Resolving SIRIUS 3TK2826 RF Fault: Feedback Loop Diagnosis and Reset

Field engineering reference for the RF (Rückführung) fault on the Siemens SIRIUS 3TK2826-CW30 safety relay. The document covers the feedback-circuit principle, the Y33 start input, the external device monitoring (EDM) loop, terminal-by-terminal wiring checks, LED interpretation, root-cause analysis, and the manual reset path required after the relay latches the fault.

1. Device Overview

The 3TK2826 belongs to the SIRIUS 3TK28 family of safety-monitoring relays. Its job is to evaluate the state of a safety sensor — an E-stop pushbutton, a guard-door interlock, a safety light curtain, or a safety mat — and to release the actuator circuit (typically one or two contactors feeding a motor or a hydraulic valve) only when the input is in the safe state. The CW30 suffix denotes the specific contact assignment, start mode, and supply variant as printed on the device faceplate; the diagnostic logic in this article applies to all CW30 units in service.

The relay monitors two independent input channels for cross-faults, supports manual-monitored or automatic start, and integrates External Device Monitoring (EDM) into the start circuit. EDM is the source of the RF fault indication. The 3TK2826 was widely deployed in machinery built between the late 1990s and the 2010s and remains in service across packaging lines, conveyors, metal-forming presses, and assembly cells.

2. What the RF Fault Means

RF is the abbreviation for Rückführung (German for "feedback" or "return"). The RF fault is raised when the feedback circuit — the EDM loop wired through the normally closed auxiliary contacts of the downstream contactor(s) — is not closed at the moment the start command is applied to terminal Y33.

In operating terms: the enabling circuit (the safety input) is closed, the operator presses the start button on Y33, but the EDM loop in series with the start button is open. The relay interprets this as "I cannot prove that the contactor I am about to energise is actually in the open state" and latches the fault. The enabling contacts (13/14, 23/24, 33/34, 43/44 as wired) stay open, the red DEVICE LED indicates the latched state, and the actuator remains de-energised.

3. EDM Feedback Principle (Rückführung)

External Device Monitoring is a mandatory function for category 3 and category 4 architectures under EN ISO 13849-1. The 3TK2826 implements EDM as part of the start circuit. The principle, step by step:

1. The relay's output contacts (typically 13/14 driving contactor K1 and 23/24 driving K2) release the contactor coils when the safety input is in the safe state and the start command is valid.
2. The normally closed auxiliary contacts of K1 and K2 (often labelled 21-22 or 31-32 on the contactor) are wired in series with the start pushbutton between the start reference (Y33) and the start return (Y34 on most CW30 variants).
3. When the safety inputs are closed, pressing the start button closes the Y33–Y34 loop only if both contactors are de-energised (i.e., their NC blocks are closed).
4. If either K1 or K2 is mechanically welded closed, its NC block stays open, the Y33–Y34 loop stays open, and the relay refuses to start — flagging the RF fault.

This cross-check guarantees that a single welded contactor cannot bypass the safety function: the relay will not release the second contactor unless it can prove the first one has dropped out.

4. Terminal Reference for the 3TK2826-CW30

The terminal assignments below are the canonical map for the 3TK28 generation. The exact layout for the CW30 sub-variant should always be cross-checked against the wiring label on the side of the device and the operating instructions listed in section 11. The terminals used in the EDM circuit are stable across the family.

5. LED Status Map

6. Common Causes — Root Cause Matrix

From field experience, the RF fault on a 3TK2826 clusters around six root causes. Work through them in order of likelihood after the safety input has been verified closed.

7. Diagnostic Procedure

Use the two-phase procedure below. Phase 1 is performed with the relay isolated; phase 2 with control supply applied. Stop at the first failed check and repair before continuing.

7.1 Phase 1 — Static checks (control supply off)

1. Lock out and tag out the machine. Disconnect the contactor coil conductors from the relay output terminals (13, 23, 33, 43) to isolate the actuator circuit.
2. Measure continuity between Y33 and Y34 with the start pushbutton released. The reading should be open circuit. If it shows continuity, the start button is wired wrong, stuck, or the start return is shorted to the start input.
3. Press and hold the start button. Measure continuity between Y33 and Y34 through the contactor NC blocks in series. Expect <1 Ω per joint and a stable reading.
4. With the start button still pressed, manually close the contactor armature (or energise the coil briefly through a test lead). The Y33–Y34 continuity must break — this confirms that the NC auxiliary block actually transfers. If continuity does not break, the block is miswired (likely a NO block installed in an NC slot) or mechanically jammed.
5. Verify the safety input: with the E-stop released and the guard door closed, measure continuity across S11–S12 and S21–S22. Both must read <1 Ω.

7.2 Phase 2 — Dynamic checks (control supply on)

1. Restore the contactor coil conductors to terminals 13/14 and 23/24.
2. Apply control supply to A1/A2. The green DEVICE LED must illuminate.
3. Reset the safety input (release the E-stop, close the guard door). Both channel LEDs must be ON.
4. Press and release the start button. The OUT LED must come on, and both contactors must pick up audibly and visually.
5. If the red DEVICE LED latches and OUT does not energise, record the LED pattern and refer to the root-cause matrix in section 6.

8. Circuit Diagram and Timing

The topology below is the canonical wiring for a CW30 variant driving two contactors with a monitored manual start. The EDM loop is the orange chain on the right-hand side; an open contact on K1 or K2 will produce the RF fault.

The expected sequence on a normal start is shown in the timing sketch below. On an RF fault, the OUT signal never rises and the red DEVICE LED latches at the moment Y33 is released.

9. Reset and Recovery

Once the underlying cause has been removed, the 3TK2826 must be reset. In manual-start configurations the reset is automatic on the next valid start edge, provided the safety inputs are closed and the feedback loop is intact. In monitored-manual-start (a.k.a. falling-edge) configurations, the start button must be pressed and released cleanly — the relay looks for the falling edge of the start command to confirm operator intent.

1. Confirm that all safety inputs are closed: E-stop released, guard door closed, light curtain clear.
2. Confirm that the feedback loop is closed: continuity from Y33 to Y34 through the contactor NC blocks, with both contactors de-energised.
3. Cycle the start button: press for at least 50 ms, then release.
4. If the relay does not release, power-cycle A1/A2 (off, wait 2 s, on) to clear any latched internal fault, then re-attempt the start.
5. If the RF fault returns immediately on every start attempt, the EDM loop is still open. Return to section 6 and re-check the root-cause matrix before re-energising.

10. Verification and Validation

After clearing the RF fault, perform a full functional safety test before returning the machine to production. The checklist below maps to EN ISO 13849-1 validation requirements for category 3/4 circuits.

• Stop function test: Press the E-stop while the machine is running. Both contactors must drop within the time specified on the relay's safety data sheet (typically ≤ 30 ms for electromechanical contactors at 24 V DC coil).
• Reset and start test: Release the E-stop, then perform a manual start. The relay must release only on a valid start edge with the EDM loop closed.
• EDM fault injection: Force a fault by opening the contactor NC block with a jumper wire. The relay must refuse to start on the next start command and must indicate the RF fault on the red DEVICE LED.
• Removal test: Remove the jumper and verify a clean restart on the next valid start edge.
• Cross-fault test: Open only one of the two E-stop channels. The relay must refuse to start and must indicate a channel-disagreement fault (not RF). This confirms the cross-fault detection is still functional after the intervention.
• Documentation: Record the date, the operator, the root cause, the corrective action, and the validation test results in the machine's safety file. Many jurisdictions require this for category 3/4 circuits under the machinery directive.

11. Migration to the 3SK Series

The 3TK2826 family has been succeeded by the SIRIUS 3SK1 and 3SK2 safety relays. New installations should use the 3SK1 or 3SK2 for spare-part availability, for compliance with the latest EN ISO 13849-1 PL e / SIL 3 targets, and for integration with the SIRIUS diagnostics web server. The RF fault principle (EDM check on the start edge) carries forward unchanged, but the terminal designations differ — never assume a 1:1 pin map when migrating.

The 3SK1 supports a single EDM loop per device; the 3SK2 supports up to four EDM loops and parameterisation through the SIRIUS diagnostics web server. For a 1:1 swap of a 3TK2826-CW30 driving two contactors, the 3SK1 in the equivalent base-unit configuration is the typical replacement. For multi-zone safety, the 3SK2 reduces panel space by replacing several 3TK28 units with a single base unit plus expansion modules.

12. Frequently Asked Questions