Despite the trend towards the miniaturisation of components and enclosures, and higher packing densities, climate control is not always addressed in a particularly rigorous way. Norman Carnt, technical manager at Finder, comments.
Most engineers intuitively know that it is good practice for electrical panels to be designed so that their internal environment is controlled. They would 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.
In certain industries and applications, particularly those that are safety critical in some way, it is almost certain that the aspect of the macroclimate will have been thoroughly considered.
For the majority of seemingly less arduous applications, however, 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.
Making the case
When justifying the modest cost and effort incurred in adding climate control to a panel, it might be best to start by considering a couple of well-respected references.
The first is the 10-Degree Rule. This rule of thumb says that for every 10°K rise in temperature, the average reliability decreases 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. One example is electromechanical components such as relays or switches, switching low or modest levels of voltage and current, which can be particularly affected by corrosion, electrolytic action, or the formation of oxides and sulphides.
The second long established source of information is the MIL-HDBK-217D reliability handbook. Here you will 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 ??T for a relay shows the improved reliability due to a 10°K reduction in ambient temperature to be 1.6 – a good corroboration of the 10-Degree Rule value of 2.
High temperatures are not necessarily just a matter of long-term equipment reliability. Exceeding a component’s maximum temperature characteristic may make it fail to function – almost immediately. An example is the case of the minimum operate characteristic for an electromechanical relay. Although specified in terms of coil voltage, it is in fact a current-driven device. Consequently, even with the appropriate coil voltage applied, under too high temperatures such as might occur in a desert oil installation, the coil resistance rise may limit the coil current such that the relay fails to operate.
The practical implementation
Some form of climate control within a panel or cabinet is therefore probably desirable, and quite possibly essential. The simplest mitigating measures that can be taken are:
• Fit a Maximum (or Ventilating) thermostat working in conjunction with a cabinet fan to limit the maximum temperature.
• Fit a Minimum (or Heating) Thermostat working in conjunction with a panel heater. Bear in mind that even if the local ambient temperature is unlikely to fall anywhere near that specified for the equipment, if the Relative Humidity and temperature are on the high side, then the water vapour held suspended in the air will be high. The temperature must not be allowed to drop too much otherwise water vapour will condense out on the equipment – to the short and long term detriment of functional and safety insulation.
Selecting a thermostat
To select a suitable thermostat, choose one 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.
The Finder 7T.81 series of thermostats offer these attributes, over a range of four models.
The first two models, both with a 0°C to 60°C setting range, are for Maximum (Ventilating) and Minimum (Heating) respectively. Then there are two further models for a setting range of -20°C to +40°C.
The 0°C to 60°C setting range will generally satisfy the majority of applications, and is particularly versatile where the Relative Humidity is generally high as it will allow the Heating thermostat to be raised towards the Ventilation setting, minimising the temperature differential and consequently the likelihood of condensation. Alternatively, 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 therefore a longer electrical life for the thermostats.
The -20°C to +40°C models are better suited to colder applications where there is no need, or even no possibility, of trying to maintain a macroclimate of 0°C or above.
Overall, controlling the climate within an electrical panel need not be expensive or a chore, whilst the benefits in terms of increased reliability, reduction or elimination of downtime and on-site rectification cost, might make it an imperative.
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