Corrugated stainless steel tubing (CSST) was first introduced in the United States in the 1980s as a less-expensive material than black steel pipe for fuel gas piping in buildings. The Japanese developed CSST after several earthquakes caused broken gas lines, resulting in explosions and fires. CSST manufacturers claimed they had cost advantages over black steel piping installations because of the reduction in the number of fittings and joints and by minimizing the labor needed for threading black steel pipe.

Soon after its introduction, CSST caught on with homebuilders. The reasons for its popularity all relate to simple economics — it was less expensive than black steel pipe, and it required less labor to install. However, was it safe? 

The first generation of CSST consisted of stainless-steel pipe that was thin enough and corrugated so that it was flexible and could be easily bent during installation. The next generation of CSST tubing designs added plastic coatings, and then later, sheaths of a tightly fitting polymer sleeve were used. There are several manufacturers of the gas tubing, and each has its own type of sheathing. 

The use of fuel gas can be dangerous inside a building or confined space. Special attention must be given to the proper design, location, installation, testing and application of the gas piping system. You should never mix the fittings from one CSST manufacturer with another because the fittings are different. 

Good engineering, installation, testing and inspections, and adhering to the manufacturer’s installation instructions should be exercised to ensure the safe operation of the fuel gas system. All installed systems should have a permit pulled for inspection, and they should be inspected by the local authorities having jurisdiction to look for conformance to installation requirements and the codes. Also, they can verify that the system holds pressure prior to being placed into service.

CSST and Lightning

Not too long after the introduction of CSST into lightning-prone areas, many fires occurred that have been attributed to the failure of CSST when the structure was struck by lightning. 

As time progressed and many installations of CSST in homes were completed, there was a noticeable rise in fires reported in those homes that were related to lightning strikes. Holes were blown into the thin wall of the corrugated stainless-steel gas tubing when arcing occurred between the CSST and other metal objects within the structure. 

This lightning-arc phenomenon contributed to the rapid growth of fires. Holes associated with lightning are very rare in fuel gas piping systems piped with Schedule 40 black steel piping. Soon after, there was a full debate over the bonding and grounding of the CSST gas piping systems. Some codes started to require CSST systems to be bonded and grounded, while other jurisdictions prohibited bonding and grounding. 

Some manufacturers require grounding and bonding; others require grounding and bonding to be in conformance with the local codes. So, make sure you get clarification from the code official having jurisdiction in your area about the bonding and grounding of CSST systems. 

When lightning strikes a building, it seeks a path from the cloud to the ground. It rarely strikes the CSST tubing in a home directly; it strikes a metal chimney or an antenna, and then the current flows through the metal objects, jumping on and off other metal objects via an arc as it seeks a path to ground. The arcing to CSST piping is typically indirect lightning arcs; the indirect currents are generally lower voltages than a direct lightning strike. 

The indirect lightning arcs between metal components, including the CSST, can instantly heat up and vaporize or spatter the metal in the wall of the CSST tubing at the point where the arc contacts the tube — even if the gas line is bonded and grounded. The metal vaporizes, resulting in a hole that allows fuel gas to escape and generally contributes to the growth of a fire. 

CSST manufacturers soon realized this was a vulnerability of the thin-walled material. They began looking at ways to arrest the arcing by installing arc suppression jackets over the CSST tubing. The arcing can occur on the jacket, thereby minimizing damage to the tube wall. 

The challenge was with the thickness of the CSST wall; it had to be thin enough to be flexible. If the walls are thicker, then the tubing loses flexibility. When the tubing wall is thin, it allows lightning arcs to blow a hole in the wall. 

Flash Density Rates

According to the National Lightning Safety Institute, each part of the country experiences a different rate of lightning strike occurrences, known as the “flash density rate.” Figure 1 is a map from the National Lightning Detection Network showing lightning density across the United States over a specific period. As you can see, some areas of the country are more prone to lightning strikes than others. This is usually associated with hot, humid weather and thunderstorms. 

If a CSST system is grounded and a building is struck by lightning, the arc can still jump onto the tubing in its path to ground. Bonding is making all the fittings and tubing electrically continuous, which helps the current travel down the CSST to ground — through the grounding clamp to the ground rod. Grounding and bonding can reduce the amount of arcing, but it will not eliminate arcing between other metal objects in the building and the potential for fires. 

My father taught me about electrical wiring, including bonding and grounding, when I worked with him as an apprentice electrician. My father was a licensed electrician and two of my brothers were electricians. CSST did not exist when I worked as an electrician. I would install ground rods, grounding conductors and grounding clamps on water lines. 

There seems to be a lot of confusion related to the ability of bonding and grounding of CSST and if it minimizes lightning arc damage. Most CSST manufacturers developed arc-resistant tubing with conductive jackets to accept the indirect arcs from lightning and reduce the risk of developing a hole. 

When I design the fuel gas systems in institutional, health-care and other high-profile buildings where evacuation of occupants could be problematic, I prefer to specify at least Schedule 40 black steel pipe and fittings. 

The National Fire Protection Association (NFPA) stated that when CSST was first considered in its fuel gas codes in 1988, lightning was not a consideration.

CSST manufacturers require an installer to take a product training and installation course. The installation courses are required as part of the CSST tubing standard ANSI LC1 and are an attempt to ensure only qualified installers put in such systems. A manufacturer’s product installation certification prevents CSST from being available for sale to the public at the big-box home improvement stores. 

CSST was first recognized by the NFPA in its NFPA 54 National Fuel Gas Code, 1988. The International Association of Plumbing & Mechanical Officials (IAPMO) approved it in 2003 after some resistance to the material in 2000. Recently, NFPA took over the fuel gas code for IAPMO. 

CSST Usage 

Today’s arc-resistant CSST can be identified by its black polymer arc-resistant jacket or expanded metal or braided jacket over the tubing and a black polymer sheath. CSST systems need to be pressure-tested in accordance with manufacturers’ requirements, industry standards and code requirements. Where there is a conflict, the more stringent language should be followed. 

The trade-offs with CSST are lower price, ease of installation, and the ability to resist building settling or earthquakes compared to black steel pipe. However, black steel pipe rarely leaks after a lightning strike on a building. Black steel pipe systems also can accommodate vibrations or structural shifts with flexible appliance connectors in appropriate locations. 

CSST products and materials are continually being improved. The plumbing and pipefitting industry has a duty to examine these new products and review their standards during the code revision process, where the proponents propose adoption of the new products with an eye on performance as well as health and safety. 

As with any new product, there will be advantages and disadvantages over old products. The disadvantages of a new product are worked out to a point where there are trade-offs in quality, cost and installation time versus traditional black steel pipe, fittings and valve materials; however, safety is an important concern. An old friend once told me, “Safety should trump labor savings, cost savings and energy savings in cases where it is warranted.” 

This is where I have a concern about a model code organization requiring a check to be placed in a box on a code change proposal form with the following question, “Will this code change increase the cost of construction?” If you check yes, you must then explain in detail how it increases the cost, or your code change can get dismissed as incomplete. Yet, there is no corresponding box to check for, “Will this improve the health and safety of the code?” with an explanation of the virtues of safety. 

I’m all for removing the cost question. Or the code organizations can add the other question and then show how it can improve safety and reduce overall life cycle costs, maintenance costs and loss of revenue. Also, costs should include medical costs associated with injuries from failed products, the value of lives lost, and the cost of replacing the building when the product fails, perhaps causing an explosion, fire, flood or other calamities. 

Manufacturers of black-jacketed, electrically conductive (arc-resistant) CSST products that have been tested and listed to ANSI LC1 Section 5.16, Arc Resistant Jacket or Covering System, may not require or include instructions for the additional direct-bonding step that is required with many of the older-style, standard yellow CSST products. However, local codes are controlling and may differ from manufacturers’ requirements. Local codes are governing and must be adhered to.

Standards for CSST Systems 

• ANSI LC1/CSA 6.26, Fuel Gas Piping Systems 

Using Corrugated Stainless-Steel Tubing (CSST)

• ANSI LC1/CSA 6.26, 25 PSI Operating Pressure Rating

• ANSI LC1/CSA 6.26 Sec. 5.16, Arc-Resistant
(AR) Jacket Rating

• ICC-ES PMG LC1027, Protective Jacketed CSST, 

A Minimum 36-Coulomb Charge Transfer

Codes Related to CSST Systems 

• ICC, International Code Council Series

• Canada, National Gas & Propane Installation 

Code B149.1

• NFPA, National Fuel Gas Code (NFPA 54)

• UMC, Uniform Mechanical Code

• UPC, Uniform Plumbing Code

So, to answer the question, is CSST safe for fuel gas distribution systems? I would say it depends on if it was the right product and was installed properly. For me, if I lived high on a hill in an area prone to lightning strikes, I might lean toward black steel for my fuel gas pipe system.