Relay Normally Open vs Normally Closed Contacts

Relays are ubiquitous fundamental components in electrical control and automation systems, and a pivotal role in circuit control is played by them. While numerous resources have been written discussing relays' "normally open" and "normally closed" contacts, these explanations are often considered to remain overly abstract, with learners being hindered from developing an intuitive understanding. This article provides a thorough and practical analysis of these two basic contact types. It does this by explaining the main points, comparing them, and looking at how they are used in the real world.

Relay Fundamentals: Working Principle and Contact Definitions

Essentially, a relay is a switch that is electrically controlled, and electromagnetic effects are used to enable small currents to control larger ones. The armature is attracted to the magnetic field generated by the energised relay coil. This attracts the armature and alters the connection state of the internal contacts. This mechanism allows relays to manage high-voltage, high-current working circuits using low-voltage, low-current control circuits, playing a vital role in automation control and safety protection systems.

In relay terminology:

Normally Open (NO) Contact: It is disconnected when the relay coil is de-energised and closes when energised.

Normally Closed (NC) Contact: It is connected when the relay coil is de-energised and opens when energised.

In this context, 'normal state' denotes the relay's initial condition after the coil is de-energised. Understanding how relays behave within circuits depends on making this distinction.

Detailed Explanation of Contact Types: Structure and Characteristics

Normally Open Contacts

Normally open contacts are typically marked as "NO" or "A" contacts. When the relay coil is de-energised, the normally open contacts stay open, which keeps the controlled circuit open. The electromagnetic force only drives the armature to move when the coil receives its rated voltage, causing the normally open contacts to close and connect the controlled circuit.

Key Characteristics:

Coil de-energized → Contact open

Coil energized → Contact closed

Suitable for "start" or "engage" control

Normally Closed Contacts

Normally closed contacts are usually labelled as 'NC' or 'B' contacts. Their behaviour is the opposite of normally open contacts: when the coil is de-energized, the normally closed contact remains closed; when the coil is energized, the contact opens instead.

Key Characteristics:

No coil voltage → Contacts closed

Coil energized → Contacts open

Suitable for "stop" or "disconnect" control

Contact Configuration Classification

Relay contact arrangements are typically described by their contact form, including common types:

Form A: Single-pole single-throw normally open relay

Form B: Single-pole single-throw normally closed relay

SPST: Single-pole single-throw

SPDT: Single-pole double-throw

DPDT: Double-pole double-throw

Practical Application Scenarios and Wiring Differences

In actual circuits, the wiring methods for normally open and normally closed contacts exhibit distinct differences. Experiments show that the two configurations may differ by only a single wire connection — moving the wire from the normally closed contact pin to the normally open contact pin. This minor change results in entirely different circuit behaviours.

Typical Applications of Normally Open Contacts

In most cases, normally open contacts are used in situations where equipment needs to be activated or when signals need to be connected.

Motor Start Control: Three-wire motor starter circuits using normally open contacts

Indicator Light Circuits: Using normally open contacts to turn indicator lights on/off

Automated Control Systems: Initiating various devices as actuators

Typical Applications of Normally Closed Contacts

The main purpose of normally closed contacts is to ensure safety and control stoppages.

Emergency Stop Circuits: Ensuring that equipment stops safely during power failure

Interlock Protection: Preventing conflicts caused by the simultaneous operation of multiple devices

Safety detection: Safety devices like door switches or light curtains

Note that during operation, NC contacts open first, followed by NO contacts closing. When the contactor coil is de-energised, the NO contacts open first and the NC contacts close to reset. The safety of the control system depends on the execution of this sequential activity.

Advanced Concepts and Considerations

Mechanical Forcing Function

Some relays have mechanical forcing mechanisms. For example, Finder's 46 series relays physically push the contacts into the 'energised' position. This feature is convenient during commissioning. However, it requires caution in industrial settings. Unintentional machine operation may cause equipment damage or personal injury.

Corresponding Concepts in Solid-State Relays

Similarly, solid-state relays distinguish between normally open (NO) and normally closed (NC) types.

Normally Open Solid-State Relay: Functionally equivalent to a mechanical relay with a "closed contact". Output terminals are open-circuited without input signal and short-circuited with input signal.

Normally Closed Solid-State Relay: Functionally equivalent to a mechanical relay with an "open contact". The output terminals are short-circuited when there is no input signal, and open-circuited when there is an input signal.

Practical Tips and Precautions

When applying relays in practice, note the following:

Coil Polarity: Make sure that the positive terminal of the power supply is connected to the correct terminal of the relay coil.

Contact Markings: Dedicated relays and sockets typically clearly indicate the positions of the common contact, normally open contact, and normally closed contact.

Protection Measures: When the relay coil is de-energised, a counter-electromotive force is generated. It is recommended to connect a freewheeling diode in reverse parallel across the coil terminals to protect the drive circuit.

Experimentation and Learning Methods

The most effective way to truly grasp the behavioural differences between relay contacts is through hands-on experimentation. Construct a simple test circuit requiring only one relay, two pushbutton switches, and a 24VDC power supply. By personally wiring the circuit and observing the relay's response under various configurations, you can develop an intuitive understanding that lays a solid foundation for future studies in industrial control systems.

During experiments, pay attention to:

Listen for the audible clicks when the relay energises and de-energises

Observe changes in indicator lights or load status

Compare outcomes from altering a single wire connection in both wiring configurations

Conclusion

Normally open (NO) and normally closed (NC) contacts represent the most fundamental yet critical concepts in relays. They each play a distinct and complementary role within control systems. Initiating and enabling functions is made possible by NO contacts, while safety protection and emergency stop systems are made perfect by NC contacts. Understanding the characteristics and differences between these two types of contacts not only aids in the correct design and connection of circuits but also forms the foundation for building safe and reliable control systems.

Once you have grasped these fundamental concepts, you can explore related advanced topics such as 'latching' and 'vibrator' circuits, as well as the 'three-wire motor starter', which is widely used in industrial control.