Relay Contact Types: NO, NC, COM in Industrial Control

Introduction (Systems Engineering Overview)

The underlying network that makes industrial automation possible consists of electromagnetic and solid-state switching devices called relays. Their key role is to restrict or allow the flow of current on the basis of signals provided through a computational device. They are used in power distribution and logical sequencing in industrial setups.

When we look at these devices from a systems engineering perspective, the relay is much more than a simple on/off button. It is a logic device that is capable of manipulating high-power circuits using low-power control signals. It does that by keeping the two sides physically separated so that the electrical currents cannot pass across them. It's called galvanic isolation. It protects both the equipment and personnel from dangerous shocks.

There are three fundamental connection states of relay that completely change the way it functions:

• Common (COM)
• Normally Open (NO)
• Normally Closed (NC)

These contact types dictate how the system thinks and makes its decision based on operational logic. It also impacts its safety profile and reliability. This guide will explain all these relay contact types in detail from the systems engineering perspective.

Fundamental Definitions and Terminal Identification

Normally Open (NO) Contacts

These contacts are often referred to as type “H”. They act like standard light switches. When the relay is at rest and receives no power, it is called a de-energized state. The relay is open and blocks the flow of electricity, called the non-conductive state.

The energization of the electromagnetic coil inside the NO relay causes the armature to close the gap. It completes the circuit and allows the electricity to flow to the load. This mechanism allows engineers to use them to turn things on, such as starting a machine, activating a device, or triggering an alarm. It is also used in applications where the machine operator has to physically keep the button pressed to operate a device. These are jogging circuits using a momentary pilot device.

Note: Never use them in emergency stops. If the wire breaks, there is no means to stop the running load.

Normally Closed (NC) Contacts

The normally closed relays (NC) are designated as type “D”. When they are in their de-energized state, the contacts are made. It allows the electricity to pass through without any external energy required to keep the circuit closed.

When the relay is energized, it creates a magnetic field that pulls the armature, breaking the circuit. The result is the stoppage of the electricity flow. They are the backbone of safety systems, particularly in circuits that contain an emergency stop button. In case of a power failure, the protection circuits have the capability to dump leftover energy into the ground.

Note: Broken wires automatically break the circuit, and the system goes to fail safe shutdown state.

The Common (COM) Terminal and SPDT Configuration

The common terminal is the main entry point for the power. It acts like a moving bridge that physically relays the power connection back and forth between the NO and NC terminals.

When a relay contains all three terminals (COM, NO and NC), it is known as a Single Pole Double Throw (SPDT) or Changeover (CO) configuration. The setup allows the single command to do two things at once: They turn one machine off by breaking the NC connection, and turn another machine on by making the NO connection. The mechanical linkage ensures that only one path can ever be active at a time.

In very fast relaying setups called high-speed changeover pairs, the NC connection is specially designated to disconnect before the NO connection closes. It prevents the electricity from taking sneak current paths or causing erratic logic known as contact race.

Standard Identification Reference

Designing Logic through Contact Interplay

Implementing AND/OR Logic Networks

Engineers use various arrangements of relay connections to create rules for the connected machines to follow.

The "AND" Rule (All Must Agree)

To make an “AND” system, the multiple normally open relays are wired one right after the other in a single continuous line. For electricity to reach the end, every single relay in the line must be closed at the same time.

Engineers use this to create conditional interlocks. For example, a motor might only run if it receives the signal that the “Safety Gate is Closed” AND “Start button is Pressed”

The "OR" Rule (Any One Will Do)

To make an “OR” system, the relays are wired side-by-side, independent of the path. It is called a parallel configuration. Closing any single contact in this setup provides a path for the electricity and completes the circuit.

It is ideal for systems that need multiple control points, such as multi-location start stations or allowing multiple different sensors to trigger an alarm.

The "NOT" Function and Safety Fail-Safes

In some cases, you want the machine not to perform certain operations. In such cases, the “NOT” function works the best.

The "NOT" Function

In case you want a single relay that is naturally on by default and turns off when activated, you use an NC contact relay. By placing one of these “NOT” NC relays directly on the power line of the motor, you guarantee that an overload warning or stop button will cut the power off instantly.

Preventing Conflicts

Circuit designers mix the use of NC and NO relays to prevent the machine from trying to do two opposite things at once. It's called electrical interlocking. For example, they will wire the Normally Closed (NC) contact of the 'Forward' relay directly into the control circuit of the 'Reverse' relay. It ensures that the motor cannot be commanded to move forward and backward simultaneously.

Advanced Engineering Considerations

Memory and Seal-In Circuits (Memory Functions)

• Memory Circuit: Machines that need to stay running even after you take your finger off the start button require the use of memory circuits. To keep the machine on, engineers wire an extra NO auxiliary contact of a relay in parallel with the momentary start button. When the system is powered on, the extra relay closes and takes over the job of supplying power. It means power continues flowing even after you let go of the button.
• Failing Safely: In this setup, the circuit is designed to forget the start command if the facility loses power. It's called an off-return memory circuit. In case of a blackout, the circuits automatically turn off, preventing the machines from springing back to life when power comes on.
• Safety Priority: When wiring these circuits, the interplay between the "Stop" NC contact and the "Start" NO contact is critical. As per industrial safety standards, the “stop” command must always win out over the “start” command.
• Constant Power vs Physical Locks: Engineers recognize two distinct ways to hold these commands. The method mentioned above requires a constant supply of electricity to stay closed. Some relays use two separate magnets to physically lock and unlock their positions. These are called mechanically held latch relays using dual coils (Latch/Unlatch).

Material Selection and Contact Durability

• Heavy-duty relaying: for relays that can handle large amounts of power without adverse impact on their operability, use silver-cadmium oxide or silver-tin oxide. They resist welding and arc erosion.
• Weak Signals: In case of dry circuits or low-level signal relaying (micro-amperes), gold-plated are considered standard. They help prevent the formation of insulating oxide layers.
• Minimum Load Rating: It's a common mistake to expect a heavy-duty relay to handle anything. It's important to see the minimal load rating. If the electrical signal is too weak, it will fail to break through microscopic surface contamination.
• Self-Cleaning Mechanism: to help fight dirt build-up, mechanical relays come with contacts that slide against each other. It's called the self-cleaning wiping action.
• Lifespan Expectation: Engineers who need to match the contact material to the specific load type. These can be resistive, inductive, or capacitive loads. If done correctly, a relay can last over 100,000+ cycles.

The Standard for Guaranteed Safety

In industrial environments, top-tier Class A safety relays should be built with strict physical design requirements. According to the European standard EN 50205, these relays should have force-guided contacts to prevent equipment failures.

Conclusion

Understanding NO, NC, and COM contacts is the absolute starting point to build a robust industrial automation system. Engineers can build a control panel that works perfectly and focus on three main things:

• Strategically Selecting Contact Materials
• Implementing Fail-Safe Logic Networks
• Utilizing Advanced Safety Configurations Like Force-Guided Relays

As these systems are working in a harsh industrial environment, they need to be made with top-tier parts. They should withstand millions of cycles without failure. In case you are implementing an automation circuit or moving towards the modern Industry 4.0 approach, consider a highly reliable solution offered by RY-ELE. They provide Miniature Intermediate Relays, Electromagnetic Industrial Relays, high-current Solid State Relays and much more.

To find the perfect contact configuration for your project needs, consider visiting https://www.ry-elerelay.com/ to explore their full product catalog. You can also contact them:

• Tel: +86-13676766839
• E-mail: sale@ruifan-ele.com
• WhatsApp: +86-18989752605
• Address: NO.5, Yonghe 3rd Road, Chengdong Street, Yueqin, Wenzhou City, Zhejiang Province, 325600, China

FAQs

1. What is the difference between an NO and an NC contact?

The NO contact remains open in a de-energized state. The contacts are not made, and electricity does not pass through. NC contact remains closed in a de-energized state. The contacts are made, and electricity can pass through them.

2. Why are force-guided contacts required in safety circuits?

Force-guide contacts help prevent the simultaneous closing operation of NO and NC contacts. It prevents dangerous equipment operations. The simultaneous closing can happen when the contact weld shuts. These ensure reliable feedback for safety control.

3. Why does a "low-level" signal require gold-plated contacts?

When you need to pass very little power through the control relay contact, slight dust or oxide layer formation can cause the system to malfunction. Gold-plating is highly resistant to corrosion and ensures low current to pass through in signal switching applications.