Most engineers intuitively know that it’s a good idea that electronic or electrical, panels or cabinets be designed such that their internal environment is both known, and controlled. They understand that there has to be limits to working temperatures for components within a panel if the overall reliability of the equipment is to be acceptable for the given application.

Of course, in many industries and applications, particularly those that are safety critical, it is almost certain that within a design process, running from an initial risk analysis of the application to MTBF reliability calculations for the final equipment, the aspect of the macroclimate will have been thoroughly considered.

However, for the majority of seemingly less arduous applications, climate control is perhaps not addressed in a particularly rigorous way, even though the trend towards more miniaturisation of components and enclosures, together with higher packing densities - probably means that it should be.

Quantifying the need

If there is any need to justify the modest cost and effort incurred in adding climate control to a panel or cubicle, then here are a couple of well-respected references that are worth reviewing:
The first is the 10-Degree Rule. This Rule of thumb says that for every 10°K rise in temperature, the average reliability is decreased by 50%. Put another way – if we can lower the temperature by 10°K we can expect the reliability to double. This is primarily a rule associated with electronics, but it applies more or less to other components where there is the possibility of failure due to electro-chemical action. A good example is electromechanical components such as relays, switches or contactors - switching low or modest levels of voltage and current. They can be particularly affected by corrosion, electrolytic action, and the formation of oxides and sulphides.
Another long established source of information is the MIL-HDBK-217D reliability handbook. Here you can find mathematical expressions for calculating the failure rate for numerous electrical and electronic components and equipment under various scenarios of application and environment. The difference here is that the part failure modes stated in the Handbook derive from experience, rather than being theoretically based. As an example, the part failure mode (ʎT) for a relay shows the improved reliability due to a 10°K reduction in ambient temperature to be 1.6. Good corroboration of the 10-Degree Rule’s value of 2. Although there is no specific part failure rate given for humidity, it is part of the environmental failure mode (πE), and the values for this distinguish between a climatically controlled and a non-controlled environment by a factor of 2.3. The importance of humidity on reliability is therefore clearly evident.
It is important also to appreciate that high temperatures are not necessarily just a matter for long-term equipment reliability. Exceeding a component’s maximum temperature characteristic may make it fail to function - almost immediately. A good example is the case of the minimum operate characteristic for an electromechanical relay. Although specified in terms of voltage, it is in fact a current driven device. Consequently, even though the appropriate coil voltage may be applied, under too high temperatures, such as might occur in a desert oil installation, the coil resistance rises too high limiting the coil current, such that the relay fails to operate. This is a scenario that does periodically occur – usually much to the surprise of the designer!

Practical implementation

It should by now be apparent that some form of climate control within a panel or cabinet is probably desirable, and quite possibly, it is essential. The three likely approaches are:

• Fit a maximum (or ventilating) thermostat working in conjunction with a cabinet fan to ensure high equipment reliability, and/or to ensure component characteristics are achieved.

• Fit a minimum (or heating) thermostat working in conjunction with a panel heater, primarily to ensure that the internal temperature does not fall below the “dew point” temperature. Whilst high levels of relative humidity (RH), particularly in conjunction with temperatures in excess of 65ºC are not desirable, it is particularly important that whatever level of a water vapour is held suspended in the air, it is not allowed to condense out on the equipment, for obvious reasons – not least being the short and long term effect on safety and functional insulation

• Fit both ventilating and heating thermostats, where it is necessary, or use a combined unit if available.

Selecting a suitable thermostat

Choose a thermostat from a range specifically designed to be used and mounted within an enclosure. It should be simple and compact, and fit directly to a 35mm rail. A bi-metal sensing element will provide a well-proven and reliable switching element completely free of any electronic circuitry, totally suited to its function and delivering a long electrical switching life. And, for convenience, select from a range that offers combined heating and ventilating thermostats within the same body.

A temperature setting range of 0ºC to 60ºC will generally satisfy the majority of applications. If the RH is generally high it will allow the heating thermostat to be raised towards the ventilation setting, minimising the temperature differential and consequently the likelihood of condensation. If the likelihood of high RH is low then it may be better to widen the two set temperatures, which, whilst still keeping the components within specification will permit a wider temperature swing and consequently a lower switching rate, and higher electrical life for the thermostats.

Heaters and fans

The amount of detail behind sizing the heater or fan would run beyond the available space for this article, but suffice to say there are a number of factors that need to be considered and characteristics that need to be known. Briefly, the data needed:

• Estimate of likely RH and appropriate target for maximum permitted internal temperature differential.

• Dimensions of the enclosure.

• Enclosure position relative to other adjacent enclosures.

• Enclosure heat transfer coefficient (dependent on material).

• The desired temperature difference (internal to external of the enclosure).

• Internal self-heating power dissipated by components.

Final thought

Overall, controlling the climate within an electrical panel need not be expensive, nor a chore once its been done a few times. Furthermore the benefits in terms of increased reliability, reduction or elimination of downtime and on-site rectification costs plus other consequential costs, might make it an imperative.