Troubleshooting of Siemens SIRIUS 3SK2 Safety Relay

Complete Guide to Troubleshooting Siemens SIRIUS 3SK2 Safety Relay Malfunctions

In the field of industrial automation, safety relays are the core components for achieving mechanical safety functions. The Siemens SIRIUS 3SK2 series safety relays are widely used in emergency stop monitoring, protective door monitoring, manual operation, light curtain evaluation, and other scenarios due to their modular design, flexible software parameterization, and safety levels up to SILCL 3/PL e/Cat. 4. However, when equipment malfunctions or systems unexpectedly shut down, engineers often need to locate the root cause of the problem and restore production in the shortest possible time. This article will provide a systematic troubleshooting guide from the dimensions of diagnostic concepts, LED status interpretation, error classification, fault response time calculation, PROFIBUS diagnostic data reading, factory reset, and equipment replacement, to help maintenance personnel quickly master the troubleshooting skills of SIRIUS 3SK2.

Overview of Diagnostic System

SIRIUS 3SK2 safety relay provides multiple diagnostic pathways, which can be flexibly selected according to site conditions:

LED indicator diagnosis: directly view the status of the DEVICE and SF (group fault) LEDs on the device panel.

Equipment display screen diagnosis: limited to models with a width of 45mm, it can display input/output terminal status and error codes.

Diagnostic display screen (3SK26): optional accessory, installed on the control cabinet door, displaying the current message, fault cause, and affected components in plain text.

Safety ES software diagnosis: Through PC/PG online connection, detailed logs, forced output, and testing modes can be viewed.

PROFIBUS diagnosis: Upload diagnostic data to the upper controller through the DP interface module (3RK35) to achieve remote monitoring.

When troubleshooting on site, it is recommended to prioritize using the diagnostic display screen (if configured) or observing the LED status to quickly identify the fault category.

Interpretation of LED indicator light status

2.1 Device LED (Equipment Status)

Suggestions for handling the meaning of LED status

Check the 24V DC power supply for no voltage, undervoltage, or overvoltage, and confirm if the power supply is within the rated range (0.85~1.2 times).

The green constant light safe mode is running normally and the system is functioning properly, no operation is required.

Green flashing (0.5Hz): The device waits for the self-test to complete during the power on self-test phase, usually entering mode after 2 seconds.

The green flashing (fast) test mode device is in Safety ES test mode, and the output can be forced.

The yellow constant light configuration mode configuration has not been released or does not exist, and a valid configuration needs to be downloaded using Safety ES.

Yellow flashing (0.5Hz) successfully restored to factory settings. The device will automatically restart and enter configuration mode.

The red constant light system has stopped internal device errors and needs to be powered off and restarted; If it still exists, replace the module.

Red flashing (0.5Hz) configuration error or wiring fault check cross circuit, output short circuit to ground, and whether the memory module is unplugged during operation.

2.2 SF LED (group fault)

Suggestions for handling the meaning of LED status

The malfunction of extinguishing the group is normal.

The group fault wiring errors, communication errors, and parameterization/configuration errors that need to be confirmed due to the constant red light need to be fixed and reset (RESET button or software confirmation) needs to be performed.

Red flashing (0.5Hz) self confirming group fault logic error (such as difference time exceeding limit, sequence error), automatically cleared after the fault disappears, without the need for reset.

Attention: When multiple faults exist simultaneously, the priority of the red constant light is higher than the red flashing. After the fault is confirmed, the system will immediately resume output when the power on conditions are met, and measures must be taken to prevent accidental restarts (such as a start button with monitoring).

Detailed explanation and handling methods for error categories

According to the diagnostic concept, 3SK2 categorizes errors into five levels, with significant differences in the handling methods for each level.

3.1 Device Error

Performance: The DEVICE LED is always red, the SF LED is always red, the device stops all outputs, and diagnostic communication cannot be performed.

Possible reasons: Internal hardware failure, overvoltage/undervoltage.

Solution: Power off and restart. If the red light remains on after restarting, the safety relay must be replaced.

Exception: If it is overvoltage/undervoltage and both Device and SF do not light up, the power supply should be checked.

3.2 System Error

Performance: DEVICE flashes red (exits safe mode) or yellow (exits test mode), SF red remains on, the device switches to configuration mode, and all outputs are turned off. But diagnostic information can still be read.

Possible reason: Serious internal logical inconsistency.

Processing: After correcting the error, perform a reset or restart before returning to safe mode.

3.3 Logic/Wiring Error

This type of error will not result in exiting security/testing mode, but will affect the corresponding functionality.

3.3.1 Wiring Error

Performance: SF red is constantly on (to be confirmed), the status of the device depends on the current mode, and the diagnostic display screen or Safety ES can locate specific terminals.

Common scenarios: feedback circuit switch time violation, cross circuit, short circuit.

Solution: Check the wiring of the sensor/actuator, correct it, and press RESET to confirm.

3.3.2 Logical Error

Performance: SF flashing red (self confirmation), no need to reset.

Common scenarios: Discrepancy time exceeding the set value, signal sequence violating regulations.

Processing: Correct the timing of sensor actions or adjust parameters, and errors will automatically disappear.

3.3.3 Group Pre warning

Performance: SF is turned off, the DEVICE status is normal, but the diagnostic display shows a pre warning (such as waiting for start testing or triggering safety sensors).

Processing: Follow the expected operation (such as pressing the start button after closing the protective door), and the warning will be automatically released.

3.4 Parameterization or configuration errors

Stage of occurrence: Only in configuration mode.

Performance: DEVICE yellow is always on, SF red is always on, and the display screen shows "PROJ ERR" (45mm model).

Possible reasons: CRC configuration error, cycle time exceeding limit, TARGET ≠ ACTUAL configuration, invalid parameter values, interconnection rule violation.

Solution: Use Safety ES to regenerate and download the correct configuration, and release the configuration.

3.5 Handshake Error

Stage of occurrence: Only in testing mode.

Performance: DEVICE yellow is always on, SF red is always on.

Reason: The connection between Safety ES and the device is interrupted (such as cable pulling out).

Solution: Reconnect, the error disappears automatically, and the device returns to configuration mode.

Quickly troubleshoot using the diagnostic display screen (3SK26)

For systems connected to the 3SK26 diagnostic display screen, fault location is the most intuitive. The display screen is directly installed on the control cabinet door, and plaintext information can be read without opening the cabinet door.

4.1 Standard Display Interface

Status display line: device identifier, operational status (security/testing/configuration), configuration status.

If there are pending messages, the display screen will automatically redirect to the message menu and prioritize them.

4.2 Messages menu

After entering the "Messages" menu, you can view by category:

Device errors

Group errors

Bus errors - such as PROFIBUS interrupts

Group warning

Group pre warning

After selecting the specific message, right-click (OK) to view the detailed reason or directly jump to the affected functional component (such as a wiring error in input terminal F-IN5). For errors that need to be confirmed, press the left soft key (Reset) after fixing them to confirm.

4.3 Status Menu

All functional components can be listed according to the following filtering criteria:

All components

There are incorrect components

Error free components

After selecting a component (such as "Emergency Stop_1"), you can see:

Input signal status (0/1/-)

Waiting to start test flag

Violation of time difference regulations

Cross circuit detection results

4.4 System Configuration Menu

Display current hardware slot information:

Slot 2: DP interface (if present)

Slot 3: Safety Relay Body (Model, Hardware Version, Firmware Version)

Slot 4-8: Expansion module (3SK1 output expansion or 3RM1 motor starter)

This can be used to confirm whether the hardware is consistent with the configuration (such as whether the expansion module is inserted in the wrong position).

Fault response time calculation - key to safety distance assessment

In the design of safety systems, it is necessary to verify the total time (fault response time) from triggering the safety function to the complete stop of the actuator to ensure that the safety distance complies with standards (such as EN ISO 13855). The response time calculation of 3SK2 involves multiple parameters, and engineers should calculate based on actual wiring and logical configuration.

5.1 Basic Definition

Response time (fault free operation): The time it takes for an input variable to change to an output action, used to define the sequence of processes and not used for safety distance calculation.

Fault response time: the time from detecting a dangerous fault to the system entering a safe state. This time must be used for safe distance calculation.

5.2 Influencing Parameters

Typical values/setting ranges of parameter meanings

TS Sensor Response Time Refer to Sensor Manual

The time it takes for tIN input to collect logic=tCYCL (program loop time)

TCYCL program cycle time is 10-60 ms (set to 15 ms when used in conjunction with DP interface)

The timer function in tTIMER logic has a time of 10 ms~655 s (must be an integer multiple of tCYCL)

TDELAY input delay (parameterizable) 0-150 ms (must be an integer multiple of tCYCL)

The maximum read back time for dark testing of tFB secure output is 3 ms (fixed) or 3~400 ms (F output delayed)

TQ output terminal response time<5 ms (semiconductor output)

TA actuator response time refer to the contactor/motor starter manual

5.3 Fault response time (including flag) under single channel actuator wiring

If a flag is used in the logic, each flag will add a tCYCL.

Formula:

tFR1=tS+tIN+2tCYCL+tTIMER1+tDELAY+tCYCL+tTIMER2+tFB+tQ+tA

t FR1=t S+t IN+2t CYCL+t TIMER1+t DELAY+t CYCL+t TIMER2+t FB+t Q+t 

5.4 Fault response time under dual channel actuator wiring (without flag)

tFR2=tS+tIN+2tCYCL+tTIMER1+tDELAY+tQ+tA

t FR2=t S+t IN+2t CYCL+t TIMER1+t DELAY+t Q+t A

Note: Under dual channel wiring, the dark test readback time tFB is not included in the fault response time, as the dual channel structure can tolerate brief pulses during single channel dark testing.

5.5 Calculation Example: Emergency stop with dual channel shutdown through two contactors

Sensor: SIRIUS 3SB3 emergency stop button, tS=10 ms (assumed)

tCYCL = 15 ms

No timer in logic, no input delay

TQ<5 ms (take 5 ms)

TA: The response time of RT20 contactor is about 20 ms

Motor inertia ignored

tFR2=10+15+2×15+0+0+5+20=80ms

t FR2=10+15+2×15+0+0+5+20=80 ms

This is the longest time from pressing the emergency stop button to the motor being powered off. When calculating the safe distance, the inertia stop time of the motor after the contactor is disconnected should also be added.

Remote diagnosis achieved through PROFIBUS

After configuring the 3RK35 DP interface module on 3SK2, the upper controller (such as SIMATIC S7) can read diagnostic data through PROFIBUS DPV1.

6.1 Available data sets

Content of access methods for devices belonging to the data group

DS0 basic unit (slot 1) reads device status, group faults, configuration errors, and operating modes

DS0 DP interface (slot 0) reads DP interface status and communication failure

DS1 basic unit reads the same as DS0 plus channel diagnostic information

DS92 Basic Unit Read Detailed Device Message List (Up to 1000 entries)

6.2 Diagnostic Frame Structure

PROFIBUS standard diagnostic section (6 bytes)+device related diagnostics (6 bytes)+status messages (12 bytes)+channel diagnostics (up to 18 bytes). The channel diagnosis carries an error number from DS92.

6.3 Common DP Error Numbers

Number Description Explanation

7. Exceeding the upper limit. Memory module is too small, reducing configuration

Too many components below the lower limit, insufficient memory, and cycle time exceeding the limit

9. Incorrect output wiring and device self-test errors

16 Parameterization Error Configuration CRC Error, Invalid Parameter Values

19 Communication error PROFIBUS or system interface error

24 actuator shutdown configuration error, handshake error, wiring/logic error

25 Logical errors in security related shutdown user programs

26 External wiring error, memory module not inserted

The upper PLC can read the data group through SFB 52 "RDREC" and trigger an alarm or automatic reset process based on the error number.

6.4 PROFIBUS bus fault handling

When the DP interface detects a bus interrupt, it will replace the PROFIBUS logic input with 0, but the safe mode will not exit. The LED BF is constantly on in red, and the display screen shows "BF".

After the bus is restored, the device automatically uses the actual value without manual intervention.

Factory reset and device replacement

7.1 Factory reset steps (applicable to all 3SK2)

Turn off the 24V DC power supply.

Press and hold the RESET button on the front panel without releasing it.

Power on again and continue holding down RESET.

Observe the DEVICE LED: first flash yellow, release RESET; If it turns red and flashes, press and hold again; Release RESET within 10 seconds when the DEVICE LED is off.

The LED starts flashing yellow, indicating that it is recovering. After completion, the device will automatically restart and enter configuration mode.

effect:

All configuration information in the internal memory has been deleted.

If an external memory module (45mm model) is inserted, the module data will also be cleared.

The DP address is restored to 126.

7.2 Replacement of defective equipment

Replace safety relay (22.5mm):

Use Safety ES to backup the configuration, or save the configuration through the diagnostic display screen.

Disconnect all terminals (removable terminals can be pulled out as a whole while keeping the wiring intact).

Dismantle the equipment.

Install new equipment of the same model.

Insert the terminal back and load the backup configuration.

Replace safety relay (45mm):

Pay attention to external memory modules: they can only be plugged in and unplugged after power failure. If the old memory module inserted into the new device contains configurations from other device families (such as MSS 3RK3), a configuration error will be returned, and the release needs to be canceled and downloaded again first.

After replacement, a complete functional test (configuration test+system test) must be performed.

Replace DP interface or diagnostic display screen:

Directly replace with the same model without the need for reconfiguration. The system automatically matches hardware after power on.

7.3 Functional testing requirements after replacement

According to the EU Machinery Directive and functional safety standards, after each component replacement, the following must be carried out:

Configuration testing: Verify that the downloaded configuration in Safety ES is consistent with the target.

System testing: Trigger each safety sensor (emergency stop, protective door, etc.) to confirm that the actuator is correctly turned off; Reset and start again, confirm that the recovery logic is correct.

For electromechanical actuators (such as 3SK1 output extensions and contactors), the functional testing intervals must comply with:

SILCL 2 or PL d: ≤ 1 year

SILCL 3 or PL e: ≤ 1 month

Common faults and countermeasures in typical application scenarios

8.1 Emergency stop circuit (dual channel NCNC, cross circuit detection activated)

Phenomenon: After pressing the emergency stop button, it cannot be turned off or reset, and the SF red light remains on.

Diagnosis: Check the diagnostic display screen and it will prompt 'Cross circuit at input n'. The usual reason is that the wiring of two channels is short circuited externally, or one of the contacts is stuck.

Solution: Check if the two NC contacts of the emergency stop button are independently wired and measure their continuity. After repairing, press RESET to confirm.

8.2 Protective door monitoring (electronic RFID safety switch)

Phenomenon: Unable to power on after closing the protective door, SF flashes red (self confirmation).

Diagnosis: Logical error, infinite time difference but sequence violation. Perhaps the feedback signal from the door lock did not change within the allowed time.

Solution: Check the timing of the locking feedback contacts (such as Lock engaged) and door position contacts of the safety switch. The difference time can be set to a finite value (such as 1 second) in Safety ES to quickly capture timing errors.

8.3 Two handed operation (Type IIIc)

Phenomenon: After pressing two buttons, the machine does not start and the SF red flashes.

Diagnosis: The synchronization time exceeds 0.5 seconds, or the crossover circuit detects abnormal signals in two channels.

Solution: Check if both buttons are pressed simultaneously (difference<500ms). If the wiring is long, shielded cables can be considered. For NONO type inputs, cross circuit detection must be activated.

8.4 3SK1 output extension controlled through device connector does not operate

Phenomenon: F-Q3-C in 3SK2 logic has output 1, but the relay for 3SK1 extension is not engaged.

Diagnosis: First, check if the position of the slide switch on 3SK1 matches the parameterized output in Safety ES ("UnderELAYED" corresponds to F-Q3-C, "DELAYED" corresponds to F-Q4-C). Secondly, confirm whether the expansion module is powered through the 3ZY12 device connector and not connected to an external power source on A1/A2 (otherwise it will bypass the safety function).

Solution: Adjust the slider switch or modify parameterization to remove external wiring on A1/A2.

Daily maintenance and preventive diagnostic recommendations

Regular optimization of dark test parameters: If there is a capacitive load (such as LED indicator lights or long cables) connected to the output terminal, the maximum readback time (3-400 ms) should be increased according to the "capacitive load curve" to avoid erroneous shutdown caused by dark test pulses.

Monitoring environmental temperature: The 22.5mm model can be installed independently with a maximum temperature of 60 ° C; it may be downgraded to 40 ° C when configured with DP interfaces or expansion modules. If the cabinet temperature exceeds the limit, the signal loop connector (3ZY1212-2AB00) needs to be used to increase the heat dissipation distance.

Establish diagnostic logs: Regularly read device logs (Logbook) through Safety ES, record timestamp fault information, and use it for predictive maintenance.

Firmware and GSD file updates: Visit Siemens Industrial Online Support to obtain the latest version to fix known issues.