Switch Logic

Normally Open vs Normally Closed

Before a motor runs, a decision must be made.

That decision happens in the control circuit.

Switches are the simplest decision-making devices in HVAC systems.

Understanding switch logic prevents serious misinterpretation of voltage readings.

What “Normal” Means

“Normally” describes the switch condition when it is:

• Not energized
  
  

• Not activated
  
  

It does not describe what the switch is doing during operation.

There are two basic types:

• Normally Open (NO)
  
  

• Normally Closed (NC)
  
  

Normal = resting state.

Normally Open (NO)

A normally open switch:

• Does not allow current to pass in its resting state
  
  

• Closes when activated
  
  

Example:

A thermostat calling for cooling.

When temperature rises above setpoint:

The thermostat closes the circuit.

From an evidence standpoint:

Voltage may be present on one side of a NO switch.

Current will not flow until it closes.

Voltage presence does not prove closure.

Normally Closed (NC)

A normally closed switch:

• Allows current to pass in its resting state
  
  

• Opens when activated
  
  

Example:

A high-pressure safety switch.

Under normal conditions:

The switch remains closed.

If pressure rises too high:

The switch opens.

Current stops.

Voltage may appear across the open switch.

Understanding state prevents misinterpretation.

Voltage Across a Switch

If a switch is closed:

Voltage drop across it should be minimal.

If a switch is open:

Full control voltage may appear across it.

Full voltage across a switch does not mean it is energized.

It means the path is broken.

HVAC Control Example

Control chain:

Transformer → Thermostat → Float Switch → Pressure Switch → Relay Coil

If the pressure switch opens:

• Current stops
  
  

• Coil does not energize
  
  

• Voltage appears across the open switch
  
  

Measurement must match logic.

Evidence Perspective

When testing switches:

• Identify whether it is NO or NC
  
  

• Determine expected state under current condition
  
  

• Measure voltage on both sides
  
  

• Confirm continuity only when de-energized
  
  

Switch interpretation requires knowing design intent.

Common Misinterpretations

• Seeing voltage on one side and assuming switch is closed
  
  

• Seeing full voltage across a switch and assuming it is powered
  
  

• Testing continuity on an energized circuit
  
  

Switch state must be evaluated safely and logically.

Stop Rule

You stop if:

• You do not know whether the switch is NO or NC
  
  

• You test continuity on a live circuit
  
  

• Voltage behavior does not match expected state
  
  

Switch logic must be understood before interpretation.

Key Takeaways

• “Normal” describes resting condition.
  
  

• Normally open switches close when activated.
  
  

• Normally closed switches open when activated.
  
  

• Full voltage across a switch usually indicates it is open.
  
  

• Switch logic must be understood before testing.
  
  

Mini-Check Quiz

1. What does “normally” refer to in switch terminology?
   
   

2. What happens to a normally open switch when activated?
   
   

3. Why might full voltage appear across an open switch?
   
   

4. Why must continuity be tested only when de-energized?
   
   

5. Why is knowing switch type critical before interpretation?
   
   

HVAC Fundamentals – Sequence

Previous Section:
Electrical Supply & Behavior (Section 2)

https://sites.google.com/view/ta-14academy/learn-hvac/hvac-fundamentals/electricity-made-simple/lesson-2-1-ac-vs-dc/lesson-2-10-transformers

Next Lesson (3.2):
Relays and Contactors
https://sites.google.com/view/ta-14academy/learn-hvac/hvac-fundamentals/electricity-made-simple/lesson-3-1-switch-logic/lesson-3-2-control-circuits-vs-power-circuits