A pressure switch does not just provide a pressure reading. It converts a mechanical pressure change into an electrical contact change, which is read by the burner's automation system. Understanding COM, NO, and NC contacts is essential for proper diagnosis.
What COM, NO and NC mean
COM is the common contact. NO stands for Normally Open, meaning it is open in the manufacturer's reference state. NC stands for Normally Closed, meaning it is closed in the corresponding reference state. The "normal" state should not be interpreted arbitrarily without the specific pressure switch's diagram.
Why confusion arises
In gas pressure switches, the electrical state changes when the pressure exceeds the set threshold. If one looks only at the letters NO/NC without knowing whether the pressure switch is for low or high pressure and what pressure is applied at that moment, they may reach a wrong conclusion.
Low pressure switch
In typical applications, the automation system requires confirmation that the pressure is above a minimum threshold. When the pressure becomes sufficient, the contact changes position, creating the required permissive signal for the startup sequence.
High pressure switch
The high-pressure switch operates with a different logic: it monitors that a maximum threshold is not exceeded. When the limit is violated, the contact changes and the automation system is led to a stop or lockout, depending on the design.
How to read the electrical diagram
We locate the pressure switch symbol, the terminal numbers, and the contact position as defined by the manufacturer. Then, we follow the circuit to the safety relay or the burner controller. This allows us to understand what electrical state the system expects before proceeding.
Testing with a multimeter
With the installation safely isolated, the technician can check for continuity between COM-NO and COM-NC and observe the change during pressure variation using appropriate test equipment. The test must be performed under pressure conditions and ranges permitted by the manufacturer.
Common diagnostic error
The fact that a contact "closes" electrically does not prove that the pressure switch is activating at the correct pressure point. For a full check, simultaneous measurement of actual pressure and observation of the switching point is required.
Why we do not bridge the contact
A permanent bridge can make the automation system consider a state safe when it is not in reality. For diagnosis, the manufacturer's prescribed procedures must be used, rather than bypassing the safety chain.
Correctly reading the COM/NO/NC terminals in combination with the burner diagram allows us to distinguish whether there is a real pressure problem, a faulty pressure switch, or an interruption in the electrical circuit.