Wherever flammable gases, dusts, or vapors accumulate in sufficient concentration, there is a risk of fire or explosion. The risk can arise in environments as diverse as a chemical plant (see Photo), a flour mill, a water treatment plant, or a vehicle paint shop. All it takes to set off an explosion is a source of ignition from an electrical spark or even a hot surface. Therefore, those responsible for electrical design, installation, and maintenance play a key role in protecting people, plant, and product from risk.
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. The major advantage of intrinsic safety is that it provides a solution to all the problems of hazardous areas (for equipment requiring limited power) and is the only technique that 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. It’s time to set the record straight because getting IS right can be a matter of life or death. Following are five common myths about IS and how to avoid them.
Myth #1: Adding an IS interface will make my equipment intrinsically safe.
This is a common but potentially dangerous belief. 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 IS 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. Other methods of explosion protection are also dependent on the system concept to some extent, but it is a fundamental requirement of IS.
Myth #2: The safety description of the IS interface is compatible with the IS device, design complete!
Assessing the safety description compatibility by checking that the voltage, current, and power outputs (Uo, Io and Po) of the IS interface is less than the voltage, current, and power inputs (Ui, Ii and Pi) of the IS device alone does not guarantee that components selected for IS environments will function effectively. The first step is to ensure that the device is fully compliant and meets the relevant safety requirements. The second step is all about the practicalities. Will the device function properly? For interfaces, this requires careful checking of operational parameters such as voltage and current.
To illustrate this point, consider an application for an analog input with a 2-wire loop-powered transmitter (see the Figure). This will have a minimum operational voltage, typically 10.5V. The IS interface must be able to supply this voltage after subtracting the voltage drop in the cabling.
Consider a passive barrier for this application, assessing the voltage drop across both channels of the barrier plus the voltage drop across the typical 250Ω safe area load using Ohm’s Law. The calculation is as follows:
VDrop Barrier = Imax x (RChannel 1 + RChannel 2) + VDiode Drop = 20mA x (333Ω + 21Ω) + 0.9V = 7.98 V
VDrop Load Resistor = Imax x RLoad = 20mA x 250Ω = 5V
VDrop Cable = VPower Supply – VDrop Barrier – VDrop Load – VOperating Voltage = 24V – 7.98V – 5V – 10.5V = 0.52V
RCable = VDrop Cable ÷ Imax = 0.52V ÷ (20mA) = 26Ω
Cable length = RCable/ROne km Cable = 26Ω ÷ 50Ω/km = 0.52km
Based on the calculations, you can see that the cable resistance could be 26Ω, which ― with a cable resistance of 50Ω/km ― could support 520m of cable. However, just a small drop (0.6V) in the power supply voltage would cause the failure of the loop.
Isolating barriers and active barriers provide a guaranteed minimum voltage, which for a typical isolator used in this application is a minimum of 16.5V at 20mA. With an available voltage drop for cabling of 16.5V - 10.5V = 6V, this solution provides a much greater operational margin.
Myth #3: Zener barriers and isolators are interchangeable.
There are two types of IS interfaces widely adopted by users of intrinsic safety: IS Zener barriers and IS isolating barriers. Typically, a site will elect to use one or the other, so it is recommended that they continue to use the same interface type. These interfaces 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 safe earth that is regularly tested, as it is a fundamental requirement of the safety of this technique.
IS isolating barriers 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: It doesn’t matter what cabling you use if the equipment is certified IS.
Since 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.
As cable faults are considered during the system analysis, the type of cable in individual installations is not closely specified in the system standard. The choice is, therefore, determined by the need for reliable system operation. Where intrinsically safe systems are combined in a multi-core, then there are special requirements. These determine which additional faults must be considered.
IS does not require mechanical protection of the cable with armor 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 are less than the capacitance and inductance allowed for the IS interface for the Gas Group in which the equipment is installed. The 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 the Ex ic protection level, 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. IS utilizes 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.
Until Ex ic was incorporated in the intrinsically safe standards, the designer had to assess the risk of different wiring options with minimal guidelines on installation. There are now clear guidelines on how intrinsically safe equipment should be installed and maintained in Zone 2 designated areas. One example is a requirement for the segregation of an exposed IS conductor of at least 50mm from a non-IS circuit, which makes the requirements for the layout of a marshaling panel clear.
Final word
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 have an appropriate level of knowledge and training and should recognize the importance of getting the analysis right. Working knowledge of IS systems is also important for those with responsibility for installation, maintenance, and service to ensure that the original IS design is not compromised.
The analysis is relatively easy and can be done by any competent professional engineer. Sourcing IS equipment from reputable manufacturers 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.
