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

New rugged indicator

Fig 1 New rugged indicator

Displaying a 4-20mA process variable or control signal in a hazardous area is a well established technology using certified indicators, often loop powered, which are available from BEKA associates and other manufacturers. These instruments are usually protected by intrinsic safety or housed in a flameproof enclosure and satisfy the requirements for most panel and field mounting applications. However there is also a need to display process variables associated with hazardous area equipment housed in an increased safety Ex e panel enclosure or cubicle for which most existing panel mounting indicators are not suitable. Ex e panel enclosures may be located in Zone 1 or 2 housing Ex e certified switches, panel lamps and terminals often associated with the control of electrically operated pumps and compressors for which a digital display may be required to show variables such as pressure, flow or temperature.

The requirements for Ex e enclosures are defined in IEC 60079-0 and IEC 60079-7. Equipment containing semi-conductors, open contacts or moving coils, such as those used in measuring instruments, are not permitted within an Ex e enclosure unless they have an additional form of protection such as Ex d flameproof, Ex m encapsulation or Ex i intrinsic safety. For equipment installed in an Ex e panel enclosure located in Zone 2, type of protection Ex n is also acceptable. Ex e enclosures are required to provide 7J impact protection and IP54 ingress protection, although many industrial Ex e panel enclosures will have greater ingress protection permitting outdoor installation.

Whilst it is possible to install a conventional BEKA intrinsically safe panel mounting instrument into an Ex e certified panel enclosure, standard instruments such as the BA307E loop powered indicator do not provide the impact protection required to maintain the Ex e panel enclosure certification. A certified Ex e external transparent cover would overcome this difficulty, but would prevent the instrument's front panel controls being operated.

Instrument enclosure test sequence

Fig 2 Instrument enclosure test sequence

For an Ex e panel enclosure located in Zone 2, where instrumentation is increasingly installed to avoid having operators in Zone 1, a certified Ex n instrument should maintain the Ex e panel enclosure's impact and ingress protection as the two protection techniques have similar ingress and impact requirements. Although for Zone 2 applications Ex n loop powered indicators provide an economically attractive alternative to intrinsic safe instruments, until the recent introduction of the BEKA 4 digit BA307NE and 5 digit BA327NE loop powered 4-20mA indicators, panel mounting models with third party Ex n certification were not readily available.

Following requests from users for solutions to these Ex e display requirements, BEKA have designed and obtained third party certification for a range of new rugged display indicators tailored for these applications. This article considers their design and installation, although these are 4-20mA loop powered indicators the philosophy applies to all similar display instruments. There are two basic design requirements:

1. As Ex e enclosures do not exclude the hazardous atmosphere in which they are installed the indicators additional method of protection must be suitable for the Zone and gas in which the Ex e panel enclosure is located.

2. The installation of the indicator in the Ex e panel enclosure must not invalidate the Ex e enclosure's certification.

For Zone 2 applications Ex nA (non-sparking) protection of the 4-20mA indicator was chosen as the most economic method of achieving safety. For Ex e panel enclosures located in Zone 1 intrinsic safety Ex ia was selected, although intrinsic safety can also be used for Zone 2 applications.

To ensure that installation of the indicator does not invalidate the Ex e panel enclosure certification, the front of the 4-20mA indicator must provide 7J impact and IP66 (IP54 minimum) ingress protection.

The new BEKA 4-20mA loop powered indicators are controlled by four front panel push buttons which are protected by an elastomeric cover and the instrument is sealed to the Ex e panel enclosure in which it is mounted with a silicone gasket. Inclusion of these plastic components requires that the instrument enclosure satisfies both the metallic and non-metallic enclosure certification requirements specified in EN 60079-0.

These are summarised in Fig 2 and involve thermal endurance at 82°C, followed by impacting at high and low temperatures, after which the enclosure must still provide the required ingress protection.

Installation of Ex nA Indicator in Ex e panel enclosure located in Zone 2
Fig 3 Installation of Ex nA Indicator in Ex e panel enclosure located in Zone 2