Use of arc suppression blankets can help reduce arc flash/blast injuries. When properly installed, arc suppression blankets absorb or deflect heat and blast energy emitted from an arc event, reducing the event's impact on workers.
Proper understanding of the elements involved in an arc flash is key to designing an installation. Heat is a major factor in an arc flash/blast event. Because of the intense heat generated in both radiation (ultraviolet and infrared) and the plasma arc (large fireball that can reach several thousand degrees), protection from this heat is the most important factor.
An important fact is that the temperature of the arc flash reduces quickly, dropping as the square of the distance from the source. As an example, if at one foot, the temperature is at 10,000 degrees, at two feet, the temperature would be (1/2) 2 X 10,000 = 1/4 X 1,000 = 2,500 degrees. Since the temperature at the arc source may be over 30,000 degrees F, the distance needed to assure safety can be many feet without barriers such as arc suppression blankets. This knowledge suggests that blanket size is a consideration.
The second factor is the pressure wave that is generated from the rapid expansion of air next to the arc source. Controlled testing shows that current level has a dramatic effect on the pressure wave. Little pressure was generated when amperages were under 15,000 amps (15KA). With amperages over 15KA, the pressure increased exponentially, as verified by high-speed photography.
This pressure wave manifests itself by the distance the blanket would distort as the amperage increased. On a test blanket, there was almost no movement at 5KA, a little at 15 KA, maybe 4 inches at 25KA, but at 40KA the distortion reached almost 18 inches. This distortion affects the suspension system by putting tremendous stress on the blanket, straps, anchors and hardware. Like the dissipation of heat, the pressure wave decreases with the square of the distance from the source, again suggesting that blanket size is included in the installation.
The effects of a pressure wave can be examined with and without an arc suppression blanket. Without the blanket, the pressure wave is known to exert a force of 400 to 500 pounds on a worker. This force will shove the worker into whatever is in the way. If an arc suppression blanket is between the fault and the worker, the blanket will absorb the force before it reaches the worker. As the area of the blanket is greater than the area of the worker, a much greater force must be supported. It is not unreasonable to estimate the pressure wave at 1 pound/in2. If the blanket is a 6 ft. X 8 ft., the total area is 6 X 8 X 144 = 6,912 square inches, resulting in a force of 6912 lbs. This much force must be accounted for in the design and installation of the blanket.
The third factor involved in an arc blast is the resulting fire after the arc blast that can consume anything combustible, including arc suppression blankets. There are four problems with fire: the first is the continued generation of heat, second the generation of toxic gasses, third the reduction in oxygen levels, and fourth is the possible melting and dripping of hot substances such as molten glass or plastic. This factor suggests that the installation include consideration of not only the blanket, but also the materials used in its installation such as straps, ropes and connectors.
Using these factors, we can determine how to best install an arc suppression blanket. The important points in installing the blanket are using the proper blanket, the best method of installation, and materials/hardware for supporting the blanket.
ASTM F2676 is the guide to selecting the proper product. The standard rates blankets by the amount of energy they are capable of absorbing before failure, evaluating them by three criteria. First is their ability to resist the arc breaking through. Blankets must not burn through within 10 cycles (10/60's of one second) at its rated amperage, either 15 KA (15,000 amps), 25KA or 40 KA. Second is its ability to stay attached. After 10 cycles at its maximum rated amperage, it must still have all of its attachment points and attachments intact. Finally is its ability to self-extinguish within 30 seconds after the arc event with no melting or dripping materials. ASTM F2676 does not, however, test or certify arc suppression blankets for wrapping.
Blanket Attachment Issues and Options
Because of the complexity and arrangement of the wiring and splicing, the variety of vault and manhole sizes and configurations, the composition of the vault’s walls and hardware and the stresses exerted on them from the environment, no one methodology outlined can be recommended solely. Each vault will have its own sets of advantages and limitations as to which technique would be most useful.
The following methods have been derived from testing done over the last several years and in conjunction with several utility companies. Each method stemmed from the fundamental concept: “When possible, channel energy – don't challenge it.”
The “J” Method
The “J” installation is so named because properly installed, an end view of the installed blanket looks like the letter J. The blanket is placed in front of the racks and splices with the bottom tucked back toward the vault wall. The blanket top is arrayed with the middle slightly loose, so that blanket bows out, acting like a funnel. The potential arc blast is redirected upwards from the bottom of the blanket and its energy is channeled up and out of an opening such as a manhole.
The Wrapped or Clamshell Method
The wrap method is a loose wrap of a large blanket around the splice and tied to the cable loosely in the middle and at the ends of the blanket. To create a “clamshell,” a regular blanket is placed over the wrapped blanket and is anchored top and bottom to the vault wall with the open end of the “C” shape pointed to the vault wall.
The intent is to channel the energy sideways away from the worker. Since one of the blankets is lying directly on the splice, a second blanket system is arrayed as a precaution. Extra attention should be used in the wrap installation as there needs to be at least two layers around the cable with an opening of six to eight inches of slack to provide room for the escaping blast.
The wrap method, although allowing quick installation, is not recommended since it can cause secondary problems. If an arc flash occurs the blanket will fall back onto the hot cable. The hot cables provide heat, which will not allow the blanket to self-extinguish.
The following pictures show a “tight” wrap as tested at Kinetrics Labs in Toronto, Canada. The first picture is of the installation. Note the tight wrapping around the cable. The next four photos show the development of the failure. During the test at 30 KA, it took only half of the first cycle to break the three Kevlar straps, open the blanket and allow the blast to proceed unimpeded.
The Suspension Wall Method
This method was invented and tested by Progress Energy and is a device made of steel piping that is assembled in the vault and attached to screw jacks. The jacks are tightened, exerting pressure on the ends and thus holding the structure in place. A blanket is attached to these supports creating a barrier wall. It can be used against the vault wall or away from it. Creating a clean and secure surface for the pressure pads is a challenge, but in testing at the Kinetrics Lab it has performed well in the test vault including the management of 40KA/10 cycle shots. However, in some applications it has proven to be difficult to get the piping pieces into the manhole or vault.
The Weave Method
This method was developed by Detroit Edison and assumed that the attachment points for the blanket would be behind the splices, which are located on racks. In testing we used the Suspension Wall Device up against the vault wall and fitted it around the splices. Of the three splices the middle was de-energized while the top and bottom were energized.
The blanket is attached at the top in the same manner as the “J” Method and then woven behind the dead circuit and then back into the front of the rack and draped down the rest of the racks and splices. At the bottom the blanket is folded back toward the vault wall and secured. This has proven to be a trouble-free and user-friendly method.
Work methods and practices chronicled here have been created by utility professionals. There are three areas considered for attachments. These are the wall or stanchion, the blanket itself and intermediate straps and related hardware.
Blankets need a strong base for attachments. These bases are usually the wall of an underground bunker or something attached to that wall. To quickly attach a blanket to a wall, either fixed or removable concrete anchors are popular for two reasons: first they are easy to install and second some models can be reused, holding down hardware costs.
Another attachment method is to use stainless steel or galvanized concrete struts and clamps. This is a common choice by many utilities as they are already widely used in their systems. Struts and clamps are also used when the integrity of the vault wall is questionable and there is a need to spread the load. In many cases, these anchoring points are already available, needing only slight modification.
Blankets tested under ASTM F2676 are certified using all of the blanket's attachment points. For that reason, you must use all of the attachment points provided on the blanket to be in compliance with the ASTM standard.
The amount of heat and stress affects the choice of attachments connecting blankets to solid supports such as walls or stanchions. Even though attachments are usually at the edge of the blanket and are not directly in the path of an arc source, they can experience brief temperatures of several thousand degrees. In combination with the heat is the force exerted on the attachments.
In testing at Kinetrics as well as in utility use, blankets are supported by either nylon or aramide straps. Adjustments are made by metal cam buckles and the ends of the straps have connectors such as carabiners. In hundreds of tests, the metal cam buckles and carabiners have never failed, but the strapping has. Non-FR nylon strapping or even "FR nylon" usually holds the initial stress, but often burns after an arc event. In general, if the potential amperage is less than 15KA with no more than 10 cycle clearing time and the blanket is larger than 4 ft. x 5 ft., nylon will hold the blanket. With higher currents and/or smaller blankets, aramide strapping should be used. Other types of adjustable strapping materials can also be used, such as stainless cable, but precaution should be used to prevent electrical problems caused by the metal.
About the Authors: Michael R. Mulvaney is the Sr. Safety & Health Engineer for DTE Energy, Detroit, Michigan. Victor L. Petrovic, Ph.D. is the Technical Director for Therm-Equip in Canton, Ohio.video
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