Ten Ways to Design for Intrinsic SafetyApril 04, 2016 by Tim Patel, Littelfuse
How to build electronic devices when a higher degree of safety is required.
How to build electronic devices when a higher degree of safety is required.
There are a lot of things to consider when designing an intrinsically safe product. Flammable gasses or vapors; and airborne dust, fibers, and filings can pose explosive hazards if sources of sparks or excess heat are present in the environment. Over the years, hazards like these have led to catastrophic losses of property and even life.
What’s even more daunting, almost every industry related to energy or basic 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; pumping stations for gas, oil, sewage, etc.).
In order to mitigate such hazardous potential, regulatory bodies from around the world worked in order to establish and refine Intrinsic Safety (IS) standards meant to minimize the risk of explosions in the potentially hazardous work environments. An IS certified device or product is designed so that it’s incapable of generating sufficient heat or spark energy to trigger an explosion.
A wide variety of products must be designed for intrinsic safety (IS) in hazardous environments. UL-913 is one standard you need to be aware of for IS.
This article touches on ten recommendations for designing circuitry for new electrically powered products intended for use in the aforementioned potentially hazardous workplaces. The following are ten things to take into consideration during the initial planning stages for these products.
1. Batteries Should be Selected Carefully
Batteries can pack a lot of energy, especially those based on high-density lithium-ion battery technology. 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 minimize the amount of electrolyte leakage that can occur under severe short-circuit conditions.
2. Be Mindful of Multiple Power Sources
It’s quite easy to overlook the implications of having one or more facilities for connection to other devices. Most often, circuits for low-voltage communication ports are 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. This could result in a condition where an excess amount of fault energy and/or power is available within the intrinsically safe device. Even when such ports are designed only for connection to associated apparatus or intrinsically devices evaluated under the “entity concept,” take care to ensure that the entire system maintains intrinsic safety protection.
3. Be Skeptical of Published Electrical Ratings for Semiconductors
The datasheets for many semiconductor components (e.g., zener diodes) will specify an absolute maximum power dissipation rating for the component. However, this rating is often based on very specific temperature and mounting conditions. Often, the given ratings in the component datasheet do not adequately reflect the conditions that the component will be exposed to in the end-use application. Always take the time to understand and evaluate the effects that the end use application will have on the component’s power rating, thermal rise characteristics, etc. This is essential when using semiconductor components as shunt voltage limiters.
4. Calculate the Thermal Rise Characteristics of Power-Dissipating Components
The maximum surface temperature of components under fault conditions must be assessed for determining the appropriate temperature class of an intrinsically safe device. To determine the maximum surface temperature of these components analytically, you’ll need information on the components’ temperature rise characteristics. In particular, a component’s case-to-ambient thermal resistance is required to compute the maximum case temperature of the component when dissipating a given amount of power at a specified ambient temperature. Unfortunately, most component datasheets do not specify this value; 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, the same 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. Energy-Storing Components Should be Limited
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.
This illustration details how an encapsulated fuse, in this case the Littelfuse PICO 259-UL913, limits the energy and temperature output that would be otherwise exposed to an explosive atmosphere.
7. Available Power in Separate Circuits Should be Limited
Although one of the main goals of intrinsic safety protection is to limit the available power in a device, it can be quite challenging to meet the industry demands for the same products. With the need for more powerful electronics to be provided in smaller packages, the functional needs of the circuit need to be balanced with the safety needs. 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 for Separation Distances
One of the most important keys to preserving intrinsic safety, especially when considering the application of faults, is providing and maintaining the required separation distances between separate circuits. When it comes to 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. Protective Components Should be Derated
The components that are selected as safety-critical or “infallible” must meet certain construction requirements; however, they must also be rated such that they do not experience stresses that could be detrimental to the reliability they provide. Intrinsic safety standards require that 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. Protective Components Should be Selected Carefully
The fundamentals of intrinsic safety, where circuits are designed to limit the amount of energy for protection against spark ignition and to limit the amount of power for protection against thermal ignition, depend heavily on the reliable operation and “failure” of protective or “infallible” components. Because these components are being relied upon to function in well-defined ways when subjected to normal operating and fault conditions within a circuit, these components must be selected wisely. 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.
To recap, preventing electrical equipment from becoming sources of ignition by limiting sources of electrical sparks and high surface temperatures, as well as maintaining required separation distances is key when thinking about the design and development of a new device or product to be used in in a hazardous workplace. 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.
Want to learn more about designing for intrinsic safety? Read the Enhancing Workplace Safety in Hazardous Locations application note.
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