Hazardous locations are areas where flammable liquids, gases or vapors or combustible dust exist in sufficient quantities to produce an explosion or fire. In hazardous locations, specially designed equipment and special installation techniques must be used to protect against the explosive and flammable potential of these substances. (OSHA Publication 3073)
There are three factors which have to be present in order for there to be a flame or explosion possibility in a hazardous environment. The three factors are fuel, oxygen, and heat (or ignition source). If anyone of the three is not present, then there is no explosion or fire hazard.
Two Approaches in Hazardous Locations
1.-Employ explosion hardening methods in the system design
- Containment- Using cabinets strong enough to hold the explosion’s energy.
- Isolation- Purging the potential explosive area of air by replacing it with an inert gas.
- Avoidance- Moving all the control equipment outside the hazardous area.
These hardening methods are sometimes too expensive or impractical to use.
2.-Use intrinsically safe components in the area to be protected
Intrinsic safety, (IS) is a protection technique for safe operation of electrical equipment in hazardous areas by limiting the energy available for ignition. In some cases signal and control circuits that can operate with low currents and voltages. For those, the intrinsic safety approach simplifies circuits and reduces installation cost over other protection methods. One reason for using intrinsically safe devices in an installation is that the devices will not generate a fire or explosion hazard due to the guaranteed limit on the stored energy.
Ethernet not Intrinsically Safe
IEEE 100BASE-TX Ethernet (“4-wire Ethernet”) is the base technology for PROFINET and is not intrinsically safe. In order to access fields of application within Process Automation in hazardous areas, it is necessary to specify an intrinsically safe version as an extension to the IEEE 100BASE-TX standard.
Intrinsic Safety with PROFINET
The current PI approach to IS uses PROFIBUS PA to meet intrinsically safety needs. PROFINET uses a proxy called a PA coupler to connect to the PA system network.
The advantages of PROFIBUS PA are:
- PROFIBUS PA works over a single twisted pair cable that provides both power and communication.
- It can communicate up to 1000 meters(1900 meters for non-IS).
- A single PROFINET to PROFIBUS PA coupler can support multiple PA segments, each with the maximum PA bus distance.
Changes are in the works with extensions to the IEEE 100BASE-TX standard and with the PROFINET standard. Even though working groups around the world are looking to solve this issue, Ethernet is currently not suitable for hazardous environments.
Editor’s note: This article is an excerpt from PROFINETuniversity.com