Relative humidity alone does not determine condensation. Moisture forms when a surface temperature falls below the local dew point. Switchgear can therefore condense after a cold delivery enters a warm room, during night-time cooling, when an idle panel is colder than the room, or when moist outdoor air reaches a cool enclosure.
The design variable is surface temperature margin
Measure ambient temperature and relative humidity to calculate dew point, then compare it with the coldest credible internal surface—not the average room temperature. Cable-entry plates, external walls, metallic partitions and unenergised busbar zones may be colder than the thermostat location. Define an alarm margin above dew point rather than relying on a single humidity threshold.
Heaters are one layer, not the whole solution
Anti-condensation heaters raise internal surface temperature. Their effectiveness depends on placement, natural convection, compartment boundaries, air leakage and control philosophy. A heater in the low-voltage compartment does not prove the cable compartment is protected. Avoid positioning that overheats wiring, electronic IEDs or nearby insulation. Prove temperature distribution under the lowest load and worst ambient condition.
Thermostat, humidistat or dew-point control?
A thermostat is simple but may energise at the wrong moisture condition. A humidistat responds to relative humidity but not directly to surface temperature. Dew-point control combines temperature and humidity and can preserve a defined margin. Whichever method is chosen, specify sensor location, switching hysteresis, failure alarm, heater supervision and behaviour when auxiliary power is unavailable.
Ventilation can help or harm
Ventilation removes heat and moisture only when incoming air is drier in absolute terms. In humid climates it can import moisture. Filters and louvers also change pressure, contamination ingress and enclosure IP performance. Outdoor kiosks may require a coordinated strategy using sealing, drainage, insulation, controlled ventilation and heating.
Storage and energisation
Cold equipment should be allowed to reach room temperature before packaging is opened. During storage, provide the manufacturer-required environmental control and inspect for moisture before energisation. Schneider Electric’s instructions explicitly warn against energising equipment when moisture or condensation is present and describe heater use in cold, damp conditions.
Commissioning checks
- Record temperature and humidity inside each relevant compartment and in the room.
- Verify heater power, protective device, wiring, thermostat/humidistat and alarm.
- Use thermal imaging or logged sensors to find cold surfaces and local overheating.
- Inspect cable entries, roof seams, drains and doors for water paths.
- Confirm ventilation and HVAC operation during light-load and de-energised periods.
- Trend environmental data before declaring the design adequate.
Failure modes often missed
Common failures include a heater fed from the panel’s own unavailable supply, one failed heater hidden by a healthy common circuit, sensor placement in the warmest zone, blocked vents, open cable trenches carrying humid air, and control settings that work only when the switchgear is loaded.
Official sources and revision control
- IEC 60068-2-30:2005 official record
- Schneider Masterclad safety precautions
- Schneider MCSeT heater instructions
- Hitachi Energy modular switchgear monitoring
Technical review date: 16 August 2026. Standards and product capabilities can change; confirm the project-specific edition, firmware, options and ordered configuration before engineering or procurement.