Institute of Measurement and Control – Ex Special Interest Group
17 June 2020
The Institute of Measurement and Control Explosive Atmospheres Special Interest Group (Ex-SIG) aims to promote good practice and support continuing professional development in the Ex discipline through a range of activities and publications.
(Click here to view article in digital edition)
The group is producing a series of briefing notes to help inform members on key topics. These are first released to members of the SIG before being made publicly available. An example of one such briefing note is included opposite.
The InstMC Ex-SIG won the Best User Application award at the Hazardex 2020 Awards for Excellence, which took place at the Hazardex Conference & Exhibition in Harrogate, UK on February 26. The Hazardex awards are designed to recognise excellence in the hazardous area sector and has become the benchmark for those supplying products, services and systems. More details can be found on pages 30 and 31 of the May issue.
The Institute of Measurement and Control also offers a professional level qualification in Ex matters: ‘Registered Explosive Atmospheres Engineer’. This is available to InstMC members who are registered with the Engineering Council as IEng/CEng, and who can demonstrate appropriate ‘competence & commitment’ in the Ex discipline, together with ongoing engagement at a professional level.
The qualification is based on peer review in the same manner as IEng/CEng, with a similar application process, and is overseen by the Institute’s Professional Registration Committee.
The Institute has a range of Special Interest Groups which members of the Institute can join for free as a membership benefit:
• Explosive Atmospheres
• Functional Safety
• Digital Transformation
Further details may be found at the Institute’s website www.instmc.org
InstMC Ex-SIG Briefing Note
Intrinsically Safe Junction Boxes
Harvey T. Dearden – HTS Engineering Group
The notion that a junction box containing more than one intrinsically safe circuit must be certified may be gaining currency and may be being actively taught on some courses.
There is however no such requirement within the IEC 60079 standard.
The EEMUA publication 214 ‘A Toolbox Guide for potentially explosive atmospheres’, Edition 7, published in 2016 says (4.4.3.v) of ‘terminal boxes’:
Where the terminal box contains more than one intrinsically safe circuit, the enclosure shall meet the requirements of IEC 60079-0. This effectively means that, under these circumstances, the enclosure should be certified.
Part 0 (General Requirements) does not specify particular requirements in relation to enclosures with more than one intrinsically safe circuit.
‘Effectively means’ is a judgement of the author(s).
Part 25 (Intrinsically safe electrical systems) and Part 11 (Equipment protection by intrinsic safety "i") do not stipulate any explicit requirement for certification of intrinsically safe junction boxes containing more than one circuit.
Part 11 6.1 says that where a non-portable enclosure meets the ingress protection and separation requirements that ‘The enclosure does not need to be subjected to the tests for enclosures in IEC 60079-0’. (E.g. impact resistance).
Part 14 16.5.3 ‘Terminal boxes with more than one intrinsically safe circuit’ says:
Except where an assessment of the combination proves that the intrinsic safety of a combination of intrinsically safe circuits is not impaired, in order to maintain the requirements for intrinsic safety, terminal boxes shall comply with the following minimum requirements:
– In addition to the enclosure requirements of 16.5.1 the enclosure shall meet the requirements as given in IEC 60079-0 for non-metallic enclosures and non-metallic parts of enclosures, metallic enclosures and metallic parts of enclosures as appropriate.
This section of the standard says in a note ‘The use of an increased safety enclosure with suitably rated increased safety terminals will satisfy the requirements of 16.5.2 (single i.s. circuit) and 16.5.3. (multiple i.s. circuits as above).
It does not explicitly require certification but points to Ex ‘e’ as one means of meeting the requirements.
Part 25 Annex B (Assessment of circuits with more than one source of power) says:
‘Where there is more than one source of power and the interconnections are made under controlled conditions so as to provide adequate segregation and mechanical stability in accordance with IEC 60079-11, then the interconnections are considered to fail to open and short circuit but not so as to reverse the connections or to change a series into a parallel connection or a parallel connection into a series one. Interconnections made within a rack or panel constructed in a location with adequate quality control and test facilities are an example of the degree of integrity required.’
So, since an exception per Part 14 16.5.3 may be made when ‘assessment of the combination proves that the intrinsic safety of a combination of intrinsically safe circuits is not impaired’ a terminal box providing ‘adequate segregation and mechanical stability in accordance with IEC 60079-11’ does not require consideration of series/parallel/reverse connections and intrinsic safety would not be considered impaired by the combination.
There is no mandatory requirement for certification of ‘simple apparatus‘ junction boxes that contain multiple intrinsically safe circuits that are of appropriate good practice design. Such a requirement could, particularly if applied retrospectively, have profound implications for existing installations, with potentially very extensive disruption and expense with little or no benefit and the introduction of risk arising from disturbance to established installations.
Note also that Part 17 (Inspection) explicitly says in the introduction:
Inspections are carried out in accordance with this standard, however for older installations the details for the equipment and installations requirements should be referenced to the standards applied at the date of the installation.
So regardless of any other consideration, a legacy i.s. junction box should not be failed just because it is not certified.
This document is distributed by the Ex-SIG as an information service to the SIG membership. No guarantee is made by the institute or the author(s) concerning the accuracy, reliability or completeness of the information provided. This document should not be construed as providing advice. Readers should satisfy themselves of the applicability of the information provided. Readers make use of the information provided at their own risk.
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