NATIONAL

TRANSPORTATION

SAFETY BOARD

WASHINGTON, D.C. 20594

PIPELINE ACCIDENT REPORT

PB2001-916501

NTSB/PAR-01/01

NATURAL GAS EXPLOSION AND FIRE IN

SOUTH RIDING, VIRGINIA

JULY 7, 1998

7366

PIPELINE ACCIDENT REPORT 

NTSB/PAR-01/01 (PB2001-916501) 

PIPELINE ACCIDENT REPORT: NATURAL GAS EXPLOSION AND FIRE 

SOUTH RIDING, VIRGINIA 

JULY 7, 1998 

• Page 17, the 3rd sentence of the second paragraph has been corrected to read 

“Evidence indicates that in this accident, the corrosion that occurred on and/or within 

one side of the splice connection increased the contact resistance between the cable end and the 

crimp connector.” 

 The original sentence used the word “reduced”. 

THESE CORRECTIONS ARE INCLUDED

IN THIS VERSION OF THE PUBLISHED REPORT:

Pipeline Accident Report

Natural Gas Explosion and Fire in

South Riding, Virginia

July 7, 1998

NTSB/PAR-01/01

PB2001-916501 National Transportation Safety Board

Notation 7366 490 L’Enfant Plaza, S.W.

Adopted June 12, 2001 Washington, D.C. 20594
National Transportation Safety Board. 2001. Natural Gas Explosion and Fire in South Riding,

Virginia, July 7, 1998. Pipeline Accident Report NTSB/PAR-01/01. Washington, DC.

Abstract: About 12:25 a.m. on July 7, 1998, a natural gas explosion and fire destroyed a newly

constructed residence in the South Riding community in Loudoun County, Virginia. A family consisting of

a husband and wife and their two children were spending their first night in their new home at the time of

the explosion. As a result of the accident, the wife was killed, the husband was seriously injured, and the

two children received minor injuries. Five other homes and two vehicles were damaged.

The safety issues identified during this investigation were (1) the adequacy of standards for minimum

separation distances between gas service lines and electrical service lines and (2) the lack of a requirement

for the installation of excess flow valves. 

As a result of this investigation, the Safety Board made two recommendations to the Research and Special

Programs Administration (RSPA) and one recommendation each to the Edison Electric Institute, the

National Rural Electric Cooperative Association, the American Power Association, and the U.S.

Department of Agriculture’s Rural Utilities Service.

The National Transportation Safety Board is an independent Federal agency dedicated to promoting aviation, railroad, highway, marine,

pipeline, and hazardous materials safety. Established in 1967, the agency is mandated by Congress through the Independent Safety Board

Act of 1974 to investigate transportation accidents, determine the probable causes of the accidents, issue safety recommendations, study

transportation safety issues, and evaluate the safety effectiveness of government agencies involved in transportation. The Safety Board

makes public its actions and decisions through accident reports, safety studies, special investigation reports, safety recommendations, and

statistical reviews.

Recent publications are available in their entirety on the Web at <http://www.ntsb.gov>. Other information about available publications also

may be obtained from the Web site or by contacting:

National Transportation Safety Board

Public Inquiries Section, RE-51

490 L’Enfant Plaza, S.W.

Washington, D.C. 20594

(800) 877-6799 or (202) 314-6551

Safety Board publications may be purchased, by individual copy or by subscription, from the National Technical Information Service. To

purchase this publication, order report number PB2001-916501 from:

National Technical Information Service

5285 Port Royal Road

Springfield, Virginia 22161

(800) 553-6847 or (703) 605-6000

The Independent Safety Board Act, as codified at 49 U.S.C. Section 1154(b), precludes the admission into evidence or use of Board reports

related to an incident or accident in a civil action for damages resulting from a matter mentioned in the report. 
iii Pipeline Accident Report

Contents

Executive Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv

Factual Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Accident Synopsis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

The Accident . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

The Residence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5

Postaccident Inspection and Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Testing and Examination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Soil Sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

NOVEC Accident Report . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Pipeline and Electric Company Utility Separation Policies . . . . . . . . . . . . . . . . . . . . . . . . . 10

Regulatory Standards and Guidance Regarding Facility Separation . . . . . . . . . . . . . . . . . . 10

Gas Pipeline Separation Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Electrical Facility Installation Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Postaccident Actions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Pipeline Accidents Involving Electric Line Failures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

The Accident . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Electrical Service Line Damage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Soil Permeation With Gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Adequacy of Standards for Minimum Utility Separation Distances . . . . . . . . . . . . . . . . . . 19

Excess Flow Valves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Findings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Probable Cause . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
iv Pipeline Accident Report

Executive Summary

About 12:25 a.m. on July 7, 1998, a natural gas explosion and fire destroyed a

newly constructed residence in the South Riding community in Loudoun County, Virginia.

A family consisting of a husband and wife and their two children were spending their first

night in their new home at the time of the explosion. As a result of the accident, the wife

was killed, the husband was seriously injured, and the two children received minor

injuries. Five other homes and two vehicles were damaged.

The National Transportation Safety Board determines that the probable cause of

the accident in South Riding, Virginia, was the corrosion and subsequent overheating and

arcing at a splice in one of the conductors of the triplex electrical service line, which,

because of inadequate separation between the electrical conductors and the gas service

line, led to the failure of the gas service line and the subsequent uncontrolled release of

natural gas that accumulated in the basement and was subsequently ignited. Precipitating

the electrical service line failure was damage done to the electrical service line during

installation of the gas service line and/or during subsequent excavation of the electrical

line.

The safety issues identified during this investigation were (1) the adequacy of

standards for minimum separation distances between gas service lines and electrical

service lines and (2) the lack of a requirement for the installation of excess flow valves. As

a result of this investigation, the Safety Board issues two recommendations to the

Research and Special Programs Administration (RSPA) and one recommendation each to

the Edison Electric Institute, the National Rural Electric Cooperative Association, the

American Power Association, and the U.S. Department of Agriculture’s Rural Utilities

Service.
1 Pipeline Accident Report

Factual Information

Accident Synopsis

About 12:25 a.m. on July 7, 1998, a natural gas explosion and fire destroyed a

newly constructed residence in the South Riding community in Loudoun County, Virginia.

A family consisting of a husband and wife and their two children were spending their first

night in their new home at the time of the explosion. As a result of the accident, the wife

was killed, the husband was seriously injured, and the two children received minor

injuries. Five other homes and two vehicles were damaged.

The Accident

At the time of the accident, South Riding, Virginia, was a growing planned

community in Loudoun County, Virginia, near Washington, D.C., and new homes were in

various stages of construction. On April 22, 1998, a Northern Virginia Electric

Cooperative (NOVEC) crew dug a trench and installed electrical service to a new home on

Rickmansworth Lane, the home that would later be involved in the accident. Underground

three-wire electrical service triplex1

 had previously been “stubbed in,” that is, installed

(buried in a trench) to within a short distance of the home site so that the service line to the

house could be spliced onto it when the structure was ready for electrical service. The

NOVEC crew spliced onto this stubbed-in triplex an additional 23-foot section of triplex

that they then attached to the house’s electric meter. (See figure 1.)

During the first week in May, a Washington Gas Light Company contractor,

Northern Pipeline Company, installed a 3/4-inch-diameter polyethylene gas service line in

a trench to the house. The one-call system was used before the excavation, and the

location of the electrical lines had been marked. The contractor foreman said that during

excavation for the gas service line, a portion of the electric service line to the house was

exposed. He stated that he understood that gas pipelines needed to be separated at least

12 inches from electrical service lines, so at the point where the electrical line had been

exposed, he moved the gas service line to maintain 12 inches of horizontal separation.

Bluestone rock dust was used as a backfill material around the gas pipelines and

electric service installed in trenches and buried within the South Riding neighborhood to

protect the utilities from damage from rocks and other materials in the local soil. Native

soil was then placed atop the imported backfill to bring the surface back to its original

grade.

1 The electrical service triplex consisted of three twined cables: two 4/0 American Wire Gauge (AWG)

current-carrying conductors (each approximately 5/8 inch in diameter) and one 2/0 AWG neutral. All three

cables were constructed of compressed, 19-strand aluminum wire covered with high-density polyethylene

insulation.
Factual Information 2 Pipeline Accident Report

Figure 1. Accident area.

The gas meter was installed on June 9, 1998, and the gas piping was inspected,

pressure-tested, and approved by the Loudoun County Department of Building and

Development by June 11. The gas meter reading at the time of installation was 1645.2

Washington Gas Light Company had verified the accuracy of the gas meter on January 14,

1997.3

 An excess flow valve (EFV)4 was not installed in the gas service line supplying the

residence.

On June 15, about 6 weeks after the gas service line was installed, a contractor

working inside the house found that the house was not receiving full electrical service. He

2 Gas meter readings are in hundreds of cubic feet. Meters are not reset when installed; the initial

reading is used as a baseline for the subsequent monitoring of gas usage.

3 On August 12, 1998, Safety Board investigators witnessed Washington Gas Light Company

representatives verifying the accuracy of the meter that had been removed from the residence after the

explosion.

4 EFVs are designed to automatically close and stop the flow of natural gas through a service line if the

flow exceeds a specified rate.

Chorley Wood Street

Rickmansworth Lane

2-inch Gas Mains

25901

25905

3⁄4-inch Gas Service Line

Electric Service Triplex

Leak Site

Not to Scale

N

Approximate Location Where Neighbor

Initially Detected Natural Gas Odor 

Sewer and Water
Factual Information 3 Pipeline Accident Report

found a NOVEC supervisor working nearby and told him of the problem. The supervisor

arranged for a NOVEC crew to go to the site when their current job was completed. Near

the end of the day, according to the foreman of the NOVEC crew, the crew came to the site

and exposed the electrical service line by hand-digging. He said they were unable to locate

the fault before the end of their shift and, because the house was unoccupied, they left the

repairs for the next day.

The next morning, June 16, another NOVEC crew was sent to make the permanent

repairs. The one-call system was not used, but the crew said they recalled seeing utility

markings on the ground near the site of the excavation. After excavating the site with

mechanized equipment, the crew located a fault near a point at which the electrical service

triplex crossed underneath the gas service line. This was also the area along the triplex

where, before the gas pipeline was installed, the electrical service to the residence had

been spliced to the preexisting triplex stub. The crew said they observed that the gas

service line crossed about 6 inches above the electrical service line. They stated that they

found that one conductor of the triplex had been damaged. They said they repaired the

fault by cutting out the damaged section, which included the original splice connection,

and splicing in a short section of new cable. They said they also examined the gas service

line to ensure that their repairs had not damaged it. After their repair, the crew stated, they

maintained or increased the 6-inch separation between the electric line and the gas service

line while they backfilled the area.

The house was purchased on June 23, 1998. Construction of the house had just

been completed, and a walk-through inspection was conducted a few days before

settlement. The inspection noted no problems with any of the appliances. Shortly after

settlement, the gas meter for the property was read so that the service could be transferred

to the new owners. At the time of the transfer, on June 24, 1998, the gas meter reading was

1,654.

After purchase, the family began moving furniture into the residence. The husband

stated that they experienced no problems with any of the appliances after they took

possession and that they did not move or modify the gas appliances, nor had they installed

new ones. The night of July 6, 1998, was to be the first night the family was to spend

together in their new home. The husband said that on the evening of July 6, the air

conditioner was working and the house was cool. He also stated that he had not smelled

gas. He said the family retired between 10:00 and 10:30 p.m. Because not all the furniture

had arrived, the parents were sleeping in the study on the main level of the three-level

house; the children were sleeping on the upper level.

Shortly before the accident on July 7, a South Riding resident standing near his

home at the corner of Chorley Wood Street and Rickmansworth Lane, about 150 feet from

the accident house, noticed what he described as a “strong” odor of natural gas. He said

that after attempting unsuccessfully to identify the source of the odor, he called

Washington Gas Light Company at about 12:19 a.m. to report it. A few minutes later, the

house at Rickmansworth Lane exploded and was engulfed in flames. (See figure 2.)
Factual Information 4 Pipeline Accident Report

Figure 2. Postaccident debris.

The children were thrown out of the house and onto the lawn, suffering minor

injuries. The husband and wife fell into the basement as the first floor collapsed. The

husband was able to crawl to safety, but the wife did not escape. The husband was burned

severely; the wife died as a result of her injuries. The adjacent, unoccupied house was

damaged by the accident and had to be demolished. Four other houses and two vehicles

were also damaged.

As a result of the explosion, the piping inside the house broke, and natural gas that

was being metered escaped through the broken piping. Firefighters partially shut the valve

to the meter upon their arrival. Natural gas continued to escape until shortly after the first

Washington Gas Light Company personnel arrived at the scene shortly after 1:00 a.m. and

closed the valve that stopped the flow of natural gas through the meter. At that time, the

gas meter reading was 1,665. About 1:20 a.m., the fire was extinguished.

The Loudoun County Fire Marshal’s Office inspected the destroyed house and

concluded that the explosion resulted when natural gas from a hole in the gas service

pipeline to the house entered and accumulated in the basement of the residence, where it

was ignited by the pilot light on the water heater.

Remains of Accident Residence Remains of Adjacent House
Factual Information 5 Pipeline Accident Report

The Residence

The house was a three-level detached home consisting of an unfinished walk-out

concrete basement with wood frame construction above. The basement was “L” shaped:

the back portion of the basement, or bottom of the “L,” was 41.7 feet long by 13.8 feet

wide, and the front portion, or top of the “L,” was 22.2 by 22.3 feet. The height of the

basement was 8.75 feet from the concrete slab floor to the subfloor of the main level. The

calculated volume of the basement was about 9,370 cubic feet.

Three gas appliances were located in the residence: a water heater, a furnace, and a

gas-burning fireplace. The water heater and furnace were installed in the basement. The

water heater was rated for a maximum gas load of 40,000 Btu per hour (about 39 cubic

feet per hour of gas).5 A technical representative of the water heater manufacturer stated

that, provided no hot water was used and assuming a basement temperature of 80° F, the

water heater would operate about four times per day for a 15- to 25-minute heating cycle

to maintain the preset water temperature of 120° F. Hot water use or a cool basement

would increase the frequency of operation. The representative also stated that the water

heater would use between 10,000 and 20,000 Btu (10 to 19 cubic feet of gas) in each

heating cycle. In addition, the pilot light at the base of the water heater consumes 400 Btu

(0.4 cubic feet of gas) per hour. The furnace and the fireplace may have been run

occasionally during the approximate 12 1/2 days of occupancy. All other appliances in the

house were electric.

Around the foundation of the house was an underground external drainage system.

This system, consisting of a 4-inch-diameter perforated plastic pipe surrounded by gravel,

was designed to collect water and direct it away from the foundation and out to the rear of

the house. Another drainage system was located under the concrete basement floor. This

system, also consisting of perforated pipe surrounded by gravel, was designed to direct

any water accumulating under the foundation into the sump pit6

 in the floor of the

basement. An automatic sump pump in the sump pit discharged the water toward the

exterior of the house. “Weep holes” approximately 1 1/2 to 2 inches in diameter had been

placed at intervals in the concrete footer around the perimeter of the foundation to allow

water to flow between the drainage systems.

Water and sewer lines passed under the foundation of the house from a point just

outside the northeast wall, near the north corner of the house. Both lines entered the

basement through the concrete floor.

5 British thermal unit (Btu)– The quantity of thermal energy required to raise the temperature of

1 pound of water 1 degree Fahrenheit at or near 39.2° F. One cubic foot of the natural gas supplied by

Washington Gas at the time of the accident would generate approximately 1,033 Btu of heat.

6 A sump pit is an opening in the floor that is designed to accumulate water for the sump pump. The

sump pit for this residence was in the east corner of the back portion of the basement.
Factual Information 6 Pipeline Accident Report

Postaccident Inspection and Testing

Testing and Examination

As part of postaccident testing to locate the source of the accumulated natural gas,

the gas pipelines were pressure-tested to 31 pounds per square inch, gauge (psig).7

 The test

revealed a leak in the 3/4-inch-diameter polyethylene gas service line leading to the house.

The leakage flow rate was measured and calculated to be about 6,500 cubic feet per hour.

Excavation of the pipeline revealed a TV cable that had been installed at a depth of

about 13 inches from the surface sometime after the electrical service line and gas pipeline

had been installed. The pipeline was found at a depth of about 45 inches, and just

underneath the pipeline was a portion of the electrical service to the house.

Examination of the gas service line revealed a roughly circular 0.38- to 0.44-inchdiameter hole in the pipeline about 7 feet from the north corner of the house. An insulated

copper tracer wire ran alongside the gas service line,8 and the electrical triplex crossed

beneath it. One of the three cables of the triplex was found to be in contact with the gas

service line. Because the electrical line was disturbed during the postaccident excavation,

the exact preaccident location of the cables in relation to the gas service line and to the

hole in the gas service line could not be determined. When excavated, one of the two

conductors (current-carrying cables) (designated “cable A” for identification) was found

touching the gas service line about 3.75 inches away from the hole along the horizontal

axis of the pipe. At this location, the other conductor (designated “cable B”)9

 was

underneath and touching cable A. The neutral cable was several inches away from the

other two. (See figure 3.)

Examination at the Safety Board laboratory revealed that the polyethylene pipe

material surrounding the hole in the gas service line was blackened and appeared thinner

and cratered, consistent with heat damage.10 Also, the thinned material formed a conical

peak around the hole, consistent with the effect of pressure on a heated and softened

pipeline wall. (See figure 4.)

7 A gas pressure of 31.5 psig for the leaking pipeline was recorded at 2:55 a.m. at the Washington Gas

Chantilly Gate Station that supplied natural gas into the distribution system serving South Riding.

8 The tracer wire is designed to carry an electrical signal that can be read by above-ground equipment

and is used to locate the polyethylene pipeline. The signal is only applied to the wire when the pipeline

location is being determined.

9 Note for reference purposes: In the Safety Board Metallurgical Laboratory factual report that is part of

the public docket for this accident (Docket DCA-98-MP-003), the conductors designated “cable A” and

“cable B” in this report are referred to as “Hot Lead 1” and “Hot Lead 2,” respectively.

10 The Vicat softening temperature for the material used in gas pipelines is 247° F (119° C). (The Vicat

softening temperature is intended to provide a standard for comparing the softening effects of temperature

on various thermoplastic resins. It is defined in American Society for Testing Materials standard D1525 as

the temperature at which a flat-ended needle of 1-mm2 circular cross section will penetrate a thermoplastic

specimen to a depth of 1 mm under a specified load using a selected rate of temperature rise.) According to

the manufacturer of the material used to construct the gas pipeline in this accident, the material becomes

sufficiently molten to fuse with itself at 375 to 400° F (190 to 204° C).
Factual Information 7 Pipeline Accident Report

Figure 3. Cables and pipeline before removal. Although the TV cable appears to lie near

the pipeline in this view, it was found at a depth of about 13 inches and was thus about

32 inches above the pipeline.

Figure 4. Hole discovered after accident in the 3/4-inch polyethylene natural gas pipeline

leading to the residence.

Neutral

Cable A

TV Cable

Hole

Hole Reflected

in Mirror

Gas Service Pipeline

Cable B
Factual Information 8 Pipeline Accident Report

The insulation on the section of tracer wire adjacent to the hole was completely

missing, and the insulation on nearby portions of the tracer wire was blackened or

otherwise discolored and showed evidence of melting. About 7 inches from the area of

missing insulation, a 2.5-inch section of tracer wire was missing insulation along one side.

The copper tracer wire in this area had separated and showed evidence of localized

damage and thinning consistent with some type of electrochemical erosion or metal loss.

Nicks and cuts typical of mechanical damage were also observed in the area of the

damaged insulation.

Laboratory examination of the triplex’s three electrical cables revealed evidence of

prior repairs, corrosion, and heat and mechanical damage. As noted previously, the service

triplex to the house had been spliced to the triplex stub at this location. Two of the three

cables still had the original in-line aluminum compression crimp connectors installed near

the damaged gas pipe. The original crimp connector in the third cable, one of the

conductors (cable A), had been removed when the conductor was repaired by installing a

short piece of new cable with two in-line crimp connectors. Both conductor cables were

damaged and parted at or near one of the crimp connectors.

Cable B was completely separated. Between the two separated ends of the cable

lay a clod of dirt. Portions of the interior of the clod showed a glassy appearance

consistent with exposure to high heat. One end of the parted cable was attached to what

remained of a crimp connector. Almost a third of the 4-inch-long crimp connector was

missing. (See figure 5.)

Figure 5. Top: An aluminum crimp connector that was attached to one of the burned

electrical cables found in proximity to the damaged natural gas pipeline. Bottom: An

exemplar crimp connector (without its cold-shrink insulation).
Factual Information 9 Pipeline Accident Report

Examination of the separated end of the connector revealed that approximately

60 percent of the surface was melted and had a glassy appearance, consistent with

exposure of the connector to high temperature.11 Removal of a small area of the melted

and glassy material revealed white aluminum hydroxide deposits underneath. The

aluminum hydroxide deposits were also observed on the remaining 40 percent of the

damaged surface and on portions of the exterior surface of the connector. Aluminum

hydroxide is a corrosion product that forms on aluminum that is exposed to moisture in a

low-oxygen environment. Cuts and tears of various sizes were found in the cold-shrink

insulation installed over the crimp connector immediately adjacent to the separation.

Examination of cable A revealed that a 1.3-inch section of the spliced-in repair

cable was missing near one crimp connector. The connector still held almost 2 inches of

the repair cable’s end. The damaged ends of the repair cable near the missing section

appeared to be undefined masses of solidified aluminum and debris, indicating that the

cable had melted and resolidified. A small portion of cable A’s insulation remained in

place between the two melted ends of the aluminum cable. The interior surface of this

section of insulation bore the imprint of the missing section of cable.

The neutral cable was intact. The insulation that had been placed over the crimp

connector after a splice showed cuts and holes. Aside from the damaged electrical

conductors, no potential sources of heat were discovered in the area around the pipeline

failure.

Soil Sampling

On the day of the accident, soil samples were collected at four locations near the

destroyed residence, including one sample from beneath the concrete basement slab near

the sump pit. Testing revealed methane and a smaller concentration of ethane in each of

the soil samples.

NOVEC Accident Report

NOVEC provided the Safety Board with its own accident report, which concluded

that the hole in the gas pipeline occurred after and as an indirect result of the explosion.

The report concluded that the gas that had accumulated in the residence and exploded

came from some unknown source inside the house, possibly “through a defect or damage

to the internal gas piping system, connection fittings, or the natural gas appliances.”

NOVEC also examined the meter base and the associated service cables from the

destroyed residence. The company reported evidence of pitting on the cables that

company representatives said they believe indicates that arcing or shorting occurred

between the cables within the meter base after the explosion.

11 The melting range is 646 to 657° C (1195 to 1215° F) for the 1350 aluminum alloy used in the cable.
Factual Information 10 Pipeline Accident Report

Pipeline and Electric Company Utility Separation Policies

Washington Gas Light Company internal procedures required that steel

transmission12 pipelines be installed at least 12 inches from other underground metallic

objects. This procedure required all other steel pipelines (those not considered

transmission pipelines) to be installed at least 6 inches away from other metallic objects.

The company had no separation requirements for polyethylene pipelines; however,

company officials stated that the company’s routine practice was to apply the requirements

for steel pipelines to polyethylene pipelines.

NOVEC trench specifications required various separation distances between the

company’s own cables and conduits. The company did not have a written procedure

requiring a specific separation between NOVEC’s buried secondary electric lines (those

serving a single residence) and other underground facilities, such as gas pipelines.

NOVEC’s procedures did require that the National Electrical Safety Code be followed.13

Several NOVEC personnel stated that, as a general practice, they maintained at least

6 inches of clearance between electrical lines and all other underground facilities.

Regulatory Standards and Guidance Regarding Facility 

Separation

Gas Pipeline Separation Requirements

The U.S. Department of Transportation’s Research and Special Programs

Administration (RSPA), through the Office of Pipeline Safety (OPS), promulgates

regulations that establish minimum safety standards for the transportation of natural gas

by pipeline. The regulations concerning underground clearance requirements for gas

transmission lines and mains are in 49 Code of Federal Regulations (CFR)

Section 192.325 and can be summarized as follows:

• Each transmission line must be installed with at least 12 inches of clearance

from other underground structures. If this clearance cannot be attained, then

the transmission line must be further protected from damage.

• Each main must be installed with sufficient clearance from other underground

structures to allow for proper maintenance and to protect it from damage.

12 In simplified terms, transmission pipelines provide large volumes of product to utility companies or

other distributors; smaller main lines then carry the product from the utility company or distributor to

different locations within the service areas where service pipelines (such as the one involved in this

accident) branch off to serve individual customers.

13 See “Electrical Facility Installation Standards” section below for more information on National

Electrical Safety Code requirements. The National Electrical Safety Code is an American National

Standards Institute code that was developed by the Institute of Electrical and Electronics Engineers.
Factual Information 11 Pipeline Accident Report

• Each plastic transmission line or main must be installed with sufficient

clearance, or must be insulated, from any source of heat so as to prevent heat

damage to the pipe.

These regulations are only applicable to gas transmission lines and mains. The

separation requirements in 49 CFR Part 192 do not apply to residential gas service lines.

For intrastate pipeline facilities within Virginia, the Virginia State Corporation

Commission has adopted and enforces the OPS Federal regulations as its minimum

pipeline safety standards. The commission also has the authority to adopt more stringent

regulations. Section 56-257 of the Code of Virginia was amended in 1996 to specifically

authorize the commission to establish minimum separation requirements, not to exceed

12 inches, between crossings of underground pipes or conduits; however, the commission

had not established any such minimum. The 1996 amendment also required that utility

pipes and conduits be installed “with sufficient clearance, or shall be insulated from any

source of heat, so as to prevent the heat from impairing the serviceability of the pipe or

conduit.” This provision applies to facilities laid in public streets and right-of-ways, not to

those located on private property.

The Gas Piping Technology Committee publishes the GPTC Guide for Gas

Transmission and Distribution Piping Systems to provide guidance for natural gas

operators to ensure regulatory compliance.14 The guide does not specifically address gas

service lines but does suggest that:

sufficient clearance should be maintained between mains and other underground

structures to provide heat damage protection from other underground facilities

such as steam or electric power lines, particularly where plastic piping is installed

in common trenches with sources of heat.

Electrical Facility Installation Standards

No Federal agency analogous to the OPS is tasked specifically with ensuring the

safety of electrical distribution systems. NOVEC, as a rural electric cooperative, falls

under the purview of the U.S. Department of Agriculture’s Rural Utilities Service. The

Rural Utilities Service does not conduct routine safety inspections of the facilities of

participating utilities. The Rural Utilities Service does require rural cooperatives that

participate in the agency’s loan program to certify that their facilities are installed in

accordance with the provisions of the National Electrical Safety Code.

Section 1, Paragraph 010, of the National Electrical Safety Code states that its

purpose is the “practical safeguarding of persons during the installation, operation, or

maintenance of electric supply and communication lines and associated equipment.” It

further states in Paragraph 012.A. that “all electric supply and communication lines and

equipment shall be designed, constructed, operated, and maintained to meet the

requirements of these rules.”

14 The Gas Piping Technology Committee is an American National Standards Institute technical

committee made up of technical specialists from both industry and government.
Factual Information 12 Pipeline Accident Report

Section 35, Paragraph 352.B.4. of the National Electrical Safety Code calls for

direct-buried electrical cables to be installed and maintained with adequate vertical

separation when crossing other underground structures “to permit access to and

maintenance of either facility without damage to the other.” This paragraph further states

that “a vertical separation of 12 inches is, in general, considered adequate, but the parties

involved may agree to a lesser separation.” The Safety Board was not able to identify any

written or oral agreement between Washington Gas Light Company and NOVEC that

governed the separation of the two companies’ lines.

Electrical distribution facilities are also subject to the Virginia State Corporation

Commission’s regulatory oversight. The commission had not adopted the National

Electrical Safety Code or developed a comprehensive set of minimum safety standards

applicable to electrical facilities.

Postaccident Actions

Washington Gas Light Company has revised its Operating Instructions Manual to

include requirements for maintaining at least 12 inches of clearance between new

polyethylene gas pipelines and existing electrical facilities. Allowances are made for

lesser separation, given specific protections. The revision also requires that existing

polyethylene gas pipelines be separated from electrical facilities as necessary whenever

those pipelines are excavated for maintenance or other reasons. Since the accident,

Washington Gas Light Company has also established a policy to voluntarily install EFVs

in all new or renewed residential service lines.

OPS representatives have met with various industry and governmental

organizations to discuss the issue of separation between underground gas pipelines and

electrical lines. These OPS representatives also notified the Safety Board staff that the

OPS plans to include the issue in the “Best Practices” initiative focusing on excavation

damage prevention.

After the accident, several utility operators, including NOVEC and Washington

Gas Light Company, formed the Utility Industry Coalition of Virginia. The coalition has

developed preliminary voluntary standards that call for 12 inches of separation between

utilities, unless an acceptable barrier is provided. NOVEC revised its policy to follow

these voluntary standards.

The Virginia General Assembly has modified Virginia law to require every utility

operator in the State that is not subject to U.S. Department of Transportation oversight to

install its utility lines “in accordance with accepted industry standards.” The new statute

also requires that the State Corporation Commission “promulgate any rules or regulations

necessary to enforce the provisions of this section as to those operators that do not comply

with such accepted industry standards.” The new laws become effective July 1, 2001.
Factual Information 13 Pipeline Accident Report

Pipeline Accidents Involving Electric Line Failures

The Safety Board is aware of two accidents in which electric line failures

reportedly generated heat that damaged polyethylene natural gas pipelines. The Georgia

Public Service Commission investigated an April 9, 1998, natural gas leak and fire at the

Oaks Shopping Center in Dublin, Georgia.15 Just before the accident, the gas department

was excavating a leaking pipeline. During the excavation, the gas ignited and a gas

department employee received first- and second-degree burns to his hands and face. The

Public Service Commission found that an electric cable serving a nearby house crossed

underneath the gas pipeline. The electric cable had a splice connector “which appeared to

have failed and melted the 2-inch PE [polyethylene] gas main that caused the leak.”

The second accident was investigated by the Illinois Commerce Commission.16 On

February 3, 1999, flames coming from the ground near two gas meters spread to a

multifamily residential building in Hanover Park, Illinois. The fire caused in excess of

$250,000 damage to the house and its contents. There were no injuries. The Illinois

Commerce Commission found that the day before the accident, a fault had been detected

in one of the electric cables that ran near the pipeline. The electric cable was marked but

not repaired that day; instead, electric service was restored to the affected house using

“jumpers” that bypassed the fault and allowed the customer to have 120-volt electric

service until the faulted cable could be repaired. Postaccident excavation of the pipeline

revealed that a section of electrical cable had melted near the hole in the polyethylene gas

pipeline. The Illinois Commerce Commission concluded that the electric cable was buried

very close to the pipeline, and it appeared “that the heat generated by the fault in the cable

melted the PE [polyethylene] service piping.”

15 Georgia Public Service Commission, Report of Natural Gas Safety Inspection No. BM98-023, City of

Dublin, Georgia, April 9, 1998.

16 Illinois Commerce Commission, Incident Report, Nicor Gas Company, Elgin, Illinois, February 3,

1999.
14 Pipeline Accident Report

Analysis

The Accident

The Loudoun County Fire Marshal’s Office inspected the destroyed house and

concluded that the explosion occurred when gas leaking from the gas service line

accumulated in the basement and was ignited by the pilot light on the water heater. Given

the extent and physical nature of the destruction, the Safety Board agrees that the

explosion occurred in the house basement and that the pilot light for the water heater, an

open flame, was a likely ignition source.

All parties to the investigation agreed that gas from some source had accumulated

inside the house. A question remained, however, about the source of the gas, with NOVEC

contending in its own report that the explosive gas accumulation resulted from a leak or

defect in the gas piping inside the house. Safety Board investigators assessed the

possibility that the accumulated gas that ignited resulted from a gas leak inside the house

rather than from the leak in the gas service pipeline that was found after the accident.

All physical evidence suggests that the gas that exploded accumulated in the

basement rather than in some other part of the house. And the only known, constantly

present ignition source was the water heater pilot light. Thus, for the explosion to have

occurred as the evidence suggests that it did, enough natural gas would have had to

accumulate inside the basement to bring the gas concentration there to at least 4.5 percent,

because the flammability limits of natural gas in air are approximately 4.5 to

14.5 percent.17 Because natural gas is lighter than air, the gas entering the basement may

have initially tended to accumulate near the ceiling before beginning to mix throughout

the basement. The pilot light for the water heater was near the floor. The volume of the

basement was calculated to be about 9,370 cubic feet.18 About 422 cubic feet of natural

gas would therefore have been needed to fill the basement from the ceiling down to the

level of the pilot light with a uniform concentration sufficient for ignition. According to

the surviving homeowner, the air conditioner was on during the evening of the explosion

and was circulating air throughout the house. This may have increased the air exchanges

within the basement and thus the volume of gas needed to fill the basement to explosive

levels.

17 The flammability limits refer to the range of concentrations of natural gas in air that can be ignited.

The limits of this range are referred to as the lower and upper explosive limits.

18 Although investigators did not attempt to determine what furniture or other household items may

have been in the basement at the time, such items would not, in the Safety Board’s view, have significantly

reduced the overall volume of the basement susceptible to gas permeation.
Analysis 15 Pipeline Accident Report

But 422 cubic feet of natural gas would have only brought the gas concentration to

the lowest explosive limit4.5 percent. According to the National Fire Protection

Association,19 “mixtures at or near the lower explosive limit (LEL) or upper explosive

limit (UEL) of a gas or vapor produce less violent explosions than those near the optimum

concentration.”20 The amount of damage inflicted on the residence suggests that the

concentration was probably higher than the lower limit. While it is impossible to

determine the exact gas concentration at the time of this accident, the violence of the

explosion indicates that the accumulation of gas in the basement of the structure was

likely greater than the 422 cubic feet that would have been required to produce the lowest

explosive level.

Gas meter readings before and after the explosion indicated that a total of

1,100 cubic feet of gas had been supplied to the house during the 12 1/2 days leading up to

the explosion. During that period, the water heater was the only continuously operating

gas appliance in the house. Information provided by the water heater manufacturer

indicated that, with no hot water usage, the water heater would have consumed about

700 cubic feet of gas in the 12 1/2 days before the explosion.21 Because the house was

occupied intermittently during this time and some hot water was used, the amount of gas

consumed by the water heater was probably greater than 700 cubic feet.

Subtracting the gas used by the water heater from the total 1,100 cubic feet of gas

supplied to the house leaves less than 400 cubic feet of metered gas not attributable to the

water heater. An unknown amount of gas escaped through the meter during the 40 minutes

or more that elapsed between the time of the explosion and the time the valve to the meter

was completely shut off. Also, some gas could have been used to heat water during the

family’s periods of temporary occupancy in the 12 1/2 days before the accident or in

testing the other gas-burning appliances as the family was taking residence. Even if the

entire 400 cubic feet of gas had leaked inside the house, this would have been less than

that necessary to reach even the lower explosive limit in the basement at the level of the

water heater’s pilot light. And as noted above, the explosive mixture was likely greater

than the lower limit. The measured gas usage thus suggests that the source of the

accumulated gas lay outside the house.

Furthermore, a resident more than 150 feet away from the accident residence

reported smelling a “strong” odor of natural gas in the 1/2 hour before the explosion. An

outdoor pipeline leak would be the most likely source of a gas odor that could be detected

at that distance. Moreover, the metered amount of gas into the house would fall even

19 National Fire Protection Association. 2001. NFPA 921, Guide for Fire and Explosion Investigations,

2001 Edition, Quincy, Massachusetts, p. 133.

20 For natural gas, the optimum concentration occurs near the middle portion of the range between LEL

and UEL.

21 Calculations use an average heating cycle consuming 12,000 Btu operating 4 times a day and the gas

usage of the pilot light at 400 Btu per hour. This is approximately comparable to the average gas usage

measured while the home was unoccupied between when the gas meter was originally installed and the gas

service transferred to the new homeowners.
Analysis 16 Pipeline Accident Report

further short of the amount required to create an explosive mixture in the basement if

enough of it had leaked out of the house to cause an odor of gas 150 feet away.

Finally, an outside source for the gas was found in the leaking gas service pipeline.

Pressure testing of this line revealed a natural gas leak of about 6,500 cubic feet per hour.

This leak rate was sufficient to have supplied the quantity of gas that would have been

required to bring the gas concentration in the basement of the house to an explosive level.

The location of the hole in the gas service lineabout 7 feet from the house

foundationwould account for the reported strong smell of gas some distance away and

at the same time facilitate the underground migration of gas into the house. The trench in

which the gas service line was installed, which was backfilled with relatively porous

material, would have provided a path to the foundation of the house along which the gas

could migrate. Tests of four soil samples collected at different sites near the accident

residence showed the presence of methane and a smaller concentration of ethane,

indicating that natural gas had migrated and permeated the soil around and underneath the

house. (See “Soil Permeation With Gas” section below for more information.)

The Safety Board therefore concludes that, based on the size and location of the

hole in the gas service line, the amount of gas that would have been needed to cause the

accident, the presence of a strong smell of gas outside the house before the explosion, and

the presence of natural gas in the soil underneath the residence, the most likely source of

the natural gas that filled the residence basement and fueled the explosion was the

damaged gas service line leading to the house.

Electrical Service Line Damage

The two damaged electrical conductors that were found crossing under the gas

service line were melted and partially consumed. The damage was consistent with

exposure to temperatures in excess of the melting point of aluminum, or 1,195 to 1,215° F.

Because clay and rock are poor conductors of heat, the temperature in the earth

environment surrounding an arcing cable will drop significantly as the distance from the

arc increases. Thus, while the nearest conductor was found 3.75 inches from the hole

along the horizontal axis of the gas service line, the conductors were disturbed during the

excavation of the gas service line and were likely closer to the hole when they failed.

The insulation on the tracer wire and the cold-shrink insulation covering the crimp

connectors on both cable B and the neutral cable contained nicks, cuts, and tears. Since

NOVEC installers and service technicians are not likely to have initially installed torn or

defective cables or insulation, the nicks, cuts, and tears found in the cables and insulation

most likely resulted from post-installation mechanical damage that occurred during

excavation or backfilling. Although a television cable was installed above the electrical

cables and gas pipeline, this cable was buried only to a depth of about 13 inches.

Furthermore, the equipment used to install such cable is incapable of reaching a depth of

45 or more inches, which was the depth at which the pipeline and electrical cables were

found at the damage location. The Safety Board therefore concludes that work performed
Analysis 17 Pipeline Accident Report

either during installation of the gas service line or during subsequent excavation of the

electrical line, or both, damaged the protective insulation for the triplex electrical

conductors and splices in the area of the hole in the gas service line.

Removal of a small area of the melted and resolidified material from the heatdamaged section of cable B revealed aluminum hydroxide deposits indicative of

corrosion. The corrosion deposits were also observed on the unmelted 40 percent of the

damaged surface of cable B, as well as on portions of the exterior surface of the connector.

This corrosion likely resulted from moisture that reached the crimp connector of cable B

through the cuts and tears in the crimp connector insulation.

Because the cable ends inside the crimp connector do not meet, the full current

load of the conductor must be transferred through the crimp connector. Unimpeded current

flow thus depends on there being firm contact between the crimp connector and the cable

ends. Evidence indicates that in this accident, the corrosion that occurred on and/or within

one side of the splice connection increased the contact resistance between the cable end and

the crimp connector. Because the rate of corrosion of aluminum can be significantly

accelerated when the material is energized,22 the amount of corrosion likely increased

fairly rapidly, reducing the metal-to-metal contact. When the contact resistance increased

sufficiently, the connector was no longer capable of supporting the required current flow

without overheating and arcing. This arcing generated heat sufficient to destroy a portion

of the conductor and the connector and to soften and weaken the nearby gas pipeline.

Therefore, given the evidence of prior excavation damage to the insulation on cable B and

the evidence of corrosion on the sides of the crimp connector near the damage, the Safety

Board concludes that the damage to the protective insulation covering the splice in the

electrical conductor identified as cable B allowed moisture to reach the crimp connector,

causing corrosion that increased electrical resistance between the crimp connector and the

cable sufficient to cause the splice to overheat and fail.

As the conductor failed under an inductive load, the additional mass of aluminum

in the connector and the cable melted, vaporized, and burned. Given that no other heat

source was present near the hole in the gas service line and that the temperature necessary

to melt, vaporize, and burn the aluminum missing from the splice connection was far in

excess of the approximately 250° F that would soften and weaken the polyethylene

pipeline, the Safety Board concludes that heat generated from the arcing resulting from

cable B’s failure under load caused the gas service line wall to soften and weaken until the

internal pressure breached the pipeline and it began to leak.

Cable A showed evidence of melting and resolidification, but unlike cable B, cable

A did not show evidence of corrosion. Also unlike cable B, a small portion of cable A’s

insulation remained intact between the two melted ends of the cable, indicating that

cable A was probably not at elevated temperatures for as long as cable B. Finally, the

22 William H. French, “Alternating Current Corrosion of Aluminum,” Paper T 73 120-3, recommended

and approved by the Insulated Conductors Committee of the Institute of Electrical and Electronics Engineers

(IEEE) Power Engineering Society (PES) for presentation at the IEEE PES Winter Meeting, New York,

N.Y., January 28 through February 2, 1973.
Analysis 18 Pipeline Accident Report

evidence of potential electrical shorting within the meter base that was reported by

NOVEC suggests that at least one conductor into the house may have been energized at

the time of the explosion. Based on this evidence, the Safety Board concludes that the

failure of the conductor identified as cable A was a secondary event that may have

occurred after the explosion as a result of short circuiting within the damaged residence’s

electrical system.

Soil Permeation With Gas

Tests of four soil samples collected at different sites near the accident residence

showed the presence of methane and a smaller concentration of ethane. The presence of

ethane in the samples confirms that the combustible gas was natural gas, not another

naturally occurring source of methane, such as sewer or landfill gas. The presence of

natural gas in these samples also verifies that the natural gas had permeated the soil

around and underneath the house.

Bluestone rock dust was used as a backfill material around the gas pipelines and

electric cables installed in trenches and buried within the South Riding neighborhood.

Because this material is more porous than the surrounding soil that was placed back on top

of the imported backfill, it likely provided a path the escaped gas could follow from the

buried utilities to the foundation of the house.

Underground drainage systems had been installed to collect water and route it

away from the foundation of the house. Both drainage systems consisted of perforated

plastic pipe surrounded by gravel. Both the perforated drain pipe and the gravel are readily

permeated by gas. Weep holes had been installed in the concrete footer around the

perimeter of the foundation to allow water to flow between the two systems. Gas could

easily pass through these weep holes from the external drainage system to the internal

system under the concrete slab.

The basement of the house offered several avenues by which gas could have

entered and accumulated. (See figure 6.) For example, the sump pit penetrated the

concrete floor and provided a possible path. Additionally, the water service line passed

under the foundation of the house from a point just outside the northeast wall, near the

north corner of the house in proximity to the hole in the gas service line. This line entered

the basement through a sleeve penetrating the concrete floor. Also, since the concrete for

new foundations is poured during discrete construction steps, cracks may have been

present in the foundation where different pours adjoined. The Safety Board concludes that

the gas present in the soil around and under the house entered the basement of the

residence either through penetrations in the concrete slab or cracks in the foundation, or

both.
Analysis 19 Pipeline Accident Report

Figure 6. Paths of possible migration of gas into the basement of the residence.

Adequacy of Standards for Minimum Utility Separation Distances

Before the accident, Washington Gas Light Company specified separation

distances for steel pipelines but not for polyethylene pipelines. Company management

indicated that the company routinely applied the requirements for steel pipelines to

polyethylene pipelines, and the installation contractor’s foreman stated that he placed the

gas service line in a trench with 12 inches of horizontal separation from the electrical line.

Nonetheless, the NOVEC crew reported that they found the gas service line only 6 inches

above the electrical cables when they excavated to repair an electrical line fault after the

gas service line was installed and before the accident.

Since the accident, Washington Gas Light Company has revised its Operating

Instructions Manual to require at least 12 inches of clearance between new polyethylene

gas pipelines and existing electrical facilities. Allowances are made for lesser separation if

specific protections are implemented. The revision also requires that existing polyethylene

gas pipelines be separated from electrical facilities as necessary if a lack of separation is

discovered during maintenance activities.

25905 Rickmansworth Lane

Not to Scale

Sump

Water and Sewer Entrances

Leak

Drain Tiles

Gas and Electric Services

Footer with

Weepholes

Foundation Wall

Floor Slab

Drain Tiles 

Plan View

View from Back

Not to Scale

Gravel
Analysis 20 Pipeline Accident Report

Before the accident, NOVEC trench specifications required various separation

patterns between their own electrical facilities; however, the company did not have a

written procedure requiring a minimum separation between its buried secondary electric

lines and other underground facilities, such as gas pipelines. NOVEC’s procedures did

require NOVEC personnel to follow the National Electrical Safety Code, which called for

electrical cables to be installed and maintained with a vertical separation of 12 inches

when crossing other underground structures. But several NOVEC personnel stated that, as

a general practice, they maintained at least 6 inches of clearance between electrical cables

and all other underground facilities, which was not consistent with the code’s separation

requirements.

Postaccident excavation revealed that one of the failed electrical conductors may

have been touching the gas service line; in any case, the conductors were close enough to

the gas service line to damage it when a splice connection in one of the conductors faulted

under load and generated an arc. The Safety Board therefore concludes that had the gas

and electrical service lines involved in this accident been adequately separated, the heat

from the arcing electrical conductor failure would probably not have damaged the gas

service line, and the accident would not have occurred. Since the accident, NOVEC has

adopted voluntary standards through its participation in the Utility Industry Coalition of

Virginia that require a minimum separation distance of 12 inches from other underground

facilities unless an acceptable barrier is provided.

While the OPS has promulgated regulations that establish minimum underground

clearance requirements for gas transmission lines and mains, no similar regulation applies

to a residential gas service line such as the one involved in this accident. Since the

accident, OPS representatives have met with various industry and governmental

organizations to discuss the issue of separation between underground gas pipelines and

electrical lines. These representatives also reported to the Safety Board staff that the OPS

plans to include the issue in the OPS “Best Practices” initiative focusing on damage

prevention. RSPA has not, however, initiated any regulatory action to ensure that

residential gas service pipelines are separated from other underground structures.

Therefore, the Safety Board believes that RSPA should require gas utility operators to

maintain a specified minimum separation distance, sufficient to protect against both

thermal and mechanical damage, between plastic gas service lines and underground

electrical facilities whenever they install a new gas service line or perform maintenance on

existing lines.

Since the National Electrical Safety Code already addresses the separation issue

for electrical facilities, the Safety Board believes that electrical industry associations and

the U.S. Department of Agriculture’s Rural Utilities Service should inform their member

utilities of the circumstances of this accident and of the need to ensure that underground

electrical facilities are installed and maintained with separation between plastic gas

pipelines in accordance with the National Electrical Safety Code.
Analysis 21 Pipeline Accident Report

Excess Flow Valves

Excess flow valves (EFVs) are available that respond to an excessive flow of

gassuch as may occur as a result of a leak by automatically closing and restricting the

gas flow. EFVs can greatly reduce the consequences of service line ruptures. They are

typically installed at the connection of the service line to the main pipeline. Depending on

the manufacturer, EFVs compatible with the operating conditions in this accident are

available that are designed to close when the flow rate exceeds about 550 to 850 cubic feet

per hourabout 1/10 of the flow rate measured in the service pipeline after the South

Riding accident. Based upon the leakage flow rate measured after the explosion and

before the pipeline was excavated, the Safety Board concludes that, had an EFV been

installed in the gas line to the residence, the EFV would have closed after the hole in the

pipeline developed, and the explosion likely would not have occurred.

In the early 1980s, the Safety Board advocated using EFVs on service lines to

schools and other buildings in which large numbers of people gather. As EFVs became

cheaper and more widely available, the Safety Board began advocating the installation of

EFVs on new or renewed residential service lines. During the 1980s, RSPA, which has

oversight responsibilities for the pipeline industry, did not require EFVs. Consequently,

the Safety Board included the use of EFVs on its 1990 list of Most Wanted safety

improvements. 23

On September 26, 1990, as a result of its investigation of five natural gas accidents

in the Kansas City-Topeka area, the Safety Board recommended that RSPA:

P-90-12

Require the installation of excess flow valves on new and renewed singlefamily, residential high pressure service lines which have operating

conditions compatible with the rated performance parameters of at least

one model of commercially available excess flow valve.

On April 4, 1995, RSPA notified Congress by letter that it had decided not to

require universal installation of EFVs and instead would issue performance standards and

customer-notification requirements for EFVs. In a September 28, 1995, letter to RSPA, the

Safety Board expressed its disappointment with this decision. The Board noted the

continued strong evidence that a way was needed to quickly restrict the flow of gas to a

failed pipe segment. On September 28, 1995, as a result of RSPA’s failure to issue EFV

requirements, the Safety Board classified Safety Recommendation P-90-12 “Closed—

Unacceptable Action.”

On March 6, 1996, as a result of its investigation of a June 9, 1994, natural gas

explosion in Allentown, Pennsylvania, the Safety Board wrote to the governors of all

23 In October 1990, the Safety Board developed the “Most Wanted” list, drawn up from previously

issued safety recommendations, to bring special emphasis to the safety issues the Board deems most critical.

The Most Wanted list is reviewed, revised, and reissued annually. The Most Wanted list is available on the

Web at <http://www.ntsb.gov/Recs/MostWant.htm>.
Analysis 22 Pipeline Accident Report

50 states and the District of Columbia asking that they require gas distribution operators to

install EFVs in all new or replaced gas service lines when operating conditions are

compatible with commercially available valves (Safety Recommendation P-96-3). Of the

States that replied, most advised that they intended to follow the lead of RSPA and had no

plans to require the installation of EFVs. The State of Virginia did not initially respond to

Safety Recommendation P-96-3.

Also on March 6, 1996, the Safety Board recommended that RSPA:

P-96-2

Require gas distribution operators to notify all customers of the availability

of excess flow valves; any customer to be served by a new or renewed

service line with operating parameters that are compatible with any

commercially available excess flow valve should be notified; an operator

should not refuse to notify a customer because of the customer’s

classification or the diameter or operating pressure of the service line.

On February 3, 1998, RSPA issued its final rule regarding EFVs. The rule requires

gas distribution operators either to install EFVs on new or replaced single-residence

service lines expected to operate continuously at not less than 10 psig or to inform

customers of the availability and benefits of EFVs and install them if the customer agrees

to pay for their installation and maintenance.

On October 6, 1998, the Safety Board classified Safety Recommendation P-96-2

“Closed—Unacceptable Action,” in part because RSPA’s final rule limits required

notifications by gas operators to residential customers even though many commercial

service lines have operating characteristics compatible with the same EFVs used for

residential service lines.

Because the Safety Board had no reasonable expectation that further action on

EFVs was likely by either RSPA or the States, the Safety Board, on May 3, 2000, removed

the recommendations regarding EFVs from its Most Wanted list. After the South Riding

accident, on August 3, 2000, the Safety Board wrote the Governor of Virginia asking for

information about Virginia’s intentions with respect to Safety Recommendation P-96-3. In

an August 16, 2000, response, the Virginia Corporation Commission noted that the State

had adopted and will enforce RSPA’s final rule. The commission further noted that it had

not identified any noncompliance relative to the EFV provisions. It also noted that many

Virginia gas operators install EFVs for existing customers upon request, provided the

customer pays for the installation, and that all Virginia operators were now installing

EFVs on all new and replaced service lines when operating conditions are compatible with

commercially available EFVs. As a result, on October 3, 2000, the Safety Board classified

Safety Recommendation P-96-3 to the State of Virginia “Closed—Acceptable Alternate

Action.”24

24 Safety Recommendation P-96-3 was classified “Closed” to 30 States. In a July 5, 2000, letter, the

Safety Board asked for updates from the States for whom the recommendation remained in an “Open”

status.
Analysis 23 Pipeline Accident Report

According to an American Gas Association survey provided to the Safety Board in

the spring of 2000, since the issuance of the RSPA final rule on EFVs, approximately onehalf of the operators of gas distribution systems have elected to install EFVs, and one-half

have developed procedures to inform customers of their availability. In the latter case, the

RSPA rule permits operators to pass along the cost of EFV installation and maintenance to

those customers who choose to have the valve installed. While the Safety Board is

encouraged that utility companies that do not provide the valves are at least making them

available to their customers, the Safety Board is concerned that customers may not fully

understand the safety benefits that EFVs can provide when they are faced with a decision

that may require that they pay for the installation and maintenance of the device.

Furthermore, Safety Board investigation of a fatal gas pipeline accident in St.

Cloud, Minnesota, (four fatalities)25 indicates that commercial establishments can also

benefit from the protection offered by EFVs. Likewise, an accident in Bridgeport,

Alabama, (three fatalities)26 may have been prevented if the gas service line had been

equipped with an EFV. Both accidents involved excavation damage to underground

natural gas service lines, and both involved gas that migrated underground to nearby

buildings, where it subsequently exploded. Although both the St. Cloud and Bridgeport

accidents involved older gas lines that would not have been subject to a RSPA requirement

unless they had required maintenance, an EFV may have prevented both accidents.

Without a requirement that, where appropriate for operating conditions, commercial gas

service lines be equipped with EFVs, further accidents involving pipeline failures leading

to fires and explosions can be expected. As noted earlier, current RSPA rules do not

require that new commercial customers be informed about the availability of EFVs, even

though their operating environments may be compatible with commercially available and

relatively inexpensive EFVs.

In light of the fact that accidents continue to occur that could have been prevented

by the use of EFVs, the Safety Board believes that RSPA should require that EFVs be

installed in all new and renewed gas service lines, regardless of a customer’s

classification, when the operating conditions are compatible with readily available valves.

25 National Transportation Safety Board, Natural Gas Pipeline Rupture and Subsequent Explosion,

St. Cloud, Minnesota, December 11, 1998, Pipeline Accident Report NTSB/PAR-00/01 (Washington, D.C.:

NTSB, 2000).

26 National Transportation Safety Board, Natural Gas Service Line Rupture and Subsequent Explosion

and Fire, Bridgeport, Alabama, January 22, 1999, Pipeline Accident Brief PAB-00-01 (Washington, D.C.:

NTSB, 2000).
24 Pipeline Accident Report

Conclusions

Findings

1. Based on the size and location of the hole in the gas service line, the amount of gas

that would have been needed to cause the accident, the presence of a strong smell of

gas outside the house before the explosion, and the presence of natural gas in the soil

underneath the residence, the most likely source of the natural gas that filled the

residence basement and fueled the explosion was the damaged gas service line

leading to the house.

2. Work performed either during installation of the gas service line or during subsequent

excavation of the electrical line, or both, damaged the protective insulation for the

triplex electrical conductors and splices in the area of the hole in the gas service line.

3. The damage to the protective insulation covering the splice in the electrical conductor

identified as cable B allowed moisture to reach the crimp connector, causing

corrosion that increased electrical resistance between the crimp connector and the

cable sufficient to cause the splice to overheat and fail.

4. Heat generated from the arcing resulting from cable B’s failure under load caused the

gas service line wall to soften and weaken until the internal pressure breached the

pipeline and it began to leak.

5. The failure of the conductor identified as cable A was a secondary event that may

have occurred after the explosion as a result of short circuiting within the damaged

residence’s electrical system.

6. The gas present in the soil around and under the house entered the basement of the

residence either through penetrations in the concrete slab or cracks in the foundation,

or both.

7. Had the gas and electrical service lines involved in this accident been adequately

separated, the heat from the arcing electrical conductor failure would probably not

have damaged the gas service line, and the accident would not have occurred.

8. Had an excess flow valve been installed in the gas line to the residence, the valve

would have closed after the hole in the pipeline developed, and the explosion likely

would not have occurred.
Conclusions 25 Pipeline Accident Report

Probable Cause

The National Transportation Safety Board determines that the probable cause of

the accident in South Riding, Virginia, was the corrosion and subsequent overheating and

arcing at a splice in one of the conductors of the triplex electrical service line, which,

because of inadequate separation between the electrical conductors and the gas service

line, led to the failure of the gas service line and the subsequent uncontrolled release of

natural gas that accumulated in the basement and was subsequently ignited. Precipitating

the electrical service line failure was damage done to the electrical service line during

installation of the gas service line and/or during subsequent excavation of the electrical

line.
26 Pipeline Accident Report

Recommendations

As a result of its investigation of the South Riding, Virginia, accident, the National

Transportation Safety Board makes the following safety recommendations:

To the Research and Special Programs Administration:

Require gas utility operators to maintain a specified minimum separation

distance, sufficient to protect against both thermal and mechanical damage,

between plastic gas service lines and underground electrical facilities

whenever they install a new gas service line or perform maintenance on

existing lines. (P-01-1)

Require that excess flow valves be installed in all new and renewed gas

service lines, regardless of a customer’s classification, when the operating

conditions are compatible with readily available valves. (P-01-2)

To the Edison Electric Institute:

To the National Rural Electric Cooperative Association:

To the American Public Power Association:

Inform your member utilities of the circumstances surrounding the July 7,

1998, explosion and fire in South Riding, Virginia, and of the need to

ensure that underground electrical facilities are installed and maintained

with separation between those facilities and plastic gas pipelines in

accordance with the National Electrical Safety Code. (P-01-3)

To the U.S. Rural Utilities Service:

Inform your participating rural utilities of the circumstances surrounding

the July 7, 1998, explosion and fire in South Riding, Virginia, and of the

need to ensure that underground electrical facilities are installed and

maintained with separation between plastic gas pipelines in accordance

with the National Electrical Safety Code. (P-01-4)
Recommendations 27 Pipeline Accident Report

BY THE NATIONAL TRANSPORTATION SAFETY BOARD

CAROL J. CARMODY

Acting Chairman

JOHN A. HAMMERSCHMIDT

Member

JOHN J. GOGLIA

Member

GEORGE W. BLACK, JR.

Member

Adopted: June 12, 2001