I&CPS 05 10
-
-
Electrical Design- Refined for Safety
Oaleep Mohla
Bruce McClung
Ben McClung
Member IEEE, ASME
Senior Member IEEE
Life Fellow IEEE
Mc2 Electrical Consulting LLC
Mc2 Electrical Consulting LLC
OeM Electrical Consulting
Charleston, wv 25312
Charleston, WV 25312
Missouri City, TX 77459
Abstract
-
Services, Inc
656 Whittington Dr
627 Pioneer Lane
A typical electrical system and equipment layout is designed to conform with certain
National Fire Protection Association's NFPA 70 or International
such as
Electrotechnical Commission's IEC 60364-1. The Occupational Safety and Health Administration
standards
also sets forth design safety standards for electrical systems under 29 CFR Subpart S 1910.302-
.308. These documents contain fundamental principles of safety for protection against electric
shock, thermal effects, overcurrent, fault currents, and overvoltage.
After an initial electrical system design is complete, and the electrical system studies, including
short circuit, protective device, and arc flash calculations are performed, current knowledge of arc
flash hazard may dictate that the electrical design be refined for safety.
An electrical design - refined for safety would consider methods to eliminate or reduce
electrical failures, to lower any fault current magnitude, and to increase speed of protective device
clearing.
The purpose of this presentation is to identify and document the benefits to be gained from the
use of such things as high resistance grounding, "see-through" rigid barriers, "fingersafe" fuses,
insulated buses and terminal covers, arc resistant switchgear and controlgear, "enhanced safety"
motor control
centers,
and/or appropriate
personal protective clothing/personal
protective
equipment.
"Electrical Design - Refined for Safety" is an innovative and cost effective way to complement
and supplement regulations and codes requirements. It can enhance employees' safety without
.
excessive reliance on administrative controls. It can eliminate or reduce the burdens associated
with obtaining special permits required for common tasks, such as meter reading, lock out/tag
out, etc.
"Electrical Design - Refined for Safety" is the careful application of engineered safety
features to enhance safety for personnel who must, from time to time, work in an environment
affected by proximity to exposed energized electrical conductors and circuit parts.
Index Terms
-
typical electrical design, electrical installation standards, prioritze safety, refined for safety,
electrical safety
I.
Presently,
the
BACKGROUND
National
Association's (NFPA)
Fire
The
Protection
Nat ional Electrical Code"
"
(NEC) addresses fundamental principles of safety
for electrical installations. The NEC enc ompa ss es
protection aga i nst electric shock, thermal effects,
overcurrent, fault cu rren ts and ovcrvoltage.
The NFPA "Standard for Electrical Safety in the
Workplace" (70E) addresses safe practices
for
employees and sets forth general requirements for
achieving and maintaining electrical safety in the
workplace.
OSHA 29 CRF Subpart S 1910.33 I -335 and
IEEE 902 provide electrical safety-related work
practices.
0-7803-9020-2/05/$20.00 ©2005
Administration
Safety
(OSHA)
has
and
Health
reinforced
the
premise that "electrical equ ipmen t when installed
,
new per recognized and accepted ·Codes' and
'Standards'
is
deterioration
considered
occurs
to
to
enclosure/conductors/insulation
be
safe
degrade
or
until
until
the
unsafe
acts are performed or until carelessness occurs."
Electrical maintenance should be performed in
compliance with NFPA 70B and 70E Chapter 2,
IEEE 902, and manufacturers' instruction booklets
for the purpose of su s t a ining equipment in "like
new" condition.
IEEE 1584-2002 and NFPA 70E-2004 prov i d e
guidance that anywhere thc incident energy at
working
IEEE.
Occupational
distance
of the
electrical
equipment
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exceeds 40 caVcm2 personnel should not perform
any energized work(l).
All of these documents have been developed to
assist and guide persons who design, construct and
commission, operate, adjust, service, and maintain
electrical equipment in commercial and industrial
facilities.
The installation and use of equipment per these
5 rna
third
party
and
certifying
agencies,
durability,
connection space,
and
-
electrical
insulation,
heating
effects under normal and abnormal conditions
likely
arise
to
classification
by
in
service,
type,
arcing
effects,
voltage,
size,
current
using or likely to come in
equipment.
contact
with the
4,300 rna
heating,
cooling,
and
moving
things
little thought to the potential hazards that confront
Cardiac arrest - burns
Employees
working
on
or
near
exposed
energized electrical conductors and circuit parts
must be protected from electrical shock, arc flash
bums,
and
arc
blast
and
other
hazards
appropriate protective measures
by
The NEC specifically add re s ses "interrupting
rating" as the highest fault current at rated voltage
that a device can safely conduct or interrupt under
m
,
generation,
the
distribution,
from contact by personnel.
faults nor do they guarantee protection from flying
debris associated with the arc blast force.
transformation,
and
utilization
III. TYPICAL ELECTRICAL DESIGN CRITERIA
of
Increased consumption of electrical power has
resulted in higher concentrations of electricity,
both higher voltage and higher currents, which
i n crea ses "shock" hazard and creates "arc flash/arc
blast" hazard.
Electrical
shock
was
recognized
first
hazard, followed by arc flash and arc blast.
as
.
a
Other
hazards, associated as by-products of the arcmg
fault, are still being cataloged.
Present
The potential hazards from electric current on
Table 1 - Effects of Electric Current on the Human
Body
Below 1 rna
Not perceptible
the
the
Equipment is sized based on interrupting rating
(available short circ u it current), with cost usually
being the primary c r iter ion in equipment selection.
Even
when
system
design
meets
recognized
"building codes", hazardous conditions can exist
during operation and maintenance.
design, often at locations geographically remote
from the customer.
its
experience.
The
own
owner
It is the rare company that
personnel
must
with
ensure
detail
that
design
the
bid
system requirements. The owner must also obtain
a
design
safety
review
by
independent
professionals experienced in industry safety.
2
30 cal/cm for new
design to allow tolerance for growth.
0-7803-9020-2/05/$20.00 ©2005 IEEE.
takes
design
specifications adequately stipulate the electrical
Faint tingle
(1) Limit incident energy to
system
"National Electrical Code"m.
retains
Reaction
electrical
customer/owner's requirements and follows
Contract engineeri ng firms do most of the detail
the human body are:
Current
Such enclosures are
not rated to contain products of internal arcing
electricity .
1 rna
pumping
energized electrical conductors and circuit parts
around, for granted. Most users of electricity give
tran smission
.
10,000 rna
rated switchgear or controlgear serves to isolate
INTRODUCTION
We take the benefits of electricity, such as
workers
Heart ceases rhythmic
standard test conditions. Enclosure of interrupting
.
II.
lighting,
Respiratory arrest
1,000-
capacity, and specific use, and other factors that
contribute to the practical safeguarding of persons
Individual cannot let-go
50-150 rna
mechanical
wi re b ending
c ontrol
9-30 rna
documents is based on suitability as determined by
strength
Slight s hock- not painful
Painful- loss of muscle
6-25 rna
(2)
NEe Article 90.1 (e) states "Intention. This
Code is not intended as a design specification or
an instruction manual for untrained persons."
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Owners must specify an acceptable amount of
outage, so that electrical application engineers
(and
p rocess engineers)
System
redundancy.
shutdown
systems)
can build
of
to
electrical
in
redundancy
systems
(and
system
enables
quickly clear electrical faults in main substations
and distribution systems to
de-energized
where
features to defeat the coordination times when
once the first pass electrical system design is
employees
complete and results of electrical system studies,
equipment.
short
circuit,
protec ti ve
reduce the incid ent
Many protective devices
energy in tho se areas.
are now available that have "maintenance switch"
It is also the owner's responsibility to request;
including
Th is effort to maintain maximum
within its area.
availability of power may have to be sacrificed to
process
work
1
incident energy still exceeds 40 cal/cm
allow
source, will be the only portion isolated for a fault
that anywhere in the electrical system incident
2
energy exceeds 40 cal/cm
reconsider the
equipment, devices, comp o nents and connections
,
working
near
or
on
electrical
Engineers and designers must make the owners
device
coordination, and arc flash hazards are available;
are
aware of their options.
The cost of an injury
resulting from a single arc flash could cost 51.8
million for a fatality or $4 million for a dis abled
survivor.
From a life cycle cost point of view, it
to lower the available fault current and increase
bec omes very easy to justify spending a little more
speed of protective device clearing time to limit
up front to have a safe electrical system.
incident energy to 40 cal/cm2 based on foreseeable
uti lization l
A.
.
Basic principles to enhance electrical system
safety:
•
IV. CRITERIA FOR USING THE ELECTRICAL
DESIGN- REFINED FOR SAFETY PROCESS
In order for electricity to benefit mankind, it
must
be
produced,
controlled.
considered
These
in
the
confined,
four
directed
concepts
design process.
be
mo st
productive area for improving electrical safety
today exists in refining the design features that
minimize/eliminate exposure to electrical shock
-
•
•
keep
The
performing
o
electri c al system.
drop
to
have a
safe
The industry is beginning to
recognize that low impedance transformers will
impedance may significantly reduce the incident
energy when a fault occurs, provided the arcing
fault current is still high enough to be detected and
quickly cleared by circuit protective devices.
Protective devices arc commonly chosen
completely
new electrical systems should be limited
maximum of 40 cal/cm2
existing
electrical
systems
should
be
cal/cm2
•
establish
PPE
selection
with
owner
and·
achieve user buy-in early in the design process
•
evaluate who will be operating the electrical
systems
o
•
•
owner or contract personnel
take all operating modes into account
if substations are equipped with tie breakers,
assume that all main breakers may be closed
have high fault currents, probably resulting in
higher incident energy. A transformer with higher
equipment
substations
refined to a limit of a maximum of 40
the voltage drop. Many times the designer needs
to c ompromise voltage
boundary
tolerance for growth, etc for recognized
electrical design must be willing to perform or
Equipment is commonly specified to minimize
1)
to a maximum of 30 cal/cm2 to allow
the
seek critical reviews of their work that upholds
safety to be as imp ortant as operability,
maintainability, and reliabi li ty.
as
minimize available incident energy
Refined for Safety" is
person(s)
close
outside of classified areas
o
isolating personnel from the detrimental effects of
hazard.
as
limits wherever possible
proactive in removing sources of hazards or in
the
equipment
keep electrical equipment outside of Class I
and arc hazards.
"Electrical Design
ele ctrical
practical to where it is utilized
Division 1 (or Clas s 2 Division
and
should
The
•
locate
while tie is closed (unless interlocked)
•
minimize shock exposure
is
minimized,
occurring
the
while
-
if shock exposure
likelihood
maintaining
of arc
flash
equipment
is
inherently minimized
to
coordinate so the smallest load, furthest from the
O· 7803·9020·2/05/$20.00 ©2005 IEEE.
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•
o
compromise may be necessary in operability
to increase the safety (e.g., coordination of
•
protective devices may have to be sacrificed) .
design must take an iterative approach
o
preliminary
o
equipment selection
o
o
o
o
o
o
•
single
scheme
line
and
operating
voltage drop study
the more the design allows for LOTO, the
more likely LOTO will be employed
•
install disconnect or polarized plug at each
•
install disconnect at base of multi-fixture poles
•
arc flash hazard analysis study
•
fixture
place lighting panels in a logical location, in
protective device coordination study
close proximity to utilization site
allow for maintenance to be performed below
8'
device selection
o
repeat studies
concentrate design effort to improve safety on
277/480V lighting circuits and medium
o
•
on
install lights that come on immediately after
power is restored
B.
Simple Example ofElectrical Design - Refined
for Safety
Lighting Design - Present
1)
Concept
o
o
Large and inexpensive lighting designs have
the human aspects.
o
injuries
occur
on
Lighting Design - Refined For Safety
In a lighting desi gn, refined for safety, designers
would consider the human interface with electrical
systems and think like the people who will use and
maintain the electrical system on a day-to-day
basis. A lighting des i gn, refined for safety, would:
•
•
use l20V,
neutral
1
phase power with dedicated
minimize arc flash hazard by using 120/20SV
systems that are fed from transformers below
•
•
125kVA
minimize the temptation to work on lighting
while "hot"
feed every other light from alternating circuits
0-7803-9020-2/05/$20.00 ©2005
IEEE.
restart typically have a lower' "T" rating
battery back-up fluorescent lights may be
a good option
provide convenient testing for battery back-up
lighting
Low
cost and continuation of process are given highest
prlonty.
Many electrical
2771480V lighting systems.
2)
•
Designs don't consider how
lighting is utilized or how it is maintained.
specify "some" lights with instant restart
be aware that lights equipped with instant
for classified areas
typically utilized 277/4S0V, 3 phase power with a
installing the maximum number of fixtures on a
single circuit. Minimal consideration is given to
equipped with a hinged mast
are switched separately and marked to be kept
•
Lighting plans are based on
install fixtures below 8'
install fixtures that can be lowered or
install "night lights" that are not switched, or
voltage Class E2 motor controllers - since this
is where most of the electrical bums occur
common neutral.
circuit will leave at least one adjacent
lighting circuit in operation
•
short circuit study
refine equipment selection and protective
lock out/tag out (LOTO) of one lighting
o
install remote test button
o
specify emergency lights
control testing feature
with
remote
•
install emergency lighting supplied from a
back-up generator, if local codes require
V.
DESIGN FEATURES FOR PROTECTION AGAINST
ELECTRICAL "SHOCK" HAZARDS
"Electrical Design - Refined for Safety"
inherently reduces the potential safety risk from
electrical shock hazard by providing a first line of
defense
to
guard
personnel
from
exposed
energized electrical conductors and circuit parts
("live parts").
Long ago the most frequently
utilized electrical design scheme to avoid contact
with exposed energized electrical conductors and
circuit parts was chain link fencing.
An early
refinement was to enclose bare bus, switches and
other electrical control and protective devices in
enclosures with hinges and latched or bolted-on
covers/ doors.
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Other "Electrical Desi gn - Refined for Safety"
concepts to be used in order to remove operators
from
in
front
interruptin g
of
rated
Table 3
- Restricted Approach Boundaries for
- an approach limit at a
Shock Protection(3)
electrical
s witchgear or controlg ear:
distance from an exposed live part within
•
"mimic panels"
which there is an i n c re a s ed risk of shock, due
•
"umbilical cords", some as long as forty feet,
to electrical arc over, combined with
having a plug on one end for connection to a
inadvertent movement, for personnel working
on
receptacle
the
switchgear/controlgear and
•
an
.
front
"remote racking" c ircu it breakers installed in
utilize
d istanc es
approach
for
boundaries
based on:
-
Table 2
Limited Approach Boundaries for
Shock Protection
Table 3
-
Restricted Approach Boundaries for
Shock Protection
Table 4
-
Prohibited Approach Boundaries for
Shock Protection
ut il iz e
Table
5
-
Basic
Minimum
Air
Insulation Distance to Avoid Flashover
Table 2 - Limited Approach lJoundariesfor Shock
Protection(J) - an approach limit at a distance
300 V and less
Avoid contact
301 to 750V
1 f1. ° in.
751 to 15,000V
2 ft. 2 in.
15,100 to 36,000V
2 ft. 7 in.
36,100 to 46,000V
2 ft. 9 in.
46,100 to 72,500
3 ft. 2 in.
72,600 to 121,000V
161,000 to 169,000V
3 ft. 3 in.
3 ft. 7 in.
4 ft. 0 in.
230,000 to 242,000V
5 ft. 3 in.
345,000 to 362,000V
8 ft. 6 in.
500,000 to 550,000V
11 ft. 3 in.
765,000 to 800,000V
14 ft. 11 in.
138,000 to 145,000V
Table 4
- Prohibited Approach Boundaries for
Shock Protection(4) - an approach limit at a
distance from an exposed live part within
from an exposed live part within which a
which work is considered the same as making
shock hazard exists
'h the rIve part
contact WIt
Nominal Voltage
Exposed
Exposed
Fixed
Nominal Voltage
Movable
Circuit
(Ph·Ph)
Conductor
Part
300 V and less
10 ft. 0 in.
3 ft. 6 in.
301 to 750V
10 ft. 0 in.
3 ft. 6 in.
10 ft. 0
15,100 to 36,OOOV
in.
10 ft. 0 in.
6 ft. 0 in.
36, iDO to 46,000V
10 ft. 0 in.
8 ft. 0 in.
46,100 to 72,500V
10 ft. 0 in.
8 f1. 0 in.
72,600 to 121,000V
10 ft. 8 in.
8 f1. 0 in.
751 to l5,OOOV
5 ft. 0 in.
138,000 to 145,000V
11 ft. 0 in.
10ft. 0 in.
161,000 to 169,000V
11 ft. 8 in.
11 f1. 8 in.
230,000 to 242,000V
13 ft. 0 in.
13 ft. 0 in.
345,000 to 362,000V
15 ft. 4 in.
15 ft.4 in.
500,000 to 550,000V
19ft. 0 in.
765,000 to 800,00OV
23 ft. 9 in.
19 ft. ° in.
23 ft. 9 in.
(3)
NFPA 70E, Table 130.2 (c)
0-7803-9020-2/05/$20.00 ©200S IEEE.
Boundary
(Ph-Ph)
button on the other end were used
panel" or "umbilical cords"
•
Nominal Voltage
open-close push
switchgear for operat i on from either "mimic
•
III
prOXImIty to the rlve part
. case
I
of
(Ph-Ph)
Boundary
300 V and less
Avoid contact
301 to 750V
Oft. 1 in.
751 to 15,000V
15,100 to 36,OOOV
Oft. 7 in.
Oft. 10 in.
36.100 to 46,OOOV
1 f1. 5 in.
46,100 to 72,500V
2 ft. 1 in.
72,600 to I2l,000V
2 ft. 8 in.
138,000 to 145,000V
3 ft. 1 in.
161,000 to 169,000V
3 ft. 6 in.
230,000 to 242,000V
4 f1. 9 in.
345,000 to 362,000V
8 ft. 0 in.
500,000 to 550,000V
10ft.9in.
765,000 to 800,000V
14 f1.5 in.
(4)
Ref ANSIIIEEE 516-2003 Guide for
Maintenance Methods on Energized Power Lines
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Table 5
-
•
Basic Minimum Air Insulation
Distance to Avoid Flashover(5)
Boundary
(Ph-Ph)
300Y and less
° [t. 0.03 in.
300 to 750Y
° ft. 0.07 in.
751 to 2,000Y
Oft. 0.19 in.
2001 to 15,000Y
Oft. l.5 in.
15,001 to 36,000V
0[1. 6.3 in.
36,001 to 48,300Y
48,301 to n,500Y
° ft. 10.0 in.
1 ft. 3.0 in.
72,501 to l 2 l ,000Y
2 ft. 1.2 in.
•
•
•
230,001 to 242,000V
4 [t. 2.4 in.
345,001 to 362,000V
7 ft. 5.8 in.
500,001 to 550,000V
1° ft. 2.5 in.
765,00 I to 800,000V
13 ft. 10.3 in.
use of improved, double insulated devices and
expose the worker to a "shock"
automatic grounding of MV feeders, capacitor
motor
•
•
employees
starters
Permanent installation of ground fault circuit
against
a t 6 rnA or less.
See Table 1 - Effects of Electric Current on
the Human Body
Note:
with today's technology, we should insist on
"dead-front" or finger safe protection (IP20)
with the equipment doors open
minimizing exposure to voltages above
240V
higher degree of ingress protection (IP40+) on
would
mmlmlze
opportunity of tools or other foreign material
•
from initiating an arc during maintenance
compartmentalizing
of
control
and
power
voltages
•
use of low voltage (below 50V) for control
•
bring test points for bus and secondary voltage
•
use of in-line resistors at test points to limit
•
power
ground fault circuit protectors for
equipment when set for 30 rnA or less are
also effective for protecting personnel
against the most severe "shock" effects.
effective methods today to protect employees
this
(provide
interrupters for personnel protection that trip
from contacting energized parts)
feed;
withdrawn
interlock with main switching device)
use of "finger safe fuses" (one of the most
incoming
are
Isolation of wireways from bus and from free
electrical "shock" hazards:
•
starters
access to breakers, disconnect switches, or
There are several electrical design concepts to
protecting
below
or
2 ft. 6.6 in.
•
glove
banks, motors, etc., when switchgear breakers
3 [t 0.0 in.
•
insulating
tools (with dual-color insulation) so damage in
138,001 to 145,00Y
•
rubber
the outer layer is readily identified - do not
161,001 to 169,00V
for
of improved
small objects (nuts) gloves rated for SOOV and
Nominal Voltage
consider
use
materials to allow finger dexterity to handle
VI. DESIGN FEATURES FOR PROTECTION AGAINST
ELECTRICAL "ARC FLASH" HAZARDS
Publication
of IEEE
1584-2002
"Guide
for
Performing Arc Flash Hazard Calculations" made
it possible to detennine incident e nergy at working
distances as well as distance from the arc source
where the incident energy is sufficient to cause a
second degree bum for electrical systems having
greater
than
SO,OOOA
available
fault
current.
Table 6 - Effects of Temperature/Heat on Human
Body
&
Materials
temperature J
heat
shows
the
effects
the
human
body
on
of
and
materials.
to front cover
power in case of arc
appropriate use of see through rigid barriers
and
vlewmg
windows
operators/electricians
electrical
terminals
(switches/circuit
to
and
view
enables
status
isolating
of
devices
breakers/contactors)
while
still providing barriers to prevent contact
(5)
Ref NFPA 70E,
Appendix C.2.1
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Table 6
Effects a/Temperature/Heat on Human
-
•
Body & Materials
•
Temperature/Heat
Reaction
145°F
Curable skin burn(o)
205°F
Human cell death
700-1,400°F
Clothing ignition
1,400°F
Clothing burning
•
1,800°F
•
•
Copper melts
2,600°F
Carbon steel melts
9,00O°F
Sun's surface
35,OOO°F
Arcing terminals
•
As more arc flash hazard calculations have been
performed
per
IEEE
1584-2002
it
has
"large"
cal/cmz
at
electrical
normal
potential arc sources.
systems
working
•
work on or near energized electrical equipment
•
•
devices (molded case circuit breakers or fuses) or
•
simply giving up protective device coordination.
It may require adding high resistance grounding,
or adding ground relays if solidly grounded, or
breaking up electrical supply systems into smaller
configurations.
Design
concepts
to
consider
for
protecting
employees against electrical arc flash hazards:
Note:
Many features, which protect personnel
against electrical "shock" hazard, also minimize
or eliminate the electrical arc flash hazard.
(6)
A curable skin burn is normally considered to be a
degree burn and can be created on most human
second
skin when a person is near an electrical arc in which the
incident energy level is 1.2 cal/cm' or 5.0 J/cm2. This is
roughly equivalent to the burn caused by exposing a
finger to the flame of a lighted match stick for one
second.
0-7803-9020-2/05/$20.00 ©200S IEEE.
phase-to-phase and phase-to-ground faults)
�
protective
clothing/personal
u se
of
up-to-date
easy-to-read
and
match cabinet wiring and control schematics
mini miz ing exposure to voltages above 240V
higher degree of ingress protection (IP40+) on
feed;
this
would
minimize the
to initiate an arc during maintenance
cal/cmz. Nonnally the des ig n alternative to reduce
incident energy requires use of current limiting
close-open
opportunity for tools or other foreign material
cal/cmz. Design alternatives to reduce the incident
work any bus calc ulated to be more than 40
with
use of "remote racking" circuit breakers
incoming
40
energy must be implemented if there is a need to
cords"
identification for all tenninals and devices that
•
exceed
of "umbilical
controls on remote end
equipment needed
With present technology for personal
would
circuit breakers or controllers;
use
working distances and appropriate personal
protective clothing it cannot be made "safe" to
energy
use of "mimic panels" for remote operation of
protecti ve
•
incident
of switchgear except
arc flash boundarylincident energy at typical
need to work where incident energy exceeds 40
the
front
pre-calculated afC flash hazard information
from
We can now calculate the
from
when breaker is in "test position"
use of electrical equipment labels, with the
modify/adjust the arc flash hazard to avoid the
where
use of provisions to avoid closing circuit
•
been
incident energy of "strong" electrical systems and
cal/cmz.
sacrificing coordination)
use of insulated low voltage buses (minimize
exceed 40
distances
faster clear i ng time for protective relays (by
•
acknowledged that 1 2% - 1 8% of the "buses" in
modern
current to downstream devices )
breakers
Aluminum melts
. 660°F
use of current limiting fuses (minimize fault
compartmentalization of control and power
voltage s
use of low voltage (below 50V) for control
power
•
•
•
•
•
bringing test points for bus and secondary
voltage to front cover
use of in-line resistors at test points to limit
power in case of arc
use of "finger safe" low voltage fuse holders
use
of
removable
terminal lugs
insulated
boot
on
all
use of insulated low voltage buses
use
of high resistance grounded electrical
systems (first fault to ground is "free")
Note:
high resistance grounding nonnally
restricts or limits the current flow for the
first ground fault to 5A or less
�
however,
a second ground fault before the first is
cleared essentially creates phase-to-phase
fault levels of current flow. It is important
to detect and clear the first ground fault
before the second occurS in order to avoid
Poge 9S
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B.
both points of grounding and flowing
Clothing
grounds,
•
Personal protective clothing is primarily worn to
prevent the person's apparel from contributing to
separate ground points.
the hazard.
in low voltage
has ratings that enable it to be utilized to match or
equipping with potential ground loops around
exceed the potential incident energy available at
line side shutters
conductive
columns/enclosures
at
all
use of clear plastic barriers around all bare
use of adequate lighting fixtures, including
lighting fixtures inside all electrical equipment
•
•
•
the working distance for given tasks from potential
arc sources.
Some protection against ignition of clothing
from heat and "fire-ball" effect has been available
use of ground fault circuit interrupters
conductors and terminals
•
Appropriately rated PPC is flame
switchgear and controlgear
use of
outdoor substations (reduces step-and-touch
•
Personal Protective
bonding
potentials)
•
for
resistant (FR) (either by fabric or treatment) and
all
•
Features
(PPC)
connections and earth between the two
through/over
•
D es ign
phase-to-phase fault current "flashing" at
in the form of flame retardant fabrics for over 25
years. Only recently has attention been directed to
identify a broad knowledge base regarding the
effects of temperature/heat on human body and
enclosures having hinged doors
materials. SeeTable 6. The identified temperature
iden�ify and label outside of enclosures having
multlple sources of electrical power so it is
synthetics, cotton, silk, and wool.
easy to locate the source disconnect devices
avoid use of earth as current return paths
current return path
Personal Protective
Most personal protective equipment is effective
against electric shock hazard.
This includes
rubber insulating gloves, insulating line hoses and
covers, insulating blankets and sheets, insulating
barriers, insulating tools, voltage rated contact­
making meters, insulating "hot sticks", etc.
Items such as rubber insulating gloves with
leather protectors provide protection against both
Transient voltage
rated meters, with either automatic ranging or
voltage
detection
capability
only,
should
be
utilized for both shock and arc hazards. The leads
of such voltage detectors are just as important as
the meters to enable safe use.
Both ends of the
leads need special attention. The meter end should
be retained in the meter case or recessed so that
pullout from the meter while in contact with
voltage will not result in inadvertent shorting and
the stab end needs to be recessed and insulating
flanges
are
required
to
provide
finger
when
applied
within
its
arc
thermal
C. Application of personnel protective
(PPC) around the individual at work
grounds
performance value (ATPy(7)).
all welding must utilize a dedicated insulated
electric shock and arc hazards.
The FR PPC
neither ignites, bums, nor exhibits break open
(EBT(7))
control voltages Less than 50Y
A. Design Features for
Equipment (PPE)
ranges for clothing ignition and burning include
Neither shock nor arc flash will occur without a
difference
of
potential
between
any
two
conductive enclosures or electrical conductors and
circuit parts. Appropriate grounding for both step
and touch potentials will avoid the differences
?etwe�n any two conductive enclosures. Stipulate,
III deSIgn specifications, grounding studs on/near
cable
terminations
in
switchgear
to
facilitate
making ground connections prior to beginning
work on de-energized equipment in areas subject
to being re-energized by fallen lines or other
abnormal contact with energized parts.
Personnel protective grounds should be installed
on either side of a work site where de-energized
electrical lines are subject to being reenergized by
falling overhead lines or by induced current from
adjacent lines.
The current flow through parallel
paths splits in the ratio of an individual to the sum
of the individual resistances.
Example:
•
For
20,OOOA
appropriate
guard
available
PPG,
fault
the
current
current
with
flow
over/through the body will only be:
against slipping.
(7) Refer to ASTM 1959-99 for complete definition of
EBT
0-7803-9020-2/05/$20.00 ©200S IEEE.
and ATPV
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20,OOOA
•
*
•
0.OOl28/l500.00128=0.017A
products to the front, the back or the ends.
This will be painful and may cause loss of
Expulsion is directed in a manner that will be
muscle control whereas current flow over the
ground conductor will be:
20,OOOA
•
Type B involves limiting expUlsion of arcing
Or 17ma.
*
o
Type A involves limiting expUlsion of arcing
1500/1500.00l28=19,999.983A
This is based on:
o
away from inhabited space.
•
Resistance
Ground
=
of
products to the front or to the back. Expulsion
is directed away from these areas.
25
ft,
250kcmil
0.00 1 28 ohms.
Resistance of Body
=
Arc resistant switchgear and controlgear are
Safety
characterized by:
•
1500 ohms total
(500 ohms at hand contact + 1 000 ohms at
•
foot contact)
•
D. Additional Features for Protecting Employees
starters
are
withdrawn
•
•
•
access
to
breakers,
blowing
expelling
products
of
the
arcing
fault
(pressurized vapors and gases, molten splatter)
disconnect
•
switches, or starters
open,
flying debris
isolation of wireways from the bus and from
unfettered
vaporizes
covers off, rupturing enclosures, and cre ati ng
(provide
interlock with main switching device)
of blowing doors
capable
banks, motors, etc. when switchgear breakers
motor
create noise, light, heat,
conductive material, generates blast forces
automatic grounding of MV feeders, capacitor
or
receiving arc resistant rating
remaining intact during internal arcing faults
which
Against Electricaf Hazards
•
receiving interrupting capacity rating
in a direction not frequented by personnel
protecting
personnel
during monitoring
or
labeling of all electrical equipment with the
while
pre-calculated arc flash hazard information -
mInlmlZeS
arc flash boundary/incident energy at typical
equipment/personal
working distances and appropriate personal
equipment (PPEIPPC) while cubicle door is
protective
clothing/personal
protective
operating
need
the
for
switchgear
which
personal
protective
protective
clothing
closed
equipment needed
E.
F.
Arc Resistant Switchgear and Contralgear
The most effective method today to protect
operators and electricians from the hazards of arc
blastlarc flash is to use arc resistant switchgear
Many potential arc flash conditions for which no
designed protection is currently available are:
•
that complies with I EEE/ANS I C37.20.7-200 1
•
"IEEE Guide for Testing Medium Voltage Metal
Enclosed Switchgear for Intern al Arcing Faults".
•
EEMA C G 14-1 1987 also covers switchgear. IEC
Standard 60298 currently covers both switchgear
and controlgear.
Three types of arc resistant switchgear and
controlgear
can
provide
varying
levels
of
protection:
•
Type C involves com ple te internal isolation of
one compartment from arc fault occurring in
adjacent compartment and fro m expulsion of
arcing products to the front, the back, the ends,
or either from the top or from the bottom;
however, one direction is still needed for
expUlsion in a manner that will be directed
away from inhabited space.
0-7803-9020-2/05/$20.00 ©2005 IEEE.
DeSign features for protection against other
potential hazards
Loud noise
Blinding light
Intense heat
•
Vaporized conductive material
•
Toxic metal vapors
•
•
Blast forces of pressure wave
Molten metal splatter/projectiles
The reader is directed to monitor progress of the
IEEEINFPA collaboration on Arc Flash Hazards
to learn more about efforts to analyze and avoid
these unusual potential hazards.
VII.
SUMMARY
The facility installation should be made safe by
design and by intent.
The commissioning period
should be utilized to inspect and confirm a safe
installation. The operation and maintenance work
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performed by operators and electricians needs to
•
Overall electrical safety requires a supportive
•
assure the equ i pment is maintained safe .
community. Electrical safety is not a proprie tary
product.
Occupational Safety and Health
Administration - 29 C FR-Pa rt 1910
OSHA 29CFR-1910-Subpart S - Electrical
Any design improvements for safety of
personnel should be announced and shared via
activities such as:
•
IEEE-IAS�PCIC Electrical Safety Committee
•
IEEE IAS�Electrical Safety Workshop
IEEE Electrical Safe ty Benchmarking
•
•
IEEE Electrical Safety Resource Center and
Virtual Community
As new concepts and materials that enhance
safety are proven, they need to be incorporated
into st anda rds , codes, gui des , and recommended
practices such as:
•
ANSIIIEEE C2
IEEE 1584
•
•
NFPA 70 and NFPA 70E
•
OSHA, both federal and state compl i ance
•
VIII.
REFERENCES
ASTM 1959-99 - Standard Test Method for
Determining Arc Thermal Performance of
Textile Materials for Clothing by Electric
Arc Method U sing I nstrumented Sensor
P anels
•
•
•
EEMAC G14-1 - Procedure for Testing the
Resistance of Metalclad Switchgear Under
Conditions of Arching Due to and Internal
Fault
lEC 60298 - AC. metal-enclosed
switchgear and control gear for rated
voltages above I kV and up to and includ i ng
52 kV
IEC 60364-1
buildings
-
-
Electrical installations of
Part 1: Fundamental principles,
assessment of general characteristics,
•
•
•
definition
IEEE-lAS Color Book Collection
IEEE 1584-2002 - IEEE Guide for
Performing Arc-Flash Hazard Calculations
IEEE/ANSI C37.20.7-2001 - Guide for
Testing Medium-Voltage Metal-Enclosed
•
•
•
Switchgear for Internal Arcing Faults
NFPA 70-2005 - National Electrical Code
NFP A 70B-2002 - Electrical Equipment
Maintenance
"
NFPA 70E-2004 - Electrical Safety in the
Workplace
0-7803-9020-2/05/$20.00 ©200S IEEE.
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