01 introduction to electrical safety

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Introduction toElectrical Safety


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a dangerous condition, such that, inadvertent orunintentional contact, or equipment failure, can resultin shock, arc flashburn, thermal burn, or blast.

NFPA 70E-19951 defines an Electrical

Hazard as


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recognizing hazards associated with the use ofelectrical energy and taking precautions so that hazardsdo not cause injury or death.

Electrical safety is defined in NFPA

70E-1995 as


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NFPA 70E

NFPA 70E requirements forsafe work practices toprotect personnel byreducing exposure to majorelectrical hazards. Originally

developed at OSHA'srequest.


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understand the nature and consequences of electricalhazards and the reasons for practicing electricalsafety.

Electrical Hazard and Electrical Safety


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First, electrical installations should be designed and

constructed to be safe by complying with the criteria ofrecognized and generally accepted good engineeringpractices.

PURPOSE OF PEC 1.0.1.1(a)Practical safeguarding of persons andproperty from hazards arising from the use ofelectricity

Four main phases of protection from

electrical hazards.


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Second, the integrity of electrical equipment shall bemaintained with particular emphasis on enclosures,insulation, operating mechanisms, grounding, andcircuit protective devices.


electrical hazards.


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NEMA Enclosures


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Overhead Insulators


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Wire Insulation


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EMT

RMC

IMC

FMC

LFMC

ENT

PVC

Conduits


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Third, unless there are serious overridingcircumstances, electrical equipment shall be placed inan electrically safe work condition before personnelwork on or near it.

Safe practices shall be used to establish an electrically

safe work condition.


electrical hazards.


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electrical hazards.

Fourth, safe work practices

and adequate protectiveequipment, tools, and testequipment shall beunderstood and used whenit is not feasible to establishan electrically safe workcondition, or when de-energizing would create agreater hazard of anotherkind.


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These electrical safety give guidance as to what needsto be done to avoid creating dangerous conditions inthe first place, how to recognize them when they doexist, and how to take appropriate precautions toavoid having anyone injured or killed.


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CPR cardiopulmonary resuscitation

NEC National Electrical Code

NEMA National Equipment ManufacturersAssociation

NESC National Electrical Safety Code

NFPA National Fire Protection Association OSHA Occupational Safety and Health Administration

UL Underwriters Laboratories

Acronyms and abbreviations


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Exposure to electrical hazards


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Electrical hazard indicates that hazards can resultfrom poor physical condition of equipment orfacilities, sometimes simply called unsafeconditions.

It also indicates that injuries can result from the

careless or inadvertent actions of people, sometimessimply called unsafe acts.


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Taking precautions means that safety considerationsregarding such conditions and actions should begiven to all aspects of electrical work, starting fromthe initial design concept, through installation andstart-up, and continuing on into the post-operational

maintenance activities.


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There are essentially three recognized kinds of injurythat may result while working on ornear electrical hazards:

a) Electrical shock;

b) Burns from contact, arcs, or flashes;

c) Impact from blasts.


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The electrical hazard of which most people are aware,and the one that most electrical safety standardshave been built around, is the electrical shock hazard.

Electrical shock affects human beings in the following

ways:

Electrical shock


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Currents as small as a few milliamperes through theheart can cause disruption of the natural electricalsignals that the heart uses to perform its normalfunctions.

Voltage levels as low as 50 V with low skin resistance

and current flowing through the chest area can causefibrillation, which can result in death.


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Electrical shock can damage human tissue where the

current enters and exits the body. Within the body,the current can also damage internal body parts in itspath.

Degree of damage amount of current,

the type of contact,

the duration of contact, and

the path of the current through the body


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Electrical shock causes the muscles to contract. Due

to muscle contraction, the person experiencing theshock may not be able to release the conductorcausing the shock (known as the let-go threshold).

This grasping leads to longer exposure.


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Several standards offer guidance regarding safeapproach distances in order to minimize thepossibility of shock from exposed electricalconductors of different voltage levels.

The most recent, and probably the most

authoritative, guidance is presented in NFPA 70E-1995.


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In fibrillation, the victim may not recover consciousness.

On the other hand, the victim may be conscious, denyneeding help, walk a few feet, and then collapse. Deathmay occur within a few minutes, or may take hours.

Detection of the fibrillation condition requires medical skill.The application of closed-chest massage, a treatment in

which blood is circulated mechanically in a fibrillationvictim, can result in the death of a subject whose heart isnot in fibrillation.


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Almost everyone is aware that electrical shock can bea hazard to life.

Many people, however, have experienced minorshocks wit h no dire consequences. This tends to makepeople somewhat complacent around electricity.

Burns from contact, arcs, or flashes


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Serious electrical injuriesinvolve burns

Electrical burns include

not only burns fromcontact, but alsoradiation burns from thefierce fire of electric arcs.

The electric arc betweenmetals is, next to thelaser, the hottest thing onearth.


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Serious or fatal burns can

occur at distances of morethan 304 cm (10 ft) from thesource of a flash.

In addition to burns from theflash itself, clothing is oftenignited. Fatal burns can result

because the clothing cannot beremoved or extinguishedquickly enough to preventserious burns over much of thebody.


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Electrical arcing is the term that is applied to the

passage of substantial electrical currents throughwhat had previously been air. It is initiated byflashover or the introduction of some conductivematerial.

Arcing involves high temperatures of up to, or

beyond, 20 000 K (35 000 F) at the arc terminals. Nomaterials on earth can withstand these temperatures;all materials are not only melted, but vaporized.


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The electric arc is widely recognized as a very high-level source of heat. Common uses are arc welding,electric arc furnaces, and even electric cauterizing ofwounds to seal against infection while deeper partsare healing.

The arc as a heat source


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The rapid expansion of air caused by a fault currenthas been recognized for some time as one of theelectrical hazards. What was not recognizedpreviously was the fact that this blast and its effectscould be calculated, and precautions against its

effects could be taken.

Impact from blasts


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People are exposed to two dangers from electricalarcs: burns and blasts.

The blast can cause falls and other injuries, as well asdamage nearby structures.

Pressures developed by arc blasts


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Case Histories


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Case no. 1 - shock

A mechanic, while workingon some equipment in therear of a power-type circuitbreaker auxiliary metering

compartment, accidentallycame in contact withadjacent energizedtransformer terminals. Heapparently had not checkedfor other energizedcomponents in the vicinity in

which he was working. Herequired medical treatmentfor shock.


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A construction electrician was assigned to a job installingwiring for lighting fixtures in a 208 Y/120 V system. The jobwas already partially completed. The electrician was toldby the foreman that everything was dead. The homerun was already installed, and those wires wereprotruding from a junction box. The electrician was on aladder and began to remove the insulation from the homerun wires using wire skinners with one hand. His otherhand was holding the wires, but was also in contact with

the box. He was not wearing gloves, and received a severeshock causing him to fall from the ladder. Fortunately, hereceived only minor bruises from the fall. He later said, Ishould have checked myself to see that it was really dead.

Case no. 2 - Shock


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A field engineer was using portable radiographyequipment to inspect the quality of medium voltage cableterminations. A construction electrician was assisting him.

The radiography equipment consisted of a cathode raytube in a metallic casing (called an X-ray head) and acontrol unit. A control cable interconnected these units.The engineer was on top of a stepladder, leaning againstthe switchgear enclosure, grasping the handles of the X-ray head, and adjusting its position. The electricianproceeded to plug the control unit into a 120 V outlet andconnect the control cable. Suddenly, the engineer receiveda severe shock and fell off of the ladder.

Case no. 3 - Severe shock


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Case no. 3 - Severe shock (cont.)

It was later determined thatthe plug-in connectionbetween the control cable andthe control unit was poorlypolarized. The electrician had

forced the connectiontogether in the wrongorientation, putting 120 V onthe casing of the X-ray head.The engineer was taken to themedical department at the site,and was observed for theremainder of the day. He saidhis grasp was locked onto thehandles momentarily until hisweight broke him loose duringhis fall. He complained ofmuscular problems for severaldays afterward.


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An electrician was part of a crew that was operating a

13 kV switch in a new electric power system. Oneswitch blade did not close properly. As other crewmembers packed up their tools and prepared toreturn to the shop to discuss the problem withsupervision, they heard a noise.

Case no. 4 - shock and burn


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They turned around to see the switch door open, and

the electrician surrounded by smoke and flames.While trying to read a part number, the electricianhad accidentally made contact with an energized partof the switch, and a flash had occurred. A coworkerpulled the man from the switch and revived him using

cardiopulmonary resuscitation. Nine months later,after having some fingers and toes amputated, theelectrician was still receiving medical treatment as aresult of the accident.

Case no. 4 - shock and burn (cont.)


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A construction electrician had been running wiring allday long for several new heating and air conditioningunits in the space above a suspended ceiling. Theelectrical feeder that supplied the new installationwas a 480 Y/277 V system, but it had been verified asde-energized earlier in the day; however, no lockouthad been installed. The electrician, at one point late inthe day, began to prepare the main feeder forconnection. He was working on a ladder and started

to remove the insulation from one of the wires. Hedid not have gloves on. He let out a yell as he becamehung up on the wires.

Case no. 5 - shock and burn


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Apparently, sometime during the day, someone had

turned on the feeder breaker. A carpenter, working rightbeside the electrician, recognized that the electrician wasfrozen to the wires due to muscle contraction. Thecarpenter used a piece of lumber to knock the electricianoff the ladder, breaking him free of the wire. Theelectrician fell onto the floor, rolled, and banged against a

wall. He was taken to the hospital and kept for two daysfor treatment and observation, having received a severeshock, second degree burns to both hands, and facial cutsfrom the fall.

Case no. 5 - shock and burn (cont.)


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A contract electrician had finished drying out a 480 V

bus duct. While working on a ladder, reinserting theplug-in units onto the bus, he encountered difficultygetting one of the units to make up properly. Hebanged on the plug-in unit and was met with a flashand blast that severely burned him and knocked himoff the ladder. He was not wearing any protectiveclothing.

Case no. 6 flash, blast, and burns


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An electrician was installing a 277 V lighting fixture in anindustrial plant. Turning the power off was inconvenient, as itinvolved winding his way around and through some vessels and

pipes, and going up two flights of stairs to the lighting panel. Apparently thinking that there was not much danger in this

minor job, he didnt bother to turn the power off. Working inan existing junction box, he made up two of the connections,and was trying to remove an existing wire nut to install the third

wire. He was having difficulty removing the nut, so he used alug crimping tool to try to pull and twist the wire nut off. Hegripped the tool too hard and it cut through the outer shell ofthe wire nut, making contact with the energized wire inside. Hewas electrocuted.

Case no. 7 - fatal shock

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