How to Wire BMS Run, Fault & Hand-Off-Auto Lighting / Switch Circuits

Introduction to Run, Fault & Hand-Off-Auto Circuits

Building Management Systems (BMS) control mechanical plant equipment like AHUs, FCUs, and pumps. Understanding how to wire run indication, fault monitoring, and Hand-Off-Auto (HOA) switching is essential for reliable operation and effective troubleshooting.

1. Run Indication Circuits

Run indication tells the BMS that a piece of equipment (e.g., a fan or pump) is actually running, not just commanded to run. This is typically done using:

• Current Switch: Detects current flow in the motor circuit
• Auxiliary Contact: Wired from the contactor to provide a volt-free signal
• Pressure Switch: For pumps, detects system pressure when running

Wiring Example (Auxiliary Contact)

The contactor auxiliary contact (normally open) is wired to a digital input on the BMS controller. When the motor runs, the contact closes, signaling the BMS.

2. Fault Indication Circuits

Fault circuits alert the BMS to issues such as motor overload, thermal trips, or loss of supply. Common fault monitoring methods include:

• Overload Relay Auxiliary Contact: Signals when the motor has tripped on overload
• Thermal Switch: Monitors motor winding temperature
• Control Circuit Fuse Monitoring: Detects loss of control voltage

Wiring Example (Overload Relay)

The overload relay's normally closed auxiliary contact is wired to a digital input. If the motor trips, the contact opens, triggering a fault alarm in the BMS.

3. Hand-Off-Auto (HOA) Switching

HOA switches allow operators to manually control equipment or place it under BMS automatic control:

• Hand (Manual On): Equipment runs continuously, bypassing BMS control
• Off: Equipment is disabled regardless of BMS commands
• Auto: Equipment operates under BMS control

Wiring HOA with Trend IQ4 Controllers

For a typical AHU fan:

1. Wire the HOA switch common terminal to the BMS digital output
2. Wire the "Hand" position to energize the contactor directly
3. Wire the "Auto" position to pass BMS control through to the contactor
4. Wire the "Off" position to de-energize the contactor

Best Practices

• Always use properly rated contactors and overload relays
• Label all terminals clearly in the control panel
• Use ferrules on all wiring terminations
• Test run, fault, and HOA operation before commissioning
• Document all wiring in as-built drawings

Common Mistakes

• BMS DO powering plant directly
• No HOA status back to BMS
• Fault wired N/O (no fail-safe)
• Mixed ELV/230V in same trunking without segregation
• No isolation for BMS control circuits
• No earthing on shielded cables
• Zero slack / bad routing / poor labelling

Troubleshooting Common Issues

Issue	Possible Cause	Solution
BMS shows "Run" but motor not running	Auxiliary contact wired incorrectly	Verify auxiliary contact is from contactor, not overload
Fault indication always active	Normally closed contact wired as normally open	Check overload relay contact configuration
HOA switch has no effect	Incorrect wiring sequence	Verify HOA switch wiring diagram matches panel design

Need help designing or troubleshooting BMS control panels? Contact Alpha Controls for expert assistance.