Why Is Power Supply Redundancy Important for ABB PLC Units?
2026-10-19
An electrical maintenance supervisor reviews a recent incident. A voltage dip on the plant's 24V DC bus caused the power supply module of an ABB PLC to fail. The PLC lost power, and the production line stopped. The restart took 6 hours because the process had to be purged, the temperatures had to be re-stabilized, and the quality team had to scrap the material in the line. The total cost was $45,000. The supervisor notes that the PLC CPU was redundant, the I/O was redundant, and the communication was redundant. But the power supply was a single module. The redundancy strategy had a gap. This guide explains why power supply redundancy is essential for ABB PLC units and how to design it correctly.
1. What Are the Failure Modes of a PLC Power Supply?
The power supply is the component that converts the plant's AC power to the 24V DC required by the ABB PLC. It is subjected to continuous load, thermal stress, and electrical transients. The failure modes include electrolytic capacitor degradation, output voltage drift, overvoltage damage from lightning or switching transients, and fan failure. The table below shows the failure modes and their typical causes.
| Failure mode | Typical cause | Effect on PLC | Mean time to failure |
| Capacitor degradation | Thermal aging; ripple current | Output voltage ripple; eventual shutdown | 5 – 8 years |
| Output voltage drift | Reference drift; load variation | PLC may reset or behave erratically | 3 – 6 years |
| Overvoltage damage | Lightning; switching transients | Immediate failure | Random |
| Fan failure | Bearing wear; dust accumulation | Overheating; thermal shutdown | 2 – 4 years |
| Fuse blown | Short circuit; overload | Loss of output | Random |
In our factory, we supply ABB PLC units with power supply modules that are rated for 10 years of continuous operation at 40°C. However, the actual life depends on the operating conditions. A power supply that is mounted in a cabinet with poor ventilation may fail in 3 to 5 years. A power supply that is subjected to frequent voltage transients may fail without warning. The failure of a single power supply module is not a rare event. It is a predictable event that can be planned for. Yueyang Tongtu E-commerce Co., Ltd. supplies ABB PLC power supply modules and redundancy kits for critical applications.
2. How Does a Redundant Power Supply Configuration Work?
A redundant power supply configuration uses two identical power supply modules connected in parallel through a decoupling diode or a redundancy module. Both modules share the load equally, or one module carries the load while the other is on standby. If one module fails, the other module continues to supply the load without interruption. The table below compares the three redundancy configurations.
| Configuration | Load sharing | Switchover time | Cost | Typical application |
| Single power supply | N/A | N/A (failure stops PLC) | Lowest | Non-critical processes |
| Parallel redundant (active/active) | Both modules share load | Instantaneous (diode decoupling) | Moderate | Continuous production |
| Standby redundant (active/standby) | One module carries load | 10 – 50 ms (relay switchover) | Moderate to high | Critical processes |
The parallel redundant configuration is the most common for ABB PLC units. Both power supplies are adjusted to the same output voltage, and the decoupling diodes prevent the failed module from draining the good module. The switchover is instantaneous because the good module is already supplying the load. In our factory, we recommend the parallel redundant configuration for continuous production lines. Yueyang Tongtu E-commerce Co., Ltd. provides redundant power supply kits that include two power modules, a redundancy module, and the necessary wiring.
3. How Should the Redundancy Capacity Be Calculated?
The capacity of each power supply in a redundant configuration must be sufficient to carry the full load of the ABB PLC and its I/O modules. This is different from a non-redundant configuration, where the power supply is sized for the load plus a margin. In a redundant configuration, each module must be sized for 100 percent of the load, plus a margin for future expansion. The table below shows the load calculation for a typical ABB PLC system.
| Load component | Power consumption (W) | Quantity | Total power (W) |
| CPU module (PM583-ETH) | 5.5 | 1 | 5.5 |
| Communication module (CM579-PNIO) | 3.0 | 2 | 6.0 |
| Digital input module (DI524) | 2.5 | 4 | 10.0 |
| Digital output module (DO524) | 3.5 | 4 | 14.0 |
| Analog input module (AI523) | 4.0 | 2 | 8.0 |
| Analog output module (AO523) | 4.5 | 1 | 4.5 |
| Total load | N/A | N/A | 48.0 |
| Redundancy requirement (each module) | N/A | N/A | ≥ 48 W (100% load) |
| Recommended module rating | N/A | N/A | 72 W (150% margin) |
Electrical maintenance note: In a redundant configuration, each power supply must be rated for the full load plus a 50 percent margin. For the example above, each module should be rated for at least 72 W. A 24V/3A module provides 72 W, which is the minimum. We recommend a 24V/5A module (120 W) to allow for future expansion and to ensure that the modules operate below their maximum rating, which extends their life. In our factory, we size the power supplies for 50 percent load sharing in normal operation. This provides a 100 percent margin if one module fails.
4. How Does Battery Backup Complement Power Supply Redundancy?
Power supply redundancy protects against the failure of a power supply module. It does not protect against a failure of the AC power source. If the plant loses AC power, both redundant power supplies will lose input, and the PLC will shut down. To protect against AC power loss, a battery backup or an uninterruptible power supply (UPS) is required. The table below shows the protection provided by different configurations.
| Configuration | Protects against power supply failure | Protects against AC power loss | Protects against voltage sag |
| Single power supply | No | No | No |
| Redundant power supplies | Yes | No | Partial (if modules have holdup time) |
| Redundant power supplies + UPS | Yes | Yes | Yes |
| Redundant power supplies + battery module | Yes | Yes (for PLC only) | Yes |
In our factory, we recommend a two-layer protection strategy for critical production lines. The first layer is redundant power supplies, which protect against module failure. The second layer is a UPS or a battery module, which protects against AC power loss and voltage sags. The UPS should be sized to provide at least 30 minutes of backup power to allow the PLC to perform a controlled shutdown. Yueyang Tongtu E-commerce Co., Ltd. supplies both redundant power supplies and battery modules for ABB PLC systems.
Frequently Asked Questions About Power Supply Redundancy for ABB PLC Units
Summary for Electrical Maintenance Supervisors
Power supply redundancy is important for ABB PLC units because the power supply is a single point of failure that can stop the entire production line. A redundant configuration uses two identical power supply modules connected in parallel through decoupling diodes. Each module must be sized for 100 percent of the load plus a 50 percent margin. The redundancy protects against module failure but not against AC power loss, so a UPS or battery module is also required for critical processes. By implementing a two-layer protection strategy, maintenance supervisors can eliminate power-related downtime and protect the production line from both internal and external electrical disturbances. Yueyang Tongtu E-commerce Co., Ltd. supplies ABB PLC power supply modules and redundancy kits for continuous production applications.
Yueyang Tongtu E-commerce Co., Ltd. supplies ABB PLC power supply modules, redundancy modules, and battery backup units. We provide capacity calculations, test procedures, and installation support for all of our products.
