The Siemens 3SK1220-1AB40 (screw terminal) and 3SK1220-2AB40 (spring terminal) modules are often classified as safety relays. They are more appropriately designated as accessories or expansion modules for the core 3SK1 safety relays.
The 3SK1220 is not a stand-alone safety relay.
This article is part of the DigiKey Field Guide for Industrial Automation
Location: Troubleshoot It → Safety Systems
Difficulty: 
Technician — difficulty levels explained
Author: Aaron Dahlen | MSEE | Senior Applications Engineer, DigiKey
Last update: 30 Mar 2026
Introduction to the Siemens 3SK1 Safety Relay
The Siemens SIRIUS 3SK1 is a family of industrial safety relays. The devices are modular and may be expanded based on machine complexity. The 3SK1220 is an important part of this family as modules may be added as necessary. For example, the base 3SK1 could monitor the E-stop, while a pair of 3SK1220 modules monitor a light curtain and a door interlock.
The 3SK1220 is not a Stand-Alone Device
The 3SK1220-1AB40 as pictured in Figure 1 is not a stand-alone device. In fact, it has no screw terminal connections for power or the output signal. This is an unpleasant surprise for customers who assumed the unit operated independently.
Figure 1: Image of the Siemens 3SK1220-1AB40 on the author’s workbench.
Tech Tip: Unlike PLC systems, the safety expansion modules are installed to the left of the primary safety relay.
The 3SK1220 Connections are Made Via a Device Connector
The power and output signals for the 3SK1220 are conducted through a card connector located in the base of the unit as shown in Figure 2. The matching 3ZY12121BA00 device connector is required to interface with the unit.
Tech Tip: Be sure to purchase the correct device connectors for all the 3SK modules in your system as they form a type of backplane to which all modules are connected.
The device connector is not always required. For instance, the 3SK11211CB42 safety relay can operate as a freestanding unit. With the restriction of monitoring a single sensor for a high SIL/PL or a pair of sensors in a low SIL/PL system.
Figure 2: Image of the 3SK1220 card connector located in the base of the unit.
About This Author
Aaron Dahlen, LCDR USCG (Ret.), is a Senior Applications Engineer at DigiKey in Thief River Falls. His background in electronics and industrial automation was shaped by a 27-year military career as both technician and engineer, followed by over a decade of teaching.
Dahlen holds an MSEE from Minnesota State University, Mankato. He has taught in an ABET-accredited electrical engineering program, served as coordinator of an electronic engineering technology program, and instructed military technicians in component-level repair.
Today, he has returned to his home in northern Minnesota, completing a decades-long journey that began with a search for capacitors. Read his story here.