Specifying the ABB TU846 termination unit is a best practice for building maintainable control systems; its correct integration is key to realizing its benefits. This guide provides a practical, step-by-step overview for panel builders, electricians, and system integrators, covering the key considerations for selection, wiring, and commissioning of the TU846 within an ABB S800 I/O system.
Warning: All installation and wiring must be performed by qualified personnel following local electrical safety regulations. Always de-energize power before beginning work.
The TU846 is designed for specific S800 I/O modules. This is the most critical step.
Module Type: You must select a TU846 variant that is compatible with your specific I/O module. For example, you need a TU846 version designed for the AI810 analog input module or the DI810 digital input module. The pinout and function of the terminal blocks are specific to the module type.
Reference the Datasheet: Always consult the ABB S800 I/O manual or datasheet for the specific I/O module. It will list the correct, compatible termination unit (e.g., "Termination Unit: TU846").
Order the Interconnecting Cable: The cable that links the TU846 to the I/O module is often sold separately. Ensure you order the correct cable part number for the specific module/TU846 combination.
Plan the Cabinet Layout: Plan the DIN rail layout so that the TU846 can be mounted directly adjacent to its corresponding I/O module. This keeps the interconnecting cable short and organized.
Mount the TU846: Snap the TU846 onto the DIN rail next to the planned location of the I/O module.
Mount the S800 Baseplate: Install the S800 baseplate on the DIN rail.
This is where the field device wires are permanently connected.
Label All Wires: Before landing wires, label each one clearly with the loop number or tag name (e.g., "PT-101 Signal", "PT-101 Shield"). This is invaluable for future maintenance.
Land the Wires: Connect the field wires to the appropriate terminals on the TU846 according to your wiring diagram. For analog signals (e.g., 4-20 mA):
Connect the signal wire (+) to the correct terminal.
Connect the return/common wire (-) to the common terminal.
Connect the cable shield to the designated grounding terminal on the TU846 (usually one ground per group of channels).
Follow Best Practices: Use ferrules on stranded wires for secure connections. Ensure all terminals are tightened to the specified torque.
Insert the I/O Module: Carefully insert the S800 I/O module (e.g., AI810) into its slot on the baseplate.
Connect the Interconnecting Cable: Plug one end of the pre-fabricated cable into the front connector of the I/O module. Plug the other end into the corresponding connector on the TU846. This cable carries the signals from the terminal unit to the module.
Visual Inspection: Double-check that all field wires are landed on the correct terminals and that the interconnecting cable is securely plugged in at both ends.
Apply Power: Energize the S800 system.
Loop Checking at the TU846: The TU846's terminals provide the perfect test point for loop checks.
For an Input (AI) Loop: Disconnect the field wires at the TU846. Use a precision current calibrator to inject a 4-20 mA signal directly into the TU846 terminals. Verify the correct value appears in the control system.
For an Output (AO) Loop: Command a 50% output from the control system. Use a multimeter to measure the mA signal at the TU846 terminals. It should read approximately 12 mA.
Continuity Test: You can also use the TU846 to check continuity of the field wiring back to the sensor without involving the I/O module.
Documentation: Keep an up-to-date loop diagram that clearly shows the connection from the field device to the TU846 terminal number and the corresponding I/O module channel.
Spare Parts Strategy: For critical systems, maintain a stock of spare I/O modules and interconnecting cables. The TU846 makes using these spares a quick and simple operation.
Common Issues:
No Signal: Check the interconnecting cable is fully seated at both ends. Verify the I/O module is properly inserted into the baseplate.
Incorrect Signal: Verify the field wiring on the TU846 matches the diagram. Check that the TU846 model is correct for the I/O module type.
Noisy Signal: Ensure the field cable shield is properly grounded at the TU846 and that the shield is not grounded at the field device end (to avoid ground loops).
Integrating the ABB TU846 termination unit is a straightforward process that yields significant long-term benefits in maintainability and reliability. By carefully selecting the compatible unit, following disciplined wiring practices, and utilizing the terminal unit for testing, panel builders and integrators can construct control systems that are not only robust but also incredibly efficient to maintain over their entire lifecycle. This approach ultimately protects the end-user's operational uptime and reduces the total cost of ownership.
