DigiKey Industrial Automation Demo: Siemens 3SK1 Safety Relay Demo

Automation and Control
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Safety is important, but implementing it correctly can be confusing. To help explain this, we built a functional safety display panel showcasing a Siemens Safety controller, redundant motor control, optical guarding, and a Banner Status Display. It provides a hands-on look at how multiple industrial safety components integrate into a cohesive, fail-safe system.

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The Hardware Layout

The demonstration board is divided into five main physical sections:

  • The Switch Panel: Contains our main rotary Power switch, a green Reset button, a black simulated Overload button, and an E-Stop.

  • The Guarded Zone: Features a simulated motor protected by a Banner SLPP14-270 safety light curtain.

  • Safety Circuit: The “brain,” featuring two safety controllers each configured by physical DIP switches.

  • Motor Circuit: The “muscle,” utilizing redundant contactors wired in series, sandwiched between a physical motor switch and an overload relay.

  • The Status Display: A Banner SD50 indicator programmed to show the exact state of the system.

This article is part of the DigiKey Field Guide for Industrial Automation

Location: Showcase It → DigiKey Projects
Difficulty: :seedling: Student— difficulty levels explained
Last update: 15 Jun 2026

The Anatomy of an Industrial Safety System

Before wiring a safety circuit, it helps to understand why specialized equipment is required. You cannot simply wire an E-Stop in series with a motor contactor coil and call it safe. Real-world components fail: contacts weld shut, wires short to ground, and mechanical springs break.

A functional safety system requires a specialized, redundant architecture divided into three parts:

  1. Inputs: Devices that monitor the hazard (e.g., dual-channel E-Stops, safety interlock switches, or optical light curtains).

  2. Logic / Controller: The “brain” that monitors the inputs for discrepancies. Options range from simple safety relays, to Advanced Safety Relays (like the 3SK1), up to full Safety PLCs.

  3. Outputs: The physical disconnects that remove power from the hazard (e.g., redundant motor contactors).

Deep Dive into the Siemens SIRIUS 3SK1

The core logic of this panel is handled by the Siemens SIRIUS 3SK1 Advanced Safety Relay (a 3SK11211CB42 basic unit paired with a 3SK12201AB40 input expansion). Unlike a standard control relay, the 3SK1 utilizes multiple microprocessors constantly checking each other. If an internal component fails, it “fails safe” and prevents a restart.

Connecting the Inputs: E-Stops & Light Curtains The 3SK1 actively monitors dual-channel signals from both the E-Stop and the light curtain simultaneously:

  • The Emergency Stop: The E-Stop utilizes two separate, mechanically linked Normally Closed (N.C.) dry contacts. The 3SK1 sends a test pulse out through one terminal, through the E-Stop contact, and back into an input terminal. If the relay detects that one channel opened but the other didn’t (a mechanical failure), it locks out.

  • The Banner EZ-SCREEN Light Curtain: Unlike dry contacts, light curtains use solid-state OSSD (Output Signal Switching Device) outputs. The light curtain supplies its own 24V redundant signals directly to the 3SK1’s inputs and performs its own internal diagnostics.

Advanced Controller Settings (DIP Switches): One of the major advantages of the 3SK1 Advanced series is that it does not require software to program. The logic is configured via physical DIP switches on the front of the unit:

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  • Cross-Circuit Detection: This switch tells the relay to look for short-circuits between its input channels. We turn this ON for physical wiring like the E-Stop.

  • Start Type (Auto vs. Monitored): You can set the system to auto-reset (where the motor starts as soon as the hazard is clear) or mandate a monitored manual start, which requires an operator to physically press and release a reset button before the contactors can pull in.

  • Sensor Connection (1 x 2-channel vs. 2 x 1-channel): This switch tells the relay how to evaluate the incoming sensor signals. When set to 1 x 2-channel (the standard for high-level safety), the relay expects both input channels to belong to a single redundant sensor (like our E-Stop). In this mode, the relay performs discrepancy evaluation, meaning it measures the time difference between channel 1 and channel 2 opening. If they don’t actuate almost simultaneously, it assumes a mechanical failure and locks out. Setting it to 2 x 1-channel tells the relay it is monitoring two completely separate, independent sensors.

  • Startup Test (With vs. Without): When activated, this feature forces a physical test of the safety sensors every time the relay powers up. Before the 3SK1 will allow the motor to start, the operator must physically actuate the E-Stop or break the light curtain, and then reset it. This proves to the controller that the mechanical contacts haven’t welded shut while the machine was powered off.

Deep Dive: If you want to understand the exact waveforms and logic these controllers are looking for, check out APDahlen’s excellent breakdown on OSSD Safety Signals: What the Waveform Tells a Safety Relay and his Introduction to the Industrial Safety Relay.

Modular Expansion and the Device Connectors: Rather than using a rat’s nest of jumper wires to connect the main controller to expansion modules, this system uses Device Connectors (3ZY12 series) that snap directly into the DIN rail. The relay modules plug mechanically and electrically into these backplane connectors, instantly sharing control power and safety logic.

  • Terminator Switch: When assembling the device connectors on the DIN rail, the final connector on the right-hand side features a tiny, easily overlooked slide switch. This is the bus terminator. It must be physically slid into the “closed” or “1” position to terminate the communication bus. If you forget to close this switch, the main 3SK1 unit will throw a configuration fault and refuse to boot!

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Part 3: The Motor Control Circuit

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Power flows through a 3RV2011 Motor Starter Protector, through two redundant 3RT2016 contactors wired in series (a strict requirement for machine safety), and finally through a 3RU2116 Thermal Overload Relay to our simulated motor load.

External Device Monitoring (EDM) To ensure maximum safety, we wire the Normally Closed (N.C.) auxiliary contacts of our motor starters back into a dedicated feedback loop on the 3SK1. Before the 3SK1 allows the system to reset, it checks this loop. If one of those contactors physically welded shut during the last cycle, the loop will be open, and the safety relay will refuse to reset.

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The Overload Simulation To demonstrate a mechanical bind or over-current scenario, we use the black Overload pushbutton. This shorts the motor output across a heavy load resistor (BRT300-10RJ8). If you hold this button down for about 30 seconds, it forces the thermal overload relay to heat up and trip. Because this relies on physical heat (bimetallic strips bending), you cannot restart the system immediately; you must wait about a minute for it to cool down.

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Deep Dive: To see exactly what is happening inside that thermal block while you hold the button, read APDahlen’s post: How Does a Motor Starter’s Thermal Overload Block Work?

Part 4: Making Sense of Faults with the Banner SD50

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In a real-world scenario, if a machine won’t start, operators need to know why. To solve this, we integrated the discrete logic version of the Banner SD50 Status Display. It is programmed to explicitly tell you the current state of the board based on the binary logic of its 5 control wires:

  • E-Stop Pressed: The E-Stop mushroom button is depressed.

  • Safety Blocked: The plane of the Banner light curtain has been broken.

  • Motor Overload: The thermal overload relay has tripped, or the physical motor switch is off.

  • Press Reset: All safety hazards have been cleared, the safety relays are fully booted, and the system is safely waiting for the operator to press the Reset button.

  • System Running: The redundant contactors are pulled in, and the simulated motor is active.

For a more in-depth look at how I set up the SD50 display, check out Adding Clear Diagnostics with the Banner SD50 Status Display

Bench Notes: Troubleshooting & Demonstration Modifications

Building a visual demonstration board often requires engineering compromises. Below are two specific modifications made to this board that highlight the messy realities of hardware integration.

1. The OSSD Trace Voltage Collision

When initially connecting the Banner light curtains to the Siemens 3SK1, the curtains immediately faulted. The Siemens relay inputs contain internal capacitance, which leaves a trace voltage on the line. The Banner’s highly sensitive diagnostic test pulses saw this trace voltage discharging backward and interpreted it as a short-circuit fault.

  • The Industrial Solution: The certified path to correct this brand-mismatch is to wire the light curtain into a Banner IM-T-9A interface module. However, doing so converts the solid-state OSSD signals into dry mechanical contacts before they reach the Siemens relay.

  • The Demonstration Compromise: To showcase the 3SK1 handling both dry contacts and native OSSD signals simultaneously, we placed a DIN-rail diode block (Phoenix Contact 277-16896-ND) in series with the OSSD lines. The diodes act as an electrical check-valve, blocking the trace voltage. Note: A series diode blinds the OSSD’s ability to detect forward short-circuits. This violates functional safety standards and should never be used as a permanent solution on a production machine.

2. Driving the SD50

To cleanly drive the SD50’s programmable states, we added the 3SK1211-1BB00 output expansion module. Dedicating a set of clean, dry contacts strictly to the status circuit gave us the extra logic state needed to tell the SD50 exactly what the safety relay was doing. While adding an expansion module solely to drive an indicator light is impractical for most field applications, it worked perfectly for this display.

  • OSSD Flicker: Because one of the SD50’s logic inputs was tied directly to the OSSD output of the light curtain, we had to make sure there was always power at one of the other inputs. OSSD outputs use continuous microsecond test pulses (dropping the voltage to 0V). Providing continuous 24V power on another pin stabilized the display and prevented it from flickering.

Complete Parts List (Bill of Materials)

If you want to replicate this logic or dig into the datasheets, here is the complete hardware list used on this board:

Power & Distribution

Sensors & Indication

Operator Interface (Siemens SIRIUS ACT)

Safety Core (Siemens SIRIUS 3SK1)

Motor Control & Overload Simulation

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