By Victor L. Petrovic, Ph.D.

More than 2,000 workers are injured yearly inarc-flash incidents. High incident electricalenergy traveling through the air results in an arcflash. Electricity jumps across a gap in millisec-

onds, heating a small quantity of air to temperatures reachingas high as 35,000 degrees F—three-and-a-half times the tem-

perature of the surface of the sun—forming an intense fireballthat vaporizes metal and burns everything combustible in theimmediate area.

When an arc flash occurs, energy is released in a numberof forms. One is UV, causing injuries similar to sunburn,which can damage the eyes. Another is the plasma arc, a com-bination of super-heated ionized air and vaporized metal fromthe conductors that can burn, melt, vaporize or degrade anymaterials near the fault. Even PPE items labeled FR burn

Page 10 September/October 2006ip

INCIDENT PREVENTION

Properly designed, constructed and installed arc-suppression blankets can help abate thedangers of arc-flash explosions and arc blasts for utility workers.

UltimateProtection

An arc explosionin a simulatedundergroundvault.

Free Subscription at www.incident-prevention.com

readily if touching a 2,500-degree sur-face. Furthermore, burning items maybe on or near a worker, causing injuryby producing noxious gases and con-suming oxygen.

An arc blast occurs when the smallquantity of air next to the fault is quick-ly super-heated and forms a pressurewave that has the energy of severalsticks of dynamite and may result in aforce of over 1,000 pounds being exert-ed on the worker. Evidence indicatesthat an arc blast is sufficiently strong toviolently throw workers, ruptureeardrums, collapse lungs and breakbones. Since most underground vaultsconsist of reinforced concrete with steelprojections to support cables, how andwhere the worker impacts also deter-mines the extent of injuries.

REDUCING DANGERArc-suppression blankets (ASBs) are

designed to reduce the danger from botharc flash and arc blast. To combat arcflash, an ASB forces the arc to travelaround the edges of the blanket before itreaches the worker, allowing greatercooling because a plasma arc decreasesin temperature as it moves further fromthe fault. ASBs also have an ablativecomponent to absorb a portion of thee n e rg y. In addition, effective A S Binstallation redirects most of the arcblast energy to a safe area, such as anopen manhole.

To offer the greatest protection, thelargest possible ASB should be used.ASBs rely on the effective interaction ofthree factors—materials, constructionand installation. Inferiority in any oneof these areas will sacrifice optimumprotection:

Materials used in an ASB must beinherently strong at high temperaturesand self-extinguish. Typical blanketsuse a combination of flame-retardantlayers to form an arc-protection system.The outer layers consist of an ablativematerial, providing protection to thestrong inner layers that protect againstthe blast and shrapnel.

Construction involves a design thatallows the blanket to resist the arc blastand provide for ease of installation.Better ASBs entail quilting to re-directthe stress of a blast to the entire blanket.Double stitching around the perimeter

enhances the strength of the ASB, towhich grommets or straps are attachedfor installation.

All proper installation methodsrequire placing the ASBs between thepotential fault and the worker. At least

Page 11

DBI Sala

1/2 ISLAND

PICKUP ADFROM MAY/JUNE

eight inches should separate the poten-tial fault from the ASB to allow forcooling before the arc blast reaches theblanket and to provide a buffer to pro-tect the worker from being hit by theASB. The recommended installationmethod, “J” installation, places the longside of the blanket vertically with thebottom of the blanket shaped like theletter “J”. The advantage of this methodis that the arc blast traveling down isredirected upward while the top of theblanket points the blast into an openarea.

Other ASB installation methodsinclude “wall,” which involves placingthe blanket between the potential faultand the worker by attaching it at the cor-ners and every two feet. While allowingthe blanket to absorb energy throughablative materials and travel longer dis-tances, this method may not provideadequate protection since the arc flash

and blast may be reflected around anenclosed bunker instead of being direct-ed to a safe opening. Similarly, the“wrapping” installation method ofrolling the blanket around a line, switchor connector leaves a high probabilityfor arc blow-through. Additionally, withwrapping the ASB may not self-extin-guish with its proximity to hot fault sur-faces, causing smoke, noxious gasesand oxygen reduction.

The cost of an arc flash incident canreach and occasionally exceed $1 mil-lion in lost time, medical bills, litigationand settlements. Though arc flash inci-dents cannot be eliminated, proper useand installation of an eff e c t i v e l ydesigned ASB can reduce injuries andtheir associated costs. ip

Victor L. Petrovic, PhD, is a recognized industry expert on the subject of arc blast protection.Currently, he serves as the TechnicalDirector for Therm-Equip, Inc. basedin Canton, Ohio.

Page 12 September/October 2006ip

INCIDENT PREVENTION

1/6 VERT

ELECTRO-MARK

NEW AD

Questions about this article?Join our online FORUM, where the

author and our other expert moderatorsare ready to provide answers to your jobsafety concerns. www.incident-prevention.com

“J” method of installation.

Blanket exposed to 25,000-amp arc blast.

Unprotected mannequin aftera chest level arc blast.