Recent Blog Entries

  •   Download OVERVIEW OF RELAY COORDINATION Relay coordination has to be carried out based on many other power system studies and Equipment Sizing calculation, are listed below Load Flow Studies Short Circuit Studies Motor Starting Studies Transient Stability Studies Harmonic Anal...
  •   Grounding transformer for neutral point connection is installed usually in vicinity of feeding transformers in substations. Consequently, overall configuration is protected with differential protection (Figure ). Depend of specific applications, current transformer is added with earthing r...
  •     It’s tempting to select a variable frequency drive based on horsepower alone. After all, if you select a drive that has the same or more horsepower than your motor you are using, then you should be good too – right? Well … not necessarily. There are six other ...
  •   Residual-current device  is protective and life-saving device. It is designed to protect   from an electric shock, if you touch phase live conductor a current of 30 milli amperes can be lethal for a person, if it persists for more than a fraction of second.  In most sce...
View All


Test

Electrical energy through the arc-flash

  •  

    Electrical energy through the arc-flash

     

    Industry is now addressing the impact to the worker of other energies transformed from electrical energy through the arc-flash event (see Figure ). These energies include electrical, thermal, acoustic, radiation, chemical, and mechanical that may cause

    • excessive noise
    • intense heat
    • flying debris or shrapnel, projected molten copper
    • expanding toxic vapor cloud
    • explosions of flammable gases, vapors, or combustible dusts
    • pressure

     

    Heat and Burns

    Clothing manufacturers have engineered PPE systems capable of limiting a burn injury to the onset of a second- degree burn when the wearer is subjected to an arc-flash event with incident energy as high as 100 cal/cm2. Thus, “barrier protection” emerged as a key strategy to mitigate the burn hazard associated with an arc-flash event.

     

    Pressure Wave

    The arc-blast pressure is related to available fault current and distance from the arc and not to the arc clearing time. This force is significant and can cause falls and injuries to the worker that are more serious than burn injuries. Research which studies the trauma from pres- sure waves is supported in part by Grant R01 OH04136-01 the U.S. Centers for Disease Control and Prevention National Institute of Occupational Safety and

    Health. The trauma from pressure waves may not be readily diagnosed in triage because of the absence of external wounds. Resulting brain injury, tissue damage  to lungs, ears, bowel, and concussions may not be accom- panied by electrical contact evidence or burns.

    Barrier protection in the form of arc-flash suits has proven effective for reducing the burn injury. Barrier protection when applied to switchgear can reduce the risk and impact of electrical burns and pressure waves. Enclosures containing primary elements are compartmentalized and grounded for maximum isolation. All live parts (where possible) are fully insulated reducing the possibility of an arcing fault to occur. All primary elements such as breakers, PTs, CPTs, etc. have disconnect means with isolating shutters.

    Workers exposed to an arc-flash event may suffer from injuries including burns, cardiac arrest, amputation, mem- ory loss, hearing loss, fracture, cataract, and blast trauma. For example, NFPA 70E-2004 requires hearing protection as part  of  PPE  where incident energy  exceeds  4 cal/cm2.

    The Flash Suit Issue

    NFPA 70E-2004 defines a flash suit as a complete FR clothing and equipment system that covers the entire body, except for the hands and feet. This includes pants, jacket, and beekeeper-type hood fitted with a face shield.

    NFPA 70E-2004 Table 130.7(C)(10) requires a flash suit hood for Hazard/Risk Categories 3 and 4. This requirement occurs when the calculated incident energy for the work task is greater than 8 cal/cm2.

    NFPA 70E-2004 Table 130.7(C)(10) requires an arc- rated face shield or flash suit hood for Hazard/Risk Cat- egory 2. The face shield must have a minimum arc rating of 8 cal/cm2, with wrap-around guarding to pro- tect not only the face, but also the forehead, ears, and neck. This requirement occurs when the calculated inci- dent energy for the work task is greater than 4 and less than 8 cal/cm2.

    Face shields are normally available with arc ratings of 10–17 cal/cm2. A flash suit hood is usually required above 10 cal/cm2.

    Heat Stress

    Flash suit hoods cover the entire head and make breath- ing more difficult than normal (without a hood). Hoods with ventilation are available and improve worker com- fort when compared to a nonventilated hood because of

    • improved ease of breathing
    • cooling effect of fresh air stream
    • reduced claustrophobic feeling
    • reduction in fogging of visor and worker’s safety

    Generally, at incident energy levels above 8 cal/cm2, a

    double layer of fabric is used for both the flash suit hood and FR clothing. When wearing two layers, there is an increased risk of heat stress.

    Heat stress can be minimized by reducing the time PPE has to be worn. Work activity should be planned such that potential arc-flash exposure time duration is identified and minimized.

     


    Most flash suits and flash suit hoods are designed for the main purpose of skin burn protection and, therefore, provide limited protection from the blast pressure wave hazards, which include:Pressure Wave

    • excessive noise
    • flying debris or shrapnel
    • projected molten

     

    Reduced Visibility and Dexterity

    The typical green tint in the flash suit visor is capable of causing dis- tortion of colors. The colors most affected are white, yellow, red, and

    blue. These four colors are commonly used to provide phase marking on electrical conductors.

    Multiple layers of fabric in flash suits and flash suit hoods also reduce worker dexterity, reduce the range of motion, and decrease the ease of movement. All of these could potentially cause problems while a work task is being performed.

     

    Reference: 

    • IEEE Recommended Practice for Protection and Coordination, IEEE 242- 2001(IEEE Buff Book).
    • Methods in reducing hazardous areas in large industrial facilities  BY MICHAEL HODDER, WILLIAM VILCHECK

     

Share this article on...