Test & measurement


Filters, cable ducts, additional equipment and fan degradation can all change the internal airflow after commissioning.


A PSU carrying exactly the same electrical load may therefore operate with much less thermal margin several years later.


USE FAULT SIGNALS AS MEASUREMENT DATA


Many industrial power supplies provide more information than a simple DC output.


Depending on the product, available signals can include: DC_OK or power-good status; warning contacts; fault outputs; output current; output voltage; temperature; redundancy status; and digital telemetry.


A DC_OK contact is often treated simply as an alarm input, but repeated changes in that signal can also reveal an intermittent problem. A machine that experiences one brief DC_OK event every few weeks should not be considered healthy simply because it automatically recovers. Recording when these events occur can help correlate them with specific operating conditions. For example, do warnings appear during machine start-up? During the operation of a high- current actuator? At high cabinet temperatures? Only after the machine has been running for several hours?


When the power supply provides digital telemetry, current, voltage and temperature can be incorporated into the same historical data system used for other process variables.


The objective is not to collect data for its own sake. The objective is to establish whether the electrical operating conditions are changing.


VERIFY REDUNDANCY INSTEAD OF ASSUMING IT WORKS


Redundant 24 VDC architectures are often installed to prevent a single power-supply failure from shutting down the control system.


However, two healthy-looking power modules do not automatically guarantee redundancy. If both modules share a load during normal operation, a failure of one supply can suddenly place the complete protected load on the remaining unit.


The measurements to watch during this transition include bus voltage, surviving PSU current, fault status and load stability.


The surviving supply should remain within both its electrical and thermal capabilities.


Load sharing should also be checked periodically where applicable. A large and unexplained imbalance between two nominally parallel modules can indicate unequal path resistance, connection problems or differences in regulation. Maintenance teams should also distinguish redundancy from backup energy.


Two PSUs can protect against the failure of one module, but if both depend on the same upstream


Instrumentation Monthly October 2026


AC source, they do not provide ride-through when that source disappears.


This distinction becomes important when interpreting system alarms: the absence of a redundant-PSU fault does not mean the control system is protected against every power disturbance.


CAPTURE TRANSIENTS THAT SLOW MONITORING CANNOT SEE Many control-power disturbances occur too quickly to be visible in routine telemetry. A PLC may reset because of a short bus-voltage dip even though the logged average voltage appears normal.


For intermittent problems, an oscilloscope or recorder with appropriate triggering can be far more informative than repeated spot measurements. Useful events to capture include: machine


start-up  large load switching  PSU failure  redundant transfer  load recovery. The engineer should look for voltage dips, overshoot, oscillation and recovery time. This is especially important when the symptoms occur only during specific transitions.


For example, a supply may operate correctly at a sustained 8 A load but show excessive voltage drop when the current jumps rapidly from 3 A to 8 A. Similarly, a redundant system may operate normally with both modules active but show a short bus disturbance when one module is removed. The measurement method should match the timescale of the event. Slow telemetry is useful for trends; fast electrical measurements are needed for transients.


ESTABLISH A BASELINE BEFORE SETTING ALARM LIMITS


One of the simplest improvements to control-power maintenance is to establish a healthy baseline. Record key values when the machine is operating correctly: PSU output voltage  voltage at critical load  total current  PSU inlet temperature  critical connector temperatures  redundancy state. Future inspections can then be compared with those values. This makes changes easier to recognise. An increase from 5 A to 7 A may be perfectly acceptable from a nameplate perspective, but it is still worth understanding if the machine configuration has not intentionally changed. Likewise, a connector that rises from 35 °C to 55 °C under the same load may deserve attention even if it remains below a published maximum temperature.


Alarm thresholds should be based on the requirements of the actual system rather than arbitrary universal values.


The acceptable voltage range depends on the connected equipment. Thermal limits depend on components, enclosure design and manufacturer specifications. Current margin depends on the


Figure 3. Example condition-monitoring workflow for a 24 VDC control-power system, combining current, voltage, thermal, fault-status and redundancy checks.


PSU rating and any derating that applies at the measured inlet temperature.


TURN MEASUREMENTS INTO PREVENTIVE MAINTENANCE The goal of monitoring is not to predict the exact day a component will fail.


A more practical objective is to detect loss of operating margin early enough for maintenance to be planned.


A useful inspection routine can combine electrical and thermal checks: Measure the present load. Compare voltage at the PSU and at critical loads. Check connector and terminal temperatures. Confirm the actual PSU inlet temperature. Review DC_OK and fault history. Verify that redundant modules are both available and operating as expected.


If these measurements are recorded consistently, they create a history of the control-power system rather than a collection of isolated readings. That history makes it easier to distinguish a normal operating variation from a developing fault.


MEASURE THE COMPLETE POWER PATH A 24 VDC power system is more than the power supply mounted on the DIN rail. Its performance depends on the complete path from the incoming supply through protection and distribution hardware to the final control load. That is why the most useful measurements are often taken away from the PSU itself. Voltage at the PLC, temperature at a terminal, current during a start-up event or a change in redundancy status can reveal problems that would otherwise remain hidden.


For maintenance engineers, these measurements provide something more useful than a simple pass/fail indication: they show how much margin remains. And when that margin begins to shrink, the opportunity exists to correct the problem during planned maintenance – before a control-power issue becomes production downtime.


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