In last month’s “Fire Side Chat” we talked about the basic requirements for hazardous areas, which include Class I, II, III and Divisions 1 and 2. We discussed the basic code requirements related to each and we ended with a brief explanation of how class and divisional classifications work together to protect an industrial facility from explosions.
This is a matter we must take time to know and understand if we are to protect our clients from risk.
To the fire alarm industry the issue of a hazardous classification pertains to the prevention of explosion with regard to our fire alarm devices. The fact is an unwanted arc within a fire pull or an exceptionally hot surface on a notification appliance can, and has, resulted in an explosion within the environment in which these devices reside.
This month, we will discuss the practical side of the hazardous classification issue as it pertains to the physical installation of fire alarm equipment. This is not a comprehensive study on the issue of hazardous area protection, thus I recommend interested fire alarm technicians follow up with a thorough reading of Article 501 as it pertains to Class 1 locations. It is also advisable to attend a manufacturer’s training program.
Basic Fire Science Review
A classified area exists when a flammable gas, combustible gas or ignitable flying fibers are present in the atmosphere. This condition commonly takes place during storage or handling operations.
Before an explosion can occur, conditions in the immediate area must be conducive to fire. There are three elements that must be present before an explosion can occur: oxygen, fuel and a means of ignition.
Another important aspect associated with the process of an explosion is temperature. In order for an explosion to occur, fuel and oxygen must achieve a temperature between the upper and lower flammable limits. The lower flammable limit (LFL) is the minimum temperature that will achieve and support ignition. When the temperature of fuel and oxygen are below this level combustion will not occur.
The upper flammable limit (UFL) is the uttermost temperature achievable that will support combustion. Experts say when the temperature is above the UFL, the environment will not support combustion because of insufficient oxygen in the atmosphere.
Purpose of Compliant Devices
As mentioned before, it is our goal as fire alarm technicians to prevent an explosion from taking place or an overheated condition in our alarm notification and initiation devices.
“The objective with regards to hazardous areas is to protect people and equipment, as well as the environment,” says Jerry Wright, commercial electrician with Abbott Electric of Canton, Ohio.
Wright says this is a two-step process. “First, we must install devices specially made to contain an internal arc, fire or explosion. And second, we use special installation techniques to assure that the combustible atmosphere cannot escape the classified area.”
To do this we must verify the compliancy of the appliances that we intend to use. Article 500.8(A)(1) in National Electric Code (NEC), NFPA 72, 2005 Edition covers this aspect of the job at hand:
“Equipment shall be identified not only for the class of location but also for the explosive, combustible, or ignitable properties of the specific gas, vapor, dust, fiber, or flyings that will be present. In addition, Class I equipment shall not have any exposed surface that operates at a temperature in excess of the ignition temperature of the specific gas or vapor. Class II equipment shall not have an external temperature higher than that specified in 500.8©(2). Class III equipment shall not exceed the maximum surface temperature specified in 503.5.”
The Class, Division and Group classifications are, thus, by code, printed on each and every device used in a hazardous classification.
Grappling With Containment
As Wright pointed out, the second step involves the containment of hazardous gases and vapor within a classified area. Although all aspects of hazardous area protection are important, this portion of the fire protection mission is more complicated than merely knowing the hazardous classifications and selecting the right equipment.
“When we install hazardous devices, such as notification appliances and fire pulls, we use rigid conduit along with sealed hazard-rated enclosures. The latter is often used to make connections and house a nonrated arcing device within in a Class I, Division 2 location. It all depends on the setting and hazard, or the classification of the area in question,” says Wright.
In this regard, when a hazardous area is classified as a Class I, Division 1 or 2, code calls for environmental containment. Article 500.8 covers a wide variety of considerations related to this issue. For example in 500.8(D), it calls for the threading of fittings and conduit to the National Standard Pipe Taper (NPT) or metric standard.
“…Conduit and fittings shall be made wrench tight to prevent sparking when fault current flows through the conduit system, and to ensure the explosion proof integrity of the conduit system where applicable. Equipment provided with threaded entries for field wiring connections shall be installed in accordance with 500.8(D)(1) or (D)(2). Threaded entries into explosion proof equipment shall be made up with at least five threads fully engaged,” (National Electric Code [NEC], NFPA 70, National Fire Prevention Association, 2005 Edition).
Again, the two code references mentioned above relate to threaded entries in listed equipment using the NPT or metric standard.
Conduits and raceways that carry detection and notification circuits from nonhazardous areas into classified ones must be sealed to contain the gases and vapors found there.
Although the manufacturer of these devices will provide an internal seal within its equipment to prevent the entry of hazardous substances, code requires you to do more to assure environment containment.
According to Article 501.15, titled “Sealing and Drainage,” fire alarm technicians must, in fact, seal the conduit that connects one device to another. This is often accomplished using what Wright calls a seal-off fitting.
“A seal-off is installed in line in a conduit or raceway to prevent any hazardous fumes or liquids to travel from classified locations to a nonclassified location. Presence of hazardous chemicals can be ignited through heat or through an arcing device. This can cause of fire or explosion,” says Wright.
According to Wright, there are two basic ways to install a seal-off device. One is to pack the fitting with fiber packing. Each conductor must be separated when doing so to help assure an adequate seal. An epoxy is then poured into the fitting to further assure the integrity of the seal. This is why there are two entry points on the seal-off fitting.
The second method involves a self-expanding epoxy that acts as a liquid when it is first administered, eventually hardening to seal the fitting. It also eliminates the need for the fiber packing. Either way, gases and vapors in a Class 1, Division 1 or 2 area are not able to migrate to a nonclassified one.
Next month, we will cover the Group designations and intrinsically safe devices, as well as the wiring methods that serve them.