Relays are one of the most fundamental components in industrial control systems, yet their naming conventions trip up engineers and technicians at every experience level. Today, we’ll be dissecting the somewhat-confusing nomenclature used to describe industrial relays, as well as going over contact configurations, types, electrical specifications, and mounting options so you can specify the right relay for your PLC-based automation system.
What Is a Relay in Factory Automation?
A relay is an electrically operated switch that acts as an interface within the I/O (Input/Output) system, using low-voltage input signals to control output devices.
Relays solve two practical problems in PLC-based systems:
- They allow output devices that operate above the PLC’s output voltage to be controlled safely
- They allow a single input trigger multiple outputs simultaneously
Without relays, system designers would be constrained by the PLC’s native output voltage and current limits. With them, the PLC becomes the brain of a much more capable system.
Normally Open vs. Normally Closed Relay Contacts
Understanding normally open (NO) and normally closed (NC) contact states is the foundation of relay selection—and one of the most common points of confusion for anyone new to relay terminology.
This topic is covered in greater depth in our guide to proximity sensors, but here’s a recap:
A switch’s “normal” state is simply its condition when no control signal is applied—its default state.
A normally open (NO) switch requires an active input to close the circuit. A normally closed (NC) switch breaks the connection when a signal is applied. In other words:
- Normally open (NO): The circuit is disconnected by default. Applying a control signal closes the circuit and allows current to flow.
- Normally closed (NC): The circuit is connected by default. Applying a control signal breaks the connection.
Real-world example: The power button on a power drill is a normally open switch—the tool only runs while the trigger is actively pressed. Release it, and the circuit opens again.
Momentary vs. Latching Relays
Most switches are momentary, requiring a continuous control signal and snapping back to their default state the moment that signal is removed.
Latching switches behave differently—they hold their switched state even after the control signal drops. Depending on the design, the latch may be mechanical (a physical locking mechanism) or electronic (using a silicon-controlled rectifier, or SCR, or a dedicated reset input).
Real-world example: Emergency stop (E-stop) buttons in factory automation are latching by design. When pressed, the button head locks in place, keeping the circuit open even after the operator’s hand is removed. This prevents machinery from being inadvertently re-energized.
Relay Terminology: Poles and Throws
Relays are functionally an array of switches sharing a single control input. They use different naming conventions to describe how those switches are arranged internally.
In the pole-throw nomenclature, the number of switches (poles) and their configuration is abbreviated as xPxT.
- Pole = the number of independent switches inside the relay
- Throw = the number of output paths available per switch
So, a normally open switch would be considered a single-pole single-throw (SPST), because there is one moving component with one potential connection path. Tie two of those switches to one control input and you’ve got a double-pole single-throw (DPST). Conversely, give one switch two possible output paths and it becomes a single-pole double-throw (SPDT).
| Abbreviation | Full Name | Description |
| SPST | Single-Pole Single-Throw | 1 switch, 1 output path |
| SPDT | Single-Pole Double-Throw | 1 switch, 2 output paths |
| DPST | Double-Pole Single-Throw | 2 switches, 1 output path each |
| DPDT | Double-Pole Double-Throw | 2 switches, 2 output paths each |
Relay Terminology: Form A, B, C Contact Configuration
Alongside pole-throw notation, relays are commonly identified using a number-letter abbreviation that combines pole count with contact configuration:
- Number = poles (number of switches)
- Letter = contact form (configuration)
The Three Most Common Relay Contact Forms
| Form | Configuration | Description |
| Form A | Single-throw, normally open | Circuit open by default; closes on signal |
| Form B | Single-throw, normally closed | Circuit closed by default; opens on signal |
| Form C | Double-throw (1 NO + 1 NC) | Transfers between two paths on signal |
Form C is the most versatile single-pole configuration—it gives you both a normally open and normally closed contact sharing a common terminal, which is useful for transfer switching and selector logic.
Other forms exist—like D, X, and Y—but they’re uncommon enough in standard automation work that you’re unlikely to encounter them outside of specialized applications.
Combining the letters and numbers gives us the following examples:
- 1A = single-pole, single-throw (normally open)
- 1B = single-pole, single-throw (normally closed)
- 1C = single-pole, double-throw (1 normally open + 1 normally closed)
- 2A = double-pole, single-throw (normally open)
- 2B = double-pole, single-throw (normally closed)
- 2C = double-pole, double-throw (1 normally open + 1 normally closed)
Electromechanical vs. Solid-State Relays: Which Should You Use?
Once you’ve determined the contact configuration you need, the next decision is relay type. The two most common in industrial automation are electromechanical and solid-state relays.
Electromechanical Relays (Ice Cube Relays)
Electromechanical relays use an electromagnetic field generated by the current in a control input to physically actuate the internal switches. They are commonly called ice cube relays due to their characteristic rectangular, translucent housing.
Advantages:
- Higher current handling capacity
- Designed to fail in a fully “open” position (adds safety)
- Lower cost per unit
- Compatible with a wide range of load types
Limitations:
- Mechanical wear over time reduces service life
- Audible click on each switching event
- Slower switching speeds than solid-state alternatives
Solid-State Relays (SSRs)
Solid-state relays are strictly electrical, using transistors rather than moving mechanical parts to change logic states.
Advantages:
- Immune to mechanical failure
- Silent operation
- Faster switching speeds
- Better suited for high-frequency switching applications
Limitations:
- Generally higher cost than electromechanical relays
- More sensitive to heat—thermal management is critical
- Typically rated for lower current than comparable electromechanical relays
Neither type is universally better. How to choose the right industrial relay depends on your current requirements, switching frequency, environment, and budget.
Key Relay Electrical Specifications
Mismatched specifications are among the most common causes of relay failure in the field. Before finalizing a relay selection, confirm the following:
Coil Voltage (Control Input Voltage)
The voltage required to actuate the relay coil. This must match your PLC or control system’s output voltage. Common logic switching levels are 12Vdc, 24Vdc, 48Vdc, and 120Vac
Contact Current Rating
The maximum continuous current the relay’s output contacts can safely carry. Exceeding this rating damages contacts and causes premature relay failure. If your connected load devices have a high power draw, you’ll need to install appropriate upstream circuit protection devices like fuses or circuit protectors before the relay contacts.
Contact Voltage Rating
The maximum voltage the output contacts are rated to switch. Verify this against the voltage of your load circuit.
Industrial Relay Mounting and Form Factors
DIN Rail Mounting
The most common installation method in industrial control panels. Relays mount on standard 35mm DIN rail alongside PLCs and other control components, keeping wiring organized and components accessible for maintenance.
PCB Mounting
Compact relays designed for direct solder-mounting onto printed circuit boards. Used in embedded control applications, custom panel builds, and space-constrained designs.
Thermal Relays
Thermal relays are specialized relays that mount directly onto contactors and circuit breakers to provide integrated overload protection. Common in motor control applications.
Terminal style varies by relay type. Ice cube relays often use solder tab contacts or plug into a dedicated screw-socket base. Solid-state relays typically use screw or plug-in terminals that accept both crimped and uncrimped wire connections. PCB and thermal relays use pin contacts. The terminal choice matters more than it might seem when you’re managing wire routing and future serviceability in a busy enclosure.
Terminal Styles by Relay Type
| Relay Type | Common Terminal Style |
| Electromechanical (ice cube) | Solder tab contacts or dedicated screw-socket base |
| Solid-state | Screw terminals or plug-in terminals |
| PCB relay | Pin contacts |
| Thermal relay | Pin contacts |
Relays aren’t glamorous, but getting them right keeps systems running cleanly and safely. Get them wrong and you’ll be troubleshooting at inconvenient hours. With a clear understanding of contact forms, electrical ratings, and relay types, you have everything needed to confidently select relays for virtually any factory automation application.
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Shop Relays & ElectricalFrequently Asked Questions: Industrial Relays
What is the difference between a relay and a contactor? Both are electrically operated switches, but contactors are purpose-built for high-current applications like motor control and include built-in arc suppression for safe interruption of large loads. Relays are used for lower-current signal and control switching.
What does 24 Vdc coil voltage mean on a relay? It means the relay’s control coil requires 24 volts DC to actuate. This is the most common coil voltage in PLC-based automation systems because most PLCs supply 24 Vdc from their digital output terminals.
Can a solid-state relay replace an electromechanical relay? Often yes—but verify that the SSR’s contact current rating, contact voltage rating, and contact form match the original relay’s specifications. Also reassess enclosure thermal management, as SSRs generate more heat under load than their electromechanical equivalents.
What does “fail open” mean for a relay? If the relay malfunctions, its contacts default to the open (disconnected) position, cutting power to the connected load. This is the safer failure mode for most automation applications, since it prevents uncontrolled machine operation on relay failure.
What is a Form C relay contact? A Form C contact is a single-pole double-throw (SPDT) configuration with both a normally open and a normally closed contact sharing a common terminal. It’s the most versatile single-pole relay contact configuration and is widely used in transfer switching and selector logic applications.
How do I know if my relay needs upstream circuit protection? If the load connected to the relay’s output contacts draws current close to or above the relay’s rated contact current, upstream protection is required. As a practical rule, if you’re switching any motor, solenoid, or other inductive load, verify the inrush current against the relay’s rating—inrush can significantly exceed steady-state draw.
