An alternative approach to Zone 1 display instrumentation
22 July 2020
Flameproof instruments are fairly simple to understand and apply, do not require Zener barriers or galvanic isolators and they do not involve users with the perceived complexity of entity and cable parameters. Worldwide, flameproof Ex d explosion protection probably remains the predominant means of preventing Zone 1 and 2 field mounting instrumentation igniting a flammable gas atmosphere.
Figure 1 – Alternative Zone 1 instrument design.
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However, for hazardous area instruments incorporating a display such as indicating transmitters and loop displays, small flameproof enclosures do not lend themselves to accommodating large windows through which displays can be easily seen. A new alternative design based on increased safety Ex e protection overcomes this limitation, allows tactile push buttons to be used and reduces the instrument cost. The resulting instrument may be installed in a Zone 1 or 2 hazardous area and used in the same way as an Ex d explosion proof instrument.
Increased safety Ex e
Increased safety Ex e is an explosion protection technique which when applied to electrical equipment, provides increased security against the possibility of excessive temperatures and against the occurrence of arcs and sparks. It has traditionally been used for protecting Zone 1 and 2 electrical machines, luminaires, trace heating and is widely used for protecting terminal enclosures. Until recently, Ex e has rarely been used for protecting low power instrumentation.
The latest edition of the IEC increased safety standard IEC 60079-7:2015 introduced two levels of increased safety Ex e protection:
Ex ec (Equipment Protection Level EPL 'Gc')
For applications in Zone 2
Intended as replacement for Ex nA protection.
Electronic components such as semiconductors and capacitors may be used.
Ex eb (Equipment Protection Level EPL 'Gb')
For applications in Zone 1 and 2
Excludes the use of electronic components such as semiconductors and capacitors.
Ex e increased safety relies heavily on the integrity of the equipment enclosure to protect the electrical equipment within the enclosure, although the explosive atmosphere is not excluded from the enclosure. Unlike most flameproof enclosures which are manufactured from aluminium or steel, an increased safety Ex eb enclosure can be compression moulded in glass reinforced plastic (GRP) with provision for a large toughened glass window. If required, an elastomeric keypad to activate internal switches for the control and calibration of the instrument while it is in the hazardous area can be incorporated.
Figure 3 – BA304SG Ex eb loop powered indicator.
To ensure that safety is maintained in an industrial environment during the lifetime of the product, the IEC increased safety standard IEC 60079-7:2015 requires non-metallic Ex eb and Ex ec enclosures to be subjected to both lengthy thermal endurance at high temperature and high humidity and to a period at a very low temperature. This is followed by subjecting the enclosure to 7J impacts at above the maximum and below the minimum operating temperature before measuring the water and dust ingress protection provided by the enclosure. Although the standard only requires a modest IP54, most certified GRP Ex eb enclosures satisfy IP66 requirements and provide similar impact protection as a metal flameproof enclosure.
Alternative Zone 1 design
This alternative Zone 1 instrument design, which is housed in an Ex eb enclosure, is shown in Figure 1. To allow installation in Zone 1, the electronic assembly within the Ex eb enclosure requires protecting by an additional method of explosion protection. Although any protection technique suitable for use in Zone 1 could be used, the low voltage and power consumption of modern electronics make intrinsic safety Ex ib and encapsulation Ex mb particularly attractive. To provide maximum flexibility, this new alternative instrument design employs both Ex mb and Ex ib explosion protection techniques.
Energy limiting components are mounted within an encapsulated Ex mb assembly with an intrinsically safe Ex ib output. The encapsulation prevents a potentially flammable gas atmosphere accessing the energy limiting components and the intrinsically safe output is nonincendive allowing a wide range of electronic components including a display to be connected to it. The capacitance and inductance of the instrument display, associated electronics and the push button switches comply with hydrogen IIC intrinsic safety limits, and power is limited to prevent excessive surface temperatures, ensuring that the circuits remain safe even under fault conditions.
Advantages of the alternative design
Olivier Lebreton, Managing Director, BEKA associates.
This alternative instrument design uses multiple well-established explosion protection techniques to produce a lower cost, drop-in alternative for a flameproof Ex d instrument. Providing a much larger display and tactile push buttons the technique is suitable for many types of hazardous area instruments including fieldbus indicators, indicating transmitters and loop powered indicators. The new design has the additional advantage that only Ex eb field wiring glands or conduit fittings are required, eliminating the need for Ex d glands. Like the explosion proof Ex d instrument they replace, instruments employing the new alternative design do not require protection by a Zener barrier or galvanic isolator. They may be installed in Zones 1 or 2 and may be connected to almost any other certified instruments having flameproof Ex d, increased safety Ex e or pressurised Ex p protection, but they cannot be used with intrinsically safe instruments or systems.
First new models
The new design technique has led to the production of two IECEx and ATEX certified Ex eb loop powered 4-20mA indicators which are a lower cost alternative for flameproof Ex d indicators. The new models are housed in a robust IP66 GRP enclosure, have a large easy to read display, tactile push buttons and do not require flameproof cable glands.
These new Ex eb models may also be used in place of Ex nA indicators for Zone 2 applications in anticipation of Ex nA non-sparking protection being replaced by Ex ec increased safety protection. This will happen when BS EN IEC 60079-15:2015, which defines Ex nA non-sparking explosion protection, is de-harmonised in April 2022.
About the author:
Olivier Lebreton CEng, MIET, is the newly appointed Managing Director of BEKA associates. Olivier joined BEKA in 2013 and became Technical Director in 2018 before becoming MD in June 2020. As Managing Director, Olivier will be responsible for all company operations, but will initially retain his Technical Director responsibilities. He is a chartered engineer and has 20 years' experience managing design projects for many different applications including, oceanographic equipment, safety critical products and hazardous area instrumentation in France, Canada and the UK.
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