Wiring Siemens 6SE7032-7EB87-2DA1 DBU X38 Fault Contact to PLC

Overview

The Siemens 6SE7032-7EB87-2DA1 is a dynamic braking unit (DBU) for the SIMOVERT MASTERDRIVES (6SE70) inverter family. In a Rubber Tyred Gantry (RTG) crane, the DBU absorbs regenerated energy from the hoist and travel drives whenever the motor acts as a generator — lowering a container, deceleration, plug-reverse, or a long down-slope travel. Without a path for that energy, the inverter's DC link rises until the drive trips on DC link over-voltage. The DBU closes an IGBT chopper across an external braking resistor when the DC bus exceeds a threshold, dissipating the energy as heat.

For crane duty, the DBU is a safety-relevant sub-system. The drive must shut down if the DBU itself is unhealthy; otherwise the next regeneration event will trip the inverter on over-voltage and drop the load. The standard practice is to wire the DBU's volt-free fault contact to a digital input on the crane PLC and interlock the drive enable (and ideally the Safe Torque Off, STO) on that input. This article documents the X38 connector on the 6SE7032-7EB87-2DA1, the behaviour of the fault contact, the disable input, the on-board voltage selection switch, and the procedure to integrate the contact with a PLC.

Module Identification

The MLFB 6SE7032-7EB87-2DA1 decodes within the MASTERDRIVES naming convention:

• 6SE70 — SIMOVERT MASTERDRIVES family.
• 32 — 400 V class, three-phase, frame / current grouping.
• 7EB87 — Function block: dynamic braking unit (chopper module).
• 2DA1 — Order-code suffix (hardware revision / option set).

Always confirm the MLFB on the nameplate before commissioning. The X38 pinout, the contact logic, and the voltage selection table (Table 5.1 in the DBU operating instructions) are keyed to the MLFB suffix. Cross-check against the matching MASTERDRIVES Compendium and DBU operating instructions on the Siemens Industry Online Support portal.

Why a Braking Unit and Not Just a Resistor

A braking resistor alone is a passive load. A braking unit is the IGBT switch that decides when the resistor is connected to the DC link. As explained in Eaton's knowledge base on braking units vs braking resistors, the chopper is the controller and the resistor is the dissipative element; they must be rated as a pair. For an RTG hoist, the duty is heavy (many cycles per shift, high regen energy) so both ratings matter. Cross-vendor references such as the Rockwell AK-DBU 600/690 VAC manual (AKDBU-UM001) and the AutomationDirect braking units and resistors catalog are useful for comparison, but the X38 pinout below applies to the Siemens 6SE7032-7EB87-2DA1 specifically. For a general background on braking unit topologies see the braking unit overview.

X38 Connector Pinout

X38 is the low-voltage control connector on the DBU. The terminals relevant to PLC integration are listed below; refer to the operating instructions for the full pin map of the -2DA1 variant.

For a single-channel PLC interlock, use terminals 3 and 4 as a normally-closed dry contact. For a dual-channel safety input (e.g. SIMATIC F-DI on an F-CPU), use both poles 3-4 and 4-5 in a two-channel configuration and check the application standard such as EN 61800-5-2 for drive safety functions.

Fault Contact Behaviour and the Ready LED

The fault contact on X38 is not a static "device is healthy" output. It is conditional on two things:

1. The DBU control electronics are powered and healthy — the green Ready LED on the front of the unit is on.
2. The drive's DC link is energised. The contact only pulls in (closes) after DC bus voltage has been applied at the drive end.

If you measure terminals 3-4 with a multimeter while the DBU is sitting on the shelf (no DC bus), the contact will read open. This is normal — the relay is held off until the chopper electronics see a live DC link. To verify the contact end-to-end, you must power the drive, wait for the DC bus to come up, and then measure.

The Ready LED is the operator's local indication that the control board is alive. It does not by itself confirm that the fault contact is closed; the two are related but not identical. Always confirm the contact electrically before trusting the interlock, and record both states in the commissioning log.

Voltage Selection Switch (Table 5.1)

Inside the DBU, a small selector switch sets the threshold and the logic convention for the "no fault" indication. This is the switch referenced as Table 5.1 in the operating instructions. The default position is matched to the DC bus voltage class of the drive (e.g. 400 V class vs 690 V class) and to whether the NC contact is interpreted as "energise to trip" or "energise to run".

If the threshold is set to the wrong DC bus class, the contact can indicate "fault" on a healthy DBU and lock the drive out. If the contact logic is inverted relative to what the PLC expects, the drive may run with a faulted DBU. Procedure:

1. Lock and tag the drive. Discharge the DC bus and verify with a meter on the drive's DC test points.
2. Open the DBU cover and locate the selector switch on the control PCB (refer to the figure in section 2.3 of the operating instructions).
3. Compare the switch position to Table 5.1 for MLFB 6SE7032-7EB87-2DA1.
4. Set the switch to the position matching your drive's DC bus class and the PLC's "healthy = closed" convention.
5. Close the cover, re-apply control power, and verify with the contact test in the commissioning section below.

Wiring the Fault Output to a PLC

Use the NC contact on terminals 3-4 as a volt-free contact. Wire it to a 24 V DC digital input on the crane PLC. For a SIMATIC-based crane, a standard SM 1221 DI works for a non-safety trip; for a SIL 2 / PL d hoist, use an F-DI module (e.g. SM 1226 / SM 1223F on an S7-1200F or S7-1500F CPU) and route the drive's STO through the safety evaluation. A typical wiring example:

• PLC DI common (24 V return) → terminal 3 of X38.
• PLC DI input (24 V sourcing) → terminal 4 of X38.
• 24 V supply rail referenced to the DI common → terminal 3 (or 4, depending on sink/source convention).

With the DBU healthy and the DC bus up, the contact closes and the DI reads "1" (DBU OK). On fault, the contact opens and the DI reads "0" (DBU fault). Ladder-logic example in a generic representation:

|--[ I_DBUnit_OK ]--[ NOT Drive_Run_Cmd ]--( M_DBUnit_Fault_latch )--|
|                                                                              |
|--[ I_DBUnit_OK ]------------------------------( Drive_Enable_OK )----------|
|                                                                              |
|--[ M_DBUnit_Fault_latch ]--------------------( Drive_Trip )------------------|

The first rung latches a fault if the contact opens while the drive is running. The second rung gates the drive enable: the drive is only allowed to start when the DBU contact is closed. The third rung drops the drive run command when the latch is set. For safety-class wiring, replace the third rung with a drop on the F-CPU's STO output, not just the run command.

Disabling the DBU via X38-1/2

The disable input is intended for service work and for cascade / interlocking logic — for example, a master controller that prevents two braking units on the same DC bus from fighting each other, or coordination with a regenerative active front end (AFE). Applying a control voltage across 1-2 inhibits the chopper, so the resistor is never connected. The drive's own DC bus over-voltage protection must still be operational in that state, or the drive will trip on the first regen event.

For an RTG retrofit where the DBU has just been replaced, the disable input is normally left open (floating). Do not strap any voltage to 1-2 during commissioning. If the new DBU sits on a shared DC bus with an AFE, wire the disable input to a PLC output that holds the DBU off while the AFE is in regen and releases it only on AFE loss-of-feedback.

PCB Relay Inspection and Bench Test

If the contact still does not close after the DC bus is up and the Ready LED is on, the fault is on the DBU side. Two checks are useful before declaring the unit failed:

1. Dry-joint / solder inspection. With the DBU de-energised and the bus discharged, remove the cover and visually inspect the relay pins and the X38 connector solder joints. Cold joints on the relay are a known failure mode on field-aged units.
2. Direct coil drive. Apply the rated coil voltage (typically 24 V DC; refer to the operating instructions) directly to the relay coil on the PCB. Listen for the click and measure the contact: it should change over. If the relay clicks but the contact is intermittent, replace the relay. If it does not click with the rated voltage applied, the PCB driver stage is faulty and the board needs repair or replacement.

After a relay or PCB repair, repeat the end-to-end contact test with the drive's DC bus up and the disable input floating.

Troubleshooting Matrix

Commissioning and Verification Procedure

1. Lock and tag the drive. Confirm the DC bus is discharged by measuring at the drive's DC test points; follow the DBU and inverter lock-out procedure in the operating instructions.
2. Open the DBU. Verify the MLFB matches 6SE7032-7EB87-2DA1 and that the voltage selection switch is in the correct position for your DC bus class per Table 5.1.
3. Inspect X38 for bent pins, broken wires, and proper crimp quality. Confirm the disable input (1-2) is floating.
4. Confirm the 24 V control supply to the DBU is present. The Ready LED should be on.
5. With the drive still locked out, measure the resistance of the X38 contact (3-4) with a multimeter on continuity. Expect open at this stage because there is no DC bus.
6. Remove the lock-out. Start the drive in a no-load condition. Confirm the DC bus rises to nominal — typically around 540-620 V DC for a 400 V class drive, around 890-1080 V DC for a 690 V class drive. Verify the exact nominal against the drive's nameplate, not the rule of thumb.
7. Re-measure the X38 contact. Expect continuity across 3-4. Verify the PLC DI reads "DBU OK".
8. Force a regeneration event — for example, command a controlled lower on the hoist with a small load, or run the long-travel down a slope. Watch the DC bus on the drive's parameter view. The bus should rise to the chopper threshold and clamp, with the DBU LED chain indicating active braking.
9. Trip the DBU intentionally (apply a voltage to the disable input 1-2, or open the resistor circuit at the DBU terminal) and confirm the PLC trips the drive within the safety reaction time.
10. Restore normal wiring and log the contact resistance, the chopper threshold, the drive's nominal DC bus, and the PLC DI state for the commissioning record.

Field Notes for RTG and Crane Duty

• The DBU in a crane sees thermal cycling. During scheduled maintenance, check the resistor's insulation resistance and the chopper's heatsink fins; an open resistor or a choked heatsink will leave you with no braking even if the fault contact is healthy.
• For a retrofit where a non-Siemens DBU is being replaced with the 6SE7032-7EB87-2DA1, the X38 pinout and contact logic are not guaranteed to match the previous unit. Re-wire to the Siemens pin map and re-validate the PLC logic — do not reuse the old cable pin-out.
• If the RTG uses regen-onto-mains (active front end) as well as a DBU, the disable input on the DBU is typically held off by the PLC while the AFE is in regen mode, and released only on AFE loss. This prevents both paths from fighting on the same DC bus.
• Use twisted, shielded cable for the X38 run. The fault contact is low-current; long parallel runs with VFD output cables will inject noise that can fool the PLC input.
• For hoists under SIL 2 / PL d, treat the DBU as a safety-relevant sub-system. The fault contact should be wired to a safety DI and the drive's STO should be released through that DI's safety evaluation, not just the run command.
• Keep a spare DBU on site. Field experience shows that once a chopper module starts to fail thermally, the next failure is usually within a few hundred operating hours, and a crane down on a working shift costs more than a spare.