Reducing Arc Flash Hazards Using Common …

1 Reducing Arc Flash Hazards Using Common Transformer Protection Methods by: Alan L. Wilks, ERMCO November, 2009 Mailing address: P. O. Box 1228 Dyersburg, TN 38025-1228 Shipping address: 2225 Industrial Road Dyersburg, TN 38024 Introduction The selection of the best protection devices and transformer designs are excellent ways to reduce arc Flash Hazards . It is the intent of this paper to address various examples of Common transformer protection methods and examine the effect each has on the arc Flash category and hence the PPE that would be required to keep any injuries to a minimum. Background Although Arc Flash Hazards have been present for over a century, it has received significant attention in the past few years, stemming from the 2007 revision of the National Electric and Safety Code (NESC). The 2007 revision of the NESC, Rule 410A3 states: Effective January 1, 2009, the employer shall ensure that an assessment is performed to determine potential exposure to an electric arc for employees who work on or near energized parts or equipment.

2 If the assessment determines a potential employee exposure greater than 2 cal/cm2 exists, the employer shall require the employee to wear clothing or a clothing system that has an effective rating at least equal to the anticipated level of arc energy. What is Arc Flash ? Arc Flash is defined by the National Fire Protection Association as a dangerous condition associated with the release of energy caused by an electrical arc. An Arc Flash is basically an electrical short circuit through the air. During an Arc Flash incident, concentrated radiant energy explodes outward, releasing a superheated ball of gas and shrapnel with a temperature of possibly four times that of the sun. In addition there is a tremendous blast force, blinding UV light and a loud noise. Arc Flash Categories Arc Flash has been categorized by the amount of heat generated at a distance of 18 from the source of the arc.

3 The primary protection used to reduce the effect of arc Flash on personnel, is the use of proper Personal Protective Equipment, commonly known as PPE . Arc Flash Hazards Page 2 The Arc Flash categories, the heat generated and the PPE required by NESC for each category is listed in the following chart: CategoryPPE RequiredLong sleeve shirtLong pantsSafety GlassesNon-melting untreated natural fiberFR long sleeve shirtFR pants with a minimum arc rating of 4 orLong pants - untreated denium cotton blue jeans of 12oz/yd2 orFR coveralls - arc rating of 4 instead of FR shirt and pantsHard HatIn addition to items listed in Category 1, useFace shield with a minimum arc rating of 8 Wrap-around guarding for forehead.

4 Ears and neckCould use Flash hood suitCotton undergarmentsNon-melting long sleeve shirt and pantsFR shirt and pantsFR coverallsHearing protectionSafety glasses or gogglesHand protectionFoot protectionCotton shirt and pantsFR shirt and pantsFlash suit and hoodHearing protectionSafety glassesHand protectionFoot protectionDangerousNo safe protection 25 cal/cm24 40 cal/cm2> 40 cal/cm2 Heat GeneratedPPE Categories0 2 cal/cm2 4 cal/cm212 8 cal/cm23It should be noted that cal/cm2 is the threshold of a second degree burn. Arc Flash protection is designed to limit the injury to no more than a just curable 2nd degree burn. You can still be burned by abiding by the rules. As a reference: X 1st degree burns affect the outer layer of skin, it is painful, but not usually permanent or life threatening X 2nd degree burns cause tissue damage and blistering.

5 The outer skin layer is destroyed. X 3rd degree burns cause the complete destruction of skin. Small areas may recover, large areas will need skin grafting. How are Arc Flash Calculations Made? The conditions that determine the amount of Arc Flash and the resultant heat, etc. depend upon the following: 1. The fault current available at the arc, which is based on the impedance of the system at the arc. The higher the impedance, the less the fault current available. 2. The time duration of the arc, determined by the back-up protection that operates, interrupting the power to the arc. 3. The surrounding environment of the arc, related to whether the arc occurs in open air or in a box . An arc in open air is not as confined and allows the energy to be dissipated in many directions. An arc in a box focuses the energy, pressure, debris, etc.

6 In one direction toward the worker. Hence, arcs in open air are less damaging as those in a box. There are two methods of Arc Flash Calculations: IEEE 1584-2002 NFPA 70E - 2004, Annex D Both methods of calculation are commonly used, but the following discussion uses the IEEE 1584 method. It should be noted that this paper is not all encompassing and only shows a few typical examples. The user must still comply with all of the requirements of these standards which contain additional details for formulas for other voltages and for consideration of current limitation not shown here. Arc Flash Hazards Page 3 The formulas for the calculations will not be discussed here, as the details can be found in IEEE 1584-2002, but the results from Using the formulas on various protection methods will be discussed.

7 What can be done to reduce Arc Flash Hazards ? 1. Wear the proper Personal Protective Equipment Wearing the proper PPE is vital to the protection of the person operating the equipment. However, it is not the intent of this paper to address the details of PPE. 2. Minimize the Fault Current Available In most instances, there is little that can be done on an existing system to reduce the fault current available. The magnitude of an arcing fault will be less than a bolted fault, due to the arc impedance and arc gap distance. Although the arc gap distance may vary, a gap of 1 (25mm) is used in the following calculations. 3. Minimize the Time Duration of the Arc The time duration of an arc is dependent upon the protection devices that are used to clear the fault. The characteristics of protection devices are typically expressed in Time-Current Curves (TCC).

8 At a given fault current level, the time to clear the fault can be easily determined Using the TCC s. Since protection devices have their own unique TCC s, the time duration of the arc may be reduced by the proper selection of protective equipment. The TCC s are available from the transformer manufacturer and the protective equipment manufacturer. 4. Change the surrounding environment of the arc There is not much that can be done with existing equipment to change the environment around a potential arc. Pole mounted transformers (both 1 and 3 ) are considered Open Air applications, which have the least arc Flash consequences. However, both primary and secondary conductors are exposed to arc Flash Hazards . Pad mounted transformers are considered to be Box which exhibit the worst arc Flash situations. A Box is defined as an 18 box with a back, a top and two sides.

9 The front is open. Unfortunately, the open front is always towards the person most likely to be injured by an arc Flash . Most pad mounted transformers have separable insulated high voltage connectors. The insulated primary help reduce the risk of arc Flash on the primary side. However, the secondary of the transformers are frequently uninsulated, causing a much greater arc Flash risk. Although single-phase pad mounted transformers with flip-top doors are considered to be a box, they do offer some degree of openness allowing the arc to dissipate in three additional directions (upward, left and right). However, there are no factors in the formulas which take this into consideration when calculating arc Flash . Three-phase pad transformers are somewhat different, in that most designs are more truly a box . Typically, there is a cabinet containing both primary and secondary conductors, each having an access door.

10 The primary door may only be opened after the secondary door is opened, but as stated earlier, the dead front construction used in most pad mounted transformers, minimizes the arc Flash risk on the primary side. However, when the secondary door is opened, it exposes uninsulated conductors causing an arc Flash risk. The secondary side of most 3 pad designs are box-like, consisting of a left-hand partition separating the low voltage from the high voltage compartments, a top covering the compartment, and a right-hand side. Only the front is open, exposing the operator to possible arc Flash Hazards . One manufacturer has a swing open top and side doors, resulting in a one-sided box. This reduces the arc Flash hazard, since there are more directions the arc Flash can be dispersed. Changes in the environment may be in the form of one or more of the following.