Preventing and Mitigating Manhole Events

Preventing and Mitigating Manhole
Events
A White Paper
Prepared by the
IEEE
Insulated Conductors Committee - C34D
July 7, 2015
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Members of the C34D Discussion Group who contributed to this White Paper are:
William Black, Chair
Jacques Cote, Vice Chair (09-14)
Stuart Hanebuth, Vice Chair (14-)
Glen Bertini
Steve Boggs
Bruce Bernstein
Jason Carnero
Sudhakar Cherukupalli
Daniel Durbak
Jeff Joy
James Justice
Mark Kelvin
Bob Malahowski
Rachel Mosier
George Murray
Kanayo Onyekwuluje
Dean Reynolds
Linda Rhodes
Brent Richardson
John Snyder
Dan Simon
Peter Tirinzoni
Rich Varjian
Lee Welch
Yingli Wen
Kevin Wright
Georgia Institute of Technology
Hydro Quebec
Power Survey Company
Novinium
University of Connecticut
Bruce S. Bernstein Consulting
Los Angeles Department of Water & Power
BC Hydro
Siemens Industry, Inc.
Seattle City Light
Stabiloc
BC Hydro
Pacific Gas and Electric Company
Power Delivery Consultants
Consolidated Edison
General Electric
EJ USA, Inc.
Commonwealth Edison
Dow Chemical
EJ Company
Geltech Solutions
Northeast Utilities
Novinium
Georgia Power Company
Consolidated Edison
EJ USA, Inc.
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Contents
Scope …………………………………………………………………….………
I. Background………………………………………………………………
II. Manhole Event Incidence……………………………………………….
A. Utility Survey………………………………………………………....
B. Data from Previous Studies………………………………………….
III. Reducing Manhole Events ……………………………..………………
A. General Methods…………………………………………………….. .
1. Cable Design and Installation …………………………….
2. Cable Operation ……………………………………………
3. Protection Devices ………………………………………….
4. Manhole and Cable Inspection……………………………..
5. Cable Testing and Evaluation ……………………………..
B.
Arc Detection Equipment …………………………………………
C.
Portable Gas Detection Equipment ……………………………….
D.
Continuous Monitoring Equipment ………………………………
E.
Installation of Vented Covers ……………………………………..
IV. Mitigating the Effects of Manhole Events ……………………………..
A.
Pressure Compensating Manhole Cover …………………….……
B.
Pivoting Pressure Relief Cover ………………………………….
C.
Explosion Mitigating Vault Restraint ………………………….
D.
Spring Mounted Covers …………………………………………
E.
Tethered Covers………………………………………………….
F.
Vented Covers……………………………………………………
G.
Quick Action Protection Systems………………………….. …..
H.
Blast-Proof Cover Design…………………………………….. …
V.
Manhole Event Modeling ……………………………………………
VI. Results of Experimental Studies ………………………………… …
VII. Literature Review ……………………………………………………
A.
General References ………………………………………..… ….
B.
EPRI Reports ……………………………………………….........
C.
Theses ……………………………………………………….…….
D.
Utility Reports ………………………………………. …………..
E.
Survey References ………………………………………………..
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Scope
This document presents a summary of the current level of knowledge of manhole events that
involve insulated cables and accessories. It includes:

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a listing of hardware and instrumentation designed to reduce the safety risk to the public
and utility maintenance crews resulting from manhole events
a consolidation of prior research along with utility best practices that may be employed
when addressing manhole events
a summary of methods of avoiding explosive events in manholes, vaults and service boxes
a summary of methods for mitigating the effects of the manhole events when they occur
the results of a utility survey relating to the frequency of manhole events and methods used
to prevent and mitigate the effects of manhole events
a summary of theoretical studies that can serve as tools to aid in the design of safer
manhole structures
a listing of available literature that address the problems that arise when events occur in
vented and non-vented enclosures
I. Background
The term “manhole event” is used to describe an abnormal condition in an underground structure
such as a manhole, vault or service box which can originate within the structure itself or in a cable
duct between structures. The typical manifestation of a minor manhole event is smoke emanating
from the underground structure or in a more extreme case, fire in the structure which can produce
large quantities of smoke and flame extending from the manhole. An example of the worst type of
manhole event is an explosion of combustible gases within the structure which can lift the top of
the structure or cause the manhole cover to be displaced.
This document is intended to summarize existing and new technologies that can be applied to the
problem of manhole events in an attempt to eliminate manhole events or mitigate their effects and
provide a safe environment for both the public and utility maintenance crews. This document will
concentrate on events that originate in insulated cables and cable accessories such as splices, joints
and terminations. Events can occur in other components in the electric distribution system such as
transformers and voltage regulators; however this document will not consider these types of
electrical-generated events.
Manhole events can be loosely categorized into three areas based on the severity of the event.
1. Smoking Manhole - An event in which smoke is visible exterior to the manhole, but no
visible flames escape from the edge of the cover or from holes in the cover
2. Manhole Fire - An event in which the cover remains seated in its frame and visible flame
escapes from the edge of the cover or from holes in the cover
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3. Manhole Explosion - An event that involves explosive forces that are sufficient to
dislodge the manhole cover from its frame or project debris into the air around the manhole
The cause of manhole events can be subdivided into three categories based on the origin of the
energy that is released during the event.
1. Electrical events – faults that occur due to failure of electrical cables, accessories or
equipment
2. Chemical events – ignition of accumulated combustible gases inside the manhole
3. Combination of electrical and chemical events – electrical failure that generates an arc
which releases electrical energy. The fault is followed by the generation of combustible
gases in the vicinity of the arc which are then ignited by the arc and generate additional
chemical energy.
While gas explosions in manholes may be less common than faults or failures of electrical
equipment, chemical-based events nonetheless release a large amount of energy when a
combustible gas or a gas mixture within its flammable limits is ignited within a manhole. The
potential for collateral damage to electrical facilities as well as the surroundings exterior of the
manhole can be significant.
For the purpose of this document, transmission voltages are assumed to range between 46kV and
500kV; distribution voltages are between 1kV and 46kV, while low voltage cables and accessory
equipment are assumed to limited to less than 600V. Secondary distribution systems include those
operating below 1kV.
Each year there are hundreds of manhole events that occur in North America. The vast majority of
these events occur in low voltage cable systems and approximately three-quarters of those events
are chemical as opposed to electrical in origin. A relatively small number of events occur at
transmission voltages and practically all of them are electrically driven events. Manhole events
appear to be increasing as electrical equipment ages and as the extent of underground installations
of transmission and distribution networks increase, particularly in highly-populated, congested
urban settings. Past events have damaged the streets and sidewalks in the vicinity of the manhole
and have caused fire and concussion damage to nearby buildings (see photos below). A manhole is
a rigid, relatively well-sealed structure that is incapable of absorbing much of the energy that is
generated during the event. As a result, the energy is mostly dissipated external to the manhole
which emphasizes the need to prevent the event from occurring in the first place or to select
manhole designs that are able to absorb the released energy if the possibility of the event cannot be
eliminated. Both approaches have been tried with varying degrees of success. This document
summarizes some methods that have been employed by U. S. and Canadian electrical utilities.
Manhole events can arise due to a number of causes. In the case of events at distribution voltages,
the root causes can often be a result of overheating or electrical failure of a splice or termination
within the manhole, a high impedance cable fault, within the ductwork or failure of switchgear
within the structure. In the case of secondary distribution, manhole events are usually caused by
failure of cable insulation within a duct between manholes or, more rarely, a phase-to-phase or
phase-to-ground fault in a duct with sufficient impedance that it does not cause power interruption.
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Since both network and radial secondary systems can deliver thousands of amperes without
interruption, substantial energy can be delivered by a fault that occurs in secondary cables. The
magnitude of the dissipated energy suggests that damage inside the manhole and external to the
structure can be significant.
Electrically driven manhole events in primary systems can release a substantial amount of energy
during a fault. The current is generally interrupted and the energy release is mitigated by the
system protective equipment. On the other hand, events driven by overheating as a result of a
defective contact evolve more slowly. Manhole events driven by a relatively low impedance
phase-to-phase fault within the duct of a network secondary system tends to evolve quickly, while
a high impedance fault caused by contaminants in a duct or manhole can evolve over many hours,
during which substantial quantities of combustible gases can be generated. These gases generally
flow toward an adjacent structure, driven by the solid-to-gas- phase change and the pressure
differences between structures. Long-duration, high- impedance faults that limit fault currents to
relatively low levels are difficult for common over-current protection systems to detect and
terminate. They may even occur intermittently which makes detection particularly challenging. As
a result, high impedance faults may continue to generate heat, decomposing the cable insulation
and generating combustible gases that are a potential hazard should they come into contact with an
ignition source.
Another major concern for manhole events is the potential for large quantities of smoke that may
be generated during an event, particularly when combustible materials ignite and burn in the
enclosed space of the structure. In these cases, the burning may proceed over a long period of time
due to the fire department’s reluctance to use water on an electrical fire. In some cases the fire may
originate in a vault located in the basement of a commercial or residential building; in this case the
hot buoyant smoke may migrate into the building and endanger occupants. Manhole events can not
only release large amounts of energy, but they also can generate substantial quantities of
combustible and toxic gases such as hydrogen, acetylene, carbon monoxide and carbon dioxide.
The photographs in Fig. 1 illustrate manhole events that resulted in various degrees of damage and
different levels of smoke and fire.
Maintenance crews and fire-fighting personnel should be trained in appropriate safety procedures
when approaching manhole fires. The color of the smoke that emanates from the manhole is
extremely important in determining the source of the event. Light gray smoke suggests a minimal
possibility of flash-over within the manhole. Heavy black, thick smoke that appears when the
cover is opened suggests there is a high possibility of sudden ignition, or flash-over, of the residual
gases that are evolved inside the manhole. Crews should be trained never to approach an area
around the cover while the manhole is still smoking. They should never look into the manhole until
it has been vented sufficiently so that only a minimal amount of smoke emanates from the
chimney. They should always approach upwind of any smoke and be equipped the proper nonflammable clothing. If it is necessary to remove the manhole cover, it should be removed quickly
and then sufficient time should be provided to allow the manhole to vent all combustion gases. All
surrounding buildings that could be influenced by the generated gases should be checked for the
presence of carbon monoxide. After the manhole has been sufficiently vented, the manhole can be
flooded with water or gel while the fire fighters maintain a safe standoff distance from the
manhole. If the cover is unable to be removed due to rusting or the accumulation of debris, extra
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caution should be used because the manhole cover can become dislodged and become a safety
hazard.
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Fig. 1 Examples of manhole events that illustrate different levels of severity and damage
There are multiple sources of combustible gases that may be found in subsurface structures that
contain electrical equipment. Cable insulations are often hydrocarbon based materials such as
cross-linked polyethylene (XLPE), ethylene propylene rubber (EPR), ethylene alkene copolymer
(EAM), neoprene, polyethylene (PE) and polyvinyl chloride (PVC). These materials generate
flammable gases if they are vaporized or pyrolyzed and mixed with air to generate mixtures within
their respective flammable limits. Transformers, voltage regulators and oil-filled switches that may
be installed in vaults utilize dielectric fluids that are often combustible when vaporized as a result
of a damaging fault. Insulating fluids employed in paper insulated lead covered (PILC) cables and
in self-contained fluid–filled (SCFF) cables are combustible and can add chemical energy to a
manhole event if they ignite during a failure.
Combustible gases in manholes can originate from multiple sources:
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
Aging cables, splices, and terminations can emit combustible gases as insulation materials
overheat and degrade. Tracking at component interfaces that may occur prior to a fault can
also generate combustible gases. If these gases are emitted in sufficient quantities such that
they reach their flammable limits, then gas explosions can occur if an ignition source is
present.
Combustible gases have been traced to salt or other chemicals that are applied to streets
during winter months. When these ionic solutions contact aging equipment and cables,
combustible gases may be generated. Low voltage cable that experience high impedance
faults are allowed to discharge through the ionic solution to ground. The resulting fault
currents generate heat that can pyrolyze the cable and cable components.
Gases can also be present in manholes when the public disposes of combustible materials
in the streets.
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Combustible gases that are a result of decaying organic material often find their way into
underground structures, and they can create a hazardous condition within the manhole.
Combustible gases can accumulate inside manholes as a result of potential leaks in
pipelines that carry natural gas that are frequently routed through or near underground
manholes.
Sections III and IV below discuss ways to increase the possibility of a safe conclusion to manhole
events and they suggest methods and mechanisms that will assist in mitigating the effects of a
manhole event.
II. Manhole Event Incidence
A. Utility Survey
IEEE PES Insulated Conductors Committee discussion group C34D commissioned a survey of
utilities in an attempt to estimate the incident rate of manhole events in North America.
Acknowledging that collection of reliable data regarding the occurrences and causes of manhole
events is difficult, at best, the data that is presented in this section should be viewed with some
caution.
The C34D survey template requested information on the number and type of manholes and the
number and type of events. It also sought information on the primary root cause of the event,
either electrically driven or chemically (flammable gas) driven.
The primary core of survey correspondents were chosen from utility employees who have attended
meetings of committee C34D. Additional potential respondents were solicited from acquaintances
and business associates. Five utilities responded with some information. The findings are
summarized in Table 1. Respondents were promised anonymity and as such were only identified
by their generic identities: urban, metro or exurban utilities. Urban utilities serve primarily a core
city and an inner ring of suburbs. Metro utilities describe a utility serving a large metropolitan
area including city, suburbs and exurbs. Exurban utilities refer to a semi-rural area undergoing
suburban development.
Table 1. Summarized Information Collected From C34D Survey Respondents
Utility ID
Service Area
Total Number of
Manholes
Events Per year
1
2
3
4
5
Urban
Urban
Metro
Exurban
Metro
250
11,200
Not Reported
607
38,037
2.5
1
3.7
9
Not Reported
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It is evident that the data is limited and sparse. Furthermore, the response from Urban 1 only
included information concerning primary system manholes in the city core.
The primary goal of the survey was to estimate the incidence of manhole events on a wider scale.
Fortunately, the survey includes a broad cross-section of utility locations covering variables of
climate, system age as well as cable and equipment types. An attempt was made to consolidate
and extrapolate the available data. Fortunately, the annual power delivered by most utilities is
available on their web-sites, on Wikipedia or calculable by knowing the population served by a
single utility and using the average per capita electrical energy consumption in the US.(1) Table 2
includes estimates of the electrical energy delivered by each of the utilities that participated in the
survey. A source for each estimate is included.
Table 2. Estimated Electrical Energy Delivered By Utilities Completing C34D Survey
Utility ID
Service Area
Estimated Energy
Delivered, Billion
kWh/Year
Source for Estimate
1
2
3
4
Urban
Urban
Metro
Exurban
57
9.5
87
5.4
Utility Website
Utility Wikipedia Entry
Utility Website
Utility Website
Est. from population served and US per capita
electricity consumption ~ 13,000 kWh/yr
5
Entire US
Metro
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4600
Wikipedia “List of countries by energy
consumption”
By normalizing the number of manholes and the number of manhole events per year per billion
kWh it is possible to obtain estimates of the number of manholes in the US, the number of
manhole events per year and the overall incident rate. The estimated data is shown in Table 3.
Table 3. Estimated Number of Manholes for Utilities Completing C34D Survey
Utility
Number Manholes
Manholes/Billion kWh/Year
Utilities Reporting Manholes
Entire US Extrapolated
50094
2,180,000
473
Utility
Events /Year
Events/Billion kWh/Year
Utilities Reporting Events
Entire US Extrapolated
16.2
464
0.101
The U.S. Homeland Security Agency estimates there are 20 million manhole structures in the US
(2). This estimate includes all utilities: water and sewer, electrical, natural gas and
telecommunications. The estimate that electrical manholes are 10% of all manholes in the US
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seems reasonable because many homes are supplied by overhead electrical service. The
aggregated data collected during the C34D survey suggests that that there are approximately two
million electrical manholes in the US and 460 manhole events per year. Using these estimates
based on the survey, there is a one-in-4700 chance that a manhole will suffer an event in any given
year.
B. Data From Previous Studies
Surveys other than the one commissioned by the C34D committee exist. There are three public
documents which contain sufficient information collected from utility files that can be used to
estimate manhole event incident rates. All three documents cover utilities in the Northeastern US:
Metro Washington DC, Metropolitan New York City and five utilities in Massachusetts.
Consolidated Edison of New York (ConEd) commissioned a study of manhole events in its system
in order to better prioritize manhole maintenance activities (3). The study involved reviewing callout tickets and orders to repair electrical failures. ConEd did not specifically track manhole events
in its record system. Instead the study team had to review the call-out tickets for keywords
suggesting a smoker, fire or explosion in a manhole structure. Call-out tickets from the years 1997
to 2006 were reviewed and identified 6670 call-outs suggestive of manhole events. ConEd has
approximately 250,000 manholes in its distribution system suggesting a one-in-375 chance of a
manhole event occurring in the ConEd system in any given year.
The other two studies were commissioned by the Public Utility Commissions of Washington DC
and the State of Massachusetts (4, 5). They were conducted by Siemens Power Technologies
International. The Washington DC study covered only the geographic area served by PEPCO
(Potomac Electric Power Company), Washington DC and portions of surrounding Montgomery
and Prince Georges counties in Maryland. The Massachusetts study included the utilities: N-Star,
National Grid, Western Massachusetts Electric Company (WMECO) and Unitil.
The Massachusetts study was conducted between mid-2004 and mid-2005. The report
summarized information supplied by the utilities and the pertinent data are reproduced in Tables 4
and 5. Table 4 contains the underground structure inventory.
Table 4. Electrical Underground Structures Reported in Massachusetts Survey
Utility
Manholes
Vaults
Service
Boxes
N-Star
Nat Grid
WMECO
Unitil
Total
38,000
20,735
3,750
192
62,677
800
1,675
250
30
2,755
0
38,089
25
1
38,115
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Total
Manholes &
Vaults Only
38,800
60,499
4,025
223
103,547
38,800
22,410
4,000
222
65,432
Table 5. Manhole Events Reported in Massachusetts Survey
Utility
N-Star
Nat Grid
WMECO
Unitil
Total
Events
44
20
30
0*
Time Period
July 04- June 05
July 04 – June 05
June 99 – June 05
1998 - 2005
Years
1
1
6
7
Events Per Year
44
20
5
0
69
*No written records exist. No Unitil employee recalled the occurrence of a manhole event during
1998-2005.
It is possible to estimate an incidence rate for all of the Massachusetts utilities surveyed from the
data indicated in Tables 4 and 5. There was an average of 69 manhole events per year reported in
the 65,432 large underground structures or a one-in-950 chance of a manhole event in any given
year.
It is interesting to note that manhole events occurred throughout the year for all the utilities
reporting events. However, each utility reported a different month of maximum event activity.
This data is recorded in Table 6.
Table 6. Manhole Events - Months of Peak Activity
Utility
Months of Maximum
Activity
Total In Maximum Month
N-Star
Nat Grid
WMECO
Unitil
April(9) & November(8)
February (8)
August(8)
Not Applicable
39%
40%
27%
Failures that peak during the summer months are normally attributed to heavy electrical loads.
Failures that peak in cooler weather months are normally attributed to high precipitation and the
presence of water and other contaminates that are able to enter the manholes. Liquid water in
contact with compromised insulation results in lower resistance paths to ground and increases the
risk of faults and events (6).
N-Star and National Grid shared sufficient data for the report authors to identify root causes of the
events. These results are summarized in Table 7.
Table 7. Root Causes of Manhole Events at N-Star and National Grid
Root Cause
Occurrence at N-Star
Occurrence at Nat. Grid
Cable Failure
Joint Failure
Other Failures
52%
36%
12%
40%
45%
15%
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The most comprehensive of the three documents described in this section involved an analysis of
the reliability of the PEPCO system in Washington DC and nearby Maryland. The report authors
had access to ten years of utility records and conducted on-site manhole inspections. The only
missing data of significance was an explicit recitation of the number of manholes in the PEPCO
system. The report mentioned a six year manhole inspection program being conducted onschedule. A total of 43,753 manhole inspections were completed in four years. If the inspections
were completed at the same rate over six years, then 65,000 manholes would have been inspected,
assuming each manhole in the system would be addressed only once.
Table 8 contains manhole event occurrences by year and severity in the PEPCO system.
Table 8. Manhole Events in the PEPCO System 2002 – 2010
Year
Smokers
Fires
Explosions
Total
2002
17
8
14
39
2003
55
6
27
88
2004
62
5
22
89
2005
62
7
25
94
2006
33
3
22
58
2007
57
12
27
96
2008
39
3
27
69
2009
53
4
25
82
2010
82
4
25
111
The number of manhole fires per year is reasonably consistent and small. The number of
explosions has remained relatively steady since 2003. Smokers have increased significantly and
are primarily responsible for the increased incidence of events over the period.
Using the latest data from 2010, a total of 111 manhole events in 65,000 manholes expose the
public to a one-in-585 chance of an event in each manhole per year. More significantly, the
chance of a manhole explosion or fire is one-in-2600 per year.
Manhole Event Incidence Rate Summary
Table 9 summarizes the overall manhole event incidence rates estimated in the previous studies
combined with the results of the C34D survey.
Table 9. Estimated Incident Rate of Manhole Events from Previous Survey and C34D Survey
Study
Estimated Incidence Rate per Year
C34D Survey
ConEd
Massachusetts Utilities
PEPCO
Overall
1 in 4700
1 in 375
1 in 950
1 in 585
Approximately 1 in 1000
Overall, the aggregated data from all sources suggests that the risk of a manhole event is one-in1000 manholes in any given year. To put this figure in perspective, the Electrical Safety
Foundation International reports that 1 in every 310 households will report a fire every year (7).
At a risk rate of one event per 1000 manholes per year, it is estimated that there will be 2000
manhole events in the US every year or 5.5 events per day. The PEPCO data suggests that 30% of
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all manhole incidents involve fire or explosion which is equivalent to 600 serious events per year
or 1.6 serious events per day in the US.
III. Reducing Manhole Events
This section summarizes methods that may be employed to reduce the number of manhole events
that occur. This approach attempts to take corrective action prior to any event in hopes of
preventing the manhole event from occurring. A wide range of methods have been employed in the
past. No single method will assure that manhole events will never occur. Each method has its own
unique set of advantages and disadvantages.
The main objectives of this approach are: identify and eliminate potential cable designs that could
lead to a manhole event; reduce the amount of energy generated by a fault by installing protection
equipment to limit the fault current and reduce the duration of a potential electrical event; and
ensure that underground systems are operated within their design limits. No attempt has been made
to determine the capital investment required to initiate any of the methods described in this section.
A. General Methods
Various methods of preventing manhole events from occurring include:
1. Cable Design and Installation

Improve level of training and cable-splicing workmanship in an attempt to reduce splice
and joint failures that result from poor workmanship. Since a reasonable percentage of
manhole events can be traced to electric faults in joints, elimination of poor workmanship
in construction should reduce the potential failures in cable accessories.

Consider the use of more robust cable designs including cables with multiple layers of
insulation, self-healing and non-flammable insulation materials.

Employ a flame retardant polymeric jacket to the cable design to reduce flammability and
reduce emission of toxic combustion products.
Conventional halogenated jacket materials like polyvinylchloride (PVC), chlorinated
polyethylene (CPE), chloro-sulfonated polyethylene (CSPE), and Bromine filled polyethylene
all produce significant smoke and toxic gases when they decompose. These gases not only are
harmful to humans but also can cause corrosive damage to components not directly associated
with the fault or fire. Halogen Free Flame Retardant (HFFR) compounds offer significant
reduction in smoke and acid gas generation along with reduced flammability.
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2. Cable Operation



For new circuits, change manhole cable routing to eliminate congestion. Separate bundled
cables to reduce mutual heating.
Ensure cable loading is within recommended ampacity limits so that cable operating
temperatures are not exceeded. De-rate cables that frequently exceed their ampacity limits.
Maintain manholes in dry and non-corrosive environments.
3. Protection Devices


Reduce fault current magnitude and the fault current duration by installing current-limiting
fuses and fast-acting protection equipment that would limit the magnitude of energy
generated during a fault.
Install phase current limiting reactors in feeders at substations to reduce fault currents.
4. Manhole and Cable Inspection




Utilize infrared cameras to locate and eliminate overheating cables and joints. Identify
steam leaks in proximity to electric facilities as a means of eliminating historically high
temperature manholes.
Institute a replacement program for aging, over-heated and degraded components. Even
relatively new equipment that has been operated under harsh environmental conditions or
heavily loaded conditions should be identified and considered for replacement.
Institute safety inspection programs to identify potential hot spots and other areas of
vulnerability.
Robotic devices could be used in an attempt to prevent manhole events by providing a
means to clean and power-wash cables, inspect manholes which show signs of smoke
generation and eliminate human entries when conditions inside the manhole are unsafe.
5. Cable Testing and Evaluation

Test cables to locate incipient faults: high voltage AC partial discharge tests; jacket
integrity tests; megohmeter tests and air-pressurized tests of insulation of secondary cables.
B. Arc Detection Equipment
If arcing in a manhole can be detected before it can generate sufficient combustible gases and
before the gases reach a dangerous level, then an early warning of potential faults or gas
explosions could reduce or eliminate some manhole events. If arcing is detected, then corrective
action can be taken to prevent a possible manhole event before it has an opportunity to occur.
Prototype equipment has been developed and undergone initial testing. The device has the
potential to recognize possible arcing from defective cables and accessories. The device is lightweight, portable, inexpensive and it can provide both audible and visual alarms when arcing is
detected. It is sufficiently sensitive to detect potential problems with minimal false alarms. A
prototype of the portable arc recognition equipment is shown in Fig. 2.
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Fig. 2 Prototype of portable arc detection equipment
C. Portable Gas Detection Equipment
Work safety standards mandate the use of atmosphere analysis equipment prior to work crews
entering a manhole or other confined space. If too much or too little oxygen, flammable gases and
vapors, or potential toxic air contaminants are detected, the source must be determined and
eliminated before personnel are allowed to enter the manhole. As required by OSHA, this method
is already required for the safety of the crew before they enter the manhole.
D. Continuous Monitoring Equipment
A wireless telemetry system has been developed that is capable of sampling and monitoring
multiple sensors and reporting data at regularly scheduled intervals. Event alerts, in the form of
email, text message, etc. can be sent immediately upon the detection of readings outside of pre-set
parameters. These alerts can include, but are not limited to, anomalous voltage and current
readings, presence of explosive gases, high temperature conditions and high water levels.
Additional sensors can include unauthorized intrusion into the manhole, PH level and the presence
of bio-chemical compounds. The system can be powered with a self- contained battery pack or
hard-wired to an external power source. The cover with integrated monitoring device is shown in
Fig. 3.
16
Fig. 3 Continuous Monitoring of Manhole Conditions with Wireless Telemetry System
E. Installation of Vented Covers
Installation of vented covers may help reduce the risks associated with cable failures that can occur
in underground electric vaults by reducing the accumulation of volatile gases that can be generated
by failures on the cable systems. The issue of volatile gases is particularly important in secondary
grid network systems due to intermittent arcing and pyrolysis of the cables that are often not
interrupted by the secondary protection system. Even when cable systems are protected by current
limiting fuses, these fuses do not prevent burning faults that can result in very significant
explosions when the generated gases reach a flammable mixture. The grated covers allow some of
the volatile gases to escape, making them easier to detect by the general public.
17
The use of vented covers entails several disadvantages that should be recognized. Test results have
shown that vented covers can still be propelled upward if the energy release during the event
reaches even modest levels. In addition, vented covers allow debris and surface water to enter the
manhole which can cause the generation of combustible gases and exacerbate the possibility of a
gas explosion within the manhole. If water that enters the manhole via a vented cover contains salt
that is applied during winter months to control ice and snow on streets, then the possibility of
combustible gases being generated is greatly enhanced.
IV. Mitigating the Effects of Manhole Events
No matter what preventive measures are taken, manhole events may eventually occur, and when
they do, procedures and safety mechanisms can be put in place that will minimize the effects of the
event. Assuming an event is unavoidable, the goal this approach is to utilize manhole designs and
new cover designs to bring the event to a conclusion without damage to the equipment inside the
manhole and without danger to the public or maintenance crews. This section lists several new
manhole covers that are designed to provide a safe conclusion to potential manhole events. Some
of these designs have been tested and installed in the field while others are still in the development
stage. Most of the covers are designed to vent high pressure gases that are generated inside the
manhole while restraining the cover so that it does not become a safety issue.
A. Pressure Compensating Manhole Cover
A controlled pressure release manhole cover (Fig. 4) restrains a manhole cover in its frame during
underground vault explosions of minor to even severe events. The cover returns to the seat of the
frame following an event. It also prevents theft, unauthorized access and potential acts of
terrorism and sabotage. The cover is available in light-weight ductile iron with an H-25 load rating
and it meets AASHTO M306-10 requirements. An optional high-security actuator bolt is
available.
The cover is designed to fit into existing frames and it is easily removed using a simple pick tool or
other traditional manhole cover removal devices.
During an explosion, the upward travel of the cover is stopped by the latch on one side of the cover
and the fixed lug on the opposite side. The upward travel of the cover is limited to two inches
initially during minor to moderate explosions. However, during more severe explosions, the cover
can automatically rise up to a maximum of three inches through the use of a pressure
compensating spring contained in the latch housing.
The pressure and flames from a vault explosion are released evenly around the perimeter of the
cover through a series of exhaust ports cast into the underside of the cover. The ports direct the
venting gases and flames downward onto the street or sidewalk surface, in close proximity to the
vault opening, minimizing potential injury to passersby. Moreover, the exiting gases create an air
dam around the perimeter of the cover preventing simultaneous incursion of outside oxygen to fuel
a more massive secondary explosion. Multiple tests of the unit during gas explosions in
18
underground test vaults have demonstrated that the cover performs to its design intent. Actual vault
explosions in cities where the covers have been installed further support the efficacy of the design.
B. Pivoting Pressure Relief Cover
Electric arc faults or gas explosions inside an underground vault can happen without warning. In
these situations a standard unrestrained manhole cover can become a dangerous projectile and
leave a hazardous open hole following the event. The pivoting pressure relief cover (see Fig. 5) is
designed to relieve built-up pressure while restraining the cover during moderate explosions and
then return to the seated position after the event is terminated. The cover is designed to tilt when
internal manhole pressure increases which permits venting of the gases to the exterior. The
pressure-relieving ability of the cover during an explosion is only limited by the area of the clear
opening and the angle to which the cover is allowed to open. Additional covers can be installed
Fig. 4 Photographs of pressure compensating manhole cover
19
on the vault to achieve increased venting. The tilting action of the cover allows for directed venting
and limits the exposure to hot gases and debris to a single preferred direction.
Fig. 5 Illustration of pivoting pressure relief cover
The standard features of this unit include ductile iron that meets heavy duty H-25 load
specifications, per AASHTO M306-10 requirements. The cover has integrated slide rails cast into
the bottom surface that are designed so that the cover can be easily removed and replaced during
vault inspections as shown in Fig. 6. Removal and replacement can be accomplished using a
variety of standard manhole cover tools.
The design allows the cover to be easily retrofitted into existing frame installations. The cover is
available with an optional security bolt for controlled access and theft deterrence. A vented cover
version is also available.
20
Fig. 6 Procedure used to remove and replace pivoting pressure relief cover
C. Explosion Mitigating Vault Restraint
Underground utility vaults may have large lift-out panels made of concrete, fabricated steel or a
combination of these materials. Many of these panels are unrestrained and rely on their own
weight to keep them in place. The pressure increase during an explosive event is capable of lifting
and launching these panels that often weigh more than 2000 pounds.
The explosion mitigating vault restraint design is intended to be used in conjunction with the
pivoting pressure relief cover. If the pressure increase from an explosion is substantial, and has
exceeded the exhausting capabilities of the pressure relief cover, the vault restraint design will
allow the concrete panel to rise and provide additional venting around the perimeter of the vault
panel. This action mitigates the possibility of structural damage to the vault. The design also
restrains the vault panel and prevents it from becoming airborne. Once the pressure is released, the
panel is returned to its original position and there is no additional hazard from a hole at the event
site.
The vault restraint mounts to the bottom of the utility vault’s lift-out panel (see Fig. 7) and utilizes
a composite leaf spring to absorb the load. The leaf spring restraint provides a lightweight
assembly that can be installed, by a single operator, which can be retrofitted to existing vaults.
21
Fig. 7 Explosion mitigating vault restrant mounted to the underside of the vault lift-out panel
The explosion mitigating vault restraint has been subjected to both static testing and actual
explosion testing at EPRI facilities. Figure 8 is a screen capture from a video taken during one of
these tests. The vault panel has fully lifted and the pressure relief cover has fully opened during
the middle portion of the explosion. Once the pressure inside the vault is exhausted, both the cover
and the panel return to their original seated positions.
D. Spring Mounted Covers
Another manhole cover has been designed to ensure that the cover does not become a projectile in
the event of a cable or joint failure. The design consists of a 600 pound lid that is bolted to a
rectangular steel plate and the assembly is installed at street level. The steel plate contains six
holes where long steel rods are inserted down into the vault. In the vault, each rod has a spring and
a large nut to secure the springs to the steel plate above (see Fig. 9). In the event of a failure, the
increased pressure inside the manhole forces the entire steel plate and the attached cover upward a
few inches above the street level. The high pressure gases vent through the open area around the
entire perimeter of the steel plate. When the pressure drops, the springs return the plate and cover
to its original position. The rod and the springs prevent any segments from becoming projectiles.
The concept of a spring mounted cover has been evaluated in a series of tests that consisted of a
precast concrete vault that contained a 138kV self-contained fluid-filled joint that was intentionally
faulted. Tests at various fault currents determined that the cover did not separate from the steel
plate and the plate did not project out of the vault area.
22
Fig. 8 Testing of vault restraint and pivoting pressure relief cover
(a)
(b)
Fig. 9 (a) Top view of spring mounted cover attached to steel plate
(b) view of springs mounted inside vault
23
E. Tethered Covers
An obvious method of mitigating the effects of a manhole event is to attach a tether to the manhole
cover so that it limits the movement of the cover during the event. Two types of tethers have been
tested and installed and their respective designs are shown in the photographs in Fig. 10. The
tethers prevent the cover from being a potential hazard to the public. Computer simulations as well
as explosion test data have shown that the tether should be made of a strong, but flexible material
that minimizes the forces on the cover and tether during the event. Both analytical and
experimental results have also concluded that the cover should not be restrained or bolted down.
Even moderate events can generate pressures inside the manhole that are capable of fracturing
bolts which are expected to keep the manhole cover in place. Attempting to hold the cover in place
also creates an unusually high internal pressure so that when the cover attachments fail, the forces
on the cover are magnified and the potential damage can exceed that which would result if the
cover had not been restrained. Computer results have also indicated that rigid tethers are not a
viable candidate for consideration because they create unusually large restraint forces and they are
likely to fail during the event.
Fig. 10 Two examples of solid covers restrained with flexible tethers
F. Vented Covers
Installing a vented cover (see Fig. 11) has several advantages in minimizing the effects of a
manhole event. The open area in the cover can permit lighter-than-air gases to escape to the
atmosphere and it also reduces the forces on the cover caused by high pressure gases that vent
through the chimney during the event. However, the application of vented covers will not
completely eliminate the possibility of a manhole event from occurring. Testing of vented covers
has shown that in the event of a high energy gas explosion or high energy fault, a vented cover will
still be dislodged from its frame, creating a potentially dangerous situation. Vented covers also
permit intrusion of water, salt, potential fuels and other debris into the manhole which can lead to
the possibility of a future manhole event. Finally, even though they will encourage light gases to
24
vent to the environment, they will not vent heavier-than-air gases that can accumulate inside the
manhole.
Previous research has shown gases that are emitted when commonly-used cable insulation
materials age are: Hydrogen, Methane, Acetylene, Carbon Monoxide and Ethylene. These gases
are either neutrally buoyant or lighter than air, so that their concentration should be decreased
when a vented cover is installed. Application of salt to streets during winter weather has been
anecdotally related to the degradation of materials that are found in manholes. The number of
manhole events that can be attributed to gas explosions has been determined to be anecdotally
related to the amount of salt that is applied to streets, and the number of manhole events increases
the following spring and summer. The number of events has been documented to decrease when
the winter is mild and the amount of salt applied to the streets is decreased.
Fig. 11 Two examples of vented covers
G. Quick Action Protection Systems
One method of reducing damage of manhole events on underground electrical systems is to
minimize the energy released at the location of the fault. Several options are available including
power assisted fuses and current limiting interrupters. Current limiting interrupters are capable of
operating in less than one half cycle and they are thereby able to greatly reduce the energy
dissipated by a fault. By reducing the energy of the fault, the temperatures and pressures that result
inside the manhole can be controlled and damage minimized. A shorter duration of the fault also
translates to other desirable effects including reducing stresses on cables and joints, thereby
reducing the chance of the manhole cover from being displaced and reducing the fault duty on
substation breakers and transformers.
Some disadvantages still exist when installing faster reacting relays and current limiting fuses.
They cannot completely eliminate or prevent explosions that can occur when combustible gases
produced by the fault are ignited by the resulting arc. Furthermore, they do not provide protection
against damage that may be caused by secondary faults and secondary arc flash. Finally, if quick
25
interruption of a fault is possible, determining the location of the fault is more difficult because
there is minimal physical damage at the fault location.
H. Blast-Proof Cover Design
To minimize the safety risk of flying manhole covers to the public and maintenance crews, a blast
proofing system has been designed for use in distribution manholes to allow the expanding gases
generated by an explosion to vent safely to the environment. The cover is secured to the
surrounding asphalt by six steel rods extend radially from the cover. The rods remain anchored in
the road base when the event occurs within the manhole. The internal pressure is able to break the
bond between the frame and the access chimney. The cover lifts vertically a sufficient distance so
that the pressurized gases can vent into the cavity that is created between the asphalt and the road
base. The cavity that is created effectively increases the vault volume and the increase in cavity
volume acts as a buffer to reduce the pressure inside the manhole. Security bolts keep the cover
attached to the frame and they are rated to restrain the cover under all anticipated internal
pressures. The six rods that extend radially from the cover are designed to withstand the generated
pressure and remain embedded in the asphalt with a reasonable margin of safety (see Fig. 12).
After the internal pressure is dissipated, the frame is designed to return to its normal position on
the chimney and the asphalt will return to rest on the gravel road base. Extreme generated
pressures may damage the road surface such that minor repair may be required.
(a)
26
(b)
Fig. 12 (a) Blast-proof cover design with radial anchor rods
(b) Schematic of blast-proof design showing deformation at top of manhole
Modification to a standard manhole design to convert it into a blast-proof design is minimal. The
manhole, access chimney, frame, cover, and back fill should be constructed according to
established construction specifications. The cover is then removed and the blast proofing rods are
driven through the flutes in the frame. This ensures a good grip between the rods and the gravel
road base. If the frame does not have the six built-in flutes, holes may be drilled. The flutes are
angled so that during installation, the rods can be inserted without contacting the top of the frame.
Other manhole assemblies can be made blast-proof by replacing the old frame with one that can
accommodate the blast proofing rods.
A small diameter hole in the center of cover allows a fire-fighting lance to be passed through the
cover without removing the bolts that secure the covers to the frame. The lance emits a water mist
into the manhole and is fed by water from a fire truck. Fire Crews can quickly drop the lance into
the center hole and retreat to a safer distance. The center hole also allows a gas sensor to be
dropped in the confined space without removing the security bolts and the cover. The gas- sensing
device will permit maintenance crews to assess the potential for gas ignition inside the manhole
and warn crews about the potential for a gas explosion. The cover is attached to the frame by
“puzzle” bolts that require a unique wrench to remove the cover in order to prevent unauthorized
entry into the manhole and to firmly attach the cover to the frame in the event of a manhole
explosion.
27
V. Manhole Event Modeling
A computer model has been developed and refined so that it is able to simulate the thermodynamic
conditions that result when an event occurs inside a manhole or vault. The model is able to
simulate electrical events, chemical events and an event that is a combination of the two. The code
was developed as a design tool so that potential mitigation schemes could be compared and
manhole designs could be evaluated from the safety standpoint. The accuracy of the code has been
verified by comparing the computer-predicted results with tests carried out on a limited number of
full-scale buried manholes during fault conditions and gas explosions. Results of the model have
led to modifications to manhole designs so that they are able to mitigate the effects of a manhole
explosion and address the issue of improving the likelihood of concluding the event in a safe
fashion. Several example outputs of the programs are included in Fig. 13 for an electrical fault and
a combination of a fault followed by gas ignition.
Figure 13a shows the pressure rise inside the manhole which follows a single peak when the
energy release is solely a result of an electrical fault. The manhole cover lifts off its frame and
begins to vent gases to the environment after 45msec, but the manhole pressure continues to rise
since the vent area through the elevated cover is insufficient to handle the increasing flow of high
pressure gases. Once the arc is extinguished after 66msec, the internal pressure rapidly decreases.
Figure 13b shows the pressure rise when the fault is followed by the ignition of the gases that are
generated as a result of the fault. In this case the pressure reaches an early, relatively moderate
value that results from the energy generated by the arc. Then the pressure begins to decrease before
the ignited gases have an opportunity to release sufficient chemical energy. As the combustion
process builds in intensity, the pressure reaches a second higher peak, and it then decreases rapidly
once all the gases in the manhole have been burned. These results illustrate that the energy
generated by the arc and the combustion gases are released in two separate time domains. The fault
is cleared very early in the event, and the gas is a much more slower-reacting event. If the fault
does not clear quickly, the two curves tend to merge yielding a more elevated pressure curve and a
more dangerous event. Any design scheme that attempts to mitigate the effects of a manhole event
must accommodate the major differences in the electrical and chemical processes that occur during
the event.
The results in the pressure curves in Fig. 13 emphasize that the timing plays an important role in
the ultimate potential damage of the event. Gas explosions are relatively long duration events
while electrical faults last only a short period of time before the system protection equipment
interrupts the energy generation that is responsible for the pressure increase. Therefore venting of
the gases through the manhole cover, or by other means, must be designed to occur rapidly before
the internal pressures increase to dangerous levels.
The accuracy of the computer model has been established by comparing the pressure rise with
pressures measured during simulated tests that involve gas explosions and faults inside manholes.
For typical manhole designs with typical manhole cover diameters, the computer predicts pressure
rises on the order of 80kPa (12psi) for reasonable conditions that occur during faults on secondary
systems. Pressures that are generated during faults in transmission circuits can be expected to
exceed these values due to the increased energy available at transmission voltages.
28
Measurements carried out during faults on full-scale cables in buried concrete manholes reached a
maximum pressure rise of approximately 60kPa (9psi). The computer model predicts pressure rises
in the neighborhood of 50-225kPa (7-33psi) when strong gas mixtures are ignited in full-scale
manholes, and tests simulating gas explosions provide pressure increases over a range of
approximately 50-175kPa (7-26psi) depending on manhole volume, gas concentration and
composition.
Experimental measurements of pressures that occur during a fault followed by ignition of the gases
that are generated by the fault are lacking. However, computer simulations have indicated that the
pressure rise can typically exceed 250kPa (35psi). The range of expected pressures that will exist
during manhole events will be capable of lifting unrestrained objects such as the manhole covers,
even vented covers, but may not lead to significant damage to the reinforced concrete of the
enclosure. Predictions of the pressure rise in a manhole during a specific event require execution of
the program for that unique set of conditions of the system geometry and the exact specifications
of the fault and gas composition.
220
Manhole: 2x2x3m
Cover: 1m dia, 140kg
Fault: 5.5kA, 12kV, 4 cycle
No gas contribution
150
140
20
130
Abs. Pressure, psi
Manhole Abs. Pressure, kPa
160
25
Fault Clears
120
110
15
100
0
0.05
0.1
Time, sec
0.15
200
Manhole Abs. Pressure, kPa
170
Manhole: 2x2x3m
Cover: 1m dia, 140kg
Fault: 5.5kA, 12kV, 4 cycles
Gas: 10% Methane
30
180
25
Abs. Pressure, psi
180
160
Fault Clears
140
20
Combustion
Complete
120
15
100
0
0.2
(a)
0.1
0.2
Time, sec
0.3
(b)
Fig. 13 Examples of output of manhole event software
(a) Internal pressure rise during an arcing event
(b) Internal pressure rise during a combination of arcing event and gas explosion
29
VI. Results of Experimental Studies
At least two test laboratories have installed underground facilities that have been used to evaluate
the potential of safety devices that are designed to mitigate the effects of underground gas
explosions. The goals of these test facilities are to understand the mechanisms and forces generated
by gas explosions within manholes and service boxes and to develop mitigation and control
methods for field testing.
EPRI’s Lenox Massachusetts facility consists of several manholes, a transformer vault, several
secondary service boxes and various runs of conduits that interconnect the underground structures.
Tests are performed by injecting known amounts of gas mixtures into the enclosure, igniting the
explosive mixture and collecting internal pressures and temperatures on a data acquisition system.
Various designs of round and rectangular covers and cover restraining systems have been tested
and evaluated at the facility over the years.
Tests have confirmed the effectiveness of some of the cover designs that are intended to minimize
collateral damage that could be caused by a manhole event. The tests have also developed a
physical understanding of the dynamics of explosive events so that new covers with enhanced
performance characteristics can be developed. The results of one testing program are summarized
in the references listed in Section VII B.
30
VII. Literature Review
A more in-depth study of manhole events is provided in the listing of available literature that
appears below. The literature has been selected so that it is relevant to events in manholes and
vaults.
A. General References (G)
1. Abdolall, Khaled and Buchholz, Vern L., B.C. Hydro 15kV Cable Explosion, IEEE
Transactions on Power Delivery, Vol. 17, No. 2, pp. 302-307, April 2002.
2. Babrauskas, V., Electric Arc Explosions, pg. 1283-1296, Interflam 2010 – Proceedings of
the 12th International Conference.
3. Boeck, W. and Kruger, K., Arc Motion and Burn through in GIS, IEEE Transactions on
Power Delivery, Vol. 7, No. 1, pp. 254-261, 1992.
4. Brzustowski, T. A., Burroughs, J. C., Chu, F. Y., and Twardus, E. M, Explosions in
Distribution Equipment, Work performed by Energetex Engineering for CEA under project
No. 149 D 269, Research Report December 1983.
5. Circuit Interruption Theory and Techniques, edited by Thomas E. Browne, Jr., Marcel
Dekker, Inc., Chaps 2, 4, 5, 1984.
6. Cress and Hatanaka, CEATI Report # T044700-5042, Detection and Interruption of Arcing
Faults on Distribution Utility Secondary Voltage Conductors, Kinectrics, Inc. Toronto,
Ontario Canada, August 2005.
7. Dasbach, A. and Pietsch, G. J., Calculation of Pressure Waves in Substation Buildings Due
to Arcing Faults, IEEE Transactions on Power Delivery, Vol. 5, No. 4, pp. 1760-1765,
November 1990.
8. Desborough, M., Pressure Rise and Burn through Predictions and the Principles of
Pressure Relief Device Design, Institute of electrical Engineers, London, 1997.
9. Drouet, Michel G. and Nadeau, Francois, Pressure Waves due to Arcing Faults in a
Substation, IEEE Trans. On Power Apparatus and Systems, Vol. PAS-98, No. 5, pp. 16321635, September/October 1979.
10. Electrical Transmission and Distribution Reference Book, Westinghouse Electric Corporation,
East Pittsburgh, PA, Chap. 21, Pgs. 703-4, 1964.
11. Friberg, F., Pietsch, G. and Schumacher, M, On the Description of Pressure Rise in the
Surroundings of High Current Arcs in Metal-Enclosed Compartments with Pressure Relief,
The Eleventh International Conference on Gas Discharges and their Application, Chuo
University, Tokyo, pp. 18-21, September 11-15, 1995.
12. Friberg, G. and Pietsch, G. J., Calculation of Pressure Rise due to Arcing Faults, IEEE
Transactions on Power Delivery, Vol. 14, No. 2, pp. 365-70, April 1999.
13. Garcez, T.V., De Almeida, A.T., A Risk Measurement Tool for an Underground Electricity
Distribution System Considering the Consequences and Uncertainties of Manhole Events,
Reliability Engineering & System Safety, Vol. 124, pp. 68 – 80, 2014.
14. Gammon, Tammy and Matthews, John, Instantaneous Arcing-Fault Models Developed for
Building System Analysis, IEEE Trans., Industry Applications, Vol. 37, No. 1, pp. 197- 203,
Jan/Feb 2001.
31
15. Hamel, Andre, Gaudreau, Andre and Cote, Michel, Intermittent Arcing Fault on Underground
Low-Voltage Cables, IEEE Transactions on Power Delivery, Vol. 19, No. 4, pp. 1862-1868,
October 2002.
16. Heberlein, G. Erich Jr., Higgins, Jack A. and Epperly Richard A., Report on Enclosure
Internal Arcing Tests, IEEE Paper No. PCIC-94-32, pp. 271-284, 1994.
17. IEEE Guide for Field Testing and Evaluation of the Insulation of Shielded Power Cable
Systems, IEEE Std 400 – 2001.
18. IEEE Guide for Field Testing of Laminated Dielectric, Shielded Power Cable Systems Rated
5kV and Above with High Direct Current Voltage, IEEE Std 400.1 – 2007.
19. IEEE Guide for Field Testing of Shielded Power Cable Systems Using Very Low Frequency
(VLF), IEEE Std 400.2 – 2004.
20. IEEE Guide for Partial Discharge Testing of Shielded Power Cable Systems in a Field
Environment, IEEE Std 400.3 – 2006.
21. Kizilcay, M. and Koch, K.-H., Numerical Fault Arc Simulation Based on Power Arc Tests,
European Transactions on Electrical Power Engineering, Vol. 4, No. 3, pp. 177-85, May-June
1994.
22. Koch, Bohdan, Tests on XLPE-Insulated Cable Arcing Faults and Arcproofing, IEEE
Transactions on Power Delivery, Vol. 3, No. 4, pp. 1289-1295, October 1988.
23. Koch, Bohdan and Carpentier, Yves, Manhole Explosions due to Arcing Faults in
Underground Secondary Distribution Cables in Ducts, IEEE Transactions on Power Delivery,
Vol. 7, No. 3, pp. 1425-1433, July 1992.
24. Koch, Bohdan and Christophe, Patrick, Arc Voltage for Arcing Faults on 25(28) – kV Cables
and Splices, IEEE Transactions on Power Delivery, Vol. 8, No. 3, pp. 779-85, July 1993.
25. Lee, Ralph H., Pressures Developed by Arcs, IEEE Trans. on Industry Applications, Vol. IA23, No. 4, July/August, 1987.
26. Lee, Ralph, H., The Other Electrical Hazard: Electric Arc Blast Burns, IEEE Transactions on
Industry Applications, Vol. 18, No. 3, pp. 146-151, May/June 1982.
27. Lutz, F. and Pietsch, G., Investigation on the Pressure Rise in the Surroundings of a HighCurrent Fault Arc, 6th International Conference on Gas Discharges and their Applications,
Part 1, pp. 270-273, September 1980.
28. Mosier, Rachel, Antoniello, Victor and Black, W. Z., Design and Test of 345 kV Cable Vaults,
Transmission and Distribution World, pp. 18-25, May 2006.
29. Rangel, E. Jr. Explosion Risks in Underground Networks, IEEE Industry Applications
Magazine, pp. 58 – 63, Sept/Oct 2014.
30. Snodgrass, Robert E. and Black, W. Z., Design of Safety Devices to Mitigate Explosions in
Underground Vaults and Manholes, IEEE Trans., Power Delivery , Vol. 23, No. 4, pp. 22622269, October 2008.
31. Snodgrass, Robert E. and Black, W. Z., Mitigating the Effects of Explosions in Underground
Electrical Vaults, IEEE Trans., Power Delivery, Vol. 20, No. 2, pp. 1767-1774, April 2005.
32. Stanback, H. I., Jr., Predicting Damage from 277-V Single Phase to Ground Arcing Faults,
IEEE Trans., Industry Applications, Vol. IA-13, No. 4, pp. 307-314, July/ Aug 1977.
33. Tarnowski, Janislaw, Cote, Jacques, Gaudreau, Andre and Gingras, Pierre, Pneumatic Testing
of the Insulation of Low-Voltage Underground Cables, Hydro-Quebec, Canada, CIGRE Paper
B1-202, 2012.
34. Walsh, Bryan P. and Black, W. Z., Thermodynamic and Mechanical Analysis of Short Circuit
Explosions in Underground Vaults, IEEE Trans., Power Delivery, Vol. 20, No. 3, pp. 22352240, July 2005.
32
35. Walsh, Bryan P. and Black, W. Z., Thermodynamic and Mechanical Analysis of Gas
Explosions in Underground Vaults, IEEE Trans., Power Delivery, Vol. 17, No. 1, pp. 8-13,
January 2002.
36. Walsh, P. J., Lama, W. and Hammond, T. J., Voltage – Current Relationship for Pulsed Arc
Discharges, Journal of Applied Physics, Vol. 52, No. 9, pp. 5476-5482, September 1981.
37. Zhang, Lily, Boggs, Steven A. et al, The Electro-Chemical Basis of Manhole Events, IEEE
Electrical Insulation Magazine, Vol. 25, No. 5, pp. 25-30, Sept/Oct 2009.
32. Non-Halogen Flame Retardant Polyolefin Compounds via Synergistic Blends of Metal
Hydroxides and Mineral Fillers, SPE International Polyolefins Conference, 2006.
33. Flame Retardant Olefinic Polymers Containing Metal Hydroxides – Fire Performance vs.
Other Properties Optimization by Surface Modification, 2010 BCC Conference on Flame
Retardant Polymeric Materials.
B. EPRI Reports (E)
The following research reports are available to EPRI members at http://epri.com. Selected reports can
be downloaded for non-EPRI members.
1. Manhole Event Risk Management – Mitigation Strategies, EPRI Report 1013886,
Technical Update, December 2007.
2. Manhole and Service Box – Explosion Suppression and Mitigation, EPRI TR-109741, Final
Report, January 1998.
3. Evaluation of Gases Generated by Heating and Burning of Cables, EPRI TR-106394, Final
Report, August 1996.
4. Manhole Cover Tests for Con Edison at the EPRI Manhole Test Facility, Nov. 2007, Product
I.D.: 1015220.
5. Manhole Event Mitigation Strategies, Dec. 2006, Product I.D.: 1012305.
6. Gas Explosion Tests on Pacific Gas and Electric Company Round Manhole Covers with the
Swiveloc CPR-II Controlled Pressure Release Manhole Cover Restraint System, March 2010,
Product I.D.: 1020559.
7. Manhole Event Gas Explosion Modeling, March 2008, Product I.D.: 1016476.
8. Manhole Event Gas Explosion Modeling, December 2008, Product I.D.: 1018404.
9. Gas Explosion Tests on Con Edison’s Prototype Covers, July 2009, Product I.D.: 1019477.
10. Manhole Event Risk Management, Dec. 2007, Product I. D. 1013886.
11. Final Report on Explosion Tests on Clogged Vented Covers for Con Edison at Lenox, Nov
2007, Product I. D.: 1015223.
12. 138-kV Maintenance Hole Restraining System Testing, Sept 1999, Product I.D.: TR-113556.
13. Manhole Event Risk Management Strategies, Nov. 2005, Product I.D.: 1010212.
14. Gas Explosion Tests on East Jordan Iron Works Rectangular composite Secondary Box, July
2007, Product I.D.:1019482.
15. Manhole Event Risk Management Strategies, Dec. 2008, Product I. D.: 1015889.
16. Underground Event Mitigation: State-of-Science Workshop Report, Nov. 2002, Product I.D.:
1001647.
17. Manhole Event Gas Explosion Modeling, Final Report, EPRI Project EP-P25698/C12429,
Phase I, March 2008.
C. Theses (T)
1. Snodgrass, Robert, Mitigation of Explosions in Underground Vaults, M. S. Thesis, School of
Mechanical Engineering, Georgia Institute of Technology, June 2002.
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2. Walsh, Bryan, Thermal and Mechanical Analysis of an Explosion in an Underground
Electrical Vault, M.S. Thesis, School of Mechanical Engineering, Georgia Institute of
Technology, January 1999.
3. Zhang, Lili, Mitigation of Manhole Events Caused by Secondary Cable Failure, University of
Connecticut, 2011.
D. Utility Reports (U)
1. Independent Assessment of Dislodged Manhole Covers, Final Report R20-05, prepared for the
Commonwealth of Massachusetts Department of Telecommunications and Energy, by
Siemens Power Transmission & Distribution, Inc., December 9, 2005.
2. Investigation of the Manhole Incidents and Explosions Occurring In and Around the
Underground Distribution System of the Potomac Electric Power Company in Formal Case
No. 991: Sixth Year Technical Audit, Report R55-11, prepared for the Public Service
Commission of the District of Columbia, by Siemens Energy, Inc., Siemens Power
Technologies International, June 30, 2011.
3. Assessment of the Underground Distribution System of the Potomac Electric Power Company,
Formal case No. 991, Investigation into Explosions Occurring in or Around the Underground
Distribution System of the Potomac Electric Power Company, Prepared by Stone & Webster
Consultants for the Public Service Commission of the District of Columbia, December 7,
2001.
4. Independent Assessment of Indianapolis Power & Light’s Downtown Underground Network,
prepared for the Indiana Utility Regulatory Commission by O’Neill Management Consulting,
LLC, December 13, 2011.
E. Survey References (S)
1. World Bank web-site, http://data.worldbank.org/indicator/EG.USE.ELEC.KH.PC
2. US Homeland Security Website, http://www.homelandsecuritynewswire. com/dr20120905many-of-the-u-s-20-million-manholes-are-in-need-of-immediate-rehabilitation-orreplacement
3. Rudin, Cynthia, et. al., “A process for predicting manhole events in Manhattan,” Mach
Learn (2010) 80: 1-31.
4. Siemens Power Transmission & Distribution, Inc., “Independent Assessment of Dislodged
Manhole Covers, Final Report R20-05, Prepared For The Commonwealth of Massachusetts
Department of Telecommunication & Energy, December 9, 2005.
5. Siemens Power Transmission & Distribution, Inc.,” Investigation of the Manhole Incidents
and Explosions Occurring In and Around The Underground Distribution System Of The
Potomac Electric Power Company in Formal Case No. 991, Six Year Technical Audit,
Report #R55-55, Prepared for Public Service Commission of the District of Columbia,”
June 30, 2011.
6. Lily Zhang, Steven A. Boggs, et. al., “The Electro-Chemical Basis of Manhole Events,”
IEEE Electrical Insulation Magazine, September/October 2009, Vol. 25, No. 5 ,pp. 25-30.
7. Electrical Safety Foundation International web-site, Error! Hyperlink reference not
valid.
ICC COMMITTEES/MITIGATING MANHOLE EXPLOSIONS/WHITE PAPER C34D
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