Safety PLCs & Safety Relays in Industrial Automation Projects

Safety PLCs & Safety Relays in Industrial Automation Projects

Safety PLCs and safety relays are not interchangeable “safety devices”; they are architecture choices that affect risk reduction, validation effort, panel design, diagnostics, lifecycle maintenance, and CE compliance. In industrial automation projects, the right selection depends on the required safety function, the target Performance Level (PL) or Safety Integrity Level (SIL), the number of safety inputs and outputs, the need for logic complexity, and the integration boundary with standard PLC, drives, remote I/O, and SCADA.

Where They Fit in the Compliance Framework

For machinery projects in Europe, the selection starts with the risk assessment and safety function definition under EN ISO 12100. The resulting control-system requirements are then engineered using EN ISO 13849-1 for PL-based design or IEC 62061 for SIL-based design. For electrical equipment of machines, IEC 60204-1 governs protective bonding, control circuits, stop functions, and emergency stop implementation. If the project includes an emergency stop circuit, ISO 13850 applies. For safety-related parts of control systems, the validation requirement in EN ISO 13849-2 is critical and is often underestimated during FAT and SAT.

In North American projects, safety architecture is frequently aligned with NFPA 79 and ANSI B11 series requirements, while functional safety performance may be documented using PL or SIL evidence for multinational OEMs. Where the system includes process safety instrumentation, IEC 61511 may also become relevant, but that is usually separate from machine safety.

How to Choose Between a Safety Relay and a Safety PLC

Safety relays are best suited to simple safety functions: one or a few emergency stops, gate switches, light curtains, two-hand controls, or a small number of contactor feedback loops. They are compact, cost-effective, and easy to commission. Safety PLCs are preferred when the machine has multiple zones, restart logic, muting, mode selection, safe speed monitoring, distributed safety I/O, or when diagnostics and lifecycle visibility matter.

A practical selection rule is this: if the safety function can be expressed with a small number of hardwired channels and no complex state logic, a safety relay may be sufficient. If the design needs interlocks across several stations, safe motion, networked remote I/O, or extensive diagnostic mapping to SCADA, a safety PLC usually reduces lifecycle cost even if the initial hardware cost is higher.

Criteria	Safety Relay	Safety PLC
Typical use	E-stop, guard door, light curtain	Multi-zone machinery, safe motion, complex interlocks
Logic complexity	Low	High
Diagnostics	Limited	Extensive
Scalability	Poor to moderate	Excellent
Engineering effort	Lower initially	Higher initially, lower lifecycle burden

Sizing the Safety Function Correctly

Sizing is not about current draw alone; it is about the achieved safety performance. For PL-based systems, the design must meet the required PLr from the risk assessment, considering category, MTTFd, DCavg, and CCF per EN ISO 13849-1. For SIL-based systems under IEC 62061, the designer must demonstrate the required SILCL and PFHd contribution of the subsystem.

For example, if a guard door requires PL d, a dual-channel safety relay with monitored contactors may be sufficient. If the same machine has 12 doors, mode selector logic, and safe speed monitoring, a safety PLC with distributed safe I/O and safe drive communication becomes the better architecture. In both cases, the final achieved performance must be verified, not assumed.

When calculating output loading, do not confuse safety output capability with ordinary relay contact ratings. If a safety relay output drives contactor coils, the coil inrush current and the duty cycle must be checked against the relay’s output specification. A basic sizing check is:

$$I_{total} = \sum_{n=1}^{N} I_{coil,n}$$

and the selected safety output must satisfy:

$$I_{total} \leq I_{rated} \times derating$$

For safety PLC outputs, pay attention to source/sink behavior, pulse testing, and whether the output is intended for direct switching or only for pilot duty. Vendor manuals often specify maximum capacitive loads, test pulse compatibility, and response times that affect the final safety chain.

Vendor Families Commonly Used in Projects

Common safety PLC families include Siemens S7-1200F and S7-1500F, Rockwell GuardLogix, Schneider Modicon M580 Safety, Pilz PNOZmulti 2, ABB AC500-S, and Omron NX-Safety. Common safety relay families include Pilz PNOZ X / PNOZsigma, Schneider Preventa, Siemens SIRIUS safety relays, ABB Jokab safety relays, and Omron G9SE.

In project practice, Pilz often appears in modular standalone safety applications, Siemens and Rockwell in larger plant-standard automation stacks, and Schneider/ABB/Omron in mixed OEM and panel-builder deployments. The “best” family is usually the one that aligns with the site standard, available engineering tools, local support, spare parts strategy, and the chosen safety network ecosystem.

Integration Rules That Matter in the Panel and Network

Safety devices should be segregated physically and logically from standard control where required by the architecture and risk assessment. Follow IEC 60204-1 for control circuit arrangements, protective bonding, and emergency stop implementation. Keep safety wiring disciplined: dual-channel inputs, monitored reset, EDM/feedback loops, and correctly documented fault exclusion assumptions.

If the project uses networked safety, confirm protocol compatibility early. Common safe networking includes PROFIsafe, CIP Safety, and FSoE. The safety PLC, remote I/O, and drive safety functions must be validated as a complete chain, including network watchdogs and reaction times. For safe motion, refer to the drive and safety function documentation for STO, SS1, SLS, or SSM behavior, and ensure the overall stop category is consistent with the machine risk assessment.

Testing, FAT, SAT, and Validation

Testing must prove the implemented safety function, not just the wiring. Under EN ISO 13849-2, validation should confirm the logical behavior, fault detection, reset behavior, response time, and absence of unintended restart. FAT should include simulated faults such as broken channels, welded contactor feedback, muted sensors, and loss of network communication where applicable.

Document the measured stop time and compare it with the safety distance requirement. For a guard interlock or light curtain, the separation distance is typically derived from the applicable standard and the measured stopping performance. A simplified timing check is:

$$t_{total} = t_{input} + t_{logic} + t_{output} + t_{machine}$$

This total reaction time must be used in the safety distance calculation, not the nominal PLC scan time alone. In practice, the longest chain often includes sensor response, safety logic processing, output de-energization, contactor dropout, and machine coast-down.

Project Decision Summary

For small machines, a safety relay is often the fastest path to compliance. For scalable systems, distributed lines, or plants with strong diagnostics and SCADA integration requirements, a safety PLC is usually the better lifecycle choice. The correct answer is determined by the safety function, not by brand preference.

If you are defining a machine safety architecture, selecting vendor families, or preparing FAT/SAT documentation for CE-marked equipment, we can help you turn the risk assessment into a compliant, testable design—please discuss your project via /contact.