CSST – The Next Generation

The International Code Council (ICC) and the National Association of Home Builders (NAHB) Building Product Issues Committee completed a joint survey of code officials across the U.S., in 2013. The survey was conducted to investigate the quality of construction in residential and commercial buildings.

The primary focus of the study was to identify practices most likely to be flagged during construction resulting in a code violation. Among various construction areas, survey respondents were asked to identify the top three most common mechanical/fuel gas system violations. The most prevalent violation was inadequate combustion air or makeup air. Improper grounding or bonding of gas piping was noted in the Top 10 causes of code violations at No. 8.

The corrugated stainless steel tubing (CSST) has been associated with the issue of proper bonding and grounding for over a decade, and CSST manufacturers have supported code proposals for the bonding of all gas piping. As part of this effort, the CSST industry has supported comprehensive research projects to validate the efficacy of bonding to mitigate the effects of arcing induced by indirect lightning strikes. These research projects have led to changes in model fuel gas codes, national product standards, and installation instructions, as well as the introduction of a new generation of CSST products design to resist damage from electrical arcing. While the direct bonding of all types of gas piping (steel, copper and CSST) would universally address the lightning arcing problem, other approaches to mitigating the damage caused by arcing are also being introduced such as arc-resistant jackets.

Code history

Requirements for bonding of gas piping systems have always been included in both the National Electrical Code (NEC) and the National Fuel Gas Code (NFGC). Going back to 1988 when CSST was first included in the NFGC, bonding of all gas piping (regardless of material) to any of the grounding electrodes was a requirement. Up to the 2002 edition, the NEC and NFGC codes both had similar bonding requirements for gas piping systems: either bonded directly to the grounding electrode system (NEC-1999) or directly to the grounding electrode (NFGC-1999). This was commonly interpreted to mean an electrical connection using a 6 AWG copper wire in a fashion similar to bonding of the copper water service.

However, in the 2002 editions of these two codes, the requirements were harmonized and modified to limit the need for electrical protection to ground fault conditions only. This required bonding only when there was a branch circuit serving the gas appliance, and the size of the bonding conductor was based on the size of the branch circuit in accordance with Table 250.122. This requirement applies to all gas piping systems regardless of the type of gas piping installed, and usually results in a 14 AWG bonding conductor based on a 15 A circuit. The unintended consequence of the code changes made in 2002 has resulted in some gas systems having no bonding connection to ground at all or an electrical connection with relatively high impedance.

During the period of 2002 to 2012, damage to gas piping systems from electrical arcing associated with lightning was being recognized more often by fire investigators, manufacturers and building officials. The gas industry engaged in research and investigations to determine the root cause of arcing between the gas piping (including CSST) and the electrical service or another metallic system induced from both direct and indirect lightning strikes. The primary causes were linked to the lack of equi-potential bonding of all metallic pathways, the use of a much smaller bonding conductor (14 AWG) on gas piping compared to copper water pipe or electric power cable (6 or 4 AWG), the close proximity of unequally bonded metallic systems (such as crossed wiring and piping), and the large-scale introduction of metallic appliance vents without a direct connection to ground.

Modifications were proposed and adopted into both the 2009 and 2012 editions of the National Fuel Gas Code (NFPA 54) resulting in new requirements for the mandatory direct bonding of all CSST gas piping systems. Both editions dictate that the CSST be bonded directly to the grounding electrode system using at least a 6 AWG copper wire (or equivalent) with a single point of attachment downstream of the point of delivery. The connections to the grounding electrode system are stipulated in NEC 250.104(B). The extra bonding requirement is in addition to bonding for ground fault protection, and has not been included in either the 2011 or 2014 editions of the National Electrical Code (Section 250.104B). However, starting in the 2011 NEC, an Informational Note was added that directs the attention of the electrical contractor to NFPA 54 for further bonding requirements for gas piping.

The discrepancy in code language between the NEC and NFGC has caused many unnecessary conflicts in the field on how and who does the bonding. Electrical contractors do not automatically assume the responsibility to install the extra bonding connection required for CSST as stipulated in the fuel gas code. The plumber is not permitted to do the bonding in most states. Code officials have called into question the efficacy of the CSST bonding. However, it was generally agreed that the new bonding requirements did not violate the NEC requirements for bonding of gas piping systems. Furthermore, the additional bonding requirements for CSST were consistent with bonding requirements for gas piping systems included in NFPA 780 Standard for Installation of Lightning Protection Systems. Bonding of all gas piping (all materials) using a 6 AWG copper wire is commonly required by electrical codes adopted in many other industrialized countries such as Canada, United Kingdom, and South Africa.

Alternative method of bonding

While bonding has been used for years as the primary means of electrical protection against arcing, it has limitations. There are many variables that can reduce the effectiveness of bonding and/or improperly bonded systems will suffer some level of deficiency in the amount of protection provided. For example, the quality of the connection to ground directly impacts the level of protection regardless of how well a system is bonded. If the grounding electrode is inadequately sized or poorly installed, its ability to dissipate the electrical energy will be severely compromised. The quality of the earth itself (such as soil type, moisture content, short and long-term weather conditions) affected the level of achievable grounding. The NEC requires no more than 25 ohms of soil resistance or a second electrode/rod must be installed. This does not insure that minimum ground resistance is achieved or maintained throughout the year.

The length of the bonding conductor is another performance factor. The rule of thumb here is “shorter is better.” However, the NEC does not provide any guidance on setting the maximum permissible length for either bonding conductors or grounding electrode conductors. Depending on the location of the fuel gas service entrance, relative to the location of the point of attachment to the grounding electrode system, the length of the bonding conductor for a CSST system can be well over 100 feet in many single family houses.

The lack of equipotential bonding as a basic requirement creates the likely scenario that large differentials in voltage potentials will exist between different metallic systems being energized. The NEC permits the use of differently sized bonding conductors for similar metallic systems within the house (such as electric wiring, gas piping, copper water service, air conditioning refrigeration lines etc.). Some metallic systems are not bonded at all (such as electrically isolated heat and cooling ducts). This situation allows a condition where the lightning energy will rise to different levels in these various pathways and the voltage rise time will be out-of-sync. The use of flexible gas piping and flexible electrical wiring also sets up the “perfect storm,” where metallic systems at different voltage potentials are put in close proximity to each other, and thus maximizing the potential for arcing.

Omega Flex recognized many of these fundamental technical issues and institutional limitations. It chose to engineer a more comprehensive solution to the arcing problem that provided built-in, arc protection along the entire length of the gas piping system. This approach is far less dependent on the vagaries of code interpretations and enforcement. In 2004, Omega Flex introduced an alternative to direct CSST bonding called CounterStrike, using an innovative conductive jacket as shown on Figure 1. In 2007, Omega Flex was able to improve the electrical performance of the CounterStrike conductive jacket. The level of protection against electrical arcing for CounterStrike is comparable to the level of protection provided by bonding to the grounding electrode system.

As shown in Figure 2, energy from a lightning arc is absorbed and dispersed within the jacket which significantly reduces the energy level that reaches the tubing wall. This makes it much more difficult for the arc beam to penetrate the CSST. Based on the performance of CounterStrike against lightning induced arcing, Omega Flex only requires that CSST be bonded like steel pipe for ground fault protection (NEC 250.104(B)), and does not require the installation of an additional bonding conductor as stipulated in the NFGC for lightning protection (7.13.2).

To date, all conductive jacket CSST products have been tested and listed to a bench standard (LC-1024) developed by ICC Evaluation Service. Currently, they are listed by ICC PMG and/or IAPMO R&T. Based on the ICC bench standard, the nationally recognized ANSI LC-1 CSST Standard has been revised and expanded to now include new testing requirements that establish the minimum level of performance for the conductive jackets. The new tests include electrical arc resistance (minimum 4.5 Coulombs) based on a lightning profile typical of many direct strikes (extremely conservative approach); extreme low temperatures elongation testing of the jacket material; the resistance of the jacket material to wear and tearing during installation; and additional corrosion testing for multi-layered jacket designs that contain metallic components. Conductive jacket CSST is expected to be tested and listed by CSA (a nationally recognized testing laboratory) during the second half of 2014. To date, 15 states have already approved the use of conductive jacket CSST without the need for the additional bonding through either a state code modification or state-wide product approval or by adopting the manufacturer’s instruction for CSST bonding requirements.

Evaluation of CounterStrike Arc-Resistant Jacket

As part of the due diligence process involving the NFGC, the NFPA Standards Council issued a Decision (D#10-2 dated 3 March 2010) requesting the CSST industry to demonstrate that the bonding methods being used would be effective by reducing or eliminating the effects of transient electrical arcing on CSST. This directive resulted in the initiation of a two-phase research project (funded by CSST manufacturers and conducted by the Gas Technology Institute) to demonstrate the efficacy of the bonding requirements imposed by the National Fuel Gas Code. This research involved a number of physical tests on CSST products to determine electrical performance characteristics, and the development of a computer model of a typical CSST piping system configured to simulate lightning strikes at various locations within the house.
Figure 3 depicts a typical piping system and lightning strike scenario. Finally, a full-size mockup was constructed and tested to validate the model results and to confirm the various outcomes from the computer simulations. The final technical report [ref 2] confirmed that the recommended bonding methods described in the NFGC 2009 and 2012 editions would provide for effective mitigation of lightning damage to CSST systems, and provides a reasonable level of safety.

Based on the results from the GTI research, there were no lightning strike scenarios where the induced energy level within any of the metallic systems within the house would have been high enough to damage the arc-resistant jacket used on CounterStrike. The highest level of arcing energy (based on the worst case lightning scenario and no bonding) was 4.75 Coulombs, which is below the damage threshold value for CounterStrike. All other lightning simulations resulted in arcing energy levels well below 4 Coulombs. It can, therefore, be reasonably deduced that arc-resistant CSST would perform equal or better than bonded yellow CSST.

In 2011, Omega Flex unilaterally announced that it would only offer its CounterStrike CSST product in the U.S. market. This bold move has not resulted in any loss of market share, and generally reflects the demand of the plumbing community for safety driven gas piping products. Over 50 million feet of CounterStrike have been sold and installed since its introduction in 2004. To date, it has established the highest possible safety record over a 10-year period of time.


The CSST industry has introduced the next generation of CSST products that are passively protected by using an arc-resistant jacket to absorb arcing energy anywhere along the length of the piping system. The installation of conductive jacket CSST avoids any gaps in code coverage and enforcement as these black jacketed CSST products will provide electrical arcing protection whether directly bonded or not. The arc-resistant performance requirements of the conductive jackets have been captured within the nationally recognized ANSI LC-1 Standard to provide CSST users assurances on product behavior.

Efforts are underway to modify state and local fuel gas codes to require that arc-resistant CSST need only be bonded the same as steel pipe and without the additional bonding conductor as stipulated in the most recent editions of the NFGC. Installation of the arc-resistant jacket will further enhance the track record of CSST as the safest type of gas piping available. Ultimately, the exceptional electrical and mechanical properties of this next generation of CSST may result in the elimination of conventional yellow CSST from the marketplace. l

Bob Torbin is director of codes and standards for Omega Flex, Inc.

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