinations with all specific use cases. Four examples are given in figure 1.įigure 1: Pressure switch symbols with subtle differences to indicate various use cases. This shows whether the switch is operated on rising or decreasing pressure. For example, the switch arm can be positioned above or below the terminals. Remember that these use cases dont mean the switches are different in design, they just show how the switch is used in the system. Some manufacturers include subtle differences in their circuit drawings that give away specific use cases. In the previous paragraph, three main functions (NC, NO, Changeover) and their symbols were explained. Switches to the other contact on pressure rise. Open in the default position and closes on pressure rise. The table below provides the most used symbols for pressure switches.Ĭlosed in the default position and opens on pressure rise. If the switch operates between two terminals, the switch is a changeover contact and can be used either NC or NO. If the arm is not connected to one of the terminals, the switch is NO. If the switch arm is in contact with the terminals, it means that the default position is closed. Each of these functions has different symbols. Changeover contacts are also referred to as SPDT (Single Pole Double Throw). Changeover means that the common terminal switches between two contacts, allowing the switch to be used in either NC or NO operation. The electrical contact of a pressure switch is typically NC (Normally Closed), NO (Normally Open), or changeover. Pressure Switch Online Selection Diagram symbols
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