Robust circuit protection for hazardous locations: a guide to industry standards and component selection
28 June 2016
Preventing electrical equipment from becoming sources of ignition is key when thinking about the design and development of a new product to be used in in a hazardous location, says Saad Lambaz of Littelfuse. This is where intrinsically safe fuses or current-limiting resistors come into their own.
According to the US Bureau of Labor Statistics (BLS), fires and explosions account for roughly three percent of all fatal injuries in the workplace. Virtually every industry related to energy or materials production has potentially hazardous locations, including energy production (such as oil/gas production/refining, storage/transportation, mining, etc.), materials processing (semiconductor fabrication, tank farms, chemicals manufacturing etc.), food production (grain milling, baking, brewing, distilling, etc.), and many others (pharmaceutical or cosmetics manufacturing, additive manufacturing, 3D printing, gasoline stations, sewage, etc.).
For anyone responsible for lowering the risk of explosions in hazardous locations where flammable liquids, vapours, gases or combustible dusts or fibres exist, one of the first steps is eliminating potential sources of ignition, such as heat or sparks produced by the normal operation of electrical equipment. For example, operating an electrical device can cause the temperature to rise above the thermal ignition threshold of the surrounding explosive atmosphere. Similarly, operating a switch for a piece of equipment can create sparks that may generate enough energy to trigger an explosion. To minimise the risks associated with operating electrical equipment in explosive environments, this equipment must employ explosion protection methods to reduce the likelihood of explosions. This applies to devices ranging from portable products like flashlights, mobile phones and gas detectors to stationary equipment like process control/automation equipment, data processing equipment and sensors/meters.
Local electrical installation codes define the protection methods that are suitable based on the type of explosive environment and the level of explosion risk associated with that environment. This is achieved by classification systems. In North America, the Classes and Divisions system is primarily used (Table 1) while the international market uses the IEC classification system. Both systems differentiate between the type of explosive atmosphere first and then further recognise multiple levels of probability that a flammable concentration of material might be present. Class I, Divisions 1 and 2 and Zones 0, 1, and 2 are locations where hazardous vapours or gases are present. Class II, Divisions 1 and 2 and Zones 20, 21, and 22 are where hazardous dusts are present.
Intrinsic safety (IS) is a protection technique for safe operation of electrical equipment in hazardous areas by limiting the energy, electrical and thermal, available for ignition. Intrinsically safe apparatus have been developed to prevent electrical equipment from becoming sources of ignition of explosive atmospheres. An IS certified device is one designed to be incapable of generating sufficient heat or spark energy to trigger an explosion. For decades, designers of circuitry for IS products had to deal with one IS standard for products designed for use in the United States (UL 913), and a second, more stringent set of requirements for meeting international standards (based on the IEC 60079-11 standard, under the IECEx program). Since the publication of the 7th edition of the UL 913 standard in 2006, which essentially references the UL 60079-11 standard (harmonized with IEC 60079-11), designers now have only a single set of harmonized requirements that allow them to create a single design that is globally acceptable from an IS standpoint.
IS-certified components for building intrinsically safe apparatus are now available, including a growing number of sealed, encapsulated fuses. The purpose of encapsulating a fuse is limiting the temperature and energy that is exposed to the hazardous environment and preventing gas and particles from entering the fuse body under normal and abnormal conditions.
In the past, IS apparatus manufacturers had to choose between sending the circuit boards on which fuses were located to third-party suppliers for conformal coating or creating their own in-house process for encapsulation in order to meet the UL 913 standard’s requirements for fuses. Neither option was particularly desirable from a manufacturer’s standpoint because both added to a product’s manufacturing cost and required extra lead time to complete. However, over the last few years, designing IS devices has become easier because of the growing commercial availability of IS compliant fuses.
How intrinsically safe fuses help prevent explosions
When a capacitor fails or an IC short occurs in an electronic circuit, an electrical fault can cause thermal runaway. If not controlled quickly, a spark can be produced or a component’s temperature can rise rapidly, posing a risk of explosion. A fuse serves as an intentionally weak link that is designed to open the faulty circuit, thereby limiting the spark energy and surface temperature. However, a regular fuse does not provide sufficient protection because arcing can occur when the fuse opens. The arcing and temperature may ignite the surrounding explosive atmosphere because the surrounding flammable environment can reach the arc/spark and accompanying temperature.
An intrinsically safe fuse or current limiting resistor is required to ensure that no component in the circuit can reach a temperature that could ignite an explosion. An intrinsically safe certified fuse will limit the current under any abnormal condition to ensure that the circuit will open without generating a spark capable of causing ignition. An intrinsically safe fuse’s surface temperature also does not reach a temperature that could ignite explosive gases or dust.
Choosing fuses for IS circuit designs
Before a circuit designer can select an appropriate fuse for an IS (or, in fact, any) application, he or she needs to gather a variety of information on the application itself and potential fuse choices:
• Normal operating current: The current rating of a fuse is typically derated 25 percent for operation at 25ºC to avoid nuisance blowing. For example, a fuse with a current rating of 10A is not usually recommended for operation at more than 7.5A in a 25ºC ambient.
• Application voltage (AC or DC): The voltage rating of the fuse must be equal to, or greater than, the available circuit voltage.
• Ambient temperature: The current carrying capacity tests of fuses are performed at 25ºC and will be affected by changes in ambient temperature. The higher the ambient temperature, the hotter the fuse will operate, and the shorter its life. Conversely, operating at a lower temperature will prolong fuse life. A fuse also runs hotter as the normal operating current approaches or exceeds the rating of the selected fuse.
• Overload current and length of time within which the fuse must open: The current level for which protection is required. Fault conditions may be specified, either in terms of current or, in terms of both current and maximum time the fault can be tolerated before damage occurs. Time-current curves should be consulted to try to match the fuse characteristic to the circuit needs, while keeping in mind that the curves are based on average data.
• Maximum available fault current: A fuse’s Interrupting Rating must meet or exceed the Maximum Fault Current of the circuit.
• Pulses, surge currents, inrush currents, start-up currents, and circuit transients: Electrical pulse conditions can vary considerably from one application to another. Different fuse constructions may not react the same to a given pulse condition. Electrical pulses produce thermal cycling and possible mechanical fatigue that could affect the life of the fuse.
• Physical size limitations, such as length, diameter, or height: Fuse dimensions are typically provided in the manufacturer’s product data sheets.
• Agency approvals required: The markets for which a product is intended will have a major impact on which agency approvals are necessary before the end product can be sold. Check current manufacturer data sheets for specific agency approval information on fuse products being considered.
• Fuse features required: Understand the specific fuse features the application calls for, such as mounting types, form factors, ease of removal, axial leads, visual indication, etc.
• Fuseholder features, if applicable, and associated rerating required: Clips, mounting block, panel mount, PC board mount, R.F.I. shielded, etc.
With this information in hand, circuit designers can use manufacturers’ data sheets or selection guides to narrow down their IS fuse options to those that match their requirements. However, trying to identify the information in dozens of data sheets can be frustrating so use of a fuse selection tool such as Littelfuse iDesign (https://littelfuse.transim.com/login.aspx) can speed and simplify the selection process.
However, no matter how useful a selection tool is, it is essential to validate the protection level and safety of the fuse selected. They should only be used as a guide to suggest a starting point in the overcurrent selection process. Application testing should always be conducted to verify the correct part selection.
An encapsulated fuse limits the energy and temperature output that would be otherwise exposed to an explosive atmosphere
To recap, preventing electrical equipment from becoming sources of ignition by limiting sources of electrical sparks and high surface temperatures, along with meeting other requirements, is key when thinking about the design and development of a new device or product to be used in in a hazardous location. Additionally, protective components need be chosen wisely, as the whole relies on the parts. Intrinsic safety standards increase the overall safety of the final product, which increases the level of protection of life and property in hazardous operating environments.
Ten Intrinsic Safety Design Tips
Keep these ten tips in mind during the initial planning stages of designing circuitry for new electrically powered products intended for use in potentially hazardous locations.
1. Select Batteries Carefully
When selecting cells or batteries for use in intrinsically safe devices, take care to ensure that the cells and batteries are robust enough to withstand the expected environmental conditions, as well as to contain or minimise the amount of electrolyte leakage that can occur under severe short-circuit conditions.
2. Be Mindful of Multiple Power Sources
Circuits for low-voltage communication ports are typically not given proper attention because designers are so focused on transmission and reception of data on those ports. However, each of these ports has the potential to be connected to equipment that wasn’t designed or evaluated for intrinsic safety protection, which could result in a condition where an excess amount of fault energy and/or power is available within the intrinsically safe device.
3. Be Skeptical of Published Electrical Ratings for Semiconductors
Data sheets for many semiconductor components will specify an absolute maximum power dissipation rating for the component, but this rating is often based on very specific temperature and mounting conditions, so they often don’t adequately reflect the conditions that the component will be exposed to in the end-use application. Take the time necessary to understand and evaluate the effects that the end use application will have on the component’s power rating, thermal rise characteristics, etc.
4. Calculate the Thermal Rise Characteristics of Power-Dissipating Components
The maximum surface temperature of components under fault conditions must be assessed to determine the appropriate temperature class of an intrinsically safe device. Unfortunately, most component datasheets do not specify a case-to-ambient thermal resistance; instead, they specify a junction-to-ambient thermal resistance. In the absence of such information, tests can be performed to determine the required values experimentally.
5. Be Aware of Voltage-Enhancing Circuits
Although switching regulators, charge pumps, and other voltage-regulating and -enhancing circuits can be useful in designing an efficient power supply, these circuits pose challenges if not provided with adequate voltage limitation. Enhanced voltage levels present at the output of such ICs can be faulted to propagate to other circuits tied to the same IC if they aren’t adequately protected with voltage limiters. This can cause issues for separation of circuits (which is based on the peak available fault voltage) and for the spark ignition assessment of other circuits.
6. Limit Energy-Storing Components
Although energy-storing components like inductors, ferrite beads, and capacitors are useful as filtering components, they pose challenges for compliance with spark ignition requirements. The energy available and stored in inductive, capacitive, and combination LC circuits must be limited such that there is insufficient energy to cause ignition of an explosive atmosphere. When coupled with the safety factors applied to the available fault voltage and/or current in the circuit, the inductance and capacitance limitations can be quite challenging. To help alleviate these challenges, encapsulation may be used to protect circuits against spark ignition.
7. Limit Available Power in Separate Circuits
Quite often, the strategy of splitting the total available power to various “separate circuits” within an intrinsically safe device is used to provide the maximum amount of power required to drive those portions of the circuit that need the power without compromising safety.
8. Keep Environmental Factors in Mind When Designing Separation Distances
The requirements for intrinsic safety equipment must take into account the environments in which such devices may be installed. These environments can contain a wide array of pollutants that can affect the insulation provided between conductive parts of circuits. It’s essential to design in sufficient space to maintain the separation of circuits necessary to preserve intrinsic safety protection.
9. Derate Protective Components
Intrinsic safety standards require that safety-critical or “infallible” components be used at no more than two-thirds of their rated voltage, current, and power when subjected to normal operating conditions and fault conditions (commonly referred to as “two-thirds derated.”) This often requires selecting components that are overrated for the application but not so overrated that they fail to provide the needed protection.
10. Select Protective Components Carefully
Although the intrinsic safety standards do not necessarily force the use of specific components, there are requirements for some widely used components that have well-known operating and failure characteristics. Fuses, current-limiting resistors, and zener diodes are commonly used to design robust voltage and current limiting circuits that can be relied on to maintain the energy and power limitation needed for intrinsic safety.
About the authors
Saad Lambaz is the Global Standards Manager for the Electronics Business Unit of Littelfuse. He joined the company in 2014 after being involved in testing and certification services in his previous role at Underwriters Laboratories (UL). Saad's current responsibilities include third party component certification, standards development and representation of Littelfuse in local and global technical committees. He received his BSEE from Southern Illinois University.
Tim Patel is the Global Marketing Manager for the Electronics Business Unit of Littelfuse and joined the company in 2013 after being involved in testing and certification services in his previous role at Underwriters Laboratories (UL). He received his BSEE from the University of Illinois at Chicago and is a licensed Professional Engineer in the state of Illinois.