KUKA KRC4 Safety Stop Acknowledge & Recovery

KUKA · intermediate · 20 min

Recover a KUKA KRC4 robot from a Safety Stop (SS1/SS2) message: find the open safety input, physically restore the circuit, then acknowledge on the smartPAD and re-enable drives.

⚠️ Never bypass a safety circuit. A Safety Stop always has a real cause — an open guard, a triggered light curtain, or a broken safety connection. Fix the cause physically; acknowledging alone does not make the cell safe.

What a Safety Stop is

On a KRC4 controller the safety controller (SafeOperation / the ESC safety system) commands a Safety Stop — typically SS1 (stop then remove drive power) or SS2 (stop but hold position with drives enabled) — whenever a monitored safety input opens. The robot stops, drives may drop, and the smartPAD shows a safety message. Unlike a program error, a Safety Stop cannot be cleared by RESET alone: the underlying safety input must first be physically restored.

Common causes

• An open safety gate / guard door in the cell.
• A triggered light curtain, laser scanner, or safety mat.
• Operator or automatic E-STOP pressed (on the smartPAD or an external station).
• A loose or damaged safety bus connector — commonly X41/X42 on the KRC4.
• Wrong operating-mode selection for the task (e.g. requesting external motion while a safety zone is violated).

Step-by-step recovery

1. Read the message. On the smartPAD, open the message window and note the exact safety message and which safety input tripped. The message text names the source (E-STOP, operator safety, safety gate, etc.).
2. Find and clear the physical cause. Close the open guard, clear the person or object out of the light curtain / scanner field, release any pressed E-STOP button (twist to release), or re-seat the loose X41/X42 safety connector. Confirm the source is genuinely restored — do not defeat it.
3. Check drive readiness. With the cause cleared, the drives should be able to return to a ready state. On the smartPAD status bar, the drive/enabling status indicator should be able to go green.
4. Acknowledge the messages. On the smartPAD press Acknowledge (or Acknowledge all) to clear the pending safety messages now that their cause is gone.
5. Re-enable motion. Select the correct operating mode with the mode switch (T1 for teach at reduced speed, AUT/AUT EXT for production). In a manual mode, press and hold the enabling switch to the middle position; the drives contactor should close and the status turns green.
6. Restart the program if needed. If an SS1 dropped power mid-motion, reposition on path and restart the program per your cell's start procedure.

If it will not acknowledge

If the message returns immediately after acknowledging, the safety input is still open. Re-check the exact source named in the message — a partially closed gate switch, a scanner in a muted/faulted state, or an intermittent X41/X42 connection are the usual culprits. Persistent unexplained safety faults call for KUKA service, not repeated acknowledge attempts.