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Nothing to get hot about

Author : Jim Munro, Chairman, IEC Committee TC31

01 August 2013

The whole subject of temperature gets complicated when it comes to Ex equipment. I don’t intend to cover the whole subject, but I will look at some aspects are part of our standards and some new work that is emerging.

TC 31 recognised the need to further explore the impact of very cold conditions on explosion protected equipment
TC 31 recognised the need to further explore the impact of very cold conditions on explosion protected equipment

Perhaps one of the most misunderstood things is the acceptable temperature ranges that apply to Ex equipment and the relationship of this to temperature classification.  Most of the key requirements related to temperature as applied to Ex equipment is contained in the general requirements document, IEC 60079-0.

In the scope it states that the standard atmospheric conditions (relating to the explosion characteristics of the atmosphere) in which it may be assumed that electrical equipment can be operated are a temperature –20 °C to +60 °C.

But in a note it states that the normal ambient temperature range for the equipment is –20 °C to +40 °C, unless otherwise specified and marked.  Why is this so?  

Well it goes on to say ‘It is considered that –20 °C to +40 °C is appropriate for most equipment and that to manufacture all equipment to be suitable for a standard atmosphere upper ambient temperature of +60 °C would place unnecessary design constraints’.  For example if might be hard to find materials for that range. It would also make it very difficult to achieve the lower temperature classifications when using a high temperature (+60 °C) as the reference.

However, there is an important reason for specifying the wider range of temperature.  It is to make it clear that all of the explosion protection techniques are assumed to be effective in that range.  Once you get outside the range it may become necessary for the standard for type of protection to introduce additional measures to ensure the technique remains effective.   In practice most of the protection technique standards remain relatively silent on the issue of extended temperature ranges and hence there might be assumed that they can be used in a wider range of temperatures.

One standard, however, that contains considerable information is IEC 60079-1 for Ex d equipment, with information to enable equipment to be properly evaluated for use at temperatures outside the range.  There are three major factors that impact on this technique.  The first is pressure that may be obtained when doing the pressure determination test. The second is the pressure at which the pressure test is done.  This is really an extension of the first as it is done at the pressure obtained in the pressure determination test times a factor of safety.  However, an additional factor of safety is possible due to the impact of the different temperatures on the strength of the material of the enclosure.  The third major factor is the non-transmission tests.  The length and gap for a flamepath may need to change to accommodate changes in the dynamics of how the flamepath operates at different temperatures.   The standard also contains information to assist in being able to make test at ambient temperature by applying factors of safety.  But there are instances where this is not predictable enough and tests need to be done at the actual temperature limits.  

With the developing frontiers for activities such as exploration and exploitation of oil in very cold areas, like those experienced in Arctic regions, TC 31 recognised the need to further explore the impact of very cold conditions on explosion protected equipment.  An ad hoc working group was established to invest and report.  Based on the report from this ad hoc working group, the ad hoc was replaced with a formal working group WG39 Adverse conditions.  The task for this group was expanded from just very low temperature to the wider of adverse conditions as follows:
To investigate the issues associated with the influence of environmental factors in adverse service conditions related to equipment, installation and maintenance in the IEC 60079 series and ISO/IEC 80079 series.

The first document produced by this group was been circulated to national committees for comment as IEC document 31/1047/DC.  These comments were considered by the working group in March this year.  The next step will be to develop the document as a technical speciation focusing initially on low temperatures.  This will be further considered by the working group at its meeting in New Delhi, India in October this year.    It is also likely that we will see additional requirements in our TC 31 standards in the future for the various types of protection for very low temperatures based on the work of this group.

There is no update on standards for this article as there has been no change since the last article. 

Standards referenced above:
1. IEC 60079-0 Explosive atmospheres – Part 0: Equipment – General requirements
2. IEC 60079-1 Explosive atmospheres –Part 1: Equipment protection by flameproof enclosures “d” 

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