For equipment installed at altitude, assess cooling and insulation separately. A supply that remains cool enough may still lack the required insulation qualification, while an electrically suitable product can overheat inside a poorly ventilated cabinet. Do not convert a general ambient-temperature rating into an altitude approval.
Why the same cabinet can behave differently at a higher site
Reduced air density changes the conditions for convective heat removal and lowers the dielectric strength of air. Manufacturer guidance therefore treats high-altitude selection as more than a temperature calculation. The useful response is a model-specific thermal and insulation review, not a universal “reduce the watts by this percentage” rule. The referenced technical explanation of altitude effects describes these two effects; its historical standards examples are not a substitute for the standards applicable to a new project.
The practical starting point is the actual site envelope: operating altitude, minimum and maximum surrounding temperature, whether the enclosure is conditioned, expected contamination, and the load’s duty cycle. Distinguish normal operation from transport and storage. Equipment shipped over a mountain pass does not necessarily have the same requirement as equipment operating continuously at that elevation.
What the MDR-20-12 sheet does—and does not—establish
The original WEHO MDR-20-12 shown here is a slim DIN-rail supply marked 12 V, 1.67 A. Its matching sheet lists a working-temperature range of −10 to +50°C and dimensions of 22.5 × 90 × 100 mm, with TS35/7.5 or TS35/15 rail installation.
That sheet does not state a permissible operating altitude or provide an altitude-dependent derating curve. It would therefore be incorrect to advertise this specific unit as qualified for operation above 2000 m—or to assume that 2000 m is its documented limit. The number is a useful enquiry trigger in many engineering discussions, not an automatically established product rating.
The product is used here to show the information an equipment builder must obtain. It is not being presented as a selected high-altitude solution. If the intended installation lacks a matching manufacturer declaration, obtain one or choose a model whose published operating envelope covers the site.
Build the thermal review around the enclosure
Ask for the permitted output at the stated altitude, inlet-air temperature, orientation and cooling arrangement. A temperature measured elsewhere in the room does not describe the air entering a densely packed cabinet. Include heat from controllers, contactors and adjacent converters, along with solar gain or other external heat sources where relevant.
Preserve the original mounting arrangement and ventilation path. For this MDR form, the clip engages the rail at the far rear of the deep blue housing; the front terminal face is not the mounting plane. Extra brackets, improvised orientation and blocked vents can change the installation from the one being assessed.
Adding a fan is a design change, not an automatic approval. Consider the real airflow through the cabinet, filter condition, fan failure, recirculation and maintenance interval. A fan’s nominal flow number outside the enclosure does not prove the airflow around the supply after ducts, filters and obstructions are present.
Review insulation requirements independently
Give the manufacturer the intended altitude and the applicable finished-equipment requirements. Ask what installation conditions and insulation assumptions apply to the exact model and revision. Do not replace this with an informal visual judgement that two internal parts “look far enough apart.”
A dielectric withstand test value in a component sheet is not an operating-altitude declaration. Likewise, reducing the load current does not by itself resolve an insulation suitability issue. Keep the electrical-safety decision separate from the heat-removal decision in the selection record.
Plan verification before the cabinet design is frozen
The qualification plan should define the intended environment, representative loads, operating states, measurement locations and acceptance criteria. Have competent engineers select the appropriate test method and evaluate the complete assembly. A conventional room-temperature run can reveal obvious issues, but it cannot silently stand in for an untested altitude condition.
Record the configuration used: supply revision, enclosure, rail position, adjacent equipment, cooling hardware and load profile. If these change after review, assess whether the conclusion still applies. “Same wattage” is not sufficient evidence that a substitute supply has the same thermal or insulation behaviour.
Frequently asked questions
Does a +50°C rating prove operation above 2000 m?
No. Ambient temperature and altitude are separate conditions. The matching MDR-20-12 sheet does not provide an altitude approval or an altitude derating curve.
Can adding a fan or reducing load resolve every altitude issue?
No. A suitable thermal change may help heat removal, but it does not by itself establish insulation suitability or a new permissible operating altitude.
A focused enquiry saves redesign later
Send WEHO the exact site altitude, local temperature envelope, required continuous and transient load, enclosure drawings, proposed orientation and applicable equipment requirements. Request the permitted operating envelope and any derating or installation conditions in writing, tied to the exact supplied model.
Use the WEHO product catalogue to identify candidates and request an application review. The right result may be a documented operating restriction, a different cooling arrangement or a different model. It should not be an invented rating added to an otherwise unchanged product label.
