24 V DC Control Power Supply RFQ: Load, Redundancy and Selectivity

An industrial control power supply RFQ should begin with the complete 24 V DC load and the failure behavior the cabinet must tolerate, not with an output-current number copied from a similar machine. A power supply may match the nominal voltage and still be unsuitable for peak current, inrush, redundancy, selectivity, ambient temperature, input variation, or the control system’s diagnostic requirements.

This checklist is for maintenance teams, system integrators, and purchasing teams reviewing a control cabinet power supply, redundancy module, selectivity module, buffer, or DC UPS. It helps collect the evidence needed for a technical review without assuming that visually similar DIN-rail units are interchangeable.

1. Build the real 24 V DC load list

Record every connected load and its operating state. Include PLC CPUs, remote I/O, HMI panels, industrial switches, sensors, transmitters, relays, solenoid valves, contactors, safety devices, communication modules, and any DC motors or actuators. For each load, record rated current, startup or inrush current, normal duty, simultaneous operation, and whether the load is continuous or intermittent.

Separate steady-state load from short-duration peaks. A cabinet can appear within the power supply rating during normal operation while still dropping voltage when several valves, relays, or communication devices start together. Include the measured current at the existing cabinet if it is available, and identify loads that may be added during a future expansion.

Also state whether a load is sensitive to a short voltage dip. PLC memory, network switches, safety controllers, and industrial PCs may have different tolerances and restart behavior. The RFQ should explain which loads must remain online and which loads may be disconnected first if the available power is limited.

2. Confirm input and installation conditions

Record the incoming AC or DC voltage range, number of phases, frequency, protective device, short-circuit level when known, and whether the supply is fed from a UPS or generator. Note phase-loss conditions, voltage variation, brownouts, harmonics, and the available control-cabinet space.

Document ambient temperature, altitude, ventilation, humidity, contamination, vibration, mounting orientation, and enclosure conditions. A unit selected for a cool control room may require derating in a hot or tightly packed panel. Include the required approvals, hazardous-area context, functional safety context, or certification class when the project specification calls for them.

Take photographs of the installed power supply label, input terminals, output terminals, fuse or breaker, cabinet ventilation, and nearby modules. A label photo can reveal the full input range, output voltage, current rating, revision, and option status that may not appear in a procurement description.

3. Decide whether redundancy is actually required

State the consequence of a single power supply failure. If a controller or network must continue operating after one supply fails, describe the required architecture: two supplies, a redundancy module, independent feeders, or a complete redundant path to the load. Clarify whether the design is 1 + 1 redundancy or N + 1 capacity redundancy.

Redundancy is more than placing two power supplies in parallel. The supplies may need decoupling so that an internal fault in one unit does not pull down the common output. Siemens describes SITOP redundancy modules as a way to decouple two power supplies so the second can continue supplying the load after one fails. The proposed wiring should be reviewed as a complete path rather than as two isolated product choices.

Ask for the proposed wiring diagram, current-sharing assumptions, alarm contacts, and test method. State whether the existing cabinet has separate AC feeds, separate DC distribution, or only one upstream source. If the two supplies share one breaker or one cable, the upstream path may remain a single point of failure.

4. Check selectivity and branch protection

List every downstream fuse, miniature circuit breaker, electronic protection module, and terminal distribution block. Identify which branch feeds the PLC, HMI, network, sensors, valves, and actuators. The RFQ should state whether the goal is to isolate one failed branch while keeping the control system online.

Siemens describes selectivity modules that disconnect an overloaded or short-circuited 24 V feeder while other consumers remain supplied. This is different from simply selecting a larger power supply. A high-current supply may continue feeding a fault instead of allowing a conventional breaker to trip quickly, especially on long or thin DC wiring.

Record branch current limits, cable lengths, conductor sizes, short-circuit behavior, reset method, and remote diagnostic signals. Include whether the maintenance team needs local LEDs, relay contacts, IO-Link, PROFINET, or another status interface.

5. Separate power failure, overload, and short-circuit needs

Describe the event that the protection system must handle. A short power dip, a several-second outage, a long outage, an overloaded feeder, and a short circuit are different problems. A buffer may bridge a brief dip, while a DC UPS with stored energy may be needed for a longer interruption or controlled shutdown.

Siemens identifies separate add-on functions for redundancy, selectivity, buffering, and DC UPS protection. The correct choice depends on the duration and consequence of the event. Do not describe all of these functions as “backup power” in an RFQ because the supplier needs to know the expected hold-up time, load priority, battery or capacitor condition, and restart behavior.

6. Check output distribution and diagnostics

Record output voltage tolerance, adjustment range, current reserve, remote sensing if used, common 0 V arrangement, grounding, polarity, and the distribution method. Identify whether the output feeds mixed loads with different noise sensitivity or whether analog instrumentation is separated from inductive loads.

List diagnostic contacts, LED states, communication interfaces, temperature alarms, and service-life monitoring requirements. Siemens SITOP pages describe options such as signal contacts, Industrial Ethernet or PROFINET integration, IO-Link, and diagnostic interfaces on selected product families. These features are relevant only when the cabinet control system can use them, so include the PLC or DCS interface and the desired alarm destination.

7. Send complete RFQ evidence

Exact manufacturer, full model, revision, input range, output voltage, current, and nameplate photo.
Complete 24 V DC load list with steady current, inrush, duty cycle, and critical-load priority.
Existing redundancy, selectivity, buffer, UPS, fuse, breaker, and distribution components.
Input source, phase, protective device, cabinet temperature, space, ventilation, and mounting constraints.
Required certifications, diagnostic interface, alarm contacts, network protocol, and restart behavior.
Quantity, destination, target delivery window, replacement reason, and available wiring drawings.

For related system context, review the KNMKS power supplies category, the automation systems section, and the industrial control systems hub. You can also use the product directory or the KNMKS contact page when the exact model or series is uncertain.

Need a control power supply checked? Send the full model, load list, cabinet photos, redundancy or UPS requirements, quantity, and destination. A complete RFQ gives the technical review a reliable starting point and makes the remaining compatibility questions visible.