bb fault in l&t drive how to reolve it

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Is your L&T drive flashing "bb"? Learn what the Base Block signal means, why it inhibits your motor, and how to troubleshoot the wiring and parameters.

If you’re working with an L&T Sx2000 or Cx2000 drive and the display is flashing "bb," you might think you’re looking at a fault code. However, in the L&T/Yaskawa ecosystem, "bb" isn't technically a "hard fault"—it is a status indicator for Base Block.

In simple terms, "bb" means the drive has cut off the output to the motor. The internal transistors (IGBTs) are turned off, and the motor is allowed to coast to a stop. The drive is essentially in a "safety pause" and is waiting for a signal before it will allow the motor to restart. Let’s find out why your drive is stuck in this mode.

Common Causes of the "bb" Signal

When an L&T drive shows "bb," it is usually responding to an external command or a missing safety link. Here are the most likely culprits:

  • External Base Block Signal: One of the digital inputs (S1-S6) is programmed to act as an "External Base Block" and that circuit is currently open.
  • Safety Circuit (STO) Interruption: On models with Safe Torque Off (STO), the H1 or H2 terminals are not receiving a 24V signal.
  • PLC Logic: Your controller or PLC is sending a command to the drive to "Pause" or "Inhibit" the output.
  • Run Command Conflict: You are trying to send a start command while the Base Block signal is still active.

Step-by-Step Troubleshooting for "bb"

1. Check the Digital Input Terminals

Most L&T drives have multi-function inputs. If an input is set to "External Base Block" (usually setting 8 or 9 in the parameters), the drive will show "bb" whenever that terminal loses its 24V signal.

  • Check the wires on terminals S1 through S6.
  • Perform a "tug test" to ensure no wires have vibrated loose.
  • Use a multimeter to see if 24V DC is present on the input that is supposed to be active.

2. Inspect the H1/H2 Safety Jumpers

If your drive has a safety terminal block (often labeled H1, H2, and HC):

  • These are for the Safe Torque Off circuit. For the drive to run, H1 and H2 must be connected to HC.
  • If you are not using an external safety relay, ensure the factory-installed jumpers are tight. If a jumper is missing, the drive will stay in "bb" mode forever.

3. Review Parameter Group H1

You need to see if a terminal has been accidentally programmed to trigger the Base Block.

  • Go to Parameters H1-01 through H1-06. These control the functions of the digital inputs.
  • Look for any parameter set to "8" (Base Block N.O.) or "9" (Base Block N.C.).
  • If you don't need the Base Block function, you can change that parameter to "F" (Not Used) to clear the "bb" signal.

4. Check the Sequence of Operation

Does "bb" only appear when you hit the "Stop" button or when a specific part of the machine moves?
Some engineers program "bb" to act as a coasting stop. If your machine is designed to coast to a stop rather than ramp down, "bb" appearing after a stop command is perfectly normal and simply means the drive is ready for the next start.

How to Clear the "bb" Signal

  1. Close the external circuit providing the Base Block signal (ensure 24V is at the terminal).
  2. If the drive is in "bb" due to a safety interruption, reset your Safety Relay or E-Stop.
  3. The Reset Button: Usually, you do not need to press "Reset" for a "bb" signal. Once the signal is restored, the "bb" will disappear, and the drive will return to "Ready" or "Stopped" mode.

Summary

The L&T drive "bb" fault is essentially a "Digital Padlock." It stops the drive's output until it is told it is safe to proceed. Check your Group H1 parameters and your H1/H2 safety jumpers first. In 90% of cases, a loose wire on a digital input is the reason your drive is stuck.

 

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