Troubleshooting Siemens 3RW55 Error 1408 Missing Load

Overview: Siemens 3RW55 Error 1408 "Missing Load"

The Siemens SIRIUS 3RW55 soft starter continuously monitors the three-phase motor current on the load side of the device using integrated current transformers on the output side. When the control unit issues an ON command but no current is detected within a defined confirmation window, the soft starter raises fault 1408 ("Missing Load") and latches the gating command off. The drive will not attempt another start until the fault is acknowledged, even if the ON input remains asserted.

Error 1408 belongs to the operating-state fault class. In the factory default configuration it is a latching fault with stop category 1 (controlled ramp-down to zero voltage, then removal of the gating pulses). The fault must be cleared either by removing the ON command, by pressing the OK / RESET button on the operator panel, or by setting the RESET bit over the fieldbus. The canonical fault list and remedial actions are documented in the Siemens Industry Online Support entry 109753752 - Errors and remedial actions of the 3RW55 and 3RW55 Failsafe soft starters.

Field experience shows that error 1408 is rarely caused by a defective soft starter. In the vast majority of service reports the device is correctly identifying an external open-circuit condition. The diagnostic effort should focus on the load path: contactor, fuses, motor cable, and the motor itself.

Affected Products, Variants, and Firmware

Error 1408 applies to every device in the 3RW5 family that performs load-current monitoring. The most common installations seen in service are:

• 3RW55 standard soft starters across the full size range, from frame-size S2 compact units up to the S12 panel-mount units for high-current applications.
• 3RW55 Failsafe variants (suffix ...F0 or ...F1) with integrated safe torque off per IEC 61800-5-2 / SIL 3 / PL e.
• 3RW52 basic soft starters, which use the same firmware-level fault list and the same root-cause categories.

The fault string is generated by the same firmware module across all 3RW5 variants. Where the behavior diverges is in the safe-stop response: on a 3RW55 Failsafe the F-DI / F-DO safety loop is evaluated before the run-up decision, so a STO demand takes priority over a missing-load trip. The operating instructions for the relevant part number are the primary source of truth for wiring and parameter locations; they are linked from the device page on Siemens Industry Online Support.

Always confirm the firmware version installed on the device before any parameter change. Some earlier firmware builds had nuisance missing-load trips on long motor cables or on soft-start sequences with high breakaway torque. The current firmware load can be read at Menu > Diagnostics > Device information > Firmware version. Released firmware images and release notes are available on the Siemens support portal under the product entry for the 3RW55.

How the Missing-Load Detection Works

The 3RW55 takes a current sample on each of the three phases through the integrated current transformers on the load side. The firmware compares the smallest of the three phase currents against the configured minimum-current threshold (displayed in the device menu under the motor-protection group) every measurement cycle, typically every 10 to 20 ms. The fault is raised when all of the following conditions are true simultaneously:

1. An ON command is present at the control input (terminal 1 / DI, the operator panel, or the fieldbus control word).
2. The thyristor bridge has been gated on, meaning the soft starter is actually attempting to deliver voltage to the load.
3. The configured run-up time has elapsed, so the starter is no longer in the soft-start ramp.
4. All three measured phase currents are below the minimum-current threshold for the configured confirmation time.

The confirmation timer is the parameter that debounces the detection to prevent nuisance trips during the natural current zero-crossing at the start of each half-cycle. The default value is short (on the order of one second) so that a real open-circuit fault is detected within roughly one second, but long enough to ride through contactor seating transients and the first few cycles of the run-up.

Figure 1 - State machine of the 3RW55 missing-load detection.

Two operating modes can produce the same 1408 alarm. The diagnostic path is different for each, and the report of the operating state at the time of the trip (in the fault buffer) tells the engineer which one was active.

• Run-up fault. The starter has finished the configured voltage ramp and is in the bypass window, but no motor current is drawn. Almost always a wiring or contactor problem upstream of the soft starter, or a contactor that has not seated.
• Steady-state fault. The motor was running and the current has now disappeared because of a cable cut, a contactor drop-out, a thermal trip, or a process event that decoupled the load.

Primary Causes and Field Frequency

Siemens categorizes the root causes of error 1408 as follows. Each cause is rated by frequency in typical service reports across pump, fan, and compressor installations.

Why the Fault Appears After a Duty-Cycle Change (Pump Alternation Case)

A common field report involves two 3RW55 soft starters that alternate every six hours through simple electromechanical timers, switching a pair of pumps. The fault is reproducible at the duty change but does not appear during steady operation. This pattern is diagnostic in itself.

When the timer rolls over it typically performs two switching actions in close succession:

1. Opens the running pump's contactor.
2. Closes the standby pump's contactor and re-asserts the ON command to its soft starter.

If both actions happen at the same instant, the second soft starter can be issued an ON command before the contactor has actually seated. The 3RW55's current monitoring samples the existing state of the line, finds no current, and latches 1408 because the confirmation timer elapses before the contactor closes mechanically. The pump stays off until an operator presses RESET on the panel.

There are three additional aggravators that are common in this exact wiring scheme:

• Single shared control signal. A single timer contact drives both contactor coils in parallel, so the standby soft starter sees ON while the running soft starter sees the same signal drop. Whichever soft starter is in the de-energized state may interpret the transition as a fault if its debounce is too tight.
• No maintained ON command. The ON command is a momentary pulse from the timer. If the soft starter requires a maintained input, the device treats the missing input as a stop command and then latches 1408 if current is still flowing when the input drops.
• Contactor dropout during the OFF half of the cycle. Some installations keep the contactor coil fed from the timer's normally-closed contact. The contactor drops out, the soft starter sees no current, and the latched 1408 prevents the next start.

In every case the underlying problem is that the soft starter's view of the process (no current) does not match the operator's intent (run the standby pump). The fix is to align the control sequence with the soft starter's monitoring logic.

Diagnostic Procedure

Use the following sequence to localize the fault. It is written for the 3-line operator panel, but the same parameters and measured values are visible in the SIRIUS Soft Starter ES engineering tool and over PROFINET, Modbus TCP, or EtherNet/IP.

1. Read the fault buffer. Open Menu > Diagnostics > Fault buffer on the operator panel. The current fault is the top entry. Note the operating hours at the time of the trip and the operating state (run-up vs. bypass).
2. Read the warning buffer. Warnings that predate the fault (undercurrent, phase failure, control voltage low) often explain why the fault was raised. The buffer is in Menu > Diagnostics > Warning buffer.
3. Measure the line-side voltage. With the device powered but in the OFF state, use a category III 600 V meter to measure L1-L2, L2-L3, and L3-L1 at the line terminals of the 3RW55. The expected value is the supply voltage plus or minus ten percent.
4. Measure the load-side voltage. With the device in the OFF state, the load terminals should be at line potential only when the contactor is closed. If you measure line voltage at the load terminals with the contactor open, you have a wiring error or a stuck contact.
5. Issue a test ON command. Force the ON input from the panel (Menu > Commissioning > Manual operation) or from the PLC. Watch the measured current in Menu > Measured values > Currents. If the three phase currents are all zero for more than the configured confirmation time, the fault is real.
6. Measure contactor coil voltage. With the ON command asserted, measure the voltage at the contactor coil terminals. The expected value is the coil nominal voltage plus or minus ten percent.
7. Listen and feel the contactor. The contactor must pull in fully. A chattering or partially seated contactor indicates a low-voltage coil or a mechanical problem. Replace the contactor if it does not seat within 100 ms of the ON command.
8. Measure the motor insulation. With the motor isolated from the soft starter, use a 500 V insulation tester to measure phase-to-phase and phase-to-ground resistance. A reading below 1 megohm indicates degraded insulation that may be opening the motor's thermal switch.
9. Verify the current transformers. If all of the above tests pass, the soft starter's internal CTs may be at fault. Compare the panel's reported current with a clamp-on meter on each phase at the motor junction box. A difference of more than ten percent between the panel reading and the clamp reading at steady state indicates a CT problem and a likely device return.

Step-by-Step Resolution by Cause

The resolution depends on which of the four primary causes the diagnostic procedure identified. Each subsection is independent and can be followed in isolation.

Cause A: Blown fuse or tripped breaker

1. Identify the affected phase from the diagnostic data.
2. Isolate the supply at the upstream disconnect and lock it out.
3. Replace the fuse with the same type and rating, or reset the breaker after clearing the cause of the trip.
4. Perform a megohm test on the motor and cable before re-energizing. A short-circuit at the load will blow the new fuse immediately.
5. Re-arm the soft starter by acknowledging the fault with the OK button on the operator panel or via the fieldbus RESET bit.

Cause B: Defective contactor or control wiring

1. With the supply isolated, visually inspect the contactor for burned contacts, broken springs, or signs of arcing on the pole faces.
2. Measure the contactor coil resistance with the wires disconnected. The resistance should match the nameplate value, typically 30 to 200 ohms for an AC coil and 1 to 5 kilohms for a 24 V DC coil. An open coil requires contactor replacement.
3. Trace the control wiring from the 3RW55 output relay (terminals 13 / 14 by default) to the contactor coil. Look for loose terminals, broken crimps, and pinched insulation.
4. Confirm that the auxiliary contact block is in the correct position. A normally-open contact used where a normally-closed is expected can hold the contactor off even with the correct control signal.
5. Replace the contactor if any contact is welded or pitted beyond the manufacturer's service limit. A contactor that has been jogging or chattering is at end of life even if it still passes a continuity check.

Cause C: Motor or cable fault

1. Disconnect the motor leads at the motor terminal box.
2. Measure phase-to-phase and phase-to-ground resistance on the motor. All three readings should match within five percent for a healthy motor.
3. Measure phase-to-phase and phase-to-ground resistance on the cable. A reading below 1 megohm indicates insulation damage and requires cable replacement or re-termination.
4. Repair or replace the damaged section. Do not bypass the 3RW55 protection. The device correctly identified an unsafe condition and the protective function must remain in service.

Cause D: Application sequence problem (pump alternation)

If the diagnostic procedure shows that the components are healthy and the fault only appears at the duty change, the application sequence is the cause. Use one of the following proven sequences.

1. Add a maintained ON command. Replace the momentary timer pulse with a maintained output that is only dropped when the soft starter is supposed to stop. The 3RW55 expects a continuous ON; missing-load detection only runs while ON is asserted.
2. Insert a sequencing delay. Configure the timer to drop the running pump's contactor first, wait at least two seconds (longer than the soft starter's debounce), then close the standby contactor and re-assert ON. The two-second delay is documented in the operating instructions as the minimum coast-down interval for the missing-load detection to reset.
3. Use the built-in application function. The 3RW5 family has dedicated application function sets for pumps. Configure pump 1 as the master and pump 2 as the slave and let the device handle the changeover. The function issues the contactor commands and the ON / OFF signals in the correct order and is documented in the device's application manual.
4. Add a sealing contact. Use an auxiliary contact from the contactor as a sealing (latching) contact around the timer output. The contactor then stays in as long as either the timer output is closed or the sealing contact is made, preventing the soft starter from seeing a momentary loss of current during the changeover.

Parameter Configuration Reference

Several parameters govern the missing-load detection. The table lists the most relevant entries with their function and a typical commissioning value for a pump application. The exact parameter path and the current firmware default can be found in the operating instructions for your part number.

Wiring Topology for a Two-Pump Alternation Panel

The diagram below shows a typical single-pump power section. The alternation is implemented in the control circuit, where two contactors (K1 for pump 1, K2 for pump 2) take turns driving the same motor feeder. The 3RW55 monitors the current on the load side; the missing-load fault is raised if the selected contactor is not closed when the ON command is present.

Figure 2 - Power section of a two-pump alternation panel with a 3RW55.

The mechanical and electrical interlocks between K1 and K2 must be rated for the motor's full load current. The 3RW55 output relay (terminals 13 / 14) drives only one contactor at a time; the second contactor is held off by the timer's logic or by a mechanical interlock. If the interlock is missing, both contactors can close simultaneously and create a cross-connection that the 3RW55 will see as a missing load on one phase.

Verification Procedure

After the resolution is implemented, perform the following verification to confirm that the fault will not return and that the protection function is still effective.

1. Clear the fault buffer with Menu > Diagnostics > Fault buffer > Clear.
2. Issue a manual ON command from the operator panel. Confirm that the motor ramps to full voltage and the measured current matches the motor nameplate plus or minus ten percent.
3. Force a simulated missing-load condition by opening the contactor manually while the motor is running. The 3RW55 should log 1408 within the configured trip time and drop the contactor command. Close the contactor again, acknowledge the fault, and confirm a clean restart.
4. Run the application through three full duty cycles (in the pump example, eighteen hours of operation). Monitor the warning buffer for any phase-failure or undercurrent warnings that could prefigure a future 1408 trip.
5. Record the operating hours and the fault buffer hash for the maintenance log. A sudden increase in 1408 events after months of clean operation is a strong signal that the contactor or cable is wearing out and should be scheduled for replacement.
6. If the device is on a fieldbus, poll the diagnostic register and confirm that the 1408 event has been cleared from the controller's alarm summary.

Related Error Codes

The 3RW55 fault list contains several alarms that interact with 1408. They are worth checking when the diagnostic procedure does not find a clear root cause, and they often appear in the same fault buffer in the minutes before a 1408 trip.

Preventive Maintenance Recommendations

A recurring 1408 in a pump-alternation installation is a strong indicator that the contactor is approaching end of life. The following preventive actions extend contactor and soft starter life and reduce the chance of an unplanned shutdown.

• Replace pump contactors on the manufacturer's service interval or every 100 000 operations, whichever comes first.
• Torque-check the line and load terminals of the 3RW55 every twelve months. Loose terminations cause intermittent contactor drop-out and arcing that destroys the contactor pole faces.
• Clean the cabinet ventilation filters. The 3RW55 derates its output current at elevated ambient temperatures; a hot soft starter can drop the contactor command to protect its thyristors, and the resulting loss of current trips 1408.
• Verify the control voltage at the 3RW55 control terminals. The device tolerates 24 VDC plus or minus twenty percent or 110 / 230 VAC plus or minus fifteen percent depending on the variant. A supply outside this range causes the output relay to chatter and produce repeated missing-load trips.
• Update the firmware to the latest release published on Siemens Industry Online Support. Firmware updates have resolved several nuisance missing-load trips in older 3RW55 builds, particularly in installations with long motor cables and high leakage capacitance.
• Log every 1408 event in the maintenance system. A weekly count above zero in a clean installation is an early indicator of contactor wear or cable damage.

3RW55 Failsafe Specific Notes

The 3RW55 Failsafe variant (suffix ...F0 or ...F1) adds a second hardware interrupt path for safe torque off per IEC 61800-5-2. The missing-load behavior is unchanged, but the safety logic takes priority: a demand on the F-DI input will open the output stage within the documented response time regardless of the missing-load state.

Two wiring pitfalls are specific to the Failsafe variant in a pump-alternation panel:

• Do not back-feed the contactor coil through a shared return with the F-DI circuit. A shared return can keep the contactor partially energized even when the safety input is open, defeating the safe-stop function and producing a confusing 1408 after the safety event.
• Wire the F-DI directly to the safety relay or safety PLC, not to a 24 VDC distribution terminal that also feeds other I/O. Cross-talk on the distribution terminal can cause nuisance safe-stop events that look like missing-load trips in the fault buffer.

The wiring diagrams in the 3RW55 Failsafe operating instructions show the correct isolation; they are linked from the device page on Siemens Industry Online Support.

Migration Notes: From 3RW40 to 3RW55

Engineers replacing a legacy 3RW40 with a 3RW55 should be aware of two behavior differences that surface as 1408 right after commissioning.

• Different default confirmation time. The 3RW40 has a longer debounce on missing-load detection than the 3RW55. A contactor that worked correctly on the 3RW40 may trip the 3RW55 if the changeover sequence was already on the edge of the 3RW40 debounce. Lengthen the timer sequencing delay as part of the swap, or reduce the missing-load sensitivity on the new device.
• Different default for response to missing load. The 3RW40 can be configured to ignore a missing-load event after the run-up time, while the 3RW55 always logs the event. The fault buffer on the 3RW55 may show 1408 entries that did not appear on the 3RW40; these are usually real events that the older device masked, not new problems.

Always commission a 3RW55 replacement with the same diagnostic procedure used in this article, regardless of whether the 3RW40 was reporting clean. The 3RW55 is a more sensitive monitor and will surface issues that the 3RW40 silently tolerated.

Frequently Asked Questions