What Do ‘Normally Open’ (NO) and ‘Normally Closed’ (NC) Mean?

In electrical work, normally open (NO) and normally closed (NC) describe the default state of a switch or relay’s contacts when it is not being actuated. A normally open contact means the electrical circuit is broken by default, and current cannot flow until the switch is activated. Conversely, a normally closed contact means the circuit is complete by default, allowing current to flow until the switch is activated. This fundamental concept of contact configuration is crucial to all forms of control circuit logic, from a simple motion sensor light switch to a complex motor starter circuit. The “normal” state always refers to the de-energized state—when the device is at rest with no power applied to its coil and no physical force acting on it. Understanding this distinction is essential for designing safe and functional systems, like a transfer switch for generator, and is a core competency for every journeyman electrician.

The “Normal” State: Understanding the De-energized Default

The single most important concept when discussing NO and NC contacts is the meaning of “normal.” In this context, “normal” refers to the state of the device when it is in its resting, de-energized state. This means no voltage is applied to the coil of a relay or contactor, and no physical force (like a person pressing a button or a cam turning) is acting on the switch. This is the state in which the device sits on a shelf before installation. Schematics always depict components in this normal state to provide a universal baseline for interpreting diagrams. For any professional, from an apprentice to a seasoned journeyman electrician, mastering this principle is the first step toward correctly interpreting and designing any control circuit logic.

Normally Open (NO) Contacts Explained

A normally open (NO) contact is like an open drawbridge. In its default state, the two sides of the contact are separated, creating a gap in the circuit. No electricity can flow across this gap. When the device is actuated—for example, when a pushbutton is pressed or a contactor coil is energized—the contacts are forced together, closing the circuit and allowing current to pass. This is an “action-to-start” configuration. A classic example is a doorbell; the circuit is open and silent until you press the button, which closes the NO contact and energizes the chime.

Normally Closed (NC) Contacts Explained

A normally closed (NC) contact is the opposite; it’s a closed drawbridge. In its default, de-energized state, the contacts are touching, allowing electricity to flow continuously through the circuit. When the device is actuated, the contacts are pulled apart, opening the circuit and stopping the current flow. This is an “action-to-stop” configuration. The most critical application for NC contacts is in an emergency stop circuit. The machine runs because the circuit is complete through the NC button. Pressing the E-stop button breaks the circuit and immediately halts the machinery, providing a vital safety function.

Reading Electrical Schematic Symbols for NO and NC

Understanding electrical schematic symbols is like learning the alphabet of circuit design. These symbols are standardized by organizations such as the National Electrical Manufacturers Association (NEMA) and the American National Standards Institute (ANSI) to ensure that any electrician can correctly interpret a control diagram. Standards like ANSI/IEEE Std 315 provide a comprehensive list of graphical symbols for electrical and electronics diagrams. Identifying NO and NC contacts is straightforward once you know what to look for.

1. The Basic Symbol: A switch or contact is generally represented by two small circles representing terminals with a line showing the path for current.
2. Normally Open (NO) Symbol: The symbol for an NO contact shows the contact path drawn separately from the circuit, indicating a gap. It visually represents that the connection is open by default. For a pushbutton, the operator is drawn above and away from the contact.
3. Normally Closed (NC) Symbol: The symbol for an NC contact shows the contact path as a line touching the circuit path. In many ladder logic diagrams, a diagonal line is drawn through the contact to clearly indicate its normally closed status, but this is not universal across all schematic types. This visually shows the connection is closed by default. For a pushbutton, the operator is drawn resting on the contact, holding it closed.
4. Combination Devices: Many pilot devices, like selector switches and pushbuttons, can have both NO and NC contact configurations in the same unit, providing greater flexibility in control circuit design.

Practical Applications in Control Circuit Logic

The true power of NO and NC contacts comes alive in their application, forming the building blocks for everything from simple lighting to complex industrial automation.

Simple Switches vs. Control Devices

At a basic level, this concept applies to common devices like a toggle switch or a motion sensor light switch. Even a device as simple as a thermostat uses these principles. As you learn how to wire a line voltage thermostat, you are working with a switch that closes a circuit (NO) when heat is needed. However, the concept’s depth is most apparent in industrial controls, where contacts are used to build logic in series vs parallel circuit arrangements to control machine operations.

The Heart of Motor Control: The Three-Wire Control Circuit

One of the most fundamental and important applications is the three-wire control circuit, used to start and stop motors. This circuit demonstrates the interplay between NO and NC contacts perfectly. It typically consists of a momentary NO “Start” button, a momentary NC “Stop” button, and a magnetic contactor. When the contactor coil is energized, it closes its main contacts to power the motor and also closes a smaller NO holding contact. This holding contact, also known as an auxiliary contact, is wired in parallel with the “Start” button. When the operator releases the “Start” button, the circuit remains energized through this now-closed holding contact, “sealing” or “latching” the circuit. The only way to de-energize the coil is to press the NC “Stop” button, which breaks the entire circuit. This design is distinct from a simple momentary vs latching switch, as the logic is created through the components themselves. This is the core of any standard motor starter circuit. While many modern systems use VFDs (Variable Frequency Drives) for advanced motor control, understanding the basic three-wire control remains essential.

Ensuring Safety with Fail-Safe Wiring

The choice between NO and NC is critical for fail-safe wiring. An NC contact is preferred for any emergency stop circuit or overload protection. The logic is simple: if the control wire to an NC E-stop is cut or a connection comes loose, the circuit opens, and the machine stops—the same result as if the button were pressed. This ensures that a wiring failure leads to a safe state. Another advanced safety technique is using interlocking contacts. In a reversing motor starter, an NC auxiliary contact from the “Forward” contactor is wired in series with the “Reverse” contactor coil, and vice-versa. This makes it physically impossible for both coils to be energized at the same time, which would cause a dead short. This same interlocking principle is used in a transfer switch for generator applications to ensure that utility power and generator power can never be connected simultaneously.

Key Considerations for Choosing NO vs. NC

Selecting the correct contact configuration is a critical design decision. Here are the key factors to consider in any control relay application:

• Application Logic: Do you need to initiate an action (Start, Jog, Activate)? Use a Normally Open (NO) contact. Do you need to stop an action or interrupt a process (Stop, Deactivate)? Use a Normally Closed (NC) contact.
• Fail-Safe Requirements: For safety-critical functions like an emergency stop circuit, always choose the contact type that will bring the system to its safest state in the event of a control power loss or wire break. In most cases, this means using an NC contact.
• Energy Consumption: In applications where a relay will be energized for long periods, consider the contact’s normal state. For example, if a light needs to be on most of the time, using an NC contact that is held open to turn it off may be less efficient than using an NO contact that is held closed.
• System Startup: How should the equipment behave when power is first restored after an outage? A three-wire control circuit ensures a motor does not automatically restart, as the holding contact will be open, requiring a manual press of the “Start” button.

Mastering the use of these contacts is essential for anyone dealing with industrial controls. See how these contacts are used in our guide to motor rated switches.

Frequently Asked Questions (FAQ)

What’s the difference between momentary vs latching switches and normally open normally closed contacts?

Momentary vs latching describes the switch’s physical action, while normally open normally closed describes its electrical state. A momentary switch (like a doorbell) returns to its normal position when released. A latching switch stays in its actuated position until acted upon again (like a standard light switch). Both momentary and latching switches have contacts that are either NO or NC.

How are NO and NC contacts used in a three-wire control circuit?

In a typical three-wire control circuit, a momentary NO “Start” button is used to energize the contactor coil. A momentary NC “Stop” button is wired in series to de-energize the coil. An additional NO holding contact from the contactor itself is wired in parallel with the “Start” button to “seal-in” the circuit after the start button is released.

Why is an NC contact used for an emergency stop circuit?

An NC contact is used for an emergency stop circuit to achieve fail-safe wiring. The control circuit is complete and the machine runs in the normal state. If a wire breaks or a connection fails, the circuit opens and the machine stops, defaulting to a safe condition. This prevents a silent failure where the E-stop wouldn’t work when needed.

Can a single pilot device have both NO and NC contacts?

Yes, many pilot devices like pushbuttons, selector switches, and relays are available with multiple contact blocks, including both NO and NC contact configurations in a single unit. This allows for complex control circuit logic, such as turning on an indicator light (NO) while simultaneously stopping a process (NC) with a single action.

Understanding these foundational concepts is a hallmark of a professional electrician. For those looking to deepen their knowledge of controls and other advanced topics, well-structured online electrical courses can provide valuable, in-depth training.