Design & installation of 4-20mA loop powered indicators for use in Ex e enclosures



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To satisfy these severe requirements, BEKA have developed a cast stainless steel panel mounting instrument housing with a 10mm thick toughened glass window. This has been assessed by Notified Body Intertek Testing and Certification who have issued IECEx and ATEX Component Certificates confirming compliance with the standards. These Component Certificates have been used to support the Ex nA non-sparking certification of the BA307NE and BA327NE and the Ex ia intrinsically safe certification of the BA307E-SS and BA327E-SS. A BA307NE is shown in Fig 1. Installation of a 4-20mA indicator in an Ex e panel enclosure located in Zone 2

Installation of a 4-20mA indicator in an Ex e panel enclosure located in Zone 2

Either the non-sparking Ex nA BA3x7NE or the intrinsically safe Ex ia BA3x7E-SS loop powered indicators may be safely installed in an Ex e panel enclosure located in Zone 2. The Ex nA certified models provide the most economic solution as an intrinsically safe interface such as Zener barrier or galvanic isolator is not required.

Fig 3 shows the circuit of a typical Zone 2 installation including a BA307NE Ex nA loop powered 4-20mA indicator. The loop may be powered from a CE marked 24V instrument power supply with acceptable isolation from the mains. If the supply is located within the Ex e panel enclosure it should have an additional form of protection such as Ex d flameproof, Ex m encapsulation or Ex nA non-sparking.

Installation of Ex i indicator in Ex e panel enclosureFig 4 Installation of Ex i indicator in Ex e panel enclosure

Each 4-20mA lead is protected from current overload by a fuse which, if installed in a fused terminal, is a convenient way of isolating the loop for maintenance, although an miniature circuit breaker (MCB) or electronic current limit are preferred by some users. The indicator's front panel push buttons, which are outside of the Ex nA instrument enclosure, are non-incendive and have been certified Ex ic [intrinsically safe in normal operation] resulting from the current and voltage limiting within the instrument.

A very useful, but frequently not appreciated feature of Ex nA equipment is that although it may only be installed in Zone 2, it may be directly connected to suitably protected non intrinsically safe apparatus located in Zone 1 as shown in Fig 3. BEKA associates application guide AG310 provides further information about the design and installation of Ex nA extra low voltage dc instrumentation.

Installation of a 4-20mA indicator in an Ex e panel enclosure located in Zone 1

Only the intrinsically safe BA3x7E-SS loop powered indicators may be installed in an Ex e panel enclosure located in Zone 1. As Zener barriers and galvanic isolators are not permitted in an Ex e enclosure located in Zone 1, these must be mounted in the safe area or in a suitable enclosure in Zone 2. Intrinsically safe apparatus, terminals and wiring should be segregated from non intrinsically safe apparatus mounted in the Ex e panel enclosure. If live maintenance is anticipated, bare intrinsically safe conductors should be protected with an IP30 cover and labelled.

Intrinsically safe BA307E-SS and BA327E-SS indicators may also be installed in an Ex e panel enclosure located in Zone 2, the installation requirements are the same as for Zone 1 except that some Zener barriers and galvanic isolators have been certified for installation in a Zone 2 enclosure and may therefore be installed in the same panel enclosure as the indicator which is useful when displaying a variable derived from within the panel enclosure.

Application Guides

BEKA associates publish two application guides which include examples of rugged loop powered indicator installations in Ex e enclosures. AG300 describes the use of intrinsically safe systems and AG310 describes the installation of [extra low voltage dc] Ex nA instrumentation. Both may be downloaded from the BEKA website.

Accessories and other applications

These new rugged indicators which included four digit instruments and five digit models with a bargraph, can be supplied with a factory fitted display backlight and dual alarm outputs. In addition to installation within an Ex e panel enclosure they may also be installed in Ex p pressurised enclosures and their rugged construction and stainless steel housing are ideal for conventional intrinsically safe applications in harsh and marine environments.

Rugged indicator

 

Based on an article written by Chris Burkitt CEng, MIET which was first published in the March 2015 edition of Hazardex.

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