ABB SAFT120PAC Power Supply Module Installation and Configuration Guide - Excellent PLC

Installing the ABB SAFT120PAC Power Supply Module can seem straightforward, but achieving a stable and noise-free DC supply for control systems requires attention to detail. I’ve installed and commissioned this module many times, both in S800 I/O racks and standalone applications, and here’s the complete step-by-step process that always delivers reliable results.

Step 1: Safety and Pre-Installation Checks

Before beginning, always disconnect the cabinet’s main DC power source. Even though the SAFT120PAC is designed for robustness, live insertion can cause contact arcing or board damage.

Then verify the following:

Input voltage rating: 24 VDC ±10%.
Ambient temperature: 0–55°C, humidity <95% non-condensing.
Proper grounding of DIN rail (continuity <0.1 Ω to PE).
Use an anti-static wrist strap and avoid touching circuit components directly.

Once verified, check the model label to ensure the correct type — “SAFT120PAC R1” or higher revision — is being installed for your DCS version.

Step 2: Mounting the Module

The SAFT120PAC mounts on a 35 mm DIN rail.
Here’s how I do it to avoid mechanical stress:

Hook the bottom edge of the module onto the rail.
Press the upper side until you hear a click.
Gently pull forward to confirm it’s locked.
Maintain at least 25 mm clearance on top and bottom for heat dissipation.

Never stack it directly above high-heat devices such as contactors or relays. ABB modules rely on convection cooling, and insufficient spacing can shorten lifespan.

Step 3: Power Input Wiring

The input terminals are located at the bottom and typically labeled:

Terminal	Function	Wire size	Note
1 (+24V)	Positive input	2.5 mm² Cu	Fused, polarity-protected
2 (0V)	Negative input	2.5 mm² Cu	Common return
PE	Protective earth	2.5 mm² Cu	Connect first

Wiring tips:

Use ferrule-crimped wire ends for a secure connection.
Tighten terminal screws to 0.5 Nm.
Avoid routing input power lines in the same duct as signal cables.

After wiring, measure the input voltage with a multimeter — it should read between 23.5 V and 25 V DC.

Step 4: Output Connection

The SAFT120PAC provides a regulated 24 VDC output to downstream devices or I/O modules.
Typical connection layout:

Output Terminal	Description
OUT+	Regulated +24V
OUT–	0V reference
PG	Protective ground

When connecting multiple loads, distribute power using separate terminal blocks rather than daisy-chaining wires directly from the module — this prevents uneven voltage drops.

Step 5: Initial Power-Up and LED Check

After confirming polarity and connections, apply DC power.
Observe the front LED indicators:

Green (PWR) – Normal operation
Yellow (WARN) – Undervoltage or overload
Red (FAULT) – Output short circuit or internal error

If the red LED appears immediately, disconnect the output load and measure terminal voltage.
If output returns to 24 V, the issue lies in the external wiring or load.

Step 6: System Verification with Code Example

Once powered, it’s a good practice to monitor the SAFT120PAC’s health through your control system.
Below is a simple IEC 61131-3 Structured Text (ST) example I often use in ABB AC 800M or Control Builder M systems to verify stable voltage supply:

(* SAFT120PAC Voltage Monitoring Logic *)

VAR

V_in : REAL;           (* Measured input voltage *)

V_out : REAL;          (* Measured output voltage *)

PowerStatus : BOOL;    (* TRUE = normal operation *)

END_VAR
(* Simulated analog input channels *)

V_in := AI_SAFT120PAC_Input();    (* Function block reading input voltage *)

V_out := AI_SAFT120PAC_Output();  (* Function block reading output voltage *)
IF (V_in > 22.0) AND (V_out > 23.0) THEN

PowerStatus := TRUE;          (* Power OK *)

ELSE

PowerStatus := FALSE;         (* Trigger alarm *)

RAISE_ALARM('SAFT120PAC_Voltage_Low');

END_IF;

This simple logic checks both input and output levels and raises an alarm if voltage drops below safe thresholds.
You can modify it to log data or trigger redundancy switching.

Step 7: Common Installation Mistakes to Avoid

Over the years, I’ve seen engineers make the same few mistakes:

Polarity reversal — connecting +24 V and 0 V incorrectly, blowing fuses.
Ground omission — skipping PE causes signal noise and module resets.
Parallel output linking without diode isolation in redundant systems.
Mounting too close to hot components, leading to thermal trips.

Avoiding these simple issues ensures stable operation and extends module life.

Step 8: Post-Installation Maintenance

After installation, I always recommend:

Recording module serial number and firmware version.
Checking temperature after 30 minutes (should be <45 °C).
Verifying system diagnostic logs show “Power Supply OK.”
Testing redundancy (if two SAFT120PAC units are used).

Keeping this data helps future troubleshooting and replacement planning.

Final Thoughts

The ABB SAFT120PAC Power Supply Module is reliable and robust when installed correctly.
A careful approach — from grounding to configuration verification — prevents 90% of startup issues.
Adding a small monitoring script like the one above makes maintenance predictive rather than reactive.

In industrial automation, the power module is the foundation — if that’s stable, everything else follows.