Uninterrupted DCS Power: We Engineer 0 ms Dual DC Transfer at ODES

TonyZhang
Dec 7, 2025
6 min read

When “One Blip” Is Enough to Stop a Plant

In the DCS process control layer, a single power interruption can escalate from a brief disturbance to a full unit shutdown. I/O modules reset, controllers reboot, communication links re-establish, and what should have been a manageable disturbance becomes a production event.

Most plants already specify dual supplies for the DCS: two independent DC sources, often backed by redundant rectifiers and batteries. The real challenge is how to transfer between those DC sources quickly enough and cleanly enough that the process layer does not see a power dip at all.

ODES has implemented this requirement as a dedicated dual DC automatic transfer unit for the DCS process layer. Installed between two independent DC buses and the segmented DCS load bus, it combines high-speed transfer, short-term energy support, and detailed alarm signalling toward the DCS or PLC. For a broader view of ODES architectures across secondary systems and control power, visit www.odes-electric.com.

System Architecture: Dual DC, Intelligent Transfer, Segmented Bus

A typical DCS process layer power scheme can be summarized as:

Within this architecture, the automatic transfer unit is responsible for:

Supervising both DC inputs (typically 110/220 V DC, extendable to 24/48 V DC).
Executing automatic transfer based on clearly defined undervoltage thresholds and recovery conditions.
Maintaining output voltage during transfer by means of internal energy storage and regulation.
Providing granular alarm contacts to distinguish between:

These dry contacts are wired into the DCS/SCADA point list with different priority levels so that operators can see not only that a disturbance has occurred, but which side of the power system caused it and how the transfer device responded.

On the DC side, key baseline characteristics include:

Rated input: 110/220 V DC (±15 %)
Transfer threshold: typically 75–80 % of rated voltage
Transfer mode: automatic main/backup with automatic return
Transfer time: 0 ms (output continuity)
Output hold-up during transfer: approximately 90–95 % of rated voltage
Rated transfer current: 16 A, with 25 A short-term capability
Short-circuit withstand: 600 A for 10 ms (half cycle equivalent)
Breaking capacity: 1 000 A, 25 operations (resistive load)
Environmental performance: −20…+55 °C, 5–95 % RH, enclosure ≥ IP40
EMC: EFT/ESD/surge immunity up to level 4, ±4 kV

This combination allows the unit to act as a controlled “front end” to the entire DCS process bus, rather than distributing dual DC and transfer logic inconsistently inside each cabinet.

Cutting Fast: 0 ms Transfer with Output Voltage Support

Ordinary auxiliary relays and simple “break-before-make” schemes are often too slow or too coarse for the DCS process layer. The most critical failure mode is a transient DC drop that forces I/O boards or communication modules to restart, even if the station batteries themselves remain healthy.

The ODES dual-source transfer unit is designed to detect a genuine DC undervoltage event and switch sources while keeping the output within the acceptable range for DCS hardware.

Key design points include:

Defined undervoltage threshold: When the source DC voltage falls below about 75–80 % of nominal, the unit recognizes that the source can no longer support the load reliably.
0 ms transfer behaviour: Internal energy storage and regulation are used so that, during transfer, the output does not drop below approximately 90–95 % of nominal, avoiding reset of CPUs and remote I/O.
Stability against minor fluctuations: The thresholds and delay elements are chosen to ride through normal ripple and brief dips, preventing nuisance transfer or oscillation between sources.

From the viewpoint of the DCS, the transfer is effectively invisible: controller power supplies see a small transient within their inherent ride-through capability, while event records and communication links continue without interruption. Compared to conventional relay logic that interrupts and then re-applies DC, this approach aligns much better with modern high-density I/O, remote I/O stations, and Ethernet-based field networks.

For AC-sensitive loads such as servers, engineering workstations, or storage systems located in the same control room, the DC dual-source transfer unit can be complemented with a Static Transfer Switch (STS) on the AC side. A well-specified STS provides 5 ms-class AC transfer (synchronous or asynchronous) at 220/380 V, ensuring that IT and networking equipment experiences continuous supply even during upstream changeover.

Carrying Heavy Loads: Cabinet Clusters, Inrush, and Fault Levels

In the process layer, the transfer device typically does not feed a single cabinet, but a cluster of DCS and I/O cabinets on a common DC bus. The aggregate load exhibits a substantial capacitive inrush when the bus is energized or when multiple cabinets are switched in simultaneously.

To design the dual-source transfer correctly, engineers must consider:

The combined steady-state current of all I/O racks, remote stations, communication switches, and gateways.
The inrush factor associated with capacitors and input filters on each device.
The possible contribution from motor-related circuits and interlocks that share the same DC bus.

The 16 A rated / 25 A short-term transfer capability of a single unit provides a conservative ceiling for one segment of the process bus. Where necessary, segmentation or parallel architectures can be used to distribute loads, while still maintaining deterministic transfer behaviour.

Short-circuit robustness is equally important. A DC fault on one cabinet or feeder should not destroy the transfer unit or propagate upstream in an uncontrolled way. The capability to withstand 600 A for 10 ms and to break 1 000 A resistive over multiple operations is a key parameter in assessing:

The unit’s ability to survive downstream faults until protective devices operate.
The selectivity of the protection and the resilience of the overall DC system.

Where the process layer interacts with high-power AC feeders—for example, large fans, circulating pumps, or drives that are interlocked with the DCS—ODES recommends applying PC-class dual-source changeover devices on the AC side. These can handle 30–5 000 A at up to 660 V AC with high short-circuit ratings, and integrate mechanical/electrical interlocking with appropriate breaking capacity. The DC transfer unit then focuses on the control and I/O loads, while the AC changeover handles the heavy motor circuits.

Making It Operable: Alarms, Settings, and Annual Testing

A dual-source transfer scheme only delivers its full value when it is embedded into operations and maintenance practices. From an engineering and O&M point of view, several elements are essential:

1. Structured Alarm Reporting

The transfer unit should provide at least the following dry contacts:

Loss of output (DCS process bus de-energized)
Device internal fault
DC source I lost
DC source II lost

These are mapped to different alarm priorities in the DCS/SCADA and, where appropriate, tied to horn, beacon, or message notification. The goal is not only to react to faults, but to build a traceable alarm sequence that explains why a particular transfer occurred and which source failed.

2. Clear Transfer Philosophy and Thresholds

In most process plants, a main-source-preferred, automatic retransfer policy is used:

The main DC source is used under normal conditions.
On undervoltage below 75–80 % of nominal, the unit transfers to the backup.
Once the main source has returned and remained stable, the unit automatically retransfers.

Maintaining the threshold in the 75–80 % range avoids frequent transfers caused by minor ripple or transient dips while still reacting promptly to genuine supply degradation.

3. Annual Functional Testing

To maintain confidence in the scheme, a yearly functional test should be part of the plant’s maintenance SOP:

Intentionally disconnect the main DC source.
Confirm seamless transfer to the backup source, with no abnormal behaviour on DCS loads.
Restore the main source and verify correct automatic retransfer and indication.
Monitor output voltage during the test; under normal conditions, it should remain slightly below the input by approximately 1–2 V due to internal regulation.

Documenting this test – including observed voltages, transfer times, and alarm behaviour – closes the loop between design and long-term operability.

Conclusion: A Structured Path to “Non-Stop” DCS Power

Keeping the DCS process layer continuously energized is not just a matter of installing more batteries. It requires a purpose-built dual-source transfer architecture that:

Executes 0 ms DC transfer with output voltage support.
Tolerates cabinet cluster inrush and process-layer fault currents.
Provides clear, graded alarm information to the DCS and maintenance teams.
Fits naturally into annual testing and operations procedures.

By applying a dedicated ODES dual DC automatic transfer unit between independent DC sources and the DCS process bus, plants can significantly enhance the reliability and maintainability of their process control supply. The result is a power system that behaves predictably under fault, maintenance, and transient conditions—so that the DCS can continue doing what it is designed to do: keep the process stable.

If you are planning a new DCS project or upgrading an existing plant, ODES can help you consult on dual-source architectures, evaluate cabinet-level loads, and request suitable transfer device ratings and settings tailored to your topology.

Share your DC load list and single-line diagrams, and our engineering team can help you learn how to segment the process bus, configure thresholds, and define annual test procedures for verifiable reliability.

To contact us for a tailored supply stability assessment and dual-source transfer proposal, please write to:

📩 tonyzhang@odes-electric.com