Standard vs. Safety PLCs: What Are the Key Differences?

Key Takeaways

A standard PLC's job is to run machinery; a safety PLC is to protect people. Safety PLCs use dual processors and constant self-checks to catch faults that standard ones miss. This controller is just one piece of a full safety system—the sensors and outputs must also be safety-rated. For hazardous machinery, following standards like ISO 13849 is mandatory and requires a risk assessment. Though the initial price is higher, safety PLCs reduce long-term costs through simpler wiring, better diagnostics, and most importantly, preventing accidents and expensive downtime.

In industrial automation, choosing the right controller is a critical decision. While standard and safety Programmable Logic Controllers (PLCs) might look similar, they are built for fundamentally different purposes. A standard PLC is designed to make things run, while a safety PLC is designed to stop things from going wrong. Knowing the difference protects your people, your equipment, and your business.

The Standard PLC: Automation's Reliable Workhorse

A standard PLC is the brain behind most automated processes. It's a rugged industrial computer that takes instructions and uses them to control everything from simple machines to entire assembly lines, replacing older, hard-wired relay systems.

What's Inside a Standard PLC?

Since a typical PLC is a modular system, you can create a controller that precisely matches your requirements. The key components cooperate to ensure the seamless operation of your business.

• Central Processing Unit (CPU): This is the PLC's processor. It supervises all other tasks, runs the user's software, and does calculations. For industrial control, it makes use of a real-time operating system to guarantee that tasks are finished in a predictable amount of time.
• Input/Output (I/O) Modules: These modules serve as the link between the PLC and the physical machinery. Sensors, switches, and buttons send signals to input modules. Motors, lights, and valves are all controlled by signals sent via output modules.
• Power Supply: The power supply transforms regular AC power into the low-voltage DC electricity required for the CPU and I/O modules to function.
• Programming Device: A PC or dedicated handheld device is used to write the control program and load it into the PLC's memory.

How a Standard PLC Operates

A PLC works by repeating a simple, four-step process called a scan cycle, frequently finishing it in a matter of milliseconds.

1. Input Scan: The PLC records the status of every input device that is connected and stores this information.
2. Program Execution: The CPU uses the stored input data to determine the next step in the user-programmed logic.
3. Output Scan: The PLC turns the output devices on or off by updating them in accordance with the logic of the program.
4. Housekeeping: The PLC interacts with other systems, such as operator screens, and conducts internal diagnostics.

Continuous repetition of this cycle enables the PLC to reliably and quickly handle a process. Operational uptime and effective control are its primary goals.

The Safety PLC: A Guardian for Hazardous Operations

A regular PLC is made to operate a process, while a safety PLC is designed to safeguard people and equipment from a process. The foundation of its design is functional safety, a discipline that focuses on ensuring that systems fail in a predictable and secure manner.

The Fail-Safe Principle

The main purpose of a safety PLC is to create a "fail-safe" state. If any part of the system fails, whether it be an internal component, a sensor, or the wiring, the safety PLC is designed to detect the issue and promptly shut down the equipment. On the other hand, during a malfunction, a normal PLC can behave erratically, which could be dangerous.

A Complete Safety System

A safety PLC is the logic-solving part of a larger Safety Instrumented System (SIS). All three components of the system must be safety-rated and functional in order for a safety function to perform properly.

1. Safety Sensors: Hazard-detecting devices such as door switches, light curtains, and emergency stop buttons.
2. Safety Logic Solver: The safety PLC that processes signals from the sensors.
3. Safety Actuators: Equipment that physically stops a machine, such as motor drives or safety contactors.

Core Differences: Standard PLC vs. Safety PLC

The different design goals of standard and safety PLCs lead to major distinctions in their hardware, software, and behavior. These differences are not about quality; they are about purpose. A safety PLC is built to assume failure is always a possibility.

Hardware Built for Failure Detection

The most important difference is in the hardware. Safety PLCs use redundancy and constant self-checks to catch faults.

• Redundant Architecture: Many safety PLCs have two processors running the same logic in parallel. They constantly cross-check each other's results. If there's ever a mismatch, it signals a fault, and the system shuts down safely. This gives the system a fault tolerance of one, meaning it can handle a single internal fault and still perform its safety function. A standard PLC has a fault tolerance of zero.
• Advanced Diagnostics: Safety I/O modules are much smarter than standard ones. They run continuous diagnostic tests on themselves and the wiring connected to them. For example, they can send out tiny electrical pulses to detect broken wires or short circuits—faults that a standard I/O module would never see.

Software with Strict Controls

The programming environment for a safety PLC is intentionally restrictive to prevent human error and unauthorized modifications.

• Certified Logic: Programmers often use pre-certified function blocks for common safety tasks like E-stops or light curtain monitoring. These blocks are already tested and validated by the manufacturer, reducing the risk of coding mistakes.
• Locked-Down Program: You cannot change a safety program while the machine is running. Any modification requires stopping the controller, downloading the new program, and re-validating the system. Access is password-protected, and any change generates a new "safety signature," creating a clear audit trail.

Safety Standards and Certification

The design and use of safety PLCs are governed by strict international standards. These rules provide a clear framework for building and verifying safety systems, ensuring they perform as expected when needed.

Key Safety Standards

IEC 62061 and ISO 13849 are the two primary standards that apply to machinery safety. These guidelines are based on IEC 61508, the fundamental functional safety standard. They give engineers methods for creating control systems that minimize risk to a manageable level. To find out how much risk reduction is required for a particular machine, a formal risk assessment is necessary.

Quantifying Safety with PL and SIL

A risk assessment's output is a performance goal that the safety function must meet.

• Performance Level (PL): Used in ISO 13849, PL is rated from 'a' to 'e' (PLa to PLe), with PLe representing the greatest performance level.
• Safety Integrity Level (SIL): Used in IEC 62061, it's a rating system for machinery that ranges from 1 to 3, with SIL 3 being the highest.

A higher PL or SIL is necessary for a higher risk. To meet these ratings, the safety PLC and every other part of the safety system need to be certified by an independent body, such as TÜV.

5 FAQs about Safety PLCs

Q1: Is it possible to perform a safety function using a standard PLC?

A: No. The internal redundancy, diagnostics, and fault tolerance required for safety application certification are absent from a typical PLC. One mistake that goes unnoticed could result in a dangerous failure. It cannot be used to achieve the necessary SIL or PL ratings.

Q2: Is a safety PLC always more expensive than using safety relays?

A: Not all the time. Safety Relays may initially cost less for a very basic circuit with one or two safety features. However, a safety PLC frequently becomes more cost-effective for systems with five or more safety functions or several safety zones since it saves a significant amount of money on installation labor, panel space, and wiring.

Q3: How can I determine the PL or SIL rating that I require?

A: A formal risk evaluation provides the required PL or SIL rating. This procedure examines the likelihood of avoiding the hazard, the frequency of exposure to it, and the seriousness of the possible harm. The necessary PL or SIL increases as risk increases.

Q4: Is it possible to have both standard and safety devices on the same network?

A: Indeed. The same Ethernet cable can be used to connect safety and standard devices thanks to contemporary safety networks like PROFIsafe and CIP Safety. Even on a typical network, the safety protocol makes communication dependable by adding additional levels of protection to the data packets to prevent loss, delay, or corruption.

Q5: What is the lifespan of a safety PLC system?

A: Usually, a "useful lifetime" of 20 years is the basis for the calculations used to certify safety components. At the end of this period, components should be replaced in order to preserve the system's certified safety level.