Meeting the Challenge

Progress Energy is a Fortune 250 investor-owned electric utility company that comprises generation, transmission and distribution businesses and a general services company. Progress Energy’s 11,000 employees serve 2.9 million customers over a 50,000-square-mile retail service area in North Carolina, South Carolina and Florida.

The generation business is made up of four nuclear, 16 fossil, four hydro and 30 combustion turbine plants. Transmission consists of high power voltage lines ranging from 69 kV to 500 kV, and various substations and switchyards. The distribution system consists of both underground and overhead lines with power voltages ranging from 13.2 kV to 23 kV phase-to-phase.

LOCATION OF UNDERGROUND DISTRIBUTION NETWORKS
Progress Energy’s underground distribution network systems are predominately located in the urban areas. These underground distribution network systems consist of primary feeders and secondaries that are terminated in vaults and manholes. These vaults and manholes serve as access points and entry points for these underground distribution network systems.

In addition to providing an access point for an underground distribution system, network vaults are large underground spaces housing transformers and other electrical equipment. Electric manholes are smaller access point spaces with narrow openings. Working in both network vaults and electric manholes is extremely dangerous.

In the St. Petersburg and Clearwater, Florida areas, there are approximately 1,200 network vaults and manholes. Many of these network vaults and manholes contain multiple electrical circuits, feeders and breakers and electrical junctions that must be maintained and serviced.

In the Carolinas, vaults and manholes are located in the Raleigh, N.C. area and contain multiple primary circuits, which can be back fed or looped. However, many additional outages are now being encountered because of the increasing difficulty in back feeding these vaults and manholes.

HAZARDS IN UNDERGROUND DISTRIBUTION SYSTEMS
Employees working in network vaults and manholes with dedicated electric utility equipment and apparatus continue to be highly concerned about safety in the event of an electric arc explosion. These employees are acutely aware of the safety hazards, as every year utility workers in the United States are seriously injured in network vault and manhole explosions. Utilities across the United States have used numerous approaches to address the problem of worker protection against electric arc hazards.

The primary hazards employees can encounter in network vaults and manholes include:
• Confined space hazards such as low oxygen levels and the presence of explosive and/or hazardous gases.
• Thermal heat hazards associated with electric arcs.
• Blast forces and shrapnel associated with electric arcs (close proximity effects).
• Pressure sound waves associated with electric arcs.

Confined space hazards such as low oxygen levels are addressed using ventilation once employees have tested the air in the network vault and manhole. Similarly, explosive gases are tested for, and monitored for the duration of the worker’s time in the network vault and manhole. However, the hazards associated with a potential electric arc in network vaults and manholes have not been well addressed by the electric utility industry.

One of the phenomenon associated with an electric arc blast is that the heat energy decreases as the square of the distance. For example, as shown in Figure 1, at 4 inches of distance from an arc blast, the heat energy is 16 times less, as compared to being one inch away from an arc blast ([1/4]2). Also from Figure 1, the heat energy at 8 inches is 64 times less as compared to being one inch away from an arc blast ([1/8]2). Therefore, distance is an important variable that can be leveraged when designing safety systems to protect against electric arc blasts.

HOW THE ELECTRIC UTILITY INDUSTRY HAS BEEN ADDRESSING ELECTRIC ARC HAZARDS
A common approach used by U.S. utilities is to wrap an engineered arc blast blanket around the electric circuits and/or electric equipment in the vault or manhole. These engineered arc blast blankets are designed to withstand specified heat energy in an electric arc. The problem with this approach is that the blast blanket, although providing some degree of protection, is positioned too close to the arc blast.

A review with other U.S. utilities revealed that many companies have also struggled about how to best protect workers in electric utility vaults and manholes. Most utilities serving major urban areas in the U.S. predominately wrap electric circuits, conductors and equipment in network vaults and manholes with arc blast blankets. In some cases, utilities have installed attachment points in select vaults and manholes, from which electric arc blankets could be suspended. But there are limitations with this approach as well:
• The ability to position the blankets in another configuration is limited by the prior placement of the attachment points.
• The surface on which these attachments would be suspended could become fatigued and degrade the integrity of interior wall surfaces.

Therefore, many utilities are reluctant to pre-install attachment points in their vaults and manholes. Unfortunately, manufacturers of electric arc blankets have not designed systems from which to suspend blankets. The manufacturers who were contacted concerning this question simply stated that, due to the various configurations of network vaults and manholes, they would not, or could not, design a blanket support/suspension system.

PROGRESS ENERGY EMPLOYEES GET INNOVATIVE
The Progress Energy network crew based in Clearwater Florida Operations realized the need to have a more effective approach, and its crew employees developed an electric arc blanket suspension assembly device. This device, shown in Figure 2, allows the arc blast blanket to be suspended. In addition, its threaded rods (also called stanchions) allow the installer to tighten and torque the plates at each end of the threaded rod to a desired tightness. This ensures a snug, rigid fit on these plates to both the floor and ceiling.

The electric arc blanket suspension assembly also allows the blanket to be positioned at various distances from electrical equipment and components. This feature of the device allows employees to more effectively leverage the distance variable to protect against the effects of electric arc blasts.

LIMITATIONS AND CONCERNS OF THE PROGRESS ENERGY SOLUTION
Due to concern regarding how well this device would work in an actual blast scenario, management did not allow its use in network vaults and manholes work. The primary concern was whether the device could withstand all the hazards associated with an electric arc blast (thermal forces, blast forces and pressure waves).

Many of the network crew employees stated that they had experienced many “near misses” over the years. These near misses have consisted of identifying network vaults and manholes that experienced electric arc explosions just a few days after a prior entry. Therefore, company management funded the testing of these network devices to determine how well they would perform in an electric arc blast simulation.

TESTING PURPOSE, PLAN DESIGN AND PROTOCOL
In December 2007, several electric arc blanket suspension assemblies were tested at Kinectrics’ High Current Lab in Toronto, Ontario. The purpose was to perform destructive testing of the device to failure. Kinectrics would simulate electric arcs based upon conditions in Progress Energy’s Florida and Carolina operations. Based upon these simulations, it would be noted if these devices withstood the thermal, blast and sound pressure forces associated with the test conditions.

Kinectrics did not measure the amount of blast force or the pressure waves, but observed the blast and its effects using high-speed photography. Some thermal energy (as measured in cal/cm2) was measured, but was not the main focus of the testing. The goal of the testing was to determine and document the failure modes and then re-engineer and redesign the device as appropriate.

The Progress Energy team, in conjunction with Kinectrics, designed the testing protocol and plan. This testing plan’s purpose was to simulate the electric arc exposure conditions on both the Florida and Carolina network vault and manhole systems. Destructive tests were performed on the electric arc blanket suspension assemblies in a variety of testing scenarios using commercially available electric arc blankets.

Figure 3 illustrates the conceptual layout of the electric arc blanket suspension assembly in relation to the position of the worker. The Progress Energy team also wanted to see how this electric arc blanket suspension assembly would perform in close proximity to an electric arc (greater blast forces, thermal energy and pressure waves) compared to greater distances away from an electric arc. Thus, the testing design included the electric arc blanket suspension assembly at one and 12 inches away from the electric arc as shown in Figure 3.

The testing protocol was designed to simulate an electric arc exposure condition in a vault and manhole system. It was not intended to replicate all possible fault conditions that may occur, but rather demonstrate a typical scenario of surface-mounted cables or connectors failing in the middle of the wall in a relatively small vault. Separate destructive tests were performed on the electric arc blanket suspension assembly to determine how the complete assembly would perform.

To create reproducible arc conditions, a jig was fabricated that allowed similar arc blasts to be produced and directed to the specific area of interest. In reality, a fault may occur in different locations and on different components within a vault. The arcing fault may be focused outwards, inward toward the wall, up, or down. The worst case, for the evaluation of the stanchions and blanket, is a direct blast in the middle of the blanket. This puts the highest blast pressure on the assembly.

Several tests were performed in this fashion. To observe the effective protection of the blanket installation in different fault orientations, the electrodes were positioned to direct the arc to different areas. This helped to evaluate the effectiveness of the blanket under different fault scenarios.

In developing the test procedure, several variables were considered. These included:
• Number of cycles of fault current.
• Type of fault (electrode orientation and gap or using Tbodies and punctured cables).
• Available fault current.
• Distance of the electric arc blanket suspension assembly from the arc.

The parameters were selected to represent conditions in Progress Energy’s Florida and Carolina operations.

TESTING RESULTS
In all the testing scenarios, the electric arc blanket suspension assembly performed without flaw and exceeded the team’s expectations in overall durability and rigidity. The electric arc blanket suspension assembly device also withstood the blast forces and pressure waves associated with the electric arc tests.

In the final tests, faulted separable insulated connectors (Tbodies) were used as a reality check to compare the results from the arc fixture to the failure within a cable connector system. In these tests, the T-body was punctured to cause a failure when the cable was energized. The T-body “explodes,” releasing pieces of rubber shrapnel. Observation from the cameras on the blanket side showed no difference in the blast pattern or performance of the stanchions or blankets. The thermal effects on the electric arc blankets were noted and documented.

Additionally, these tests showed and illustrated the importance of having an electric arc blanket suspension assembly in place when work is being performed in network vaults and manholes. Figure 6 highlights the importance of this safety protection system and shows the effects of an arc flash on a mannequin with, and without, an electric arc blanket suspension assembly.

TESTING DATA POINTS
The table on page 20 outlines the various testing scenarios that were conducted at Kinectrics’ lab facility. In addition to these test dimensions, the stanchions were each torqued at 20 lb/ft on the handles of the screw jack. The value was used to approximate typical hand tightening of the stanchions.

OTHER TESTING OUTCOMES
The following activities took place as a result of this successful project:

The electric arc blanket suspension assembly as shown in Figure 2 was placed on Progress Energy’s internal distribution engineering standards. This allows the device to be used officially anywhere in Progress Energy’s system to ensure a consistent reproducible device will be available to employees. Data and specifications were obtained from the local fabricator who developed the prototypes used for the Kinectrics testing.

A video presentation was also developed for employees that included several key messages, including:
• General understanding of how electric arcs are formed and behave.
• Understanding the importance of the inverse square rule as it relates to distance.
• Proper use of PPE.
• Importance of following proper work practices.
• Importance of employee input as it pertains to submitting new ideas to make work safer.
• Importance of the hot line tags and operational safeguards of proper instantaneous settings.

SAFETY ENHANCEMENTS
The project team also examined additional ways to safely perform network vault and manhole work, and recommended the following:
• Increasing the widespread use of infrared guns to allow employees to better identify/note hot spots of potential cable failures prior to entering the hole.
• Using specially designed hearing protection. The network team evaluated several types of hearing protection that would allow normal conservations to be heard, but also provide hearing protection in the event of an electric arc blast.

As the network vaults and manholes are expanded in other parts of Progress Energy service areas for additional distribution circuit capacity, the information (safety considerations) learned from this project will be incorporated into the design philosophy for the configuration of new network vaults and manholes.

ASTM TEST METHOD DEVELOPED
Progress Energy was the first electric utility to perform electric arc testing on an electric arc blast blanket in this configuration. Consequently, Kinectrics, with Progress Energy’s permission, submitted the testing protocol that the Progress Energy team developed and used at Kinectrics to the ASTM F18.65 committee. The ASTM F18.65 subcommittee used this testing protocol to develop a draft Standard Test Method for Determining the Protective Performance of an Arc Protective Blanket for Electric Arc Hazards. This draft standard test method was submitted for ASTM ballot in September 2008.

The following employees associated with the design and prototype development of this device should be commended for their efforts and ingenuity. These employees include Patrick Haines, Network Specialist; Tom Fahrman, Network Specialist; and Rafael Abadal, Supervisor, Network Operations.

This project illustrates the importance of asking questions and soliciting input from employees on ways to do their jobs safer. The initiative and determination of these employees, coupled with the company’s commitment to safety, resulted in the development of a device that is not only beneficial for its workers, but for the electric utility industry as well. It is hoped that this device and its use will be a major step forward in improving the protection of employees working in network vaults and manholes. iP

video