What is the best method to know the fan running status ? in Ventilation

The main types of fans used for ventilation in industrial or commercial building parking areas are

·       Jet fans,

·       Axial fans, and

·       centrifugal fans ( Sometimes)

The Available Methods

·       Status Indication Contacts/Relays​

·       Differential Pressure across the pressure

·       Current Sensing

·       Tachometer Output

·       BMS/PLC Integration

·       Smart Fan Controllers

Method	Detects Fan Powered	Detects Actual Airflow	Detects Mechanical Failure	Complexity/Cost	Typical Use
Differential Pressure Switch	Yes ​	Yes ​	Yes ​	Moderate	Safety-critical systems
Current Sensing	Yes ​	No	No	Low-Medium	General status
Relay/Auxiliary Contact	Yes ​	No	No	Low	Simple monitoring

How Current Sensing Works

• A current sensor (typically a CT or electronic current switch) is installed around the supply wire of the fan motor.

• When the fan is operating, current flows through the supply wire; the sensor detects and measures this current.​

• If the current exceeds a preset threshold (adjustable to match the fan’s rated load), the current switch provides an output—usually a relay contact closure—indicating "fan running."

• For fans using three-phase power, sensors may be fitted to all three phases for detailed analysis or just one phase for simple status.​

• The status signal from the current switch is sent to a BMS, control panel, or remote indicator for real-time monitoring

Key Points

• The current sensing method is non-intrusive and reliable for fan ON/OFF status.​

• It cannot detect airflow loss if the fan is mechanically failed but electrically powered—combining with other methods (like differential pressure switches) gives fail-safe confirmation.​

• Installation is fast and safe since the CT or current switch usually clamps onto the wire, requiring no direct wire cutting.​

This method is widely used for industrial/commercial applications due to its simplicity, safety, and reliability

How Differential Pressure methods Works

A differential pressure switch installed across a fan is a well-established method for confirming actual airflow and verifying fan running status in industrial and commercial ventilation systems.​

Working Principle

• The switch senses the pressure difference between the fan's inlet and outlet, triggering when airflow (and hence pressure rise) exceeds a set threshold.​

• If the fan is ON and functioning, the differential rises above the setpoint—the switch actuates (typically from NC to NO)—signaling "fan running."

• If the fan is OFF, jammed, or there's no real airflow (like a broken belt or impeller), the pressure difference is negligible, and the switch does not actuate.​

Benefits

• Positive Proof of Flow: Unlike just monitoring motor current or relay contacts, a differential pressure switch confirms actual airflow, not just motor power.​

• Fault/Trouble Detection: Detects problems even if the motor is running but airflow is not (slipped couplings, belt breakage, blocked impeller).​

• Direct Integration: Output can be wired to BMS, PLC, or remote alarm panels for real-time monitoring and event logging.​

Limitations & Considerations

• Maintenance: Being a mechanical device, it needs periodic inspection to prevent clogging of pressure ports and tubes.​

• Sensitivity: Needs proper sizing/setpoint and placement relative to fan size and airflow; less suited for very low flow systems.​

• Complementary: For best reliability, often used alongside electrical (current/relay) status monitoring for redundancy.​

Summary

Differential pressure switches across fans are effective for providing "proof of flow" in parking ventilation, supplementing electrical status indicators for greater reliability in safety-critical applications. This method is widely used in India and internationally for BMS and compliance in industrial/commercial settingsngs.

Expert Recommendation

• Differential pressure switch is the most reliable method for confirming that the fan is not only powered but actually moving air, providing proof-of-flow and detecting hidden mechanical issues like broken belts or impellers.​

• Current sensing and relay contacts can signal when a fan is powered, but cannot ensure airflow—these should be used as additional or backup signals, not the sole status indicator for safety scenarios.​

Best Practice

For critical parking ventilation, use a differential pressure switch for primary status (proof of airflow) and supplement with current sensing or relay contacts for power status. This dual approach ensures both electrical and mechanical integrity of the system.​

This combination is standard for compliant, modern systems, particularly in India’s industrial and commercial sector where safety and reliability are paramount

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