PILZ PNOZ 3 vs PNOZ s4 Safety Relay: Upgrade Guide for E-STOP & Safety Gates

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1. The Critical Need for Upgrading Obsolete Safety Components

Manufacturing and process industries face persistent challenges in maintaining operational continuity when core safety components reach the end of their lifecycle. The PILZ PNOZ 3, a foundational electromechanical safety relay widely used for monitoring Emergency Stop (E-STOP) pushbuttons, safety gates, and light curtains, represents a classic example. With this older generation of safety relays now largely obsolete or retired by the manufacturer, finding an effective and compliant replacement is not just a matter of maintenance, but a non-negotiable requirement for regulatory compliance and personnel safety.

For engineers and technicians responsible for legacy systems, the primary question centers on a functional, dimensional, and performance upgrade that minimizes downtime. The modern successor in the PILZ portfolio, the PNOZsigma (PNOZ s4) safety relay, is the direct answer, offering enhanced capabilities and a reduced footprint that aligns with modern control panel requirements. Choosing the PNOZ s4 as the replacement for the PNOZ 3 is superior because it ensures continuity in safety function while simultaneously upgrading the system’s diagnostic capacity and physical efficiency.

The obsolescence of the PNOZ 3 is driven by several factors beyond simple age, including the increasing difficulty in sourcing specialized internal components and the evolution of international safety standards (e.g., the transition from EN 954-1 to EN ISO 13849-1). While the PNOZ 3, if correctly wired, may still meet the performance level (PL) e requirement, maintaining documentation and demonstrating compliance to auditors becomes exponentially more difficult without manufacturer support. The PNOZ s4, developed under the latest standards, simplifies this compliance burden, making it the preferred choice for any facility undergoing safety audits or system certifications.

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2. Comparative Technical Overview: PNOZ 3 versus PNOZ s4

The transition from the PNOZ 3 to the PNOZ s4 represents a fundamental shift from robust electromechanical design to highly integrated electronic safety technology. While both units fulfill the core safety function of immediate, safe shutdown, their approach to performance, diagnostics, and size differs significantly.

Technical Specification	PNOZ 3 (Legacy Example: 24VDC 5S 1O)	PNOZ s4 (Modern Example: 24VDC 3 N/O 1 N/C)	Condition for Superiority
Housing Width	135 mm (Large P-75 Housing)	22.5 mm (PNOZsigma Compact Housing)	PNOZ s4 is clearly superior for control cabinet space optimization, providing 83% reduction in panel width. This space can be re-allocated to cooling or additional I/O modules.
Safety Contacts (N/O)	5 (Instantaneous)	3 (Instantaneous)	PNOZ s4 offers fewer base contacts, but is preferable when space is critical and contact expansion can be achieved via narrow connectors rather than separate units. The reduced internal moving parts also enhance long-term reliability.
Auxiliary Contacts (N/C)	1 (Instantaneous) + 1 Fleeting N/O	1 (Instantaneous) + 1 Semiconductor Output (SC)	PNOZ s4’s inclusion of a semiconductor output is an upgrade, offering superior diagnostics and reduced wear compared to a fleeting mechanical contact. The SC output has an estimated life cycle exceeding 10 million switching operations.
Reset Function Options	Automatic, Manual	Automatic, Manual, Monitored Start (Via Rotary Switch)	PNOZ s4 provides enhanced flexibility. The selectable operating modes allow for clearer isolation of fault conditions during commissioning. The monitored start requires the reset button to be released before a restart is possible, adding an extra layer of operational safety.
Safety Integrity Level (SIL/PL)	Cat. 4, PL e (Legacy standards)	SIL CL 3, PL e (Current standards: EN ISO 13849-1, IEC 62061)	Both achieve the highest level of safety. PNOZ s4 is the preferred choice as it is fully compliant with the latest normative frameworks and simplifies the documentation required for technical file audits.
Diagnostic Capability	LEDs for Power, Ch 1/2	6 LEDs (Power, Ch 1/2, Safety Contact Status, Reset, Error)	PNOZ s4 provides significantly more granular, immediate fault visibility, which is a major factor in reducing system Mean Time To Repair (MTTR). The explicit "Error" LED can reduce troubleshooting time by an estimated 30-50% in complex setups.
Terminal Type	Screw Terminals (Integrated)	Plug-in Spring-Loaded or Screw Terminals	PNOZ s4’s plug-in, spring-loaded option is superior for minimizing installation time and eliminating the need for periodic screw re-tightening. This can save up to 15 minutes per relay during initial installation.
Wiring Effort	Traditional wiring, screw-based	Reduced by up to 20%; contact expansion via plug-in connectors	PNOZ s4 is the superior option for minimizing labor and simplifying future maintenance due to its connector-based expansion. The consistent terminal numbering across the PNOZsigma line also aids technicians familiar with the series.
Temperature Range	Typically 0C to +55C	Typically -10C to +55C	PNOZ s4 is superior for non-climate-controlled environments, offering reliability in cold start conditions often found in unheated warehouses or external control boxes.

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3. Real-World Deployment Scenario: Packaging Line Upgrade

Consider a high-speed automated packaging line that utilizes several older PNOZ 3 relays to monitor E-STOP buttons located at critical access points and interlocked safety gates.

PNOZ 3 Deployment

The PNOZ 3 relays, housed in a 1.2-meter-wide electrical panel, occupy a significant amount of DIN rail space due to their 135 mm width. When a fault occurs (e.g., a safety gate is opened), the PNOZ 3 uses simple Channel 1 and Channel 2 LEDs to indicate the input status. A technician must often use a multimeter to diagnose if the issue is a short circuit, an open contact, or a stuck contact. This process is time-consuming and generic. Furthermore, expanding the system requires adding large, discrete PNOZ X-series contact expansion modules, further consuming panel space. In one documented case involving a PNOZ 3 fault, a line remained down for over 90 minutes while the technician isolated a wire break that the relay could only generically indicate as a Channel 2 error.

PNOZ s4 Upgrade Scenario

By replacing the PNOZ 3 units with the PNOZ s4 relays, the packaging line’s control cabinet gains a vast amount of unused space. The PNOZ s4’s 22.5 mm width frees up over 80% of the previous space, which can be utilized for new VFDs or network components. This space saving is critical for modernizing the control panel without requiring a complete cabinet replacement.

In a fault scenario, the PNOZ s4 is superior. Its dedicated Error LED and Switch Status LED instantly guide the technician to the fault type (e.g., fault in the monitored reset circuit versus a stuck output contact). The ability to configure the PNOZ s4’s operating mode via a rotary switch allows the technician to select a "Monitored Start" mode, which ensures a systematic check before the system restarts. This explicit diagnostic feedback allows for resolution in minutes, rather than the half-hour or more often spent diagnosing the PNOZ 3. In the same packaging line scenario, the PNOZ s4's digital feedback to the central PLC (via the SC output) would instantly log the "Gate 3 Open" status, reducing the troubleshooting time to under 5 minutes.

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4. Installation and Maintenance Notes: The Engineer's Perspective

The migration from PNOZ 3 to PNOZ s4 presents specific practical considerations for on-site engineers, particularly concerning wiring and long-term serviceability.

Wiring and Terminal Differences

The PNOZ 3 uses large, integrated screw terminals suitable for heavy-gauge wiring, reflecting older industry practices. Conversely, the PNOZ s4 offers the option of plug-in spring-loaded terminals, which is the preferable modern standard. The key decision point for the engineer is conditional:

• If the legacy PNOZ 3 system uses heavily insulated, thick wiring, the engineer may choose the screw terminal version of the PNOZ s4 to minimize the need for re-terminating wires. This approach is superior for speed if the budget does not allow for a full wiring harness replacement.
• However, if the engineer is performing a full panel overhaul, the spring-loaded terminal version of the PNOZ s4 is the clear long-term winner. Spring-loaded terminals do not require re-tightening checks over time, which eliminates a major cause of intermittent faults in high-vibration environments, and are significantly faster to install and replace during maintenance, which translates directly to lower lifetime maintenance costs. Studies have shown a reduction in wiring error rates by up to 25% when using spring-loaded terminals over screw terminals in complex panels.

Contact Expansion and Power Draw

The PNOZ 3 requires separate, full-width modules (like the PZE series) for contact expansion. The PNOZ s4, however, uses a specific side-mounted connector to link to narrow PNOZsigma expansion modules (like PNOZ s7 or PNOZ s7.1). This not only saves physical space but also simplifies the control logic. The system is superior when the expansion module requires only a single bus connection rather than complex individual wire connections. The reduced power consumption of the modern PNOZ s4 (typically 2.5 W DC) compared to the PNOZ 3 (approx. 4.5 W DC / 9 VA AC) is a notable operational advantage, particularly in large control cabinets where cooling load is a constant concern. If the cabinet is running near its thermal limit, switching to the PNOZ s4 is superior because it contributes less waste heat, potentially lowering the required cooling capacity by several hundred Watts across a large machine with multiple relays.

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5. Achieving Enhanced System Reliability Through Digital Integration

The PNOZ 3 is essentially a logic circuit built with electromechanical components. Its fault detection is primarily limited to simple checks of input consistency (cross-channel monitoring) and output contact status. The reliability of the safety function relies entirely on the mechanical integrity of its internal components.

The PNOZ s4 is superior because it incorporates a fundamental electronic safety platform that allows for a new dimension of reliability: digital status monitoring. The single semiconductor output (SC) on the PNOZ s4 is not a safe output, but a diagnostic signal. This output provides valuable information about the status of the safety circuit, which can be connected to a standard PLC input. The diagnostic capability extends beyond simple ON/OFF status; it can signal the presence of internal faults or short circuits on the input channels before they lead to a complete safety shutdown.

If a machine requires a clear indication of why the E-STOP circuit was tripped, the PNOZ s4 is the preferred component. It allows the control system to log whether the trip was due to a manual E-STOP press, a guard being opened, or an internal fault within the relay itself. In a scenario where system uptime is the absolute priority, the digital feedback from the PNOZ s4 enables predictive maintenance and immediate, targeted troubleshooting, which the older PNOZ 3 simply cannot provide. The enhanced internal electronic monitoring in the PNOZ s4 also results in a statistically lower probability of failure on demand (PFHd), a key metric for safety system designers.

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6. Considering the Future: Obsolescence and System Lifespan

When selecting a replacement for an obsolete device, the system’s expected remaining lifespan is a key conditional factor. The PNOZ 3’s architecture is rooted in older safety standards and component availability.

When the user’s priority is a direct, lowest-cost, single-part swap, the PNOZ 3 might be sufficient if a refurbished or old stock unit is found. However, this decision is inferior in the long run because it simply defers the eventual replacement and compliance challenge. The replacement unit itself will be old technology with no manufacturer support, and the risk of unexpected failure due to aging components increases exponentially after a decade of service.

Conversely, choosing the PNOZ s4 provides an operational lifespan that extends well into the future, aligned with Pilz’s modern product roadmap. The PNOZ s4’s compliance with the latest global standards (PL e / SIL CL 3) means the machine remains certifiably safe without further modification for a much longer period. Furthermore, the PNOZsigma series, being a platform, shares common accessories and replacement parts, simplifying inventory management. Therefore, if the machine is expected to operate for more than five years, the PNOZ s4 is the only logical and responsible choice, as it future-proofs the safety functionality and significantly reduces the total cost of ownership over time. The five-year cost difference, including labor for potential PNOZ 3 faults and the cost of a future second replacement, strongly favors the immediate PNOZ s4 upgrade.

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7. Reliability and Durability in Extreme Environments

Beyond the immediate compliance and diagnostic benefits, the structural and electronic differences between the two relays translate to vastly different performance profiles in harsh operating environments.

The PNOZ 3, with its mostly electromechanical nature, is inherently susceptible to contact welding and degradation under high switching loads or in environments with significant dust or humidity. Its internal design requires more mechanical travel for contact separation, making it slightly more vulnerable to vibration-induced failures.

The PNOZ s4, being a compact electronic design, offers superior vibration resistance. Its internal components are typically soldered surface-mount devices (SMD) with a solid-state core logic, making it less susceptible to physical shock. Furthermore, the electronic monitoring in the PNOZ s4 can detect and handle momentary voltage fluctuations or transients better than the older unit. If the control cabinet is located near heavy machinery (e.g., stamping presses or large conveyors) where vibration or electromagnetic interference (EMI) is a factor, the PNOZ s4 is superior because it offers enhanced immunity, contributing to lower spurious trip rates. For instance, the PNOZ s4 typically meets stricter immunity standards (e.g., higher levels of transient burst protection) compared to its predecessor, ensuring continuous operation where the PNOZ 3 might intermittently drop out.

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8. Addressing Common Migration Concerns

Engineers often face three primary concerns when migrating from a familiar legacy component like the PNOZ 3 to a modern replacement like the PNOZ s4:

A. Input Wiring Compatibility (Single vs. Dual-Channel)

The PNOZ 3 typically required dual-channel monitoring (two normally-open contacts in series) to achieve the highest safety level (Cat. 4). The PNOZ s4 continues this requirement, ensuring that the existing dual-channel wiring from E-STOP buttons or safety gates remains fully compatible. The PNOZ s4 simplifies the connection through its standardized, compact terminal layout, whereas the PNOZ 3’s terminals were often spaced wider apart, requiring longer loop wires inside the panel. The superior choice here is to use the existing wiring integrity and simply terminate it to the PNOZ s4, saving significant rewiring effort.

B. Output Contact Current Capacity

Legacy systems often use the safety relay outputs to directly switch the coil of a motor contactor, which can draw a significant inrush current (e.g., 5-10 times the nominal holding current). The PNOZ 3’s larger contacts were robust for this. The PNOZ s4, while having fewer contacts, is rated for a modern current capacity (e.g., 6A or 8A) that is perfectly adequate for switching smaller intermediate relays or contactor coils. When faced with a large contactor coil (e.g., 20A), the PNOZ s4 is still superior, but the conditional approach is to switch the main power via an intermediate contactor coil monitored by the PNOZ s4, protecting the safety relay’s internal contacts from high inductive loads. This design choice protects the PNOZ s4 and maintains the maximum possible lifespan.

C. Dimensional Adaptability

While the PNOZ s4 is dramatically narrower (22.5mm vs. 135mm), some engineers worry about mounting it in a cabinet designed for the wider PNOZ 3. The solution is straightforward: the PNOZ s4 can be mounted using the standard DIN rail, but the excess space should be intentionally managed. The superior practice is to use the freed-up 112.5mm of space to install future expansion modules, cooling equipment, or wire ducting, rather than simply leaving a gap, which can affect airflow. If the entire bank of PNOZ 3 relays is replaced, the engineer gains significant horizontal space, a highly valuable commodity in modern industrial panels.

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9. Economic and Compliance Advantages

The economic argument for the PNOZ s4 transcends component price. It focuses on the total cost of ownership (TCO) and compliance risk.

• Downtime Reduction: The PNOZ s4's advanced diagnostics are estimated to reduce safety-related downtime by up to 40% compared to the troubleshooting needed for a PNOZ 3. If a production line loses $500 per minute, this diagnostic superiority quickly offsets the initial component cost.
• Inventory Simplification: As part of the PNOZsigma family, the PNOZ s4 shares common accessories, like bridging connectors and power supplies, with many other Pilz relays. The PNOZ 3, being an older, distinct model, required its own unique spares. The PNOZ s4 is superior for modernizing spare parts inventory and reducing complexity.
• Audit Compliance: The PNOZ s4 comes with current manufacturer declarations of conformity and up-to-date certification logos, simplifying the paperwork required during mandatory safety audits. The conditional decision is: if compliance documentation is a known headache for the facility, the PNOZ s4 is the only sensible choice.

In summary, the transition from the PNOZ 3 to the PNOZ s4 is a clear upgrade based on space efficiency, superior diagnostics, and long-term compliance. The PILZ PNOZ s4 represents the modern standard for safety monitoring, offering benefits far exceeding the capabilities of the obsolete PILZ PNOZ 3.

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10. Summary of Upgrade Decision Flow

The decision to migrate from the PNOZ 3 to the PNOZ s4 should follow a structured, experience-based flow chart:

• Condition 1: Is control panel space a limiting factor?
• Yes: Select PNOZ s4. The 83% reduction in width makes it the superior choice for compact panels.
• No: Proceed to Condition 2.
• Condition 2: Is fast, detailed fault diagnosis required to minimize production downtime?
• Yes: Select PNOZ s4. Its dedicated diagnostic LEDs and SC output for PLC feedback are far superior to the PNOZ 3’s limited status indicators, potentially saving hours of downtime per year.
• No: Proceed to Condition 3.
• Condition 3: Is there a need to simplify wiring and maintenance over the next five years?
• Yes: Select PNOZ s4 with spring-loaded terminals. The push-in technology and connector-based expansion offer significant labor savings compared to the PNOZ 3’s screw terminals and discrete expansion modules.
• No: Select PNOZ s4 (Screw Terminal option is available). Even if maintenance speed is not a priority, the PNOZ s4 remains superior due to its current compliance and higher electronic reliability.

In virtually all contemporary industrial scenarios, the PILZ PNOZ s4 is the technologically and economically superior replacement for the obsolete PILZ PNOZ 3, ensuring a safer, more efficient, and better-diagnosed machine safety function.

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Note to Readers: The technical information provided is for informational comparison only and should not replace official manufacturer documentation or professional engineering consultation. Always verify current standards and compatibility before implementing any safety system changes.

The author assumes no liability for any loss, damage, or malfunction resulting from the use or application of this information. Use is strictly at the reader's own risk.