A control cabinet may appear sealed from the outside and still develop moisture problems inside. Many users assume that an enclosure with a high IP protection rating is automatically protected against all moisture damage. In practice, this is only partly true. The IP rating describes protection against the ingress of foreign bodies and water from the outside. However, it does not automatically prevent moisture from accumulating inside or condensing during temperature changes.
Closed control cabinets in particular can become problematic when temperatures fluctuate. If warm, humid air is trapped inside the enclosure and the control cabinet subsequently cools down, the relative humidity can rise sharply. If the dew point is reached, condensation forms on enclosure surfaces, terminals, circuit boards, relays, power supplies or connectors. The consequences can be leakage currents, corrosion, insulation problems, sporadic faults or complete failure of the electronics.
This article explains why moisture can occur inside a control cabinet despite IP protection, what role temperature changes, control cabinet heaters, ventilation, cooling units, humidity sensors and dew point monitoring play, and how critical conditions can be detected at an early stage. Suitable solutions can be found, among others, in the areas of humidity data loggers, humidity sensors / dew point sensors, transmitters as well as devices such as the testo 175 H1 temperature and humidity data logger or the testo 176 H1 temperature and humidity data logger.
Table of contents
- Why moisture in the control cabinet is an underestimated problem
- Why IP protection alone is not enough
- How condensation forms inside the control cabinet
- Temperature changes, weekend shutdowns and night-time cooling
- Dew point: The decisive factor for condensation
- Typical consequences for electronics, terminals and control systems
- Control cabinet heater: When it is useful
- Ventilation, filter fans and pressure compensation
- Cooling units and temperature control inside the control cabinet
- Humidity sensors, data loggers and dew point monitoring
- Alarming and limit values for critical conditions
- Where humidity should be measured inside the control cabinet
- Table: Problem, cause and possible solution
- Practical example: Failure after weekend shutdown
- Suitable measuring instruments and product areas
- Conclusion: Humidity monitoring protects against invisible causes of failure
- FAQ: Frequently asked questions about moisture in control cabinets
Why moisture in the control cabinet is an underestimated problem
Moisture inside a control cabinet is often not a sudden visible water ingress, but a gradual problem. At first, the system runs without any noticeable issues. Only after some time do sporadic faults occur: a power supply does not start reliably, a relay sticks, a frequency inverter reports a fault, a controller fails after switching on or terminals show the first signs of corrosion.
These sporadic faults make diagnosis particularly difficult. When the service technician arrives on site, the control cabinet may already be warm and dry again. The moisture has evaporated, the system runs again and the actual trigger remains hidden. Without temperature and humidity recording, it is then almost impossible to trace whether critical conditions occurred during the cooling phase, at night or over the weekend.
Moisture becomes especially problematic where electronics react sensitively to leakage currents, corrosion or changes in insulation. Modern controllers, communication modules, frequency inverters, power supplies, safety relays and sensor systems are powerful, but not indefinitely tolerant of condensation. Even thin films of moisture on circuit boards or terminals can be enough to cause fault patterns.
The problem is therefore not limited to outdoor installations. Control cabinets in production halls, cold areas, washing systems, waterworks, pumping stations, food processing plants, warehouses or unheated technical rooms can also be affected. The decisive factor is not only whether water enters from the outside, but whether dew point conditions occur inside.
Why IP protection alone is not enough
The IP rating describes how well an enclosure is protected against the ingress of solid foreign bodies and water. A control cabinet with a high IP rating can therefore be very well protected against dust, splash water or water jets from the outside. However, this does not automatically mean that the air inside remains dry.
A sealed enclosure can even trap moisture. If the control cabinet absorbs humid air during installation, maintenance or while the door is open and is then closed, this air initially remains inside the enclosure. When the control cabinet heats up during operation, the air can absorb more moisture. When it cools down again, the relative humidity rises. If the dew point is reached, water condenses inside.
In addition, control cabinets are rarely completely hermetically sealed in real life. Cable glands, seals, door gaps, pressure differences, membranes, ventilation elements or ageing of the seal can allow slow air exchange. With changing temperatures, the control cabinet can effectively “breathe”: warm air expands, and later colder air is drawn back in. Over time, this can introduce additional moisture.
A high IP rating is therefore important, but only one part of the protection concept. For electrical reliability, temperature control, moisture management, ventilation, pressure compensation, heating and monitoring must also be considered.
How condensation forms inside the control cabinet
Condensation occurs when air is cooled so much that it can no longer hold all the moisture it contains. The relative humidity then reaches 100 %, and excess moisture is deposited as water on colder surfaces. In a control cabinet, these surfaces can include enclosure walls, mounting plates, metal parts, terminals, connectors, circuit boards or device surfaces.
Cold surfaces inside the cabinet are particularly critical. When the outside temperature drops, the enclosure wall often cools faster than the air inside the control cabinet. As a result, droplets can first form on the inside of the enclosure wall. From there, moisture can run onto components or enter terminal areas. In other cases, an invisible film of moisture forms directly on electronic assemblies.
Condensation always depends on the interaction between temperature and humidity. A relative humidity of 60 % may still seem uncritical in warm air. However, if the same air cools down significantly, it can quickly reach a critical range. That is why humidity measurement alone, without considering temperature, is only of limited value.
For a reliable assessment, temperature and relative humidity should therefore always be considered together. These two values can be used to estimate whether conditions are approaching the dew point and when condensation is likely.
Temperature changes, weekend shutdowns and night-time cooling
Many moisture problems do not occur during normal operation, but during shutdown periods. During operation, power supplies, controllers, frequency inverters, relays or transformers generate heat. The control cabinet remains warm inside, and the relative humidity may appear low. When the system is switched off, this waste heat disappears. The control cabinet cools down, and the relative humidity rises.
Faults after weekends, public holidays, night shifts or longer shutdowns are typical. On Friday, the system is still running without problems. Over the weekend, the control cabinet cools down. On Monday morning, faults occur when the system is switched on. After some operating time, the control cabinet warms up again, the moisture partly evaporates, and the fault can no longer be clearly reproduced.
Outdoor installations are also affected. During the day, the control cabinet warms up due to ambient temperature, sunlight or operation. At night, the temperature drops. If humidity is high, condensation can form. Similar effects can occur in cold stores, washing areas or unheated halls.
Temperature changes are therefore an important indicator during troubleshooting. If electronics mainly fail in the morning, after shutdown, after a temperature drop or during weather changes, moisture inside the control cabinet should be checked specifically.
Dew point: The decisive factor for condensation
The dew point is the temperature at which the air is saturated with water vapour. If this temperature is reached or undercut, the air can no longer hold the moisture, and water condenses. For control cabinets, the dew point is therefore often more meaningful than relative humidity alone.
An example shows the connection: warm air inside the control cabinet can contain a certain amount of moisture. When the system switches off and the temperature drops, the relative humidity rises. Even if no additional water enters, the air inside can suddenly reach critical values. As soon as a surface is colder than the dew point, condensation can form there.
Dew point monitoring considers temperature and humidity together. This makes it possible to detect whether the conditions inside the control cabinet are moving into a critical range. This is particularly useful for systems that operate only temporarily, outdoor installations, unheated rooms or processes with strong temperature fluctuations.
In practice, a complex climate system is not always necessary. A suitable temperature and humidity data logger alone can show whether humidity rises sharply during certain time windows and whether there is a risk of condensation. For permanent monitoring, humidity sensors, dew point sensors or transmitters with alarm functions can be used.
Typical consequences for electronics, terminals and control systems
Moisture can cause different faults inside a control cabinet. Visible water damage is only the most obvious form. More common are gradual effects that are initially difficult to identify. These include corrosion on terminals, connectors and conductor tracks, increased contact resistance, leakage currents, insulation problems or sporadic communication errors.
Electronic assemblies react particularly sensitively to moisture when dust, dirt, production residues or conductive deposits are also present. A dry dust layer is often less critical than the same dust layer with a moisture film. Conductive paths can then form and lead to malfunctions.
Relays, contactors and terminals can also be affected. Corrosion increases contact resistance and can lead to heating. Connectors can develop contact problems. Sensor and bus signals can be disturbed. This is particularly critical for safety functions, because not only availability but also reliable function must be assessed.
| Affected area | Possible effect | Typical fault pattern |
|---|---|---|
| Circuit boards | Leakage currents, corrosion, short-circuit risk | Sporadic electronics faults |
| Terminals | Contact resistance and corrosion | Heating, contact problems |
| Connectors | Unreliable contact | Communication or sensor signal errors |
| Power supplies | Start-up problems or failure | System does not start after shutdown |
| Frequency inverters | Faults due to moisture or insulation problems | Fault message when switched on |
| Relays and contactors | Corrosion or contact problems | Unreliable switching |
The consequences are therefore not just short-term faults. Moisture can significantly shorten the service life of components and lead to failures that are difficult to identify over the long term.
Control cabinet heater: When it is useful
A control cabinet heater is not only used to keep devices warm. In many applications, its most important task is to keep the temperature inside the control cabinet above the dew point. This prevents condensation from forming on components and enclosure surfaces.
A control cabinet heater is particularly useful for outdoor installations, unheated rooms, pumping stations, water and wastewater plants, systems with weekend shutdowns, high humidity or strong temperature fluctuations. Even if enough self-heating is generated during operation, heating may be required during shutdown periods.
Correct control is important. A heater that runs permanently can consume unnecessary energy and thermally stress the control cabinet. Control via thermostat, hygrostat or combined temperature and humidity control is often more sensible. The heater is then only activated when temperature and humidity suggest critical conditions.
The heater must also match the size of the control cabinet, installation situation and air circulation. Warm air should be able to distribute sufficiently. If cold corners, poorly ventilated areas or areas behind mounting plates are not reached, condensation can still occur there.
Ventilation, filter fans and pressure compensation
Ventilation can help remove heat and moisture from the control cabinet. At the same time, however, it can also introduce humid ambient air. It must therefore always be assessed whether ventilation is helpful or problematic in the specific case.
Filter fans are often used to dissipate waste heat. If the ambient air is dry and clean, this can be useful. In humid, dusty, oily or aggressive environments, however, the air drawn in can create new problems. Filters must be maintained regularly; otherwise cooling deteriorates and airflow decreases.
Pressure compensation elements can prevent strong pressure differences from forming due to temperature changes. This reduces the uncontrolled intake of humid air through seals or cable glands. Such elements can be particularly important for sealed enclosures and outdoor installations.
Ventilation is therefore not a universal solution against moisture. It must suit the environment. In some applications, a controlled cabinet heater or a cooling unit is more suitable than unfiltered air exchange.
Cooling units and temperature control inside the control cabinet
Cooling units are used when the waste heat inside the control cabinet is too high or the ambient temperature does not allow sufficient passive cooling. They can stabilize the internal temperature and thus also influence the moisture behaviour. However, temperature control must be set carefully.
If a control cabinet is cooled too strongly, cold surfaces can form. If these come into contact with humid air, condensation can also occur. This applies especially with strongly fluctuating environments, poor airflow or when warm, humid air enters the control cabinet.
With cooling units, the combination of temperature and humidity must therefore also be considered. A constant temperature is good, but it alone does not guarantee dry conditions. The decisive question is whether surfaces inside the control cabinet can fall below the dew point.
For critical applications, it can be useful to record temperature and humidity inside the control cabinet permanently. This makes it possible to check whether the heater, ventilation or cooling unit actually creates the desired conditions or whether humidity peaks still occur during certain operating phases.
Humidity sensors, data loggers and dew point monitoring
Humidity sensors and data loggers help make moisture problems visible. Without measured values, there is often only suspicion. Long-term recording makes it possible to detect whether relative humidity rises critically at night, during shutdowns or during temperature changes. Recording temperature and relative humidity together is particularly meaningful.
A data logger is well suited for troubleshooting. It can be placed inside the control cabinet for a few days or weeks and document the trend. This makes it possible to check whether the control cabinet reaches critical ranges during the cooling phase. If faults always occur at certain times, the logger can make these connections visible.
For permanent monitoring, humidity sensors or dew point sensors with an output signal are useful. These can be connected to a controller, control system, data logger or alarm module. Depending on the design, analogue signals such as 4…20 mA or 0…10 V, digital interfaces or relay outputs are possible.
Dew point monitoring is particularly helpful when the aim is not only to display a humidity value, but to prevent a critical condition. An alarm can be triggered, for example, if relative humidity rises above a limit, the dew point approaches the control cabinet temperature or a combination of temperature and humidity becomes critical.
Alarming and limit values for critical conditions
Humidity monitoring is particularly useful when an action is derived from it. A measured value alone does not prevent a failure. Only alarming, control or maintenance measures make the monitoring effective. Limit values should therefore not be chosen arbitrarily, but should match the application.
A simple limit value can be set, for example, for high relative humidity. In many cases, however, considering the dew point is more useful. If the internal temperature is only slightly above the dew point, the risk of condensation increases. In such cases, a control cabinet heater can be switched on or a message can be sent to maintenance.
For critical systems, multi-stage alarming can be useful. A pre-alarm indicates that conditions are becoming unfavourable. A main alarm indicates an acute risk of condensation or permanently high humidity. This makes it possible to react before electronics fail.
| Monitoring variable | Benefit | Typical reaction |
|---|---|---|
| Relative humidity | Shows moisture load inside the control cabinet | Check alarm, ventilation or heating |
| Temperature | Shows cooling, heating and shutdown phases | Assess temperature control |
| Dew point | Assesses condensation risk | Activate heating or issue warning |
| Humidity trend over time | Identifies critical time windows | Narrow down cause and plan measures |
| Limit value violations | Documents critical conditions | Trigger maintenance or system inspection |
The limit values should be reviewed regularly. If the environment, operating mode or control cabinet equipment changes, the moisture behaviour may also change.
Where humidity should be measured inside the control cabinet
The position of the sensor determines whether the measurement is meaningful. A humidity sensor should not be placed just anywhere inside the control cabinet, but where critical conditions can realistically occur. These are often areas with poor air circulation, colder enclosure surfaces, lower cabinet areas, areas near cable glands or areas where corrosion or moisture traces have already been visible.
At the same time, the sensor should not be placed directly in the airflow of a heater, fan or cooling unit if this means that only a local special condition is measured. The aim is to measure the control cabinet air representatively or to monitor a known risk point deliberately.
For larger control cabinets or cabinet rows, a single measuring point may not be enough. Temperature and humidity can differ between the upper and lower areas. Areas behind mounting plates or near cold outer walls can also show different values. In such cases, several measuring points or a targeted comparison measurement are useful.
For troubleshooting, a mobile data logger can initially be used at different positions. Once a critical area has been identified, a permanent sensor system can be installed there.
Table: Problem, cause and possible solution
Moisture inside the control cabinet can have different causes. The following table shows typical fault patterns and possible measures.
| Problem | Possible cause | Possible solution |
|---|---|---|
| Electronics fail after weekend shutdown | Control cabinet cools down, relative humidity rises, condensation forms | Record temperature and humidity trend, check control cabinet heater |
| Condensation on enclosure wall | Inner wall falls below dew point | Use dew point monitoring, heating or better temperature control |
| Corrosion on terminals | Permanently high humidity or repeated condensation | Find moisture source, install sensor system, check seals and cable glands |
| Faults only in the morning | Night-time cooling and humidity peaks | Use data logger over several days and evaluate shutdown phases |
| High humidity despite sealed enclosure | Humid air trapped or pressure changes draw in ambient air | Check pressure compensation, controlled ventilation or drying concept |
| Moisture after cleaning the system | Splash water, steam or humid ambient air enters the area | Check IP protection, seals, cleaning concept and sensor position |
| Cooling unit present, but condensation still occurs | Surfaces too cold or unfavourable airflow | Check setpoints, airflow and dew point conditions |
| Fault cannot be reproduced | Moisture evaporates after heating during operation | Use long-term data logger and compare events with fault times |
Practical example: Failure after weekend shutdown
In a production plant, a controller repeatedly fails on Monday morning. During the week, the system runs mostly without problems. Maintenance initially replaces a power supply and checks several terminals, but finds no clear cause. The only noticeable pattern is that the faults occur after longer shutdown periods.
A temperature and humidity data logger is then placed inside the control cabinet. The logger records values over two weeks. The measurement shows that during operation, the temperature inside the control cabinet is significantly higher and the relative humidity remains uncritical. After shutdown over the weekend, however, the control cabinet slowly cools down. During the night before Monday, the relative humidity rises sharply. At times, conditions are reached where condensation inside is plausible.
A more detailed visual inspection reveals slight corrosion marks on individual terminals and traces of moisture on a cool enclosure wall. The cause is therefore not a single defective component, but the climate inside the control cabinet during the shutdown phase.
As a measure, a controlled cabinet heater is installed to keep the control cabinet above critical dew point conditions. In addition, a humidity sensor with alarm function is planned so that unusually high humidity is detected early. After the modification, the Monday morning failures no longer occur, and the measured values show significantly more stable conditions.
The example shows that moisture problems are often time-dependent. Without data recording, the cause would have been difficult to find because the control cabinet appeared dry during normal operation.
Suitable measuring instruments and product areas
For troubleshooting suspected moisture in the control cabinet, humidity data loggers are suitable. They record temperature and relative humidity over a longer period and make visible whether critical conditions occur during night-time cooling, shutdowns or weekends.
The testo 175 H1 temperature and humidity data logger is a suitable solution when temperature and humidity trends in buildings, rooms or enclosed areas need to be documented. For more extensive measurement tasks with several probes, the testo 176 H1 temperature and humidity data logger may also be of interest.
For permanent monitoring and integration into controllers or control systems, humidity sensors / dew point sensors are useful. They can measure continuously and, depending on the version, provide limit values, dew point or output signals for further processing.
If measured values need to be standardized and transmitted to controllers, data loggers or control systems, transmitters can be used. They convert sensor signals into suitable output signals and enable structured integration into existing automation or monitoring systems.
Conclusion: Humidity monitoring protects against invisible causes of failure
Moisture inside the control cabinet is a serious risk for electronics, terminals, connectors and control systems. A high IP rating may prevent water from entering from the outside, but it does not automatically solve the problem of trapped moisture, temperature changes and condensation inside.
Cooling phases, weekend shutdowns, outdoor installations, unheated rooms and systems with changing operating states are particularly critical. When the temperature drops and the relative humidity rises, the dew point can be reached. Condensation then forms exactly where sensitive electronics are supposed to function reliably.
Control cabinet heaters, suitable ventilation, cooling units, pressure compensation, humidity sensors, data loggers and dew point monitoring help detect and reduce the risk. Long-term recording is especially valuable because moisture problems often occur only at certain times and are no longer visible during later troubleshooting.
Anyone who wants to avoid failures caused by condensation should not leave the control cabinet climate to chance. Temperature and humidity should be monitored where critical conditions can occur. This makes it possible to detect moisture problems before they lead to corrosion, leakage currents or electronics failures.
FAQ: Frequently asked questions about moisture in control cabinets
Why does moisture occur inside a control cabinet despite IP protection?
IP protection mainly prevents the ingress of foreign bodies and water from the outside. However, moisture may already be trapped inside the control cabinet or introduced by temperature and pressure changes. When the temperature drops, this moisture can condense.
What is the most common cause of condensation inside a control cabinet?
Condensation is often caused by temperature changes. Warm, humid air inside the control cabinet cools down, relative humidity rises, and when the dew point is reached, water forms on colder surfaces.
Why do moisture-related faults often occur after weekend shutdowns?
During operation, electrical components generate heat. After shutdown, this heat disappears and the control cabinet cools down. This increases relative humidity, and condensation can form. Faults then occur when the system restarts.
What damage can moisture cause inside the control cabinet?
Moisture can cause corrosion, leakage currents, insulation problems, contact faults, controller malfunctions, power supply failures, relay problems and communication errors.
Does a control cabinet heater help against condensation?
Yes, if it is correctly designed and controlled. A control cabinet heater can keep the internal temperature above the dew point and thus prevent condensation.
When is a hygrostat useful?
A hygrostat is useful when heating or ventilation is to be switched depending on humidity. A combined assessment of temperature, humidity and dew point can be even more meaningful.
Is a filter fan enough to prevent moisture?
Not always. A filter fan can remove heat and moisture, but it can also introduce humid ambient air. Whether it is useful depends on the environment, humidity, dust load and control cabinet concept.
What is the dew point inside a control cabinet?
The dew point is the temperature at which the air is saturated with moisture. If this temperature is reached or undercut at a surface, condensation can form.
How can I prove moisture problems?
The best method is long-term measurement of temperature and relative humidity. A data logger shows whether critical values occur during night-time cooling, shutdowns or weekends.
Where should a humidity sensor be placed inside the control cabinet?
The sensor should be mounted at a representative or deliberately critical location, for example in poorly ventilated areas, near cool enclosure surfaces or where moisture traces have already appeared. It should not be placed directly in the special airflow of a heater or fan if this would not provide a representative value.
Which measuring instruments are suitable for troubleshooting?
For troubleshooting, humidity data loggers are suitable, for example the testo 175 H1 or the testo 176 H1. For permanent monitoring, humidity sensors / dew point sensors and suitable transmitters are useful.
Can a control cabinet be too tightly sealed?
A sealed control cabinet provides good protection against external influences, but it can also trap humid air inside. Without pressure compensation, heating or humidity monitoring, temperature changes can still lead to condensation.