HAZARDOUS AREAS & SAFETY FEATURE DISPELLING SAFETY MYTHS


Roger Highton, MTL product line manager for intrinsic safety at Eaton, shares his pick of the most common misunderstandings connected with designing for Intrinsic Safety


P


rocessing environments are potentially hazardous places in which


to work. Intrinsic safety (IS) is an electrical design approach that prevents explosions from occurring by ensuring that the energy transferred to a hazardous area is well below the energy required to initiate an explosion. It provides a solution to all the problems of hazardous areas (for equipment requiring limited power) and is the only technique which meets this criterion. In addition, the IS technique is universally accepted and is incorporated under all local legislation: including ATEX and OSHA. Despite this, common misunderstandings about IS persist. Here are my top five:


the relevant safety requirements, then check the practical aspects, such as will the device function properly? For interfaces, this requires careful checking of operational parameters such as voltage and current. Myth #3 – Zener barriers and


isolators are interchangeable IS Zener barriers and IS isolating barriers


have very different installation and maintenance requirements, so it is important to understand them. Most IS Zener barriers are simple, versatile, loop powered interfaces that require a tightly controlled power source, with limited voltage available in both hazardous and safe area connections. They require a


‘The IS system designer must accept responsibility for the adequacy of the design and the safety implications of the use of the system in association with hazardous areas’


Myth#1 – Adding an IS interface will


make my equipment intrinsically safe This is a common myth, and a


potentially dangerous one. Adding an IS interface (isolator or barrier) does not make any equipment intrinsically safe. Mitigation of explosion risk can only be achieved by installing equipment that has been specifically designed to meet IS requirements, along with a suitable IS interface. Where intrinsically safe equipment is interconnected by wiring, the safety of each piece of equipment is affected by the performance of the other pieces of apparatus in the circuit. The IS technique relies on the system being correctly designed and intrinsic safety becomes a system concept. Myth #2 – The design is complete if


the safety description of the IS interface is compatible with the IS device Checking that the voltage, current and


power outputs of the IS interface is less than the voltage, current and power inputs of the IS device alone does not guarantee that components selected for IS environments will function effectively. System designers must first ensure that the device is fully compliant and meets


safety earth that is regularly tested as it is a fundamental requirement of the safety of this technique. IS isolators are more complex, with a shorter Mean Time To Failure (MTTF). They are application specific, can be used with a wide range of power supplies, can provide higher voltage in both hazardous and safe area connections, and simplify regular inspection requirements. Myth #4 – If the equipment is


certified IS, it doesn’t matter what cabling you choose Because cables have inductance and


capacitance, and hence energy storage capabilities, they can affect system safety. Consequently, the system design imposes restrictions on the value for each of these parameters: but only rarely is there a serious limitation placed on the available cable. Intrinsic safety does not require mechanical protection of the cable with armour or conduit, permitting the use of conventional instrumentation cables and thus reducing costs. The cable parameter checks are straightforward, simply requiring that the capacitance and inductance of the cable and field devices is less than the capacitance and


/ PROCESS&CONTROL


The IS technique is universally accepted and is incorporated under all local legislation: including ATEX and OSHA. Despite this, common misunderstandings still persist


inductance allowed for the IS interface for the Gas Group in which the equipment is installed. Usual practice is to calculate the maximum cable length allowed for a particular installation, ensuring this is not exceeded when designing the cable runs. Myth #5 – Under Ex ic, the IS system


rules are not as strict in Zone 2 as they are for Zone 1 This myth arises from a


misinterpretation of the Ex designations. Intrinsic safety utilises three levels of protection, ‘ia’, ‘ib’ and ‘ic’ which balance the probability of an explosive atmosphere being present against the probability of an ignition-capable situation occurring. The use of these levels of protection ensures that equipment suitable for each level of risk is available (normally ‘ia’ is used in Zone 0, ‘ib’ in Zone 1 and ‘ic’ in Zone 2). The Zone 2 designation indicates that the risk of an explosive is infrequent. There are clear guidelines on how intrinsically safe equipment should be installed and maintained in Zone 2 designated areas. The IS system designer must accept


responsibility for the adequacy of the design and the safety implications of the use of the system in association with hazardous areas. The designer must therefore have an appropriate level of knowledge and training and should recognise the importance of getting the analysis right. The analysis of simple systems is


relatively easy and can be done by any competent professional engineer. Sourcing IS equipment from reputable manufacturers such as Eaton MTL may provide further reassurance and expertise. For more complex systems – such as those using a combination of non-linear and linear sources of power where a greater degree of experience is required – it may be desirable to approach an ‘approved certification body’ to provide an analysis for such a system.


Isolating barriers and active barriers such as Eaton’s MTL5541 provide a guaranteed minimum voltage, which allows a greater operational margin in IS applications


Eaton www.eaton.uk.com


PROCESS & CONTROL | OCTOBER 2020 17


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