Evaluation of CSST Gas Piping Subjected to Electrical Insult

Transcription

1 Evaluation of CSST Gas Piping Subjected to Electrical Insult Rising Tide Consulting, LLC Brian Kraft, PE and Cutting Edge Solutions, LLC Robert Torbin, PE October 2009 The research data presented within this document represents a small (but representative) sub-set of the complete set of test variables associated with both the normal use of electricity and voltage surges associated with lightning strikes. The objective of the research was to determine (based on the limited set of data collected and analyzed) the adequacy of CSST to withstand the electrical forces associated with both fault and insult conditions within a typical residential electric power system. There are no stated or implied guarantees that CSST (or any other gas piping material) can be safely used under all circumstances and operating conditions associated with over-voltage surges in connection with direct and indirect lightning strikes on or near the premise. The bonding of CSST must be performed in accordance with the National Electrical Code, the National Fuel Gas Code and the manufacturer s installation instructions to provide the maximum achievable protection from arcing damage resulting from differences in potential between metallic systems in close proximity to each other, and based on the manner in which these systems are bonded to the grounding electrode system. Copyright 2009 Kraft/Torbin

2 Executive Summary Recent events have shown the proclivity of corrugated stainless steel tubing (CSST) to become physically damaged (through electrical arcing) when installed in close proximity to other metallic systems that are simultaneously energized by a lightning strike but not equally bonded to the grounding electrode system. This has raised concerns within the electrical community about the general capability of CSST to withstand other electrical insults associated with the normal use of electricity. This report summarizes the engineering analysis performed to address these concerns regarding the performance of CSST under electrical insult. Considering current installation practices and code requirements for residential construction, electrical service and fuel gas piping systems, the most probable (and severe) electrical insults were determined to be fault current and indirect lightning strikes. The damage modes associated with CSST and these insults were identified as: resistance heating in conductors, resistance heating in connectors, mechanical damage in connections, and arcing between conductors. CSST (1/2 inch nominal size) was subjected to a fault current (250 amps, 4 seconds) as determined through analysis of building characteristics and the UL-1 Standard with no ill effects to the material. CSST fittings and commercially available bond clamps were tested for connection resistance and were found to be comparable to the resistance of the bond clamps in their currently listed application (clamp mounted to copper water pipe). Following guidance in the UL-467 Standard, commercially available bond clamps mounted to CSST fittings were subjected to a pull test (150 lb) with no loosening of the mechanical connection. Using widely accepted lightning industry standard test methods (10 µs x 300 µs waveform) a test rig was designed to model CSST piping installed in close proximity to another electrically conductive material/system. The effects of direct bonding (as well as no bonding) on the creation of perforations through the CSST wall induced by electric arcing were evaluated. It was determined that while a non-bonded configuration would transfer upwards of 97 percent of the lightning energy through the arc, direct bonded configurations reduced this amount to 20 percent or less. The amount of energy transferred through an arc is highly indicative of damage to the material, and the testing showed that while arcing in non-bonded configurations caused perforations in the tubing, arcing in direct bonded configurations did not visually damage the tubing. In many instances, direct bonding prevented arcing altogether. By performing testing simulating the possible electrical insults that may occur in a CSST gas piping system, data was collected and evaluated. It was concluded that CSST is a robust product capable of maintaining product integrity and safety even when subjected to the types of electrical insult that may be found in a residential or light commercial building during a normal malfunction of the electrical system and insults from indirect lightning strikes. Based on these studies, the following conclusions are drawn: CSST can withstand (without failure) expected ground faults imposed by the electrical system. CSST will provide an effective, low-impedance conductive pathway to ground when it is energized by the electrical service or an indirect lightning strike. For indirect lightning strikes, direct bonding will reduce or eliminate the damage resulting from electrical arcing between the CSST and another metallic system in close proximity by eliminating or reducing the difference in electrical potential. All CSST systems should be directly bonded to the grounding electrode system using a 6 AWG or larger copper wire in addition to any self-bonding provided through the equipment grounding conductor. Copyright 2009 Kraft/Torbin 2

3 1.0 Introduction and Qualifications Recent events have shown the proclivity of corrugated stainless steel tubing (CSST) to become physically damaged (through electrical arcing) when installed in close proximity to other metallic systems that are simultaneously energized by a lightning strike but not equally bonded to the grounding electrode system. Two of the key design factors associated with this type of damage is the relatively thin (0.010 inch) wall thickness of the CSST and the lack of equipotential bonding between all metallic systems. This has raised concerns within the electrical community about the general capability of CSST to withstand other electrical insults associated with the normal use of electricity. This report summarizes the results from different electrical properties tests that were performed by third party testing organizations. These results are intended to address these concerns and provide validation that CSST can be installed and operated safely and reliably within typical electrical power systems that are intended for residential and commercial structures. The impact of a lightning strike (whether a direct or indirect strike) to any building can result in catastrophic damage to the structure and its contents depending on several factors. Bonding of metallic systems to the grounding electrode system alone is not considered a sufficiently effective means to eliminate the potential damage caused by lightning. A more comprehensive approach to protecting the structure is required including the installation of a lightning protection system, providing equipotential bonding for all metallic systems contained within the building, and maintaining adequate amount of physical separation between these metallic systems. Corrugated stainless steel tubing is both a gas piping material and product used for the conveyance of fuel gas (natural gas and propane gas) between the gas source (utility supplied meter or second stage LP gas regulator) and the connected appliances and/or equipment. The construction, wall profile and wall thickness of CSST is very different compared to the more traditional rigid piping materials (black iron pipe and copper pipe/tubing). The thinner wall thickness provides the CSST product with its characteristic flexibility that enables its installation advantages compared to conventional rigid plumbing systems. At the same time, the thinner wall also makes the CSST more vulnerable to damage from a variety of mechanical threats and electrical insults. However, CSST is considered to be mechanically robust and capable of successfully resisting (through design or installation) typical mechanical and electrical threats prevalent during normal operation and electrical upset conditions. CSST systems have been installed in millions of homes and building and have operated safely for over 20 years. As a point of comparison, data provided by the NFPA indicate that a lightning-induced fire is more likely to occur in a run of electrical conductor than in a run of CSST gas piping. In order to ascertain if CSST constitutes a real and significant safety hazard, an evaluation of the product performance under electrical insult was performed. This evaluation required the identification of potential electrical insults, the characterization of the insults, and the determination of their potential energy levels. Once the nature of the electrical insults were defined, the most likely damage modes to the CSST material were then determined. The characterization of the most probable electrical insults and damage modes permitted the generation of test protocols that were then used to evaluate the performance of the CSST. Copyright 2009 Kraft/Torbin 3

4 The objective of the engineering assessment was to evaluate and document the performance of CSST when exposed to those electrical insults that may routinely occur within a residential or light commercial building or when exposed to voltage surges associated with indirect lightning strikes. The following qualifications apply to the testing program that was performed, the data resulting from that research, and the conclusions drawn: Regarding lightning protection, there is no absolute measure of safety. There are no established and accepted threshold values for resistance to lightning energy, and bonding alone will not insure protection from damage. Therefore, the research was focused on determining if an acceptable level of performance could be achieved based on predictable electrical threats. The testing performed was not considered an exhaustive study of all of the possible variables affecting the lightning profile and the magnitude of the associated voltage surge. Rather, the testing was intended to provide a general confirmation that CSST is a safe and reliable product when exposed to both routine and upset operating conditions within typical residential electrical systems. It was the intent of all of the testing organizations to work within the established framework of existing product standards and to use (to the degree practical) conventional and accepted testing methods and protocols related to electrical properties. Where necessary, modifications were made to existing standards and procedures to accommodate unique product characteristics or usage. The design of the testing requirements was in recognition of, and in compliance with, the current National Electrical Code (NFPA 70) and National Fuel Gas Code (NFPA 54), and related UL product standards for installation practices, electrical bonding, and material properties. The NFPA 54 and NFPA 70 codes both contain bonding requirements for gas piping systems. However, additional bonding requirements specific to CSST were adopted in the 2009 edition of NFPA 54 that go beyond the bonding required by the NEC. It is in accordance with the NFGC requirements that the CSST electrical insult program was designed and the testing conducted. The basis for the conclusions drawn from this research was founded on the test data/results combined with the application of accepted principles of engineering and the observations and opinions of recognized experts in the field. Although a more comprehensive study could be commissioned to examine a greater range of electrical insult, the basic conclusions and recommendations are expect to remain the same. Copyright 2009 Kraft/Torbin 4

5 2.0 General Background 2.1 Gas Piping A residential fuel gas system is composed of three sub-systems including the gas utility service (meter riser, shut-off valve, service regulator, and meter); the gas-fueled appliances and equipment including their vents; and the piping network (pipe/tubing, fittings, valves and flexible connectors) within the building. The modern gas utility service is comprised entirely of underground polyethylene (PE) piping leading from the distribution network/main in the street to the exterior of the building where the piping is brought above ground through a gas meter riser that connects to the utility gas meter. The meter riser, which provides the transition from underground PE piping to above ground metal piping, is almost always electrically conductive. However, these risers have only a minimum length (less than 3 feet) of steel pipe in direct contact with the earth and are not considered part of the grounding electrode system. In the past, the underground utility gas service piping was commonly metallic pipe (typically steel). Depending on local conditions, it was not unusual for the metallic service lines to be cathodically protected. For this reason, and to prevent inadvertently creating a second electrical ground point, a dielectric union has been traditionally installed (by the gas utility) in the service line prior to the connection point with the service regulator. The purpose of the dielectric union is to electrically isolate the underground portion of the gas delivery network from the interior gas piping system. Even though non-conductive polyethylene piping is now used in virtually all new underground gas service line installations, the dielectric union continues to be installed for protection from stray electric currents and static discharges from the PE piping. The meter riser, pressure regulator and meter are commonly installed outdoors on the side of the building for service convenience and other practical considerations. Curb-side or indoor meter locations were not considered as part of this study. The impact of and on underground metallic service piping from a LP gas container, which is covered under a different code (NFPA 58), was also excluded from the study. House gas piping may consist of rigid steel pipe, copper pipe/tubing, corrugated stainless steel tubing or a combination of these three materials. Routing within the building varies greatly depending on the floor plan, gas appliances installed, and installation methods employed by the piping installer. Depending on when the gas piping is installed during the construction schedule and given the limited space available to install utilities within conventional wood-frame construction, it is not unusual for interferences to occur between different utilities. Therefore, it is not uncommon for gas piping to be routed in close proximity to, or even in direct contact with electric wiring, heating/cooling ducts, copper water piping, steel structural supports, appliance enclosures and flue-gas vents. This proximity can have a contributory effect on the likelihood and impact of electrical insult to these various metallic pathways during electrical storms. All gas piping systems (regardless of material) are required to be electrically continuous by the electrical and fuel gas codes. All gas piping is installed per the requirements of the locally adopted fuel gas code, which is commonly a modified version of the National Fuel Gas Code (NFPA54), International Fuel Gas Code (IFGC), or the Uniform Plumbing Code (UPC). Furthermore, CSST must also be installed in accordance with the manufacturer s instructions because it is a listed system (per the ANSI LC-1 Standard). When it is likely to become Copyright 2009 Kraft/Torbin 5

6 energized, gas piping systems must also be bonded to the grounding system of the premise in accordance with the appropriate sections of both the NEC and the local residential and fuel gas codes. In more recent years, the means for bonding of gas piping (when required) has been the use of the equipment grounding conductor of the electrically powered appliance to which it is connected. Under the current (and commonly applied) interpretation of the NEC, bonding would not be required if there were no electric power required to operate the appliance (such as a natural draft water heater). CSST is manufactured in a variety of sizes ranging between 3/8 to 2 inches in internal diameter. The nominal size of 1/2 inch represents a significant percentage of currently installed CSST in the residential market and also presents one of the smallest volumes of material. The lower volume of material provides a near-worst (and representative) case scenario for resistance to electrical insult and damage. For this reason, 1/2 inch CSST was used throughout the investigation. The electrical performance would be expected to improve for the larger sizes of CSST. 2.2 Electrical System Similar to the gas utility service, a residential electrical system consists of three sub-systems including the utility electrical service entrance (the main conductors, electrical panel/enclosure and circuit breakers); the connected electrical appliances and loads; and the house wiring within the building. Utility mains are generally run via overhead power distribution lines. However, newer housing developments often employ underground distribution systems. Both overhead and underground systems are locally earth-grounded to provide protection from stray voltage, voltage imposed by lightning, line surges and unintentional contact with higher voltage lines. Residential electrical systems also incorporate a grounding system, commonly provided through a grounding electrode conductor and a variety of possible electrodes including buried rods/pipe or an Ufer system. This provides the electrical ground specific to that building, and is referred to as the house ground. Referring to NEC/NFPA-70 and common electrical installation practices, appliances are generally supplied by, and grounded to, the electrical system through the circuit wiring. The circuit power conductors are routed through an over-current protective device (circuit breaker) at the electrical service panel, and continue on to each appliance or electric load. Specifically dedicated to the safety ground, the third conductor (equipment grounding conductor) in the circuit wiring is generally the same gauge as the power conductors. The most common copper wire gauge sizes for gas-fueled appliance circuits are 14 AWG, 12 AWG, and 10 AWG. Direct bonding (as explained later) generally involves the use of larger conductors. Other than a three-foot minimum separation spacing that may be required in some jurisdictions, few requirements exist for the co-locating of the gas and electric utility service entrances relative to each other or the premise. It is generally left up to the two utilities to determine the placement of their facilities based on their own requirements and other considerations. While it may be efficient for the builder to have both utility entrances located in close proximity, installations resulting in the electrical service entrance being on the opposite side of the building from the gas service entrance are quite common. It will be shown that the separation distance between the electric panel and/or the grounding electrode system and the gas service entrance is an important Copyright 2009 Kraft/Torbin 6

7 factor in the effectiveness of the bonding connection. Both the gas piping system and the electrical wiring system provide conductive pathways that run throughout a building. Other metallic systems provide additional pathways including communications/coax wiring, metallic plumbing systems, gas appliance vents, structural steel, and HVAC cooling and heating ducts. Often these various pathways can be interconnected at various locations throughout the building. For example, a gas-fired warm air furnace will provide a common electrical connection point for electric wiring, HVAC control wiring, gas piping, appliance vent, ductwork, and plumbing feed lines. In addition, it is common for all of these conductive pathways to be in close proximity to each other at various points throughout the building, depending on building type/design and utility layout. However, depending on many different factors, not all of these conductive pathways are bonded, or they are not bonded in the same manner or with the same sized conductor. While these types of bonding variations are not problematic for ground fault protection (the stated purpose for bonding in the NEC), these variations set up potential problems when providing protection from lightning energy (not within the scope of the NEC). 2.3 Grounding and Bonding Grounding is divided into two areas: system grounding and equipment grounding. Grounding is the intentional connection of a current carrying conductor to the earth to limit the voltages caused by accidental contact of the supply conductors with conductors of higher voltage or a surge from a lightning strike. When provisions for grounding and bonding exist in a building, there is always an effective low-impedance ground path for any stray currents that could be caused by an electrical fault. It is the bonding of the equipment grounding conductor (EGC) bus to the neutral bus (and not the connection to earth) that is intended to operate the over-current protective devices. This flow of current during an electrical fault allows the circuit breakers in the electrical system to operate, terminating the current flow, and thus, clearing the fault and eliminating any electrocution hazard. However, voltage surges caused by lightning energy will not activate the over-current protective devices, and therefore, a direct connection to earth is required to dissipate the energy flow. Bonding is the electrical connection between conductive systems. The purpose of bonding is to maintain an equal voltage between these systems. This prevents both shock hazards and the possibility of electrical arcing between systems. Bonding of metallic piping systems (including gas piping) is required in modern electrical construction. This bonding may be achieved through the use of the equipment grounding conductor of electrically fed gas-fired appliances (Self Bonding), or through the use of a dedicated bonding conductor (Direct Bonding). Self Bonding: When the equipment grounding conductor is used as the bonding means, no additional steps are required by the electrical contractor or the piping installer to achieve this minimum level of protection from ground faults. The use of the equipment grounding conductor is generally referred to as self-bonding. However, for situations where the gas piping system is connected to a non-powered gas appliance, it would be unusual for the piping system or appliance to be bonded to the electrical system grounding system at all. Based on conventional electrical practices, appliances that do not have an electrical connection are considered not likely to become energized, and there is no equipment grounding conductor available or required. Any Copyright 2009 Kraft/Torbin 7

8 metallic system that is not bonded increases the potential for a hazard (to the building occupants) to exist whenever and however the system might become energized by some other external force. Direct Bonding: It is not uncommon for piping systems, including steel gas piping and metallic water piping, to be bonded to the electrical grounding system through the use of a dedicated wire and clamp. As this method provides a direct connection between the piping system and the electrical ground system, it is referred to as direct bonding. For direct bonding, a listed clamp is attached to a single location on the piping system to provide an electrical contact. The bonding conductor is then connected to the clamp and routed to either the electrical panel or to the grounding electrode system. Direct bonding of all metallic supply lines entering a building is a critical, but often overlooked, approach when considering protection of a building and its contents during an electrical storm. Lightning strikes near buildings (with ground current transfer distances from to 1 to 3-km from the strike center) can induce differences in potential between the electrical system and any nonbonded mechanical system. Direct bonding of these systems (using a 6 AWG or larger copper wire) to the building grounding electrode system allows the mechanical systems to be energized at (or near) the same rate as the electrical system and in unison with the voltage wave induced by the lightning strike. Use of the equipment grounding conductor as the bonding means does not achieve the same effect. The EGC (which is typically a 12 or 14 AWG copper wire) does not allow (nor was it intended to) the mechanical system to be energized at the same rate as the electrical system. The path to ground through the EGC is typically much longer (and with greater impedance) than the direct bonding distance (near the service entrance) between each mechanical system and the grounding electrode system. When energized by lightning, this situation permits the electrical potential in the many conductive pathways to become unbalanced, and thus arcing is more likely to occur. Bonding through the equipment grounding conductor is only intended for personnel safety in the event of an electrical fault occurring in the premise wiring system, and has been shown to be inadequate when dealing with lightning energy. 2.4 Sources of Electrical Insult There are various sources of electrical energy that must be investigated to fully evaluate the performance of a material throughout the lifetime of a building. These energy sources include the installed electrical system and external sources such as the utility power distribution system and lightning. Fault Current: A properly designed and maintained electrical system does not impose voltage or current on systems not intended for that purpose. Gas piping (of any material) is not intended to be a conductor or current carrying pathway. However, electricity can be applied to the various conductive pathways in a building during a fault current event. A fault current event can occur if electrical equipment fails or if the insulation/isolation mechanisms of the electrical wiring system fail. As the possible sources of these failures are random and highly variable within a normal building, it is impossible to predict which system(s) will be energized. For this reason, it must be assumed that any, or all, conductive pathways can become energized. Copyright 2009 Kraft/Torbin 8

9 If there is no effective path to ground for the imposed energy, any contact with an energized system can result in arcing or electrocution. Thus, a significant electrical hazard to the occupants of the building would exist. This hazard is largely negated by the bonding of these conductive pathways to the electrical ground system of the building. This bond provides a safe lowimpedance path to neutral earth. Lightning Strikes: Lightning is, by its very nature, a massively powerful natural phenomenon, capricious and impossible to predict. While significant research has been performed on the lightning phenomenon, general engineering solutions to mitigate its impact have not been widely implemented in the residential market. The occurrence of lightning varies from intense (millions of strikes per year) to light (thousands of strikes per year) on a state-by-state basis. Given that the magnitude, frequency and duration of the energy involved with each lightning strike varies widely, the design of the most effective protection should take local conditions into consideration. Lightning energy is transferred to a structure whether the strike hits the building directly or indirectly through another medium (such as the earth or overhead power lines). A direct strike to a building occurs when the ground-side termination of the lightning bolt attaches to any portion of the building. Side flashes (where the lightning bolt strikes a nearby object but a secondary bolt arcs from that object to the building) are also considered direct strikes. The attachment point of the lightning bolt can connect to any portion of the building, but is especially likely to strike high metallic systems. Such likely contact points include gas appliance vents, metallic plumbing vents, antennas or satellite dishes, and communications wire mounted to the roof area. Once lightning enters the building, the lightning energy will travel to ground through all of the available, inter-connected conductive pathways and not just the path of least resistance. There is sufficient power to generate multiple arcs within the building, thus "connecting" systems that are normally isolated, or taking short cuts between conductors. Lightning energy does not discriminate against CSST gas piping, but rather energizes any type of gas piping material (steel or copper) including the stainless steel flexible appliance connectors at the end of the piping system. All types of gas piping have suffered damage from lightning strikes based on numerous anecdotal reports. Observations from real world lightning strikes on residential structures clearly indicate that the CSST will conduct the lightning energy without suffering a failure, but is vulnerable to damage through arcing. This arcing is thought to occur in a building lacking any type of lightning protection system, because of direct contact (or close proximity) between energized metallic systems, and the limited capability of the currently required bonding method (self-bonding) for gas piping systems. Similarly, lightning energy will use all available ground points, whether designed as grounds or merely connected in some way with the earth. In this way, lightning will use all available paths to disperse its electrical energy to neutral earth. In the case of a direct strike to a structure, energy will travel indiscriminately through all of the conductive systems within the building including electrical wiring, communications cables, ventilation ducts, water piping, gas piping, steel structural members and gas appliance vents, and often with a destructive effect. Due to the high levels of electrical power inherent in the lightning flash, no single ground point can provide a sufficiently low impedance connection to earth to fully and immediately discharge Copyright 2009 Kraft/Torbin 9

10 all of the lighting energy. Practically speaking, this means that a direct lightning flash to the ground or to a structure will saturate the earth around the immediate path to ground at the object struck. The lightning strike will proceed to conduct overflow energy to neutral ground by generating very high voltages and currents in the surrounding earth, objects and structures, commonly to very large distances. An indirect strike occurs when the energy from the main bolt enters the structure through another pathway such as the overhead power line, through the electrical system grounding electrode system or underground metallic piping. Generally speaking, the full force of the lightning bolt from an indirect strike is spread throughout many systems. In the case of a flash near the structure, lightning energy will travel into the structure and through the conductive pathways if they provide a significant path to neutral earth. The energy level in this situation is such that systems or objects that would not normally be considered electrically continuous and/or conductive can be, and are used as paths to ground. For example, water in trees or wooden framing members can provide lightning with a path to ground, but with significant damage to the insulating cellulose. Re-bar in unburied footings will respond in the same fashion, with similar explosive damage to the surrounding concrete. The dielectric insulator at the gas service entrance is commonly arced over during a lightning event, with the lighting using the underground meter riser as a ground point. Any of the conductive systems within the residence that pass near a steel structural support, such as a Lally column in a New England basement, will provide a potential path to ground, even if a short arc through the air is required to make the connection. 2.5 Damage Modes Conductors exposed to extremes of current and voltage can be damaged in a variety of ways. The most common damage modes were examined in these studies to evaluate the possibility of damage to the conductive pathway represented by the CSST gas piping, CSST fittings, and the bonding clamp. Resistance Heating in Conductors: When a current is passed through a conductor, the natural resistance of the conductor to current flow generates heat. If sufficient heat is generated at a rate that cannot be effectively conducted away, melting or other heat damage effects will occur in the conductor. As the possibility for damage is based on the rate of heat generated, the amount of current applied and the amount of time it is applied will determine whether the conductor will be damaged. For this investigation, the primary conductor is the gas piping or CSST. Currents applied by a fault are in the range of 200 to 300 amps over a period of 4 to 6 seconds required for a circuit break to trip, or a rough value of 1800 amp seconds. Current applied by an average indirect lightning strike should be in the range of 30,000 amps over a period of roughly 75 microseconds, or a rough value of 2.25 amp seconds. As a lightning strike occurrence is so much faster than a fault current event, the fault current is significantly more likely to cause the resistance heating type of damage to a conductor or CSST. Resistance Heating in Connections: When current is passed between two connected objects there is the possibility of additional heating at the connection interface. This heating would be controlled by the quality of the electrical contact, which is commonly measured by the resistance of the connection. The resistance of the connection is partially controlled by the contact area Copyright 2009 Kraft/Torbin 10

11 between the two conductors and the mechanical stability of that contact. If there is significant current or a high contact resistance, significant heat will be generated at the connection point. This heat can cause the expansion of the conductors, possibly further increasing the contact resistance. With sufficient heating, either from high current over prolonged periods or from a poor quality connection, the conductors/connectors could be damaged. Mechanical Damage in Connections: Where two conductors are joined at installation to form a high quality electrical connection, there is the possibility that through normal service the electrical connection may be undermined. In the case of a metallic clamp on a metallic fitting, this would generally occur if the clamp were to be loosened on the fitting, or be knocked loose from the fitting. Therefore, an important criterion is the mechanical strength of the connection between conductors. Arcing: When two conductors are in close proximity, but are not bonded together, voltage rises in one conductor due to impressed current can cause an arc to form between the two conductors. The voltages required to generate these arcs are fairly high, on the order of 30,000V/cm. For this reason, arcing is primarily caused by lightning strikes to the building. CSST gas piping can be damaged by this arcing to/from other conductors in close proximity. This arc causes intense, localized heating at the arc termination points, sometimes creating enough heat to melt through the tubing wall which can create various sizes/shapes of perforations as illustrated below. The existing research shows that this type of damage is most closely related to charge delivered, that is, the amount of charge transferred by the arc and the duration over which the arc is active. Charge delivered (or the area under the curve of amperes-versus-seconds) is highly predictive of material damage. Copyright 2009 Kraft/Torbin 11

12 Various Examples of Arcing Damage to CSST Copyright 2009 Kraft/Torbin 12

13 3.0 Resistance Heating in CSST as a Conductor 3.1 Background Circuit Breakers Operation and Trip Curves: The purpose of the over-current protective devices (i.e. circuit breakers) is to detect excess current levels (i.e. greater than the rating of the circuit) that are indicative of a circuit fault. When this happens, the circuit breaker disconnects the circuit to prevent damage and/or a safety hazard that may occur due to this fault. The time required for the circuit breaker to trip is dependent on the amount of over-current, and is specified in trip curves supplied by the circuit breaker manufacturer. The trip curves supplied by Square-D (a widely used product) are designated by circuit rating. The maximum rating for circuits using 14, 12 and 10 AWG copper wire is 15, 20 and 30 amperes respectively. Review of Applicable Standards UL-1: It is a generally accepted engineering practice to refer to and compare with existing product standards when determining how to evaluate a new product or requirement. In this case, none of the product standards for any of the approved gas piping materials contain fault current or conductivity requirements. For this reason, it was necessary to turn to other products (and their standards) that are somewhat similar to the CSST product, and where the applicable standard contains these types of requirements. Due to its visual similarity to CSST, Flexible Metal Conduit (and its product standard UL-1) was selected to provide further guidance to the investigation. Pertinent excerpts from the Scope of the UL-1 Standard include the follows: 1 Scope 1.1 These requirements cover flexible aluminum and steel conduit designed for use as metal raceway for wires and cables in accordance with the National Electrical Code, NFPA 70. The majority of the UL-1 Standard addresses either similar construction and/or performance requirements also covered by the CSST product standard (ANSI LC-1). However, not present in the LC-1 Standard, but present in the UL-1 Standard, are performance requirements for Resistance Testing and Fault Current Testing. The Fault Current Testing requirements were of specific interest to the investigation. The Fault Current Testing in UL-1 requires that 3/8 inch and 1/2 inch products withstand 470 amps for 4 seconds. Larger sizes must withstand 750 amps for 4 seconds. However, as mentioned previously, Flexible Metal Conduit is not used for or in the same manner as CSST. A specific performance issue, particular to this investigation, was whether or not either of these two products can be used as an equipment grounding conductor. Flexible Metal Conduit is permitted to be used as an equipment grounding conductor in accordance with NFPA 70 Section Equipment Grounding Conductors. CSST, on the other hand, is not permitted to be used as a grounding conductor in accordance with NFPA 54 Section 7.13 Electrical Bonding and Grounding. This difference in application (as a grounding conductor) significantly impacts the fault current level expected to be carried by the material. The Flexible Metal Conduit, when used as an Copyright 2009 Kraft/Torbin 13

14 equipment grounding conductor, may be the only path to ground for fault currents. Conversely, given that the CSST cannot be used as an equipment grounding conductor, another conductor must be present that is intended to carry the fault current. In other words, CSST will never be intentionally exposed to the full current generated by a fault. These circumstances necessitated a reduction in the amount of test current (as specified in the UL-1 Standard) applied during the fault current testing. Further analysis of the fault current circuit was required to determine the most appropriate levels for testing CSST. 3.2 Determination of Test Current The various installation techniques and configurations involved in the bonding of gas piping must be considered when determining realistic fault current levels. This information includes bonding techniques for CSST, grounding methods of appliances (likely to generate a fault), and the electrical characteristics of the components that will be subjected to the fault current. The equipment ground conductor and the conductive pathway provided by the gas piping (in series with the bonding conductor) lead back to the same physical location, specifically to the electrical system ground (refer to Figure 1). For this reason, the gas piping/bond conductor will be at least as long as the equipment ground conductor. In fact, these two conductive pathways form a parallel circuit between the appliance and ground (refer to Figure 2). The information provides a rough picture of the electrical circuit formed by these conductors, and allows the performance of a circuit analysis of the expected fault current environment. Figure 1 Bonded CSST Figure 2 Fault Circuit The resistance of the conductors in the circuit is also required for a circuit analysis. The electrical resistances of 14, 12, 10 and 6 AWG copper wire are frequently used physical constants which are consistently reported as 2.6, 1.6, 1.0 and 0.4 mω/ft, respectively. The electrical resistance of CSST was not generally known prior to this study. However, independent testing of 1/2 inch CSST did establish a value of 21 mω/ft. This resistance testing was performed on a CSST assembly consisting of a segment of tubing with attached end fittings. The conductivity testing results (see Table 6 in the Appendix) compare favorably to the maximum allowable resistance values as currently proposed to the ANSI LC-1 Standard. These data also confirm that CSST meets the code requirement that all gas piping be electrically continuous. Copyright 2009 Kraft/Torbin 14

15 3.3 Applicable Fault Current Levels With the guidance provided by the UL-1 Standard, and the development of the fault current electrical circuit, it is possible to analyze this circuit to estimate the level of fault current (amps) that may be imposed on the CSST and its duration. The four second duration designated in the UL-1 Standard is appropriate to this electrical insult and was adopted as a test requirement. In determining the fault current applied to the CSST, consideration was given to the nature of the circuit formed and the relative resistances of each parallel leg of the circuit. The CSST is between 8 and 21 times less conductive than the equipment grounding conductor. Even when three-quarters of the length of the CSST/bonding conductor leg is made up of 6 AWG copper bonding conductor, the combined resistance of this leg is still between 2.1 and 5.6 times less conductive than the equipment grounding conductor by itself. Given that this configuration would require a much longer length of CSST plus bonding conductor than the equipment grounding conductor alone (to be practical in real construction), this can be used as a worst case scenario. A rudimentary understanding of parallel circuits and the two-to-one resistance ratio between the equipment grounding conductor and the CSST/bonding conductor allows for a reduction of onehalf of the applied current specified in the UL-1 Standard. In fact, a circuit analysis shows the CSST/bonding conductor leg carrying only one-third of the full current. However, by maintaining the one-half level factor will supply an additional safety factor beyond that allowed by the UL-1 Standard. This resulted in a fault current acceptance criteria that required 3/8 inch and 1/2 inch CSST to withstand 250 amps for 4 seconds, and larger sizes (up to 2 inch) of CSST to withstand 375 amps for 4 seconds. Referring to the trip charts for 15, 20 and 30 ampere circuit breakers, and using a time of 4 seconds, it was determined that a maximum current of 67.5, 90 and 150 amperes was needed, respectively. These values are well within the already proposed current levels, further supporting their adoption. 3.4 Testing Results and Interpretation The resistivity of the CSST material is significantly higher than the resistivity of Flexible Metal Conduit. The effect of this difference would be a significant increase in required voltage to drive the test current through the sample. This higher voltage presents both a safety and available equipment issue to the laboratory performing the testing. The solution to this issue was to decrease the length of the sample from the six feet (as required by the UL-1 Standard) to a one foot segment. As the same current is being passed through the material regardless of length, there is no real change to the validity of the testing. The UL-1 Fault Current Test, with fault current levels modified to 250 amperes and 375 amperes, was performed on 1/2 inch and 3/4 inch CSST (0.010 inch wall thickness) by a nationally recognized testing laboratory (Intertek ETL-Semko, Montreal, CN). All of the test samples withstood the imposed current levels without deformation or visible damage to the fittings or tubing. Copyright 2009 Kraft/Torbin 15

16 At completion of the testing, all test samples were returned to the manufacturer for leak testing in accordance with the requirements of the ANSI LC-1 Standard. All samples tested demonstrated tight fitting connections with the tubing and none of the samples exhibited any leakage. There is sufficient similarity and guidance available within the UL-1 Standard Flexible Metal Conduit to develop a CSST-based testing protocol for fault current with appropriately adjusted values for current magnitude (test duration remaining the same). This allowed the development of a test protocol that could be used to verify the suitable performance of CSST when subjected to the electrical insult expected from an electrical fault. The performance of the material subjected to the testing protocol at both designated current levels on the appropriately sized CSST test segments (including fittings) clearly demonstrated the ability of the CSST material to withstand the expected fault currents that may be imposed on it by an electrical system fault. Test data is provided in the Appendix of this report. Copyright 2009 Kraft/Torbin 16

17 4.0 Resistance Heating and Mechanical Stability in Connectors 4.1 Background Bond Clamps: The intended purpose of attaching bonding clamps to the CSST fittings is to provide a dedicated, permanent connection between the gas piping system and a low impedance conductor that is connected to the electrical ground system. For a bonding clamp to perform its intended purpose, it must provide low electrical resistance and a stable mechanical connection to the CSST fitting. It had been previously determined that a bond connection directly to the tubing itself would prove to be unacceptable because of the lack of mechanical robustness associated with the thin tubing wall and the lack of a uniform corrugation profile between the different CSST brands. The brass, hex-shaped fittings provide a much more mechanically stable and electrically conductive platform for making the bonding connection. The connection between the clamp and the CSST system, specifically the robustness and electrical properties of this connection, are of particular interest. The currently listed bonding configuration for a water piping system is a bond clamp attached to round copper pipe. The CSST configuration is a bond clamp connection made on the hex-shaped nut of the CSST fitting. Upon investigation, it was found that the common design of commercially available bonding clamps (those reviewed at the time) will significantly mitigate the differences in geometry (between the clamp and the clamping surface). Clamp Design and Hex-Shaped Fitting: The commercially available clamps examined are manufactured in an octagonal-shaped configuration, with a variety of ridges on the inside of the clamp for establishing mechanical contact with the sub-surface. This design makes the clamp size adaptive by allowing it to be used over a wide range of pipe sizes. When installed properly, this design results in four points of contact (where the clamp ridges contact the pipe) between the pipe and the clamp as shown in Figure 3. This leads to a relatively narrow width contact patch between the clamp and the pipe measuring no more than approximately 1/8 inch. When these same clamps are applied to a CSST fitting, four similar points of contact are formed where the ridges contact the hex nut as shown in Figure 4. Visual examination of these various connections (based on many different clamp products) shows no significant variation or differences. Even more illustrative of the substantial mechanical character of this connection is the low electrical resistance (which is discussed in greater detail) which corroborates the visual conclusions. Test methods were developed to evaluate the mechanical-electrical connection of a bonding clamp on a CSST fitting. These evaluations allowed a comparison of the connection of the clamp on a hex-shaped brass CSST fitting to the currently listed configuration on a round rigid pipe (either steel or copper). Copyright 2009 Kraft/Torbin 17

18 Figure 3 Clamp on Round Pipe Figure 4 Clamp on Hex Nut Review of Applicable Standards UL-467: When evaluating products for a new application, it is standard engineering practice to examine the existing product standard and test methods, and employ or adapt similar test methods to the new application. The product safety standard referenced by the National Electrical Code for bonding clamps is UL-467, Grounding and Bonding Equipment. This standard has been used for preliminary guidance in evaluating bonding clamps for use with hex-shaped CSST fittings. Pertinent excerpts from the Scope of the UL-467 Standard include the following: 1 Scope 1. 1 These requirements cover grounding and bonding equipment for use in connection with interior wiring systems in accordance with the National Electrical Code, NFPA 70. These requirements also cover hospital grounding jacks and the mating grounding cord assemblies. 1.2 These requirements cover ground clamps, bonding devices, ground mesh, grounding and bonding bushings, water-meter shunts, armored grounding wire, ground rods, and the like. It is clear from the scope that UL-467 is a general product standard and that many of its requirements will not be applicable to bonding clamps. In fact, a thorough review of the document identifies only two requirements that are applicable to evaluating a currently tested and listed bonding clamp for attachment to a new product/geometry (e.g. the CSST fitting). The first requirement is a Pull Test to evaluate the ability of a clamp intended for use with armored cable to withstand a load of 150 lb. While this requirement is intended and designed to evaluate the robustness of the mechanical connection of the cable armor to the clamp, it can also be used for guidance in developing testing requirements specific to the mechanical interface between the clamp and the CSST fitting. The second applicable requirement concerns the electrical resistance of the connection between the clamp and fitting or pipe and has been replicated for further understanding: Copyright 2009 Kraft/Torbin 18