1746-OX8 AB Output Module 1746-0X8 1746OX8

Allen-Bradley 1746-OX8 | SLC 500 Isolated Relay Output Module — 8-Point, 5A, 265VAC / 125VDC

Part Number: 1746-OX8

Manufacturer: Rockwell Automation / Allen-Bradley (USA)

Product Type: Digital Output Module — Isolated Relay Outputs

Platform: SLC 500

Output Points: 8 

Isolated Relay Outputs for the SLC 500

The 1746-OX8 is the isolated relay output module for the Allen-Bradley SLC 500 PLC family. It provides eight individually isolated relay output points — each a normally open contact rated at 5A continuous, suitable for switching loads up to 265V AC or 125V DC.

The "isolated" designation is the key differentiator between the 1746-OX8 and standard SLC 500 relay output modules. In a standard relay output module, output points typically share a common reference. In the 1746-OX8, each relay contact is isolated from all others — there is no common reference across points. This isolation allows different output points to switch loads at completely different voltages, different phases, or different power sources from a single module. An output controlling a 24V DC valve can sit alongside an output switching a 120V AC contactor coil on the same module without any electrical coupling between them.

This capability is essential in machine control wiring where diverse load types — contactors, solenoids, pilot lights, signal relays — operate at different voltages and must be controlled from a compact PLC I/O assembly.

Electrical Ratings in Context

The 5A / 265V AC rating makes the 1746-OX8 directly suitable for switching three-phase motor starter contactor coils (typically 120V AC or 240V AC), full-voltage motor starter auxiliary contacts, and pilot devices on standard North American or European industrial power systems. The 125V DC rating extends the same capability to DC control circuits — common in older plant equipment, switchgear, and HVAC systems.

Key Specifications

FAQ

Q1: One relay output on the 1746-OX8 is failing to energise its load even though the SLC program shows the output bit as ON. What should be checked?

With the output bit confirmed ON in the processor, check the relay's driven state by measuring voltage across the output terminals with the load disconnected. If voltage appears (contact is open when it should be closed), the relay coil circuit inside the module is energising but the contact has failed — common after high inrush loads or overloads. If no voltage from the output driver, verify the module is correctly seated and the slot address in the program matches the physical slot.

Q2: The 1746-OX8 is discontinued. Can the 1746-OW16 (16-point relay output) substitute?

The 1746-OW16 provides 16 relay outputs but uses a common return arrangement — outputs share a common terminal rather than being fully isolated. If the application relies on the 1746-OX8's per-point isolation to switch loads at different voltages from the same module, the OW16 cannot substitute directly. Confirm the application's isolation requirements before selecting an alternative.

Q3: Several output relays on the 1746-OX8 are welded closed (contacts stuck). What caused this?

Welded contacts result from excessive inrush current at switching — typically from motor loads without surge suppression, or from capacitive loads that draw very high peak current at turn-on. The 5A continuous rating applies to resistive loads; inductive or motor loads at startup can momentarily draw 5–10× their run current. Install surge suppressors on inductive loads and review the load current profile against the module's rated make/break current.

Q4: The SLC 500 rack with the 1746-OX8 is in a high-vibration environment. Are there any precautions?

The SLC 500 chassis should be secured to the panel with all mounting hardware torqued to specification. Verify the I/O module locking tabs are engaged. In high-vibration environments, relay contacts can develop intermittent connections due to connector movement — check all field wiring terminal connections are correctly torqued, particularly on output circuits carrying load current.

Q5: Where is the discontinued 1746-OX8 sourced?

Specialist Rockwell Automation / SLC 500 spare parts dealers and industrial PLC aftermarket suppliers hold exchange stock. Confirm the 1746-OX8 designation specifically — adjacent relay output modules (OW16, OA8, OA16) have different contact configurations. Functional test on a SLC 500 rack before acceptance is advisable for refurbished units.