The use of 4-20mA indicators in hazardous areas



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Figure 3 illustrates a typical application of an 'nA' certified indicator and the following notes consider the majority of factors, which have to be taken into account when considering such a system.

Certified 'nA' indicators mounted in Zone 2 can provide indication of the 4-20mA signal from equipment in the safe area and equipment in the hazardous area using any method of protection [Ex d, Ex e, Ex m, Ex p and Ex n] other than intrinsic safety. The essential difference in the principles of the method of protection used in intrinsic safety from that used in the other methods of protection prevent the use of 'nA' instruments in intrinsically safe circuits. Similarly, Intrinsically safe instruments cannot be readily used with equipment using other methods of protection.

Reference 2, provides a detailed explanation of how certified 'nA' indicators may be used to display the output from hazardous area instrumentation protected by other techniques, such as flameproof and increased safety.

Maintenance
Indicators are usually mounted in readily accessible places in Zone 2 for operational reasons. This combined with the fact that the indicators usually have separate terminal compartments makes this a convenient point to fault find. Intrinsically safe circuits permit any action to assist fault finding and the removal of equipment without isolation. Other types of circuit permit fault finding on live circuits in Zone 2 providing that a risk assessment has demonstrated that the process cannot cause ignition. For example measuring voltage with an intrinsically safe meter is acceptable but disconnecting wiring is not. The use of switch-fused terminals in the circuits using 'nA' indicators enables them to be isolated when maintenance requires it. This is important since 'gas clearance' certificates are difficult to obtain when the hazardous area equipment is dispersed.

Conclusion
The use of certified 'nA' indicators to provide digital indication of 4-20mA signals derived from hazardous area equipment using a method of protection other than intrinsic safety, together with the more common intrinsically safe solution, means that an acceptable solution to all such requirements exists.

Epilogue
The IEC intend to convert the 'Ex nA' requirements into 'Ex ec' requirements for no very good reason. Fortunately it will probably take another four years, which should give time to organise a smooth changeover. Since it is not anticipated that there will be any significant changes in requirements this changeover should not be too difficult. However it is possible that the light current aspects will tend to be neglected in favour of the more economically significant heavy current applications.

 

If the changeover follows the pattern of the 'nL' to 'ic' changeover then the problem will be the ATEX requirements rather than IEC Ex requirements. Clear guidance of the acceptability of both methods of protection during the changeover period to end users and manufacturers at an early date would make all the difference.

References
1  BEKA associates Application Guide AG300
    Intrinsically safe loop powered indicators.

2  BEKA associates Application Guide AG310
    Installation of [extra low voltage d.c.] Ex nA
    instrumentation.

These references and also the Ex certification of the indicators can be downloaded from the website
www.beka.co.uk.

The author

Chris Towle

Chris Towle BSc, CEng, MIMech.E, MIET, Hon FInstMC.

This article was written by Chris Towle who has many years experience working with hazardous area instrumentation. His involvement in standards began in 1961 on the intrinsic safety committee of BSI and was followed shortly after by joining the main and code of practice committees. From 1970 he became involved in both IEC and CENLEC intrinsic safety committees and was secretary of both for a fifteen year period. He is still actively involved in all three organisations.

 
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originally printed in Hazardex, March 2013