Overview
The question of whether a main contactor (or main circuit breaker) must sit upstream of a SINAMICS S120 Line Module is one of the most common wiring decisions on a Booksize or Chassis cabinet. The answer is conditional: for Basic Line Modules (BLM) and Active Line Modules (ALM) the line contactor is documented as optional, while for Smart Line Modules (SLM) the contactor is effectively mandatory because the module's diode bridge cannot be pulse-blocked to interrupt regenerative current safely. The SINAMICS S120 Function Manual describes a dedicated "line contactor control" function that closes and opens an external line contactor, and evaluates the feedback contact to detect welded or failed contacts.
Is a Main Contactor Mandatory in Front of an S120 Infeed?
According to Section 2.8 of the 6SL3097-2AC00-0BP3 Booksize Equipment Manual, the line contactor is shown as optional in every standard wiring diagram. However, "optional" only means that the infeed module itself will function without it; the contactor is still strongly recommended, and in some topologies it is required for safe operation. The decision matrix is driven by three considerations:
- Infeed topology – BLM, SLM, or ALM behave differently under DC-bus load.
- Precharge behavior – the contactor is needed to isolate the line until precharge completes on SLM and ALM.
- Functional safety and maintenance isolation – a service disconnect is almost always required by the end user's electrical standard (e.g. IEC 60204-1).
Line Contactor Behavior by Line Module Type
| Line Module | Line Contactor Required? | Reason |
|---|---|---|
| Basic Line Module (BLM) | Optional, but recommended | Diode bridge; DC bus is unidirectional. Pulse disable (EP low) collapses the bus only after the connected Motor Modules are inhibited. A line contactor provides clean galvanic isolation for service. |
| Smart Line Module (SLM) | Effectively mandatory | Diode motoring bridge + IGBT regenerative bridge. Precharge must complete before main contactor closes. There is no electronic way to "pulse disable" the diode path; the only safe way to isolate is the line contactor. |
| Active Line Module (ALM) | Optional, but recommended | IGBT bridge in both directions. Can regulate the bus to a controlled voltage during precharge. The line contactor isolates for service and is still closed by the firmware once precharge is complete. |
SINAMICS S120 "Line Contactor Control" Function
The SINAMICS S120 firmware exposes a built-in sequencer for an external line contactor. The function:
- Closes the line contactor when the Line Module enters precharge and the DC bus reaches a defined percentage of nominal.
- Opens the contactor after the drives have been ramped down, pulses inhibited, and the DC bus has discharged.
- Monitors a feedback contact on the contactor so a welded or stuck contactor is detected (r0825 / r0863 internal status bits).
Wiring is on terminal strip X9 of the Line Module:
| Terminal | Signal | Direction | Use |
|---|---|---|---|
| X9:1 | P24 | Output | +24 V supply for contactor coil |
| X9:2 | M | Output | 0 V return for contactor coil |
| X9:3 | DI (contactor in) | Input | Line contactor feedback (NO) |
| X9:4 | DI (contactor in) | Input | Line contactor feedback reference |
| X9:5 | DO (close cmd) | Output | Close command to line contactor |
| X9:6 | DO (close cmd) | Output | Close command reference |
EP Terminal X41 and the Off-Delay Requirement
Terminal X41:1 / X41:2 on the Line Module is the EP (Enable Pulses) input. This is the same hardware signal that gates the Motor Modules' pulses. When EP is removed:
- The IGBTs in the Line Module and all Motor Modules turn off within a few milliseconds.
- The DC bus begins to discharge through the motor EMF and any braking resistor.
- If the upstream line contactor drops out at the same instant, the BLM/ALM will draw a small residual current from the grid that can cause the line-side fuses to clear or the contactor to chatter.
For this reason, an off-delay timer must be inserted between the EP drop-out and the line contactor drop-out. A typical value is 1–3 seconds for Booksize BLM/ALM and up to 5 seconds for larger Chassis units with high bus capacitance. The timer is realised either:
- Externally with a pneumatic or electronic timing relay on the contactor coil, OR
- In the PLC by latching the "OFF1" command and dropping the line contactor output (X9:5/X9:6) only after the timer expires.
Control Strategy: When Should the Infeed Be Energised?
Three practical operating modes are used in the field. All three are valid; the choice depends on the duty cycle and the cabinet thermal design.
| Mode | Trigger to close contactor | Trigger to open | Pros | Cons |
|---|---|---|---|---|
| Always on (24 V present) | Cabinet 24 V healthy | Cabinet 24 V removed | Drives ready instantly; no warm-up time | Continuous precharge resistor and fan wear |
| PLC RUN | PLC enters RUN and no faults | Drives inhibited, timer expires | Coordinated with control program; precharge only when used | Slight delay on first motion command |
| On-demand (motion request) | Any drive receives ON/OFF1 | All drives OFF1 low, EP delay expires | Lowest idle losses; best for rarely used axes | Precharge current on every start; thermal cycling of contactor |
For most production machines, the PLC RUN mode offers the best balance. The contactor closes on the rising edge of "PLC_Run" combined with no active fault, and opens only after the drives have completed OFF1 and the EP off-delay has expired.
Recommended Wiring Topology
The diagram below summarises the safe wiring between the PLC, the SINAMICS S120 Line Module, and the line contactor. Use a 24 V signal from the PLC to drive the contactor; the Line Module's X9 contact handles the actual close command and monitors feedback.
Commissioning Procedure
- Wire the contactor coil to X9:5/X9:6. Wire the auxiliary NO contact to X9:3/X9:4.
- In STARTER or Startdrive, open the Line Module configuration and confirm
p0860 = 1(line contactor present) andp0861 = r0863.0for the close command source. - Set the EP off-delay in the PLC or external timer to 1–3 s (Booksize) or 2–5 s (Chassis).
- Power up the cabinet with the contactor forced open. Verify that the Line Module enters "Precharge running" (r340 bit pattern) and the contactor does not close.
- Release the force. The contactor should close within ~1 s of precharge complete (typically 90% of nominal DC bus).
- Issue ON/OFF1 from the PLC. The contactor must hold closed while drives run.
- Drop ON/OFF1 and wait for OFF1 ramp + EP delay. The contactor should drop out cleanly with no F30021 or F30027 fault.
- Force the contactor to remain closed by simulating a welded contact; the Line Module should raise F07300 "Line contactor feedback error".
Fault Codes Related to the Line Contactor
| Fault / Alarm | Meaning | Typical Cause | Corrective Action |
|---|---|---|---|
| F07300 | Line contactor feedback error | Aux contact not closing when expected, or not opening when expected | Check wiring of X9:3/X9:4; replace contactor if welded |
| F06310 | Line contactor not closed at end of precharge | Coil not energised, control wiring broken | Check X9:5/X9:6 and 24 V supply |
| F30021 | Ground fault on DC bus | Contactor opened under load, motor EMF pumped into open DC bus | Increase EP off-delay; verify the contactor is not being dropped by OFF1 |
| A06310 | Line contactor reached end of switching cycles | Contactor wear (typical >1,000,000 ops) | Plan preventive replacement; consider solid-state relay if duty cycle is high |
Functional Safety Integration
When SINAMICS Safety Integrated (STO, SS1, SS2, SLS, SOS) is used to reach PL d / SIL 2 or higher, the safety function lives inside the drive. The line contactor is not part of the safety chain for the drive pulses because EP already cuts them. However, the line contactor is still part of the Energy Isolation function required by ISO 13849-1 and IEC 60204-1 for maintenance. Many machine builders therefore use a safety contactor with mirror contacts and a separate STO channel that drops the contactor after a Cat-1 stop sequence.
Troubleshooting Matrix
| Symptom | Likely Root Cause | Where to Look |
|---|---|---|
| DC bus drops to 0 the moment OFF1 is issued | EP off-delay missing | X41 wiring and external timer |
| Line fuses clear at start | Contactor closes before precharge | X9 wiring; check p0860 = 1 |
| F07300 only on shutdown | Contactor opens before DC bus collapses below hold voltage | Increase off-delay |
| Drives trip on F30002 on first motion after power-up | Precharge incomplete when ON/OFF1 issued | Wait for r340.2 = 1 before releasing the drives |
| SLM regenerates, then F30021 trips | Contactor dropped while motor is in regen | Off-delay too short; SLM cannot pulse-block the diode path |
Verification Checklist
- After commissioning, capture an trace in Startdrive of r340 (Line Module status word) and r0863 (contactor control word) through one full start/stop cycle.
- Confirm the contactor closes only when r340.2 (precharge complete) transitions to 1.
- Confirm the contactor opens only after r340.6 (operation enabled) clears, the OFF1 ramp finishes, and the EP off-delay elapses.
- Verify the feedback contact bit matches the coil state within 50 ms. If it does not, F07300 is expected.
- Run the Safety Integrated acceptance test if STO or SS1 is in use; the line contactor must not be considered part of the STO chain.
FAQ
Is the main contactor in front of a SINAMICS S120 infeed module mandatory?
It depends on the Line Module type. For BLM and ALM the line contactor is optional per the Booksize Equipment Manual (6SL3097-2AC00-0BP3, Section 2.8) but strongly recommended for service isolation. For SLM it is effectively mandatory because the diode motoring bridge cannot be pulse-disabled, so only the line contactor can interrupt current safely.
Which terminals control and monitor the line contactor on a SINAMICS S120 Line Module?
The Line Module's X9 connector is used. X9:5 and X9:6 drive the close command to the contactor coil, and X9:3 and X9:4 read the auxiliary feedback contact. The firmware's "line contactor control" function sequences these signals based on precharge state and EP.
Why is an off-delay needed between EP and the line contactor?
EP removes the IGBT gate signals and the DC bus begins to discharge. If the line contactor drops at the same instant, the Line Module can draw residual current from the grid, blow fuses, or trip F30021. A 1–3 second off-delay (Booksize) lets the bus collapse under controlled conditions before the mains is disconnected.
When should the main contactor be energised during normal operation?
For most production machines, energise the contactor when the PLC enters RUN and no faults are active. De-energise it only after all drives complete OFF1, pulses are inhibited, and the EP off-delay expires. This avoids the bus-loss faults seen with on-demand operation while still powering the cabinet down when idle.
Does the line contactor need to be part of the safety chain for SINAMICS Safety Integrated?
No. Safety Integrated functions such as STO and SS1 already remove drive pulses through the EP path. The line contactor provides energy isolation for maintenance, which is a separate function under ISO 13849-1 and IEC 60204-1. Use a dedicated safety contactor if a category-1 stop is required.