The use of 4-20mA indicators in hazardous areas



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Figure 2 illustrates a typical intrinsically safe system. The permitted location of the field equipment is determined by the system analysis and is unaffected by the inclusion of the indicator. The requirements of intrinsically safe systems are widely documented elsewhere and hence this document gives a more extensive treatment to the less well documented use of 'nA' indicators. Reference1 gives a detailed account of the application of intrinsically safe indicators.

The use of 'nA' indicators
The application of 'nA' indicators is less common than intrinsically safe indicators but they do solve the problem of providing indication in hazardous areas for all other methods of protection. 'nA' instruments use the same basic circuit as intrinsically safe instruments but must meet the more stringent enclosure requirements of the 'nA' standard. There are requirements for impact resistance, ingress protection and in addition for plastic requirements for anti-static properties.

 

The majority of intrinsically safe indicators meet these requirements for operational reasons but they are not all required for compliance with the standard. In practice the only significant difference between the two types of indicator is the incomprehensible label.

'nA' indicators are usually certified by a 'Certified Approval Body' for IECEx and ATEX purposes. The ATEX Directive permits self-certification of Category 3 equipment such as these indicators but increasingly end-user's preference is for third party certification. 'nA' indicators may only be mounted in Zone 2 but since Zone 2 is the logical place for indicators this is not a severe limitation. 'nA' certification is independent of the gas classification but requires temperature classification. Indicators are usually certified T5 or T6, which requires that a maximum input current [100mA], and voltage [30V] are specified.

cont..

Fig 3
 
Figure 3 Typical 'nA' System

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