The document summarizes research on understanding and preventing manhole events caused by electrical failures underground. It discusses that over 95% of manhole events are caused by low voltage cable failures. The document outlines the electrical properties of these failures and different event outcomes. It also discusses strategies that utilities are taking for prevention, detection, mitigation, and response to reduce risks and costs from manhole events.

1. Understanding Manhole Events
Stuart Hanebuth
Power Survey Company
shanebuth@powersurveyco.com
973-344-7116
Jodie Lane National Conference
Stray Voltage Detection, Mitigation & Prevention

2. Importance of a Better Understanding
• There has been a lot of research into causes, prevention
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detection, mitigation and response
Little of this work has been consolidated to identify
beneficial strategies and best practices
Significant maintenance and capital spending to prevent
and respond, oftentimes with limited benefit
Events represent risk to employees and the public
Industry and public do not always speak the same
language on these events
Consolidation of prior research, along with utility best
practices will save utility and ratepayer dollars and
improve public safety
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Stray Voltage Detection, Mitigation & Prevention

3. Manhole Event Sources
• Transmission and primary cable initiated events
• High fault energy
• Interrupted quickly by protective systems
• Incipient fault detection may be possible
• Transformer initiated events
• High fault energy
• Large fuel source
• Primary faults interrupted by protective systems, secondary faults
may or may not be interrupted
• DGOA, visual inspection, and cathodic protection are effective
preventative measures
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Jodie Lane National Conference
Stray Voltage Detection, Mitigation & Prevention

4. Manhole Event Sources
• Natural gas initiated events
• Large fuel source
• Potential for collateral damage to electric facilities
• Methane and chemical related events
• Less common
• Hard to predict
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Jodie Lane National Conference
Stray Voltage Detection, Mitigation & Prevention

5. Manhole Event Sources
• Low voltage cable events (>95% of manhole events1)
• Limiters designed to prevent damage to adjacent sections but not
to interrupt fault
• Large potential fuel source from cable insulation
• Toxic gases produced during decomposition of insulation
• Visual inspections provide limited benefit (less than 5% reduction)
• Arc fault detection may not provide adequate sensitivity
• Contact voltage detection locates conductive faults
• Venting demonstrates some benefit in reducing severity
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Stray Voltage Detection, Mitigation & Prevention

6. Electrical Properties
• Low Current
• Gas generation with a little as 1.28 W/cm1
• Sustained Combustion at 18W/cm
• Distinctive waveform2-4
• May persist for short intervals of seconds to minutes
with long quiescent periods5
• Arcing events can energize street level objects and
can be detected with mobile contact voltage detection
systems6
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Stray Voltage Detection, Mitigation & Prevention

7. Protective Systems
• Limiters
• Designed to protect adjacent
sections from thermal overloads
during three phase faults7
• Typically limit in the 1,000-5,000
amp range
• Not generally effective in
preventing or mitigating gas
producing faults
• Arc fault detection
• Most sensitive systems seem to
be in the 5 amp – 50 amp range6
• May not be able to detect gas
producing faults
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Stray Voltage Detection, Mitigation & Prevention

8. Chemistry
• A variety of materials have been used for low voltage cable insulation8
• Paper Insulated Lead (PILC)
• Kerite
• Styrene Butadiene Rubber (SBR) 1940 - 1960
• Butyl Rubber
• Neoprene
• PVC
• EPR
• Variety of duct materials have been used9
• Wood
• Cellulose-Tar
• Concrete
• PVC
• Each represents different risks and benefits
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Jodie Lane National Conference
Stray Voltage Detection, Mitigation & Prevention

9. Cable Failure Outcomes
• Majority of failures result in power
quality and contact voltage
related issues
• Smoking manholes are the most
probable manhole events as a
result of insulation degradation
• Manhole fires and explosions can
be further subdivided
Manhole
Fires and
Explosions
Smoking Manholes
Energized Objects
• Electrically driven events
• Chemically driven events
Flickering Lights
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Stray Voltage Detection, Mitigation & Prevention

10. Collateral Damage Concerns
• Primary damage from
low voltage faults
• Damage to nearby
natural gas facilities10
• Building explosions from
carbon monoxide
accumulations
Danny Iudici/for New York Daily News
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Stray Voltage Detection, Mitigation & Prevention

11. Event Prevention Strategies
• Post installation testing11
• Duct sealing to minimize
airflow12
• Filling manholes with inert
materials to minimize gas
accumulation
• Contact voltage testing to
find incipient faults
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Jodie Lane National Conference
Stray Voltage Detection, Mitigation & Prevention

12. Visual Inspections
• Analysis of over 55,000
Event Probability
visual inspections found
small reduction in
secondary related
events such as13
• Smoking manholes
• Contact voltage
• Power Quality Events
Structure Category
• No reduction in manhole
fires or explosions
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Stray Voltage Detection, Mitigation & Prevention

13. Mitigation Strategies
• Several cover designs and restraining approaches have been implemented14
• Tethering
• Self restraining
• Venting
• Deployment strategies not well established
• Is 100% installation the optimal approach?
• High density areas
• Dense structures
• Duct or Cable driven installation
• Analysis needed on impacts of deployment
• Water
• Primary Joints
• Customer basements
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Debris accumulation
Access
Civil design
Increased duct airflow
• At least one “standard” exists for deployment of vented covers15
• Office of the Telecommunications Authority Hong Kong. (2010, June, 30). Implementation
Guidelines on Mitigating the Risk of Gas Explosion in Telecommunications Manholes [Online].
Available: http://tel_archives.ofca.gov.hk/en/report-paper-guide/guidance-notes/gn_201003.pdf
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Stray Voltage Detection, Mitigation & Prevention

14. Response Strategies
• Area of little consideration by most utilities
• Fire department approaches problem different than utility
• Variety of agents are available on the market
• Some firefighters have written on the topic
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Battalion Chief Frank C. Montagna, “Manhole Fires”, http://www.chiefmontagna.com/Articles/manhole%20fires.htm
Battalion Chief Frank C. Montagna, “What You Should Know About 10-25 Reponses”, http://www.chiefmontagna.com/Articles/pdf/Manhole.pdf
Boston Fire Department, “SOP #49B Special Procedures and Precautions for Incidents Involving Manholes”, http://www.firesops.com/wpcontent/uploads/2012/02/BFD_SOP_49B_Special_Procedures_and_Precautions_Involving_Manhole_Incidents.pdf
D. Leihbacher, “Managing Manhole Fires”, Fire Engineering Magazine, January 2008, http://www.fireengineering.com/articles/print/volume-161/issue1/features/managing-manhole-fires.html
• Conflicting strategies
• Remove covers or leave in place
• Flood Structures or leave dry
• Apply other firefighting agents
• Isolation
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Stray Voltage Detection, Mitigation & Prevention

15. Conclusion
• General need to define common terms to discuss the
issue
• Low voltage cable failures are at root of most of these
events
• Need wider focus than simply mitigating manhole events,
also need to consider:
• Prevention
• Early detection
• Maintenance
• Response
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Stray Voltage Detection, Mitigation & Prevention

16. Bibliography
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L. Zhang, “Mitigation of Manhole Events Caused by Secondary Cable Failure,” Ph.D. dissertation, Dept. of Elect. and Electronic Eng., Univ.
of Conn., Storrs-Mansfield, CT, 2011
D.G. Ece, F.M. Wells and H.G. Senel, “Analysis and Detection of Arcing Faults in Low-Voltage Electrical Power Systems,” in 7th
Mediterranean Electrotechnical Conference., Antalya, Turkey, 1994, pp. 929-935
W. Charytoniuk et al., “Arcing fault detection in underground distribution networks feasibility study,” in Industrial and Commercial Power
Systems Technical Conference, Clearwater Beach, Fla, 2000, pp.15-20
B. Koch and Y. Carpentier, “Manhole Explosions Due to Arcing Faults on Underground Secondary Distribution Cables in Ducts,” IEEE
Trans. Power Del., vol. 7, no. 3, pp. 1425-1433, Jul 1992
Hamel, A. Gaudreau, and M. Cote, “Intermittent Arcing Fault on Underground Low-Voltage Cables” IEEE Trans. Power Del., vol. 19, no. 4,
pp. 1862-1868, Oct 2004
N. Weisenfeld, Y. When, “Arcing Fault Detection Projects”, 2010 Jodie S. Lane Public Safety Conference, New York, NY
F. Heller, and I. Matthysse, “Limiters, Their Design Characteristics and Application” IEEE Trans. Power App. Syst., vol. 74, pp. 924-950, Oct
1955
C. Zuidema et al., “A Short History of Rubber Cables,” IEEE Electr. Insul. Mag, vol. 27, no. 4, pp 45-50, Jul/Aug 2011
L. Zhang et al., “The Electro-Chemical Basis of Manhole Events,” IEEE Electr. Insul. Mag, vol. 25, no. 5, pp. 25-30, Sep/Oct 2009
T.J. Parker and D. J. Ward, “Insuring Adequate Spacing Between Underground Distribution Conductors in Conduit and Gas Lines,” IEEE
Trans. Power Del., vol. 18, no. 1, pp. 291-294, Jan 2003
J.Côté, “Manhole Explosions Discussion Group Hydro-Québec Experience”,
http://www.pesicc.org/iccwebsite/subcommittees/subcom_c/C34/Presentations/2011Spring/C17.pdf
L. Zhang, S.A. Boggs and S. Livanos, “Manhole Events Caused by Secondary Cable Insulation Breakdown,” in Annual Report Conference
on Electrical Insulation Dielectric Phenomena, Quebec City, Canada, 2008, pp. 107-110
Consolidated Edison Co. of NY, Inc. Stray Voltage Test and Inspection 2010 Annual Report 04-M-0159, Consolidated Edison Co. of NY,
Inc., New York, NY, 2011
W. Black, J.Côté, “Mitigating Manhole Explosions”
http://www.pesicc.org/iccwebsite/subcommittees/subcom_c/C34/Presentations/2012Spring/C-21.pdf
Office of the Telecommunications Authority Hong Kong. (2010, June, 30). Implementation Guidelines on Mitigating the Risk of Gas
Explosion in Telecommunications Manholes [Online]. Available: http://tel_archives.ofca.gov.hk/en/report-paper-guide/guidancenotes/gn_201003.pdf
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Jodie Lane National Conference
Stray Voltage Detection, Mitigation & Prevention