Most engineers intuitively know that it is good practice that electrical panels should be designed such that their internal environment is controlled. They would understand that limits to working temperatures for components within a panel are essential if the overall reliability is to be acceptable for the 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. However, for the majority of seemingly less arduous applications, climate control is perhaps not rigorously addressed , even though the trend towards more miniaturisation of components and enclosures, together with higher packing densities, suggests the need.
Making the case
Justifying the modest cost and effort incurred in adding climate control to a panel, might best start with considering a couple of well-respected references.
The first is the 10-Degree Rule, which suggests that for every 10°K rise in temperature, the average reliability decreases by 50%. Put another way – if the temperature is lowered by 10°K , the reliability should 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, which shows 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. This is a scenario that does periodically happen – usually much to the surprise of the designer!
The practical implementation
Some form of climate control within a panel or cabinet is therefore probably desirable, and quite possibly essential. And 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 and 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 suitable thermostat
Chose a thermostat 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 4 models.
Firstly there are two models, both with a 0°C to 60°C setting range, for Maximum (Ventilating) and Minimum (Heating) respectively. Then there are two further models but 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, 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.
