In harsh industrial environments, the presence of gases, airborne dust, and petroleum products can be highly explosive when a spark or high temperatures are affecting the surrounding environment. Underwriters Laboratories (UL) and other standards bodies had to step in to minimize these potential hazards to protect life and property.
In 1971, UL 913 was issued to establish the standard for "Intrinsically Safe Apparatus and Associated Apparatus for Use in Class I, II, and III, Division 1, Hazardous (Classified) Locations." The standard made sure that there would be no ignition possible in Division 1 Hazardous (Classified) locations by limiting exposure to high surface temperatures along with dust-tight enclosures. The construction and testing of electrical apparatus, or parts of such apparatus is clearly specified in UL 913.
Table 1: UL 913 hazardous locations.
Intrinsically Safe (IS) devices
A mandatory requirement of operating electronic equipment in a hazardous area requires that certified IS devices must be used. (See figure 1.)
Figure 1: Some environments are hazardous and some are not.
PICO259-UL913 fuse design and encapsulation are certified for use in intrinsically safe apparatus for applications up to voltages of 125Vrms (190V peak) -- see figure 2. The encapsulant eliminated the extra step of conformal coating of the device.
Figure 2: The surrounding encapsulant is more than 1-mm-thick in 259-UL913 fuses. They keep a good seal and limit the energy and temperature generated when the fuse operates. Dust is also prevented from entering the fuse body.
Electronic devices in hazardous areas
Motor controllers, lighting, flow meters, communication handsets, some sensors and process control and automation products may be used in a hazardous area. While in operation, there are possibilities of small internal sparks from devices such as motor brushes, connectors or switch contacts. The energy of these sparks must be contained to avoid ignition of explosive materials in the environment.
Average time current curves show fast fuse response.
The use of an intrinsically safe certified fuse must be used to limit the current under abnormal conditions so that the circuit will open with no spark generated that may cause ignition. The surface temperature of the fuse need also be kept below ignition temperatures. The fuse must also help prevent the temperature of a component, like an IC or a capacitor, in such devices as meters, valves or gauges from rising to an unsafe level. The solution is to use a fuse that has an intentionally weak link so that it will open the faulty circuit and limit the spark energy and surface temperature. (See figure 3.)
Figure 3: Fuse operation to limit spark and temperature dangers.
For more information visit www.littelfuse.com.
Fuses like this need to be very fast-acting to be effective -- Design News readers need to act fast as well because the Littelfuse Speed2Design contest deadline is quickly approaching.
I assure you, having been to NASA myself, that you will be absolutely astounded with the talented engineers and incredible programs to which you will be exposed if you are selected as a winner. October 2, 2013 is the deadline -- don't miss it!
so you can visit NASA Johnson Space Center and see such programs as:
- International Space Station (ISS) National Laboratory
- Orion program
- Project Morpheus
- Robonaut and Robotics
- Extravehicular Activity Space Suit Design
- Human Health and Performance Center
- Avionics and Electronics for Space Environments (Micro-Wireless Instrumentation Systems)
- Safety and Mission Assurance
- Chief Technologist (R&D efforts and collaboration)