Test & measurement


WHAT TO MEASURE BEFORE A 24 VDC CONTROL-POWER PROBLEM BECOMES DOWNTIME


By Steven Zhang, Product Manager, Powernexu A


24 VDC control-power problem rarely announces itself with a clear warning that a power supply is about to fail. More often, the first visible symptom is a PLC reset, an industrial PC reboot, a communications dropout or an I/O fault that appears intermittently under certain operating conditions.


By that point, the underlying problem may have been developing for weeks or months. Increasing load, voltage drop, restricted airflow, connector resistance and loss of redundancy can all reduce the operating margin of a 24 VDC system without immediately causing a shutdown. For maintenance and control engineers, the challenge is therefore not simply to confirm that 24 V is present. It is to identify which measurements reveal that the system is moving away from its normal condition.


MEASURE VOLTAGE WHERE THE LOAD ACTUALLY SEES IT


The most obvious measurement is DC voltage, but the measurement point matters.


A power supply may maintain 24.0 V at its output terminals while equipment several metres away receives noticeably less because of resistance in cables, terminal blocks, fuses and connectors.


The relationship is straightforward: Vdrop = I × R


A combined path resistance of 0.1 Ω carrying 8 A produces a voltage drop of 0.8 V. If the supply output is 24.0 V, the load may see only 23.2 V. That difference may still be within the equipment specification, but the available margin becomes smaller during start-up or transient events. Useful measurements should therefore include both the power-supply output and the voltage at critical loads such as PLCs, industrial PCs and communications equipment.


Measurements taken only at the PSU can hide problems further down the distribution path. A particularly useful test is to monitor the far-end voltage while the machine moves through its normal operating cycle. If voltage falls whenever several actuators or auxiliary loads switch simultaneously, the cause may be


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TEMPERATURE CAN REVEAL PROBLEMS BEFORE VOLTAGE DOES


Electrical faults frequently produce heat before they produce a complete loss of power. Connector resistance is a good example.


Conduction loss follows the relationship: P = I²R


Figure 1. Recommended measurement points in a


24 VDC control-power system, including PSU output, distribution point and critical downstream loads.


an overloaded branch, undersized wiring or increasing connection resistance rather than poor regulation inside the PSU.


TREND CURRENT INSTEAD OF CHECKING IT ONCE


A current measurement becomes much more useful when it is compared with previous measurements. Control cabinets evolve. Additional I/O, network switches, gateways, sensors and computing hardware are often added after commissioning. Each individual addition may be small, but over time the total current can approach the original supply capacity.


A system that operated at 55 per cent of rated current when commissioned may be running at 80 per cent or 90 per cent several years later. The absolute value is important, but the trend often tells a more useful story.


A gradual increase can indicate: added loads; changing operating cycles; deteriorating equipment; additional auxiliary devices; and reduced redundancy margin.


Start-up current should also be considered separately from steady-state current. Industrial PCs, capacitive loads, communications equipment and some electronic protection modules can draw significantly different current during power-up. If several devices start simultaneously, the peak may be high enough to pull down the DC bus even though the average operating current is acceptable. A clamp meter is sufficient for many routine checks, but capturing fast start-up or transient behaviour may require a current probe and oscilloscope or a suitable data acquisition system.


Even a modest increase in resistance can therefore generate significant local heating at higher current.


A terminal or connector may continue operating while its temperature gradually rises because of loose fastening, damaged contact surfaces, contamination or conductor problems. Comparing similar connection points is often more useful than evaluating one absolute temperature. If several terminals carry comparable current but one operates 15 °C or 20 °C hotter than the others, that difference deserves investigation. Thermal imaging is especially effective for this type of comparison because it allows maintenance teams to inspect multiple connection points without interrupting operation. The same principle applies to the PSU itself. A rising inlet temperature can reduce available output capability, particularly if the manufacturer specifies thermal derating above a certain operating temperature.


Cabinet temperature should therefore be measured at the air entering the power supply, not only at the enclosure door or somewhere outside the cabinet.


Figure 2. Electrical and thermal indicators of declining 24 VDC power margin, including load growth, voltage drop, connector heating and elevated PSU inlet temperature.


October 2026 Instrumentation Monthly


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