What Are Arc Fault Hazards in Underground Power Systems and How to Prevent Them?

An arc fault in an underground power system can generate temperatures exceeding 35,000 degrees Fahrenheit, four times hotter than the surface of the sun. In open-air electrical systems, that kind of energy is dangerous enough. Below ground, where gases have nowhere to escape and faults can go undetected for weeks, the consequences are far worse.

According to data from the National Fire Protection Association (NFPA), arc flash incidents cause roughly 30,000 workplace injuries each year across all electrical environments. Underground systems account for some of the most catastrophic events, including manhole explosions that launch steel covers into the air and send toxic gases into nearby buildings. Most of these incidents trace back to preventable causes: degraded cable insulation, poor conduit sealing, improper grounding, or damaged connections that were never caught during installation.

This guide explains how arc faults develop in underground power systems, what makes them so dangerous below grade, and how the right conduit materials, fittings, and installation practices reduce the risk of a fault occurring in the first place.

What Is an Arc Fault in Underground Power Systems?

An arc fault occurs when electrical current jumps across a gap between two conductors or from a conductor to ground. Instead of following its intended path through insulated wiring, the current creates an uncontrolled electrical discharge, a plasma arc, that produces extreme heat, intense light, and rapid air expansion.

In above-ground systems, arc faults often trip protective devices quickly and dissipate into open air. Underground systems present a different set of problems. Conduit runs are enclosed, cables are buried in ducts surrounded by soil, and the confined spaces of manholes and vaults trap heat and gases. A small arc that would self-extinguish in open air can sustain itself for minutes or even hours underground, slowly burning through cable insulation and generating flammable gases that accumulate in sealed spaces.

There are two primary categories of arc fault events in underground infrastructure. Electrically driven events release their energy directly from a high-current electrical arc, typically during a catastrophic short circuit. Combustion-driven events, which are more common, result from low-level arcing that smolders cable insulation over time, producing carbon monoxide, methane, and hydrogen. These gases collect in manholes and vaults until a secondary arc or spark provides ignition.

Research published by the IEEE Working Group P2417, documented in the IEEE Guide for Early Detection, Mitigation, Preventative Measures, and Response to Smoke, Fire, and Explosions in Underground Electrical Structures, confirms that roughly 95% of manhole events occur on low-voltage cables operating below 600 volts. Of those events, approximately 75% are combustion driven.

Common Causes of Arc Faults in Underground Systems

Understanding why arc faults occur is the first step toward preventing them. Several factors contribute to fault development in underground cable and conduit systems.

• Insulation degradation is the primary driver. Cable insulation breaks down over time due to thermal cycling, chemical exposure, and simple age. Older cables with styrene butadiene rubber insulation begin to oxidize at temperatures as low as 150 degrees Celsius, making them significantly more vulnerable than modern ethylene propylene rubber (EPR) insulated cables, which resist oxidation up to 200 degrees Celsius. Once insulation cracks, even microscopic gaps allow current to arc across the breach.

• Moisture and water ingress dramatically reduce the dielectric strength of cable insulation. Water entering through damaged sheaths, failed joint seals, or unsealed conduit ends creates conductive paths that enable arcing at voltages far below the cable's rated capacity. In underground duct systems, standing water combined with soil contaminants creates a particularly aggressive environment for insulation breakdown.

• Mechanical damage during installation accounts for a significant percentage of premature cable failures. A Canadian utility study cited in IEEE P2417 documentation found that roughly 5% of underground cables examined after a series of secondary cable failures had been damaged during installation. Cables dragged across joint edges, pulled through ducts containing debris, or bent beyond their rated radius develop insulation weak points that become arc fault origins months or years later.

• Loose or corroded connections at splices, terminations, and junction points introduce resistance into the circuit. Increased resistance generates localized heat, which accelerates insulation degradation in the immediate area and creates conditions favorable to arcing.

• Rodent and pest damage is an often-overlooked contributor. Rodents chew through cable jackets and insulation to access the warmth of energized conductors, exposing bare copper or aluminum that can arc to adjacent conductors or grounded conduit walls.

Why Underground Arc Faults Are Particularly Dangerous

The confined nature of underground electrical infrastructure amplifies the danger of arc faults in several critical ways.

When cable insulation smolders inside a sealed duct, the burning material produces large volumes of carbon monoxide, methane, hydrogen, and carbon particulate. Because the duct system is enclosed, hydrogen tends to escape through porous duct walls, but carbon monoxide and methane accumulate in manholes, vaults, and service boxes at the low-pressure ends of the system. Carbon monoxide is both highly toxic and extremely flammable, with an explosive range of 2% to 74% concentration in air.

Standard overcurrent protection devices often fail to detect underground arc faults because the fault current is too low to trigger a breaker or fuse. Research conducted by Hydro-Quebec and Consolidated Edison demonstrated that arcing faults in wet, contaminated ducts can sustain as interrupted arcs drawing less than one amp, well below the detection threshold of conventional protection systems. These low-level faults can persist for days or weeks, generating gases the entire time.

The physical consequences of an undetected accumulation are severe. When flammable gas concentrations reach explosive levels and a secondary arc provides ignition, the resulting explosion can launch manhole covers weighing several hundred pounds into the air. Smoke and toxic gases can also migrate through underground conduit pathways into adjacent buildings through electrical service entrances, creating exposure risks far from the original fault location.

Workers entering manholes and vaults for routine maintenance face additional hazards. These are classified as confined spaces with potential for oxygen deficiency, toxic atmospheres, and explosive gas concentrations. A manhole guard rail with shield provides surface-level fall protection, but atmospheric testing and confined space protocols are essential before any entry.

How Proper Conduit Selection Reduces Arc Fault Risk

The conduit material surrounding underground cables plays a direct role in arc fault prevention. Conduit protects cables from the mechanical damage, moisture exposure, and environmental stresses that cause insulation failure.

Aluminum rigid conduit manufactured to UL6A and ANSI C80.5 standards provides both physical cable protection and an inherent equipment grounding path. In underground systems where grounding continuity is critical for fault clearing, metallic conduit eliminates the risk of a missing or damaged wire-type equipment grounding conductor.

For nonmetallic conduit installations, NEC Section 352.60 requires a separate equipment grounding conductor (EGC) inside the raceway to provide a reliable fault-return path. Without this conductor, a ground fault may not generate enough current to trip the overcurrent device, allowing the fault to persist and develop into an arcing condition.

Regardless of material, conduit must be properly sealed at all openings to prevent moisture ingress. Fiberglass end caps with gaskets provide watertight seals at termination points, while vinyl end caps protect conduit openings during construction and in spare conduit runs.

Grounding, Bonding, and NEC Compliance

Proper grounding and bonding form the primary electrical defense against sustained arc faults. NEC Article 250 establishes the requirements for creating an effective ground-fault current path, one with low enough impedance to carry fault current at a magnitude sufficient to quickly trip the overcurrent protective device.

Several NEC provisions apply directly to underground arc fault prevention.

NEC 250.4(A)(5) requires that electrical equipment and wiring be connected to the earth through a grounding electrode system. For underground conduit installations, this means the grounding electrode conductor must establish a reliable connection between the system ground and the earth, stabilizing voltage and providing a path for lightning and surge currents.

NEC 250.118 identifies which conduit types qualify as equipment grounding conductors. Rigid metal conduit (RMC), intermediate metal conduit (IMC), and electrical metallic tubing (EMT) all qualify when properly installed with tight, continuous connections. 316 stainless steel compression connectors maintain electrical continuity at conduit joints in corrosive underground environments where standard connectors degrade over time.

NEC 501.15 addresses sealing and drainage requirements for conduit systems in hazardous (classified) locations. Sealing fittings prevent flammable gases and vapors from traveling through the conduit system and reduce the risk of an arc at one location igniting gases at a distant point. An Armor Guard coupler kit provides bonding and grounding continuity at coupling points, maintaining the low-impedance fault path that enables fast protective device operation.

Loose connections are among the most common causes of high-impedance faults that standard protection cannot detect. Every threaded conduit joint must engage a minimum of five full threads (per ANSI/ASME B1.20.1) to ensure both mechanical strength and electrical continuity.

Installation Practices That Prevent Arc Faults

The quality of the original installation determines whether a conduit and cable system remains arc-fault-free for its intended service life. Several field practices directly reduce the risk.

• Handle cables with care during pulling. Cable insulation damage during installation is a leading cause of premature faults. Use proper cable pulling equipment, apply manufacturer-recommended lubricant, monitor tension with a tensiometer, and never exceed the cable's rated maximum pulling force. Avoid dragging cable across sharp edges, duct joints, or debris.

• Run a mandrel through every conduit before pulling cable. A properly sized mandrel confirms that no obstructions, joint offsets, or deformations exist inside the conduit. A mandrel that encounters resistance indicates a problem that would damage cable insulation during installation.

• Seal every conduit opening immediately. Leaving conduit ends open during construction allows water, soil, insects, and debris to enter the duct system. These contaminants create the wet, dirty conditions that sustain low-level arcing faults. Cap or plug every opening at all times during installation.

• Maintain proper conduit spacing in duct banks. Keep a minimum of 3 inches between power conduits to allow heat dissipation and reduce the risk of thermal degradation in adjacent cables. Use duct spacers at 5- to 6-foot intervals to maintain alignment.

• Install warning tape 12 inches above buried conduit. This NEC 300.5(D)(3) requirement protects against future excavation damage, one of the most common mechanical causes of cable insulation failure in underground systems.

• Inspect and test after installation. Perform insulation resistance testing on all conductors before energizing. Document results and compare against baseline values during future maintenance cycles to identify insulation degradation before it progresses to an arcing condition.

Detection and Monitoring of Underground Arc Faults

Even with proper installation, underground systems require ongoing monitoring to catch faults before they escalate.

Arc fault circuit interrupters (AFCIs) monitor circuits for the irregular current waveforms characteristic of arcing. While AFCIs are primarily associated with residential branch circuits per NEC 210.12, the underlying detection technology has applications in commercial and industrial settings where conventional overcurrent devices miss low-level arc signatures.

High-voltage detection equipment allows workers to verify de-energized conditions before entering manholes or vaults. An HVD1 high voltage detector rated for 50kV to 765kV provides non-contact voltage detection for medium- and high-voltage underground systems.

Partial discharge testing identifies insulation defects before they develop into full arc faults. Partial discharges are small electrical pulses that occur within insulation voids or at defect sites. Detecting these pulses during routine testing provides an early warning of insulation failure.

Mobile electric field detection systems, used by utilities in cities like New York and Toronto, survey underground infrastructure from street level to identify energized manholes, contact voltage conditions, and cable faults without excavation. These systems detect the low-level electric fields that emanate from structures with underground distribution faults.

Dedicated manhole monitoring systems use infrared cameras and gas sensors to detect cable hot spots, stray voltage, rapid temperature increases, and early-stage carbon monoxide and methane accumulation. These systems provide real-time alerts before conditions reach dangerous levels.

About Utility Pipe Supply

Utility Pipe Supply has served contractors, utilities, and engineering firms since 1997 with a complete inventory of conduit, fittings, grounding products, safety equipment, and installation tools for underground infrastructure projects. As a certified WBE/DBE/FBE, the company delivers reliable products and responsive service to keep projects safe, compliant, and on schedule.

Frequently Asked Questions

What temperature can an arc fault reach?

Arc faults can generate temperatures ranging from 5,000 to 35,000 degrees Fahrenheit. At these temperatures, copper and aluminum conductors vaporize, insulation ignites instantly, and nearby combustible materials catch fire. In underground systems, the heat also decomposes cable insulation and duct materials into flammable gases that accumulate in confined spaces.

What causes arc faults in underground cables?

The most common causes include insulation degradation from aging or thermal stress, moisture entering through damaged cable sheaths or unsealed conduit ends, mechanical damage during cable installation, loose or corroded connections at splices and terminations, and rodent damage to cable jackets. Wet and contaminated duct conditions make sustained arcing more likely.

How do arc faults cause manhole explosions?

Low-level arcing in underground ducts smolders cable insulation, producing carbon monoxide, methane, and hydrogen gases. These gases travel through the duct system and accumulate in manholes and vaults. When concentrations reach explosive levels and an ignition source is present, the resulting explosion can launch manhole covers and send flames and toxic smoke to the surface.

What NEC requirements address underground arc fault prevention?

Key provisions include NEC Article 250 (grounding and bonding), NEC 352.60 (equipment grounding conductor in nonmetallic conduit), NEC 300.5 (underground installation requirements including burial depth and warning tape), NEC 501.15 (conduit sealing in hazardous locations), and NEC 514.8 (underground wiring in hazardous locations requiring threaded rigid metal or intermediate metal conduit).

How can I detect an arc fault in an underground system?

Detection methods include arc fault circuit interrupters (AFCIs) for branch circuit monitoring, partial discharge testing for insulation defect identification, insulation resistance testing during routine maintenance, high-voltage detection equipment for verifying de-energized conditions, and dedicated manhole monitoring systems that detect temperature changes, gas accumulation, and stray voltage in real time.

Does conduit material affect arc fault risk?

Yes. Conduit protects cable insulation from the mechanical damage, moisture, and environmental stress that cause arc faults. Metallic conduit (RMC, IMC, aluminum) also provides an equipment grounding path per NEC 250.118, which helps ensure fault current is high enough to trip protective devices quickly. Nonmetallic conduit (PVC, HDPE, fiberglass) requires a separate equipment grounding conductor inside the raceway.

Get the Right Products for Safe Underground Installations

Utility Pipe Supply carries conduit, connectors, grounding products, sealing fittings, safety equipment, and detection tools to help you build underground systems that stay arc-fault-free. With nationwide shipping and in-stock availability, we keep your crew working and your project compliant. Call (815) 337-8845 or request a quote to get started.