Protecting electrical enclosures against cold and condensation

An enclosure heater is an electrical heating device installed inside a control cabinet or equipment housing to reduce the risks of low temperatures and condensation. Its purpose is to maintain suitable conditions for the PLC, HMI and other components, without unnecessarily overheating the enclosure. Typical applications include outdoor cabinets, humid industrial environments and control panels that cool down overnight.

Why do water droplets form inside a cabinet?

Condensation occurs when a surface exposed to humid air falls below the dew point. For example, air at 20 °C and 80% relative humidity has a dew point of approximately 16.4 °C. If a metal surface inside the cabinet cools to 15 °C, moisture can condense on it even while the air remains at 20 °C. Where necessary, a humidity sensor helps assess relative humidity alongside temperature.

The heater warms the air and internal surfaces to reduce the likelihood of condensation. Heat released by operating equipment may be sufficient while the machine is running, but this heat source disappears during shutdown. Moisture problems can therefore become apparent at the first start after an overnight shutdown, rather than during production.

PTC, resistance and fan-assisted heaters

In a PTC heater, the element's resistance increases as its temperature rises, helping to limit its heating output. This behaviour does not perform the same task as a thermostat maintaining a selected cabinet temperature. Check the specified supply voltage, heating capacity and the ambient temperature at which that capacity was measured.

Fanless heaters commonly rely on natural convection. Fan-assisted heaters help distribute warm air more quickly and evenly, which can benefit larger or densely populated cabinets. A fan cannot compensate for poor positioning. Blocked airflow or fan failure can impair heat distribution.

Should you use a thermostat or a hygrostat?

A thermostat responds to temperature, while a hygrostat responds to relative humidity. For heating, select and connect the thermostat so that it calls for heat as the temperature falls. The difference between a temperature sensor and a thermostat matters here: the sensor measures temperature, while a suitable temperature controller or thermostat generates the control action.

A hygrostat can switch on the heater when relative humidity exceeds its setting. Combined devices can monitor both variables in industrial humidity control applications. No single temperature or humidity setting suits every enclosure. Settings depend on the environment, condensation risk and the permitted operating conditions of the installed equipment.

How do you select the heating capacity?

Cabinet volume alone is not enough to determine the required wattage. Consider the minimum ambient temperature, target internal temperature, enclosure material, effective surface area, insulation, installation arrangement and heat released by internal equipment. Account for reduced internal heat during machine shutdown. In larger enclosures, several appropriately distributed heaters can provide more even heating than one concentrated heat source.

Verify the capacity selected using the manufacturer's calculation method through temperature measurements under actual operating conditions. As a simple energy example, a heater drawing a constant 100 W for eight uninterrupted hours consumes 0.8 kWh. Thermostatic cycling reduces its operating time; energy consumption in humidity control depends on both operating duration and actual power draw.

Installation and operation

Natural-convection heaters are commonly positioned in the lower part of the cabinet, but mounting orientation and minimum clearances must follow the manufacturer's instructions. Keep the warm-air path clear and maintain suitable separation from cables and temperature-sensitive components. Position the thermostat or sensor so that it measures representative cabinet conditions rather than only the heater's warm-air plume.

Check the supply voltage, control contact load rating and fuse selection. Some PTC heaters have an initial inrush current considerably higher than their normal operating current. Protective earthing requirements depend on the device's electrical protection class. If protection is required during machine shutdown, the panel design must allow the heater to operate in that condition.

If moisture persists while the heater is operating, increasing the wattage alone may not resolve the cause. Inspect seals, cable entries, airflow, control settings and cold surfaces, considering the complete humidity control installation and maintenance requirements. Coordinate heating and cooling to avoid unnecessary simultaneous operation.