Pressure switch symbol standards explained

You may come across different symbols for pressure switches. The most commonly used symbols are defined in the following standards:

• IEC 60617 (also known as British Standard BS 3939).
• NFPA / JIC. Also adopted by the NMTBA (National Machine Tool Builders Association).
• ANSI Y32.2-1975 (also known as IEEE Std 315-1975).
• IEEE Std 91.

More information on different standards can be found on Wikipedia. If you want to learn more about pressure switches, read our main article on pressure switches or how to adjust a compressor pressure switch.

Diagram symbols

The electrical contact of a pressure switche is typically NC (Normally Closed), NO (Normally Open), or changeover. Changeover means that the common terminal switches between two contacts, allowing the switch to be used in either NC or NO operation. Changeover contacts are also referred to as SPDT (Single Pole Double Throw). Each of these functions has different symbols.

If the switch arm is in contact with the terminals, it means that the default position is closed. In other words, the switch is NC. If the arm is not connected to one of the terminals, the switch is NO. If the switch operates between two terminals, the switch is a changeover contact and can be used either NC or NO.

The table below provides the most used symbols for pressure switches.

Function	Symbol NFPA	Symbol IEC	Explanation
Normally Closed			Closed in the default position and opens on pressure rise.
Normally Open			Open in the default position and closes on pressure rise.
Changeover (SPDT)			Switches to the other contact on pressure rise. Can be used either NC or NO.

Functions further explained

In the previous paragraph, three main functions (NC, NO, Changeover) and their symbols were explained. Some manufacturers include subtle differences in their circuit drawings that give away specific use cases.

Remember that these use cases dont mean the switches are different in design, they just show how the switch is used in the system. For example, the switch arm can be positioned above or below the terminals. This shows whether the switch is operated on rising or decreasing pressure. Four examples are given in figure 1.

This leads to multiple possible combinations with all specific use cases. For each of the four symbols, a use case is given.

Symbol A from Figure 1 is an NC switch. A pressure rise will cause the switch to open. This would typically be applied as a high-pressure switch. If the pressure gets too high, the switch will open and shuts off the pump that is adding pressure to the system.

Symbol B from Figure 1 is an NC switch with the switch arm below the terminal. When the pressure drops, the switch will change to the open position. This would be applied as a low-pressure switch. A typical application is the monitoring of oil pressure. If the pressure drops below a safe level, the switch will open and stop the motor.

Symbol C from Figure 1 is an NO switch with the switch arm above the terminal. It will close with a pressure drop. This is called a high-pressure switch. A typical application is to maintain pressure in the compressor tank. If the tank pressure drops below a minimum, the switch is activated and turns on the pump to sustain the pressure.

Symbol D from Figure 1 is a NO switch with the switch arm below the terminal. This is a low-pressure switch and it can be used for instance to start a motor when the pressure gets too high.

FAQs

What is the symbol for a pressure switch?

Different types of symbols are in use. The most commonly used symbols are the ones defined by the IEC and the NFPA (JIC). There are different symbols for normally closed, normally open, and changeover contacts.

Why are electrical symbols important?

Symbols make it easy to understand circuit diagrams. They are quick to draw and read. Standards in symbols help to exchange and read diagrams globally.