Electrical Safety Design

7/31/2019 Electrical Safety Design

1/110

ELECTRICAL SAFETY

DESIGN

General Requirements

Wiring Design and Protection

Wiring Methods and Components

Equipment for General Use.


2/110

Introduction

This module deals with

Occupational Safety andHealth Administration (OSHA)standards for electrical safetydesign.

These standards cover only

electrical system parts that anemployee would use or withwhich she/he could makecontact.

The modules purpose is tominimize potential workplacehazards by specifyingelectrical equipment andsystems designcharacteristics.


3/110

Development of The Code

The OSHA electrical standards were based on the National

Fire Protection Association's standard NFPA 70E, "ElectricalSafety Requirements for Employee Workplaces." The NFPA70 Committee derived Part I of their document from the 1978edition of the National Electrical Code (NEC).

The standards extracted from the NEC were those consideredthe most applicable to employee safety and least likely tochange with each new edition of the NEC.

OSHA's electrical standards are performance oriented;therefore they contain few direct references to the NEC.However, the NEC contains specific information as to how therequired performance can be obtained.


4/110

Introduction - Objectives

At the end of this module, you should beable to:

Identify the general requirements for electricalsafety design

Define proper wiring design and methods

Define proper installation and protection of

electrical equipment and components Identify unsafe wiring design, methods,

equipment, protection, and components.


5/110

General Requirements -

Introduction

The General Requirements

section, based on standard1910.303, describes the generalrequirements for electrical safetydesign.

The topics for this section arelisted on the left.

The conductors and equipmentrequired or permitted by 1910.303shall be acceptable only ifapproved.

Electrical equipment shall be safefrom recognized hazards that are

likely to cause death or seriousphysical harm to employees.

Examination and properinstallation are important methodsfor ensuring safety from hazards.


6/110

Equipment Examination,

Installation, and Use The following considerations are

examined to determine ifequipment is safe:

Suitability for installation and use,in conformity with electricalstandard provisions. Suitability ofequipment for an identified

purpose may be evidenced bylisting or labeling for that identifiedpurpose

Mechanical strength anddurability, including the adequacy

of the protection by partsdesigned to enclose and protectother equipment

Electrical insulation


7/110

Equipment Examination,

Installation, and Use Heating effects under conditions

of use

Arcing effects Classification by type, size,

voltage, current capacity, andspecific use

Other factors which contribute tothe practical safeguarding of

employees using or likely to comein contact with the equipment.

Listed or labeled equipment shallbe used or installed in accordancewith any instructions included in

the listing or labeling and with therequirements on the followingpages.


8/110

Splices and Arcing Parts Conductors shall be spliced or joined

with suitable splicing devices.

Another option is for conductors to bespliced by brazing, welding, orsoldering with a fusible metal or alloy.

Soldered splices should bemechanically and electrically securebefore soldering.

All splices, joints, and the free ends ofconductors shall be covered with aninsulation equivalent to that of theconductors, or covered with aninsulating device suitable for thepurpose.

Parts of electric equipment which in

ordinary operation produce arcs,sparks, flames, or molten metal shall beenclosed or separated and isolatedfrom all combustible material.


9/110

Marking Electrical equipment may not be used

unless the manufacturer's name,

trademark, or other descriptive markingby which the organization responsiblefor the product may be identified isplaced on the equipment.

Other markings indicating voltage,current, wattage, or other ratings shallbe provided as necessary.

Marking is almost always provided onthe name-plate of reputablemanufacturers equipment.

The markings shall be of sufficientdurability to withstand the operatingenvironment.

Violations usually occur when thenameplate is covered, removed, orobliterated by painting or other abuse.

One example of abuse is shown to theleft. Importance of Marking


10/110

Importance of Marking Marking is very important.

If a piece of equipment is connected to a voltage higher than its rating, a violent failurecould occur.

If it is connected to a voltage below its rating, it may attempt to perform its intendedfunction and overheat badly, leading to failure.

If AC equipment is energized with the wrong frequency, or with direct current, it willprobably fail violently. These occurrences can cause fires and burns to employees.

If equipment is connected to a circuit not adequate for the load, conductors may overheat

and deteriorate. Over current protective devices should prevent serious damage in such a case. However,

these devices may have been tampered with, reducing protection capability.

If the manufacturer's name is not marked on the nameplate, there will be difficulty intracing the reasons for faulty performance and in preventing future failures.

Sometimes it is necessary to know the identity of the manufacturer in order to determine

if equipment is approved for a particular purpose. The manufacturer's name may also be necessary so that the user can obtain information

or replacement parts.


11/110

Disconnecting Means and

Circuit Identification Identification (as used in OSHA standards) means equipment is

labeled to indicate its purpose. Required disconnecting means for motors and appliances shall give

legible indication of its purpose.

If the purpose of the circuit is obvious, no identification of thedisconnect is required.

A disconnecting means is a switch used to disconnect the circuitconductors from the electric current source.

Disconnect switches protect workers and equipment by enabling acircuit to be opened to stop the flow of electricity.

Each service, feeder, and branch circuit shall have its purpose legibly

identified (unless clearly evident) at its disconnecting means or overcurrent device.

Identification should be located at the circuit origination point. Identification Example


12/110

Identification Example On a panel that controls several

motors or on a motor controlcenter, each disconnect must beclearly marked to indicate themotor to which each circuit isconnected.

In this figure, the Number 2 circuitbreaker in the panel box suppliescurrent only to disconnect Number2, which in turn controls thecurrent to motor Number 2.

This current to motor Number 2can be shut off by the Number 2circuit breaker or the Number 2disconnect.


13/110

600 Volts, Nominal or Less 1910.303 also addresses the safety of

persons qualified to work on the

equipment, but who do not removeguards and enclosures in order to workon live parts.

Table S-1 of 1910.303(g)(1)(i) lists theminimum dimensions for working spacein the direction of access to live partsoperating at 600 volts or less and likelyto require examination, adjustment,servicing, or maintenance whileenergized except where required orpermitted in Subpart S.

In addition, work space may not be lessthan 30 inches wide in front of theelectrical equipment measured fromexposed live parts, or from theenclosure front or opening if the liveparts are enclosed.


14/110

600 Volts, Nominal or Less Concrete, brick, or tile walls are

considered to be grounded.

Working space is not required in backof assemblies such as dead-frontswitchboards or motor control centerswhere there are no renewable oradjustable parts such as fuses orswitches on the back and where allconnections are accessible fromlocations other than the back.

Working space shall not be used forstorage.

In addition the working space access,illumination, and headroom shall beadequately maintained.


15/110

600 Volts, Nominal or Less Except as required or permitted elsewhere in this standard, live parts of

electric equipment operating at 50 volts or more shall be guarded againstaccidental contact by approved cabinets, enclosures, or by:

Location in a room, vault, or similar enclosure only accessible to qualified persons

Suitable permanent, substantial partitions or screens so arranged that only qualifiedpersons will have access to the space within reach of the live parts and that anyopenings shall be so sized and located to prevent accidental contact with the live

parts or to bring conducting objects into contact with them Location on a suitable balcony, gallery, orplatform

Elevation of 8 feet or more above the floor or other working surface.

In locations where electric equipment would be exposed to physical damage,

enclosures or guards shall be so arranged and of such strength as to preventsuch damage.

The purpose of guarding equipment is to protect any person, not just qualifiedelectricians, against accidental electrical equipment contact.
http://c/PPT/Glossary.ppthttp://c/PPT/Glossary.ppt


16/110

Over 600 Volts, Nominal Electrical equipment operating

above 600 volts, nominalcreates extremely dangerousworking environments.Therefore, it is important toheed the followingrequirements.

The entrances to all buildings,rooms, or enclosurescontaining exposed live partsor exposed conductors

operating at over 600 volts,nominal shall be under theobservation of a qualifiedperson or locked at all times.


17/110

Over 600 Volts, Nominal Electrical installations in the

following areas where accessis controlled by lock and keyor other approved means areconsidered sufficient toprevent unqualified access:

Area surrounded by a wall,screen, or fence 8 feet orhigher unless other featuresprovide a degree of isolationequivalent to an 8 foot fence

Vault Room

Closet


18/110

Over 600 Volts, Nominal Electrical installations operating

over 600 volts, nominal havingexposed live parts shall beaccessible to qualified personsonly.

Sufficient space shall be provided

and maintained about electricequipment to permit ready andsafe operation and maintenanceof such equipment.

Where energized parts are

exposed, the minimum clear workspace may not be less than 6 feet6 inches high (measured verticallyfrom the floor orplatform), or lessthan 3 feet wide (measuredparallel to the equipment).
http://c/PPT/Glossary.ppthttp://c/PPT/Glossary.ppthttp://c/PPT/Glossary.ppt


19/110

Over 600 Volts, Nominal The minimum depth of clear

working space in front of electricequipment is given in Table S-2 of1910.303(h)(3)(i).

Unguarded live parts aboveworking spaces shall be

maintained at elevations not lessthan specified in Table S-3 of1910.303(h)(3)(iii).


20/110

Over 600 Volts, Nominal At least one entrance not less than 24 inches wide and 6 feet 6 inches high

shall be provided to give access to the working space about electricequipment.

On switchboard and control panels exceeding 48 inches in width, there shallbe one entrance at each end of such board where practicable. Where bareenergized parts at any voltage or insulated energized parts above 600 voltsare located adjacent to such entrance, they shall be suitably guarded.

Adequate illumination shall be provided for all working spaces about electricequipment operating over 600 volts, nominal.

The lighting outlets shall be arranged so persons changing lamps or makingrepairs on the lighting system will not be endangered by live parts or otherequipment.

The points of control shall be so located that persons are not likely to come in

contact with any live part or moving part of the equipment while turning on thelights.

Permanent ladders or stairways shall be provided to give safe access to theworking space around electric equipment installed on platforms, balconies,mezzanine floors, or in attic or roof spaces.


21/110

Quiz Question:

One of the primary purposes ofsoldering is to mechanically and

electrically secure two conductors.

True

False


22/110

Quiz Question:

Identification of "disconnecting means" is

used to:

Mark the power rating of the device

Mark the purpose of the device Mark the manufacturer of the device

Mark the current and voltage rating of a device


23/110

Quiz Question:

A wall, screen, or fence less than 8 feet in

height is not considered sufficient to prevent

access to electrical equipment operating at

over 600 volts, nominal.

True

False

Wi i D i d


24/110

Wiring Design and

Protection Circuits connected

improperly can poseserious shock or burnhazards.

The Wiring Design andProtection section, based

on standard 1910.304,describes the requirementsfor proper wiring designand protection for workerand equipment safety.

The topics covered in thissection are listed on theleft.

G d d d G di


25/110

Grounded and Grounding

Conductors Grounded/grounding

conductors of an electricalcircuit must be marked orcolor coded in a way thatallows employees to identifyand distinguish them fromeach other and from the other

conductors in the circuit. This section describes

identification procedures fortwo such conductors.

Circuit Grounded Conductors Equipment Grounding

Conductors

G d d d G di
http://c/PPT/Glossary.ppthttp://c/PPT/Glossary.ppthttp://c/PPT/Glossary.ppthttp://c/PPT/Glossary.ppthttp://c/PPT/Glossary.ppt


26/110

Grounded and Grounding

Conductors Color coding is one means by

which conductors areidentified from other circuitconductors.

Acceptable color codingincludes the method requiredby the National ElectricalCode, Section 210-5.

Color Coding Example


27/110

Color Coding Example The graphic illustrates color

coding on a distribution panelboard.

The National Electric Code,Section 210-5 states:

"The circuit groundedconductor of a branch circuitshall be identified by acontinuous white or naturalgray color. The equipmentgrounding conductor of abranch circuit shall be

identified by a continuousgreen color or a continuousgreen color with one or moreyellow stripes unless it isbare."


28/110

Color Coding Example Bare copper or aluminum wire

is permitted for use as agrounding conductor.

Distinct identification isimportant to maintain correctpolarity between theungrounded (hot) and

grounded (neutral) conductorwhen plugs, receptacles, andconnectors are used in anelectrical branch circuit.


29/110

Wiring Design and Protection -

Grounded and Grounding Conductors

The correct polarity between the ungrounded(hot) and grounded (neutral) conductors whenusing plugs, receptacles, and connectors isimportant because incorrect polarity cancreate dangerous situations. There are twobasic guidelines:

No grounded conductor may be attached toany terminal or lead which would reversedesignated polarity

A grounding terminal or grounding-type device

on a receptacle, cord connector, orattachment plug may not be used forpurposes other than grounding.

Many pieces of equipment will operateproperly even though the supply wires are notconnected in the order designated by designor the manufacturer. Improper connection ismost prevalent on the smaller branch circuit

typically associated with standard 120 voltreceptacle outlets, lighting fixtures, and cord-and plug-connected equipment.


30/110



In a common 120-volt outletwith a portable hand toolattached, the black conductorsupplies power to the motor.

The grey (or white) conductoris used as a circuit groundedconductor for the motor.

This ground is needed totrigger the circuit breaker, thusstopping the motor if a faultoccurs.

The green wire is used as the

equipment groundingconductor to protectemployees handling metalenclosures that may beenergized by an internal fault.


31/110



The graphic at the leftillustrates the propergrounding for a hand tool.

An incorrectly connectedcircuit can cause dangeroussituations.

You can inspect threeexamples and their outcomesby clicking on the examplesbelow. Reversed Polarity with Circuit

Ground

Reversed Polarity withEquipment Ground

Reversed Circuit andEquipment Ground

Reversed Polarity with


32/110


Circuit Ground In reversed polarity the black (ungrounded) and white (circuit

grounded) conductors are reversed.

Although a shock hazard may not exist in this situation, othermechanical hazards can occur.

For example, if an internal fault occurs in the wiring, the equipmentwould not stop when the switch is released or would start as soon asthe supply cord is plugged into the improperly wired outlet. This couldresult in serious injury.



33/110


Equipment Ground It is extremely dangerous when the black (ungrounded) and green

(equipment grounding) conductors are reversed.

The metal case of the equipment is at 120 volts with reference to thesurroundings. The equipment will not work with this wiring, however,it is possible for a person to attempt to troubleshoot the problembefore unplugging the power cord.

Physical contact with a conductive equipment surface will result in aserious, or even deadly, shock.

Reversed Circuit &


34/110

Reversed Circuit &

Equipment The figure below shows the white (circuit grounded) and green

(equipment grounding) conductors reversed. Although it is not considered by OSHA or code terminology to

be reversed polarity, a hazard can still exist. In this case, dueto the wiring error, the white wire is being used to provideequipment grounding. Under certain conditions, this could be

dangerous

Wiring Design and


35/110

Wiring Design and

Protection - Service Entrance The service entrance is where

the serving conductors enter abuilding.

Means shall be provided todisconnect all conductors in abuilding or other structure fromthe service entrance

conductors. One disconnecting means for

conductors is required to belocated at a point near theservice entrance.

The disconnecting means canbe a switch or circuit breaker,and must be capable ofinterrupting the circuit from thesource of supply.

Wiring Design and


36/110

Wiring Design and

Protection - Service Entrance The disconnecting means

shall plainly indicate whether itis in the open or closedposition.

Additional service entrancerequirements exist for servicesover 600 volts, nominal:

Service-entrance conductorsinstalled as open wires shallbe guarded to make themaccessible only to qualifiedpersons

Signs warning of high voltageshall be posted where otherthan qualified employeesmight come in contact withlive parts.



37/110


Overcurrent Protection Conductors and equipment shall

be protected from overcurrent in

accordance with their ability tosafely conduct current.

Feeders and branch circuits over600 volts, nominal shall also haveshort-circuit protection.

The requirements described onthe following screens apply toovercurrent protection of circuitsrated 600 volts, nominal or less.

An important, basic requirementfor all overcurrent devices is toensure they have adequateinterrupt ratings for the circuit inwhich they will be installed. Conductors and Overcurrent

Devices

Conductors and Overcurrent


38/110

Conductors and Overcurrent

Devices Electric current is the flow of electrons through a conductor.

The size of the wire is the main determinate in deciding how much current can safely flow

through a conductor. A larger wire can carry more current safely. If too much current flows through a

conductor, excess heat is produced.

If the circuit is not protected, the heat may continue to build until it reaches a temperaturehigh enough to destroy the insulation and cause a fire.

The National Electrical Code specifies the allowable current flow permitted in certain-

sized conductors. Ampacity is the term used to describe the current-carrying capacity of a conductor. The

size of the fuse or circuit breaker required to provide protection is determined by theampacity of the conductor in the circuit to be protected and the type of load that is on thecircuit.

Fuses and circuit breakers are designed to protect equipment and facilities againstovercurrent and overheating, but they also provide considerable protection against shock

in most situations. The basic idea of a protective device is to make a weak link in the circuit. A fuse acts as a

weak link that is the first piece of the system to be destroyed.

In the case of a circuit breaker, a set of contacts open the circuit and stops the flow ofcurrent. Unlike a fuse, a circuit breaker can be re-used by re-closing the contacts.



39/110


Overcurrent Protection

Except for motors runningoverload protection,overcurrent devices may notinterrupt the continuity of thegrounded conductor unless allconductors of the circuit areopened simultaneously.

Unless excepted, overcurrentdevices are always placed inthe "hot" side of a circuit(usually a black wire) and inseries with the load, so that all

the current in the circuit mustflow through overcurrentdevices.



40/110


Overcurrent Protection

Except for service fuses, allcartridge fuses which are

accessible to other thanqualified persons and all fusesand thermal cutouts on circuitsover 150 volts to ground shallbe provided with

disconnecting means. This disconnecting means

shall be installed so that thefuse or thermal cutout can bedisconnected from its supply

without disrupting service toequipment and circuitsunrelated to those protectedby the overcurrent device.



41/110


Overcurrent Protection Overcurrent devices require

proper operation and maintenance

due to the tremendous amounts ofenergy they are likely to handleduring a fault.

Device location must allow forimmediate access by all employeeor authorized building

management personnel whentrouble occurs.

Overcurrent devices may not belocated where they will beexposed to physical damage.

Physical damage often resultswhen they are located where theycan be struck by lift trucks or othervehicles, by crane hooks, bymaterials being handled, etc.



42/110


Overcurrent Protection Most fuses and circuit breakers

have the potential to emit flashes

or sparks. As a result, easily ignitable

material should not be storednearby.

Since fuses and overcurrentdevices can sometimes operate atelevated temperatures, they shallbe located or shielded so thatemployees will not be burned orotherwise injured by theiroperation.



43/110


Overcurrent Protection Circuit breakers shall clearly

indicate whether they are in

the open (off) or closed (on)position.

Where circuit breaker handleson switchboards are operatedvertically rather than

horizontally or rotationally, theup position of the handle shallbe the closed (on) position.

If used as switches in 120-volt,fluorescent lighting circuits,

circuit breakers shall beapproved for the purpose andmarked "SWD."
http://c/PPT/Glossary.ppthttp://c/PPT/Glossary.ppthttp://c/PPT/Glossary.ppthttp://c/PPT/Glossary.ppt


44/110

Quiz Question:

The circuit grounded conductor shouldbe colored :

Green with yellow stripes Blue

White

Green


45/110

Quiz Question:

Most equipment will NOT operate if the

supply and grounding wires are not

connected correctly.

True

False



46/110


Grounding Requirements Grounding electrical circuits and

electrical equipment is required to

protect employees againstelectrical shock, safeguard againstfire, and protect against damageto electrical equipment.

There are two kinds of groundingwhich correspond to the two types

of grounding conductors (circuitgrounding conductors andequipment grounding conductors)described earlier in this module. Electrical Circuit or System

Grounding

Electrical Equipment Grounding

Circuit and Equipment GroundingSchematic

Schematic of Circuit and
http://c/PPT/Glossary.ppthttp://c/PPT/Glossary.ppthttp://c/PPT/Glossary.ppthttp://c/PPT/Glossary.ppthttp://c/PPT/Glossary.ppt


47/110

Equipment Grounding

This is an example of circuit and equipment grounding.

Wiring Design and


48/110

g g

Protection - Grounding Path The path for electrical circuit and

equipment grounding shall be

permanent and continuous.

This requirement was extractedfrom NEC 250-51, EffectiveGrounding Path.

NEC 250-51, Effective GroundingPath

Wiring Design and


49/110

g g

Protection - Grounding Path Remember the following items

regarding safe grounding paths.

The fault current in AC circuits willbe limited by the sum ofresistance and reactance. Theonly low- reactance path is thatwhich closely follows the circuitconductors.

If a metallic raceway system isused, make sure that the metallicsystem is continuous andpermanent.

In cases where a metallicraceway system is not used,

provide a green or bareequipment grounding conductorclose to the supply conductors toassure that all enclosures arebonded together and to thesource.

Support, Enclosure, and


50/110

Equipment Grounding Proper grounding is important

to prevent shock, stabilize

voltage, and channel faultcurrents.

Metal enclosures for serviceequipment shall be grounded.

Metal cable trays, metalraceways, and metalenclosures for conductorsshall be grounded, except for:

Metal enclosures such assleeves that are used toprotect cable assembliesfrom physical damage



51/110

Equipment Grounding Metal enclosures for

conductors added to existing

installations of open wire,knob-and-tube wiring, andnon-metallic-sheathed cablewhich meet the followingconditions: Runs are less than 25 feet

Enclosures are free fromprobable contact withground, grounded metal,metal laths, or otherconductive materials

Enclosures are guardedagainst employee contact.



52/110

Equipment Grounding Frames of electric ranges, wall-

mounted ovens, counter-mountedcooking units, clothes dryers, and metaloutlet or junction boxes which are partof the circuit for these appliances shallbe grounded.

Exposed non-current carrying metalparts of fixed equipment which maybecome energized and meet the

following conditions shall be groundedif: Within 8 feet vertically or 5 feet

horizontally of ground or groundedmetal objects and subject to employeecontact

Located in a wet or damp location andnot isolated

In electrical contact with metal In a hazardous (classified) location

Supplied by a metal-clad, metal-sheathed, or grounded metal racewaywiring method.



53/110

Equipment Grounding If exposed non-current carrying metal

parts of fixed equipment operates withany terminal at over 150 volts to groundwhich may become energized it mustbe grounded.

However, there are exceptions to thissituation. These exceptions are:

Enclosures for switches or circuit

breakers used for other than serviceequipment and only accessible byqualified person

Metal frames of electrically heatedappliances which are permanently andeffectively insulated from ground

The cases of distribution apparatussuch as transformers and capacitors

mounted on wooden poles at a heightexceeding 8 feet above ground orgrade level.



54/110

Equipment Grounding Exposed non-current-carrying metal parts of cord- and plug-connected

equipment which may become energized shall be grounded:

If in a hazardous (classified) location If operated at over 150 volts to ground, except for guarded motors and metal

frames of electrically heated appliances when the appliance frames arepermanently and effectively insulated from ground

If the equipment is of the following types: Refrigerators, freezers, and air conditioners

Clothes-washing, clothes-drying, dishwashing machines, sump pumps, andelectrical aquarium equipment

Hand-held motor-operated tools

Motor-operated appliances of the following types: hedge clippers, lawn mowers,snow blowers, and wet scrubbers

Cord- and plug-connected appliances used in damp or wet locations or byemployees standing on the ground or on metal floors or working inside of metaltanks or boilers

Portable and mobile X-ray and associated equipment

Tools likely to be used in wet and conductive locations

Portable hand lamps.



55/110

Equipment Grounding If metal parts or tools are likely to

be used in wet or conductive

locations, they do not have to begrounded if their power is suppliedthrough an isolating transformerwith an ungrounded secondarywinding of not over 50 volts.

Listed or labeled portable toolsand appliances protected by anapproved system of doubleinsulation, or its equivalent, neednot be grounded.

The equipment shall bedistinctively marked to indicatethat the tool or appliance uses anapproved system of doubleinsulation.



56/110

Equipment Grounding The metal parts of the following

non-electrical equipment shall be

grounded:

Frames and tracks of electricallyoperated cranes

Frames of non-electrically drivenelevator cars to which electricconductors are attached

Hand operated metal shiftingropes or cables of electricelevators

Metal partitions, grill work, andsimilar metal enclosures aroundequipment of over 750 volts

between conductors.



57/110

Grounding Methods "Non-current-carrying metal

parts of fixed equipment shall

be grounded by an equipmentgrounding conductorwhichruns with or encloses thecircuit conductors, if required.

One option for grounding is to

contain the equipmentgrounding conductor known asa third wire.

For D-C circuits only, theequipment grounding

conductor may be runseparately from the circuitconductors. Third Wire Example


58/110

Third Wire Example A third wire and prong are commonly

provided in the cord set to ground cord-connected equipment.

The third wire serves as an equipmentgrounding conductor which isconnected to the metal housing of aportable tool and a metal grounding businside the service entrance equipment.

The third wire provides a path for fault

current should an insulation failureoccur.

In this manner, dangerous fault currentwill be directed back to the source (theservice entrance) and will enable circuitbreakers or fuses to operate.

This opens the circuit and stops the

current flow. Click here to see the potential danger

and benefit of a third wire systemcontrol.


59/110

Third Wire ExamplePotential shock hazards exist when an equipment grounding conductor is not used. If a

fault occurs, most of the current will follow the path of least resistance. The worker can

provide that path to ground as an alternate to the white circuit grounded conductor

(neutral) for some portion of the current. The shock severity will depend on the amount ofcurrent that flows through the worker.

Cord- and Plug-Connected Equipment Without Equipment GroundingConductor (Short circuit flows through worker)


60/110

Third Wire ExampleIf there is a properly connected grounded conductor the current in a fault is not sent

through the worker. It should be noted that properly bonded conduit and associated metal

enclosures can also serve as a grounding conductor.

Cord- and Plug-Connected Equipment With Equipment GroundingConductor (Short circuit flows through equipment ground)

Wiring Design and


61/110

Protection - HigVoltage High voltage refers to circuits

of 1000 volts and over. If high

voltage systems aregrounded, they shall complywith all applicable provisionspreviously discussed in thismodule as well as additionalrequirements andmodifications contained in theWiring Design and Methodssection.


62/110

Quiz Question:

Reactance is not considered when

calculating AC fault current for

conductors.

True

False


63/110

Quiz Question:

The third wire in a cord is used for

equipment grounding.

True False

Wiring Methods and


64/110

Components The Wiring Methods and Components section is based on standard

1910.305. This section describes general and specific requirements

for various wiring methods and components. The topics included inthis section are listed below.

The wiring method provisions described on the following pages donot apply to the conductors that are an integral part of factory-assembled equipment.

Wiring Methods and Components -


65/110


Wiring methods must adhere tocertain general requirements:

Metal raceways, cable, armor,and other metal enclosures forconductors shall be metallically

joined together into a continuouselectric conductor and shall be soconnected to all boxes, fittings,and cabinets to provide effective

electrical continuity No wiring systems of any type

shall be installed in ducts used totransport dust, loose stock, orflammable vapors

No temporary or permanentwiring system may be installed in

any duct used for vapor removalor for ventilation of commercial-type cooking equipment, or in anyshaft containing only such ducts.



66/110

Temporary Wiring

Temporary wiring can be an easysolution to wiring problems.

However, the use of temporarywiring for electrical power andlighting installations of 600 volts,nominal or less is restricted to thesituations listed here.

During remodeling, maintenance,repair or demolition of buildings,structures, or equipment andsimilar activities.

For experimental or developmentwork.

For a period not to exceed 90days for Christmas decorativelighting, carnivals, and similarpurposes.



67/110

Temporary Wiring

Temporary wiring over 600 volts,nominal may be used only during

periods of tests, experiments, oremergencies.

Temporary electrical power andlighting wiring methods may be ofa class less than would berequired for a permanent

installation. However, all other requirements in

this module for permanent wiringshall apply to temporary wiringinstallations.



68/110

Temporary Wiring

This page and the next one describe general requirements fortemporary wiring conditions.

Feeders shall originate in an approved distribution center. Theconductors shall be run as multi-conductor cord or cable assemblies, orthey may be run as open conductors or insulators not more than 10 feetapart if they will not be subject to physical damage.

Branch circuits shall originate in an approved power outlet or panelboard.Conductors shall be open conductors or multi-conductor cord or cable

assemblies. If run as open conductors, branch circuits shall be fastenedat ceiling height every 10 feet. No branch- circuit conductor may be laidon the floor, and each branch circuit that supplies receptacles or fixedequipment shall contain a separate equipment grounding conductor.

Receptacles used in temporary wiring circuits must provide a connectionfor an equipment grounding conductor. Unless the receptacle is supplied

by a metallic raceway that provides a continuous grounding path back tothe source, a separate equipment grounding conductor must be placed inthe branch circuit. There must be good electrical connection between thereceptacle grounding terminal and the equipment grounding conductor. Receptacle Temporary Wiring Schematic

Receptacle Temporary
http://c/PPT/Glossary.ppthttp://c/PPT/Glossary.ppthttp://c/PPT/Glossary.ppthttp://c/PPT/Glossary.ppthttp://c/PPT/Glossary.ppthttp://c/PPT/Glossary.ppthttp://c/PPT/Glossary.ppt


69/110

Wiring Schematic This figure illustrates the proper wiring of receptacles used with a temporary

wiring system.



70/110

Temporary Wiring More general requirements for temporary wiring conditions are shown here.

No bare conductors nor earth returns may be used for the wiring of anytemporary circuit. Earth returns are prohibited because they are not alwayseffective and may present a serious hazard on temporary work sites.

Description of Earth Returns

Suitable disconnecting switches or plug connectors shall be installed to permit thedisconnection of all ungrounded conductors of each temporary circuit.

Lamps for general illumination shall be protected from accidental contact orbreakage. To be adequately protected, lamps shall be elevated at least 7 feet fromnormal working surfaces or by a suitable fixture or lampholder with a guard.

Flexible cords and cables shall be protected from accidental damage. Sharp

corners and projections shall be avoided. Flexible cords and cables shall beprovided with protection to avoid damage when passing through doorways or otherpinch points.


71/110

Description of Earth Returns Earth returns use the earth to provide a current path back to the supply source. This is

done by implanting a grounding electrode at the equipment being served and connectingthe equipment to the ungrounded conductor and to the grounding electrode.

Since one side of the supply source is also connected to ground through a groundingelectrode, a return path exists; however, its effectiveness is dependent on varying soilconditions.

The graphic to below shows an earth return, which is not allowed, in contrast to a properreturn system


72/110

Conductors for General Wiring Temporary or permanent

conductors can create shock andfire hazards that usually can beprevented through insulation.Insulating material should be theappropriate composition andthickness as rated for:

Voltage the conductor will carry

Current the conductor will carry

Temperature extremes and otherenvironmental factors

Location of the conductor.

Insulated conductors must also beeasily identifiable, color coding ismost often used.


73/110

Conductors for General Wiring The colors to be used are:

White or natural gray forneutralconductors

Green or green with yellow stripesforequipment groundingconductors unless bare wires areused.

Other types of circuit wires may beany color except those listedabove.
http://c/PPT/Glossary.ppthttp://c/PPT/Glossary.ppthttp://c/PPT/Glossary.ppthttp://c/PPT/Glossary.ppthttp://c/PPT/Glossary.ppthttp://c/PPT/Glossary.ppthttp://c/PPT/Glossary.ppt


74/110

Cabinets, Boxes, and Fittings In addition to shock and fire hazards,

conductors could be damaged if theyrub against the sharp edges where they

enter cabinets, boxes, or fittings. To protect them some type of clamp or

rubber grommet must be used.

The device used must close the holethrough which the conductor passes aswell as provide protection fromabrasion.

If the conductor is in a conduit and theconduit fits tightly in the opening,additional sealing is not required.

The knockouts in cabinets, boxes, andfittings should be removed only ifconductors are to be run through them.

However, if a knockout is missing or ifthere is another hole in the box, thehole or opening must be closed.


75/110

Cabinets, Boxes, and Fittings All pull boxes, junction boxes, and

fittings shall be provided withapproved covers. If metal coversare used, they shall be grounded.

In completed installations, eachoutlet box shall have a cover,faceplate, or fixture canopy. Outlet

box covers having holes throughwhich flexible cord pendants pass,shall be provided with bushings orshall have smooth, well-roundedsurfaces on which the cords maybear.

Boxes shall provide a completeenclosure for the containedconductors or cables.


76/110

Cabinets, Boxes, and Fittings Boxes shall be closed by suitable

and securely fastened covers.

Underground box covers thatweigh over 100 pounds meet thisrequirement.

Box covers shall be permanentlymarked "HIGH VOLTAGE" on theoutside of the box cover and shallbe readily visible and legible.

If cabinets, boxes, fittings, cutout

boxes, and panelboard enclosuresmust be located in a damp or wetlocation, they shall be installed soas to prevent moisture or waterfrom entering and accumulatingwithin the enclosures.


77/110

Switchboards and Panelboards Switches, switchboards, and circuit

breakers shall be weatherproof in wetlocations.

A switchboard that has exposed liveparts must be located in an area that isnot subject to wetness or dampness.

One purpose of 1910.305 is to lessenthe chance of severe shock if a workeraccidentally comes into contact with thelive parts.

Additionally, only qualified persons mayhave access to switchboards withexposed live parts.

To limit access, the switchboard shouldbe located in a locked room or within alocked cage or fenced area.

Keys to the locks should be controlledto ensure that only properly trainedpersonnel are allowed to enter thearea.


78/110

Switches Switches provide a means

for turning power on and off

to equipment or circuits.

Three type of switches arecovered in the WiringMethods and Components

section.

Single-Throw Knife Switch

Double-Throw Knife

Switch Flush Snap Switch


79/110

Single-Throw Knife Switch Switches provide a means for turning power on and off to equipment or circuits.

Single-throw knife switches have one energized (closed or "ON") position and one open(dead or "OFF") position. The switch must be designed so that when it is in the open

position, the blades are not energized (connected to the load side, not the supply side ofthe circuit).

Switches must also be installed so that if the switch falls downward, it will not fall into itsenergized position. However, some single-throw knife switches are designed to beinstalled so that they open upward, but these must have a latch or other locking deviceused to secure the switch in the open position.


80/110

Double-Throw Knife Switch Double-throw knife switches

are knife switches that have

two energized (closed or"ON") positions and one open(dead or "OFF") position.

These switches can bemounted vertically so that theyare moved up and down, orhorizontally so that they aremoved left and right. Ifswitches are mountedvertically, they must have alocking device (such as a

spring-loaded device) that willhold the switch blades in theopen position.


81/110

Flush Snap Switch Flush Snap switches that

are mounted in ungrounded

metal boxes and locatedwithin reach of conductingfloors or other conductingsurfaces shall be provided

with faceplates of non-conducting, non-combustible material.

Motor Protection


82/110

Switches are used as disconnecting means for motors.Specific motor disconnection requirements are listed here:

A disconnecting means shall be located in sight from thecontroller (within 50 feet of the controller). However, a singledisconnecting means may be located adjacent to a group ofcoordinated controllers that are mounted adjacent to each other

on a multi-motor continuous process machine.

The controller disconnecting means for motor branch circuitsover 600 volts, nominal may be out of sight of the controller, ifthe controller is marked with a warning label giving the location

and identification of the disconnecting means which is to belocked in the open position

Motor Protection


83/110

The disconnecting means shall disconnect the motor and thecontroller from all ungrounded supply conductors and shall be so

designed that no pole can be operated independently

If a motor and the driven machinery are not in sight from thecontroller location, the installation shall comply with one of thefollowing conditions:

The controller disconnecting means shall be capable of being lockedin the open position

A manually operable switch that will disconnect the motor from itssupply source shall be placed in sight from the motor location.

Motor Protection


84/110

Additional motordisconnecting meansrequirements are listed here:

The disconnecting means shall plainly

indicate open (off) or closed (on) The disconnecting means shall be

readily accessible. If more than one isprovided for the same equipment, onlyone must be readily accessible

An individual disconnecting meansshall be provided for each motor, but asingle disconnecting means may beused for a group of motors under anyone of the following conditions: If a number of motors drive special

parts of a single machine or piece ofapparatus, such as a metal orwoodworking machine, crane, or hoist

If a group of motors is under theprotection of one set of branch-circuitprotective devices

If a group of motors is in a single roomin sight from the location of thedisconnecting means.

Motor Protection


85/110

Motordisconnecting means provideone method for meeting the followingrequirement.

Motors, motor-control apparatus, andmotor branch-circuit conductors shallbe protected against overheating due tomotor overloads or start failure, andagainst short-circuits or ground faults.

Overload protection is not required ifstopping a motor could cause additionalor increased hazards.

An example is where motor operation isnecessary for a safe shutdown of

equipment or process and motoroverload sensing devices areconnected to a supervised alarm

Motor Protection


86/110

Stationary motors are not required toguard commutators, collectors, andbrush rigging located inside of motor

end brackets and not conductivelyconnected to supply circuits operatingat more than 150 volts to ground.

Exposed live parts of motors andcontrollers operating at 50 volts or morebetween terminals shall be guardedagainst accidental contact by:

Installing live parts in a room orenclosure only accessible to qualifiedpersons

Installing live parts on a suitablebalcony, gallery, or platform, elevatedand arranged as to exclude unqualifiedpersons

Elevating live parts 8 feet or moreabove the floor.

Motor Protection


87/110

Suitable insulating mats or platformsshall be provided and used byattendants to stand on if they are

required to work near live controller ormotor parts operating at over 150 voltsto ground where parts are guardedagainst accidental contact only bylocation.

Transformers


88/110

Contact with transformers occurs less often then with motors; however,transformers still pose a serious shock and fire hazard. The requirementsbelow apply to most transformers.

To prevent accidents, follow these requirements: The operating voltage of exposed live parts of transformer installations shall be

indicated by warning signs or visible markings on the equipment or structure

Dry-type, high fire point liquid-insulated, and askarel-insulated transformersinstalled indoors and rated over 35 kV shall be in a vault

Oil-insulated transformers installed indoors shall be in a vault if they present a firehazard to employees

Combustible material, combustible buildings along with parts of buildings, fireescapes, and door and window openings shall be safeguarded from fires which mayoriginate in attached or adjacent oil-insulated transformers

Transformer vaults shall be constructed so as to contain fire and combustible liquidswithin the vault and to prevent unauthorized access. Locks and latches shall be so

arranged that a vault door can be readily opened from the inside Any pipe or duct system foreign to the vault installation may not enter or pass

through a transformer vault

Materials may not be stored in transformer vaults.

Capacitors


89/110

Capacitors are used in most electricalsystems to store electrical charge;therefore, they can be a source of

severe shock unless that charge isdrained when the capacitors aredisconnected from the power source.

Unless some type of automaticdischarge is designed into a system,devices such as resistors must bepermanently attached across the

terminals of the capacitors to drain thecharge when the circuit is open (de-energized).

Most capacitors are manufactured withthis type of discharge resistor. Surgecapacitors, which act like lightning rods,do not require an automatic means for

draining the charge.

Storage Batteries


90/110

Storage batteries (which areusually lead-acid or alkali) presentan explosive hazard in addition to

a shock danger because they canproduce explosive gases,including hydrogen, if they areovercharged.

These explosive gases must notaccumulate in quantities that mayform an explosive mixture with air.

A spark or open flame could ignitethe mixture and cause anexplosion.

To prevent this accumulation,

good ventilation must be provided.

Quiz Question:


91/110

Temporary electric power and lighting

wiring may be of a class less thanwould be required for permanent

installations.

True

False

Quiz Question:


92/110

Temporary wiring over 600 volts,

nominal may NOT be used for:

Emergencies Experiments

Tests Remodeling


93/110

Flexible Cords and Cables


94/110

The standard for safe use offlexible cords is one of themost frequently violatedelectrical standards,particularly in smaller plants.

There is a temptation tomisuse flexible cords becausethey seem to offer a quick andeasy way to carry electricity towhere it is needed.

The basic problem is thatflexible cords generally aremore vulnerable than the fixed

wiring of the building.



95/110

Flexible cords should not be: Used as a substitute for the

fixed wiring of the structure

Run through holes in walls,ceilings, or floors

Run through doorways,windows, or similar openings

Attached to building surfaces

Concealed behind buildingwalls, ceilings, or floors.

Uses of Flexible Cords andCables

Uses of Flexible Cords and

Cables


96/110

Flexible cords and cables shall be approved and are suitable for thefollowing:

Pendants Fixtures Wiring

Portable lamp or appliance connection

Elevator cables

Wiring of cranes and hoists (where flexibility is necessary)

Connection of stationary equipment to facilitate their frequentinterchange

Prevention of the transmission of noise or vibration. (vibration mightfatigue fixed wiring and result in a situation more hazardous than flexiblecord.)

Appliances where the fastening means and mechanical connections are

designed to permit removal for maintenance and repair (e.g. watercoolers, exhaust fans)

Approved data processing cables.

Equipment for General Use -



97/110

There is usually not muchquestion about using the shortflexible cord attached to anapproved appliance or tool.

Likewise, there is usually noquestion about an extensioncord used temporarily topermit the use of the

appliance or tool in itsintended manner at somedistance from a fixed outlet.

However, there are questionswhen the usage is not

obviously temporary, and aflexible cord is used to avoidproviding a fixed outlet whereneeded.




98/110

If a flexible cord is usedimproperly, it can be damagedby: Door or window edges

Staples or fastenings

Abrasion from adjacentmaterials

Aging.

If the conductors becomepartially exposed over time,they will create a danger ofshocks, burns, or fire.




99/110

Flexible cords shall only beused in continuous lengthswithout splices or taps.

Hard service flexible cords,No. 12 or larger, may berepaired if the splice retainsthe insulation, outer sheathproperties, and usage

characteristics of the cord. Flexible cords shall be

connected to devices andfittings so that strain relief isprovided to prevent pull from

being directly transmitted tojoints or terminal screws.

Portable Cables Over 600

Volts, Nominal


100/110

Multi-conductor portable cable foruse in supplying power to portableor mobile equipment at over 600

volts, nominal shall consist of No.8 or larger conductors employingflexible stranding.

Cables operated at over 2000volts shall be shielded for thepurpose of confining the voltage

stresses to the insulation. Grounding conductors shall be

provided.

Connectors for these cables shallbe of a locking type with

provisions to prevent their openingor closing while energized.

Portable Cables Over 600

Volts, Nominal


101/110

Strain relief shall be provided atconnections and terminations.

Portable cables may not beoperated with splices unless thesplices are of the permanentmolded, vulcanized, or otherapproved type.

Termination enclosures shall besuitably marked with a highvoltage hazard warning.

Termination enclosures shall beaccessible only to authorized andqualified personnel.

Lighting Fixtures, Lampholders,

Lamps, and Receptacles


102/110

Fixtures, lampholders, lamps, rosettes, and receptacles may not have liveparts exposed to employee contact during normal operation. However,rosettes, and cleat-type lampholders and receptacles located at least 8 feet

above the floor may have exposed parts. Portable type handlamps powered through flexible cords shall be equipped

with a handle of molded composition or other material approved for thepurpose, and a substantial guard shall be attached to the lampholder or thehandle.

Screw-shell type lampholders shall be installed for use as lampholders onlyand must not be used with screw-base socket adapters. These adaptersscrew into the existing lamp socket and convert lampholders into receptacles.These adapters are not permitted because equipment grounding connectionscannot be made through the two-blade adapters and because the fixture hasbeen designed only for lighting.

Only weatherproof lampholders may be installed in wet or damp areas.Unprotected lampholders might allow moisture to enter the lampholder socket,creating an electrical shock hazard.

Cord Conductors


103/110

Cord connectors are used to join twosections of electrical cord together.

Attachment plugs are devices that arefastened onto the end of a cord so thatelectrical contact can be made betweenthe conductors in the cord and theconductors in a receptacle.

Connectors, plugs, and receptacles areuniquely designed for different voltagesand currents, so that only matching

plugs will fit into the correct receptacleor cord connector.

In this way, equipment rated for onevoltage-current combination cannot beplugged into a power system that is of adifferent voltage or current capacity.

The only exceptions to this are 125-volt

and 250-volt, 20-Ampere, T-slotreceptacles.

The 125-Volt, 20-Ampere T-slotreceptacle is illustrated in the graphic tothe left.

Cord Conductor


104/110

An electrical appliance that israted for 15 amperes will notoverload a 20-ampere circuit, and

the 20-ampere breaker will stillprovide overcurrent protection for15-ampere equipment.

Note that the opposite is notnecessarily true; therefore, a 20-ampere plug is made not to fit into

a 15-ampere receptacle or cordconnector.

The National ElectricalManufacturer's Association(NEMA) has established standardplug and receptacle connector

blade configurations.

Cord Conductor


105/110

Each has been developed tostandardize the use of plugs andreceptacles for different voltages,

amperages, and phases from 115volts through 600, from 15amperes through 60, and forsingle- and three-phase systems.

A receptacle installed in a wet ordamp location shall be suitable for

the location.

Appliances


106/110

Along with receptacles and lightfixtures, appliances are found in mostworkplaces.

Electrical appliances such as portableair conditioning units, coffee-makers,and fans must not have any exposedlive wires or electrical parts that mightcreate an electric shock hazard.

Exceptions to this are appliances suchas space heaters or toasters that must

have exposed current-carrying partsthat operate at high temperatures totransfer heat.

The heat generated by these partsminimizes the possibility of directcontact and resultant electric shock.

Each appliance shall be marked with itsrating in volts and amperes or volts andwatts.

All appliances must have adisconnecting means.

Quiz Question:


107/110

Appliances may have live parts exposed if:

The heat produced by the live parts protectsagainst contact

A proper disconnecting means is installed

A warning sign is present An insulating material protects against

accidental contact

Quiz Question:


108/110

Flexible cords may be:

Run through doorways, windows, or similaropenings

Attached to building surfaces

Used to prevent transmission of noise orvibration

Used as a substitute for the wiring of a structure

Quiz Question:


109/110

Portable cables may not be operated

with splices unless the splices are ofthe permanent molded, vulcanized, orother approved type.

True

False

Summary


110/110

During this module we have discussed the

scope and philosophy of Subpart S of theOSHA standard and presented specific andgeneral electrical safety design

requirements for:


Wiring Design and Protection Wiring Methods and Components

Equipment for General Use.

Documents

Electrical Safety Design