Corrugated stainless steel tubing

Corrugated stainless steel tubing (CSST) is tubing made of stainless steel with corrugation on the inside or outside.

CSST is not FAC (Flexible Appliance Connector) tubing. Presently, CSST and FAC tubing both use corrugated stainless steel tubing. In the past, FAC used corrugated brass tubing. However, FAC tubing is made only in lengths of 1 to 6 feet, FAC tubing is made with connectors on each end, and FAC tubing does not have a plastic sheath. On FAC tubing the corrugations are visible. CSST's plastic sheath covers its corrugations.

Variants

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The most common CSST is a type of conduit used for fuel gas distribution in buildings. It has a flexible corrugated stainless steel inner layer and an outer yellow or black plastic jacket. Yellow-jacketed CSST was developed first and is the most common. It has a non-conductive plastic yellow jacket. Black-jacketed CSST is relatively new. Its black jacket is electrically conductive. Manufacturer information indicates this conductive jacket dissipates the energy of indirect lightning strikes that might otherwise pierce or damage the yellow-jacketed CSST.

A less common type of CSST does not have a plastic jacket. It is mainly used in heat exchangers.

Usage

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CSST is used as an alternative to the older standard “black iron” (steel) gas pipe. The inner stainless steel layer of typical residential CSST is .2 to .3 mm thick while the wall thickness of typical residential “black iron” gas pipe is 3 to 4 mm thick. CSST is more expensive than “black iron” gas pipe. However, CSST requires fewer joints, may be safer during earthquakes, and may be easier to install.

Properties

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CSST is alleged to have an increased risk for lightning related fires. Some believe that CSST is more likely to be pierced by lightning than “black iron” pipe. There are varied opinions over CSST's fire risk and it has been involved in litigation.

Installation

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Buildings with CSST should be inspected by experts (licensed plumbers and electricians) to verify proper bonding and installation. The CSST experts should be qualified to assure compliance with the requirements of the manufacturer and local codes. Be aware that most home inspections are not done by licensed plumbers and electricians. Building owners are advised to assure that their CSST system complies with their insurance company's requirements.

To reduce the possibility of CSST lightning damage: (1) Yellow-jacketed CSST should be bonded (connected to the electrical service panel's ground bus) at its entrance into a building. Many local codes require this. (2) All CSST should be installed away from metal objects such as metal duct work, metal pipes, electrical wiring, metal beams, or metal conduit.

CSST should not make contact with sharp objects or edges to reduce the possibility of leaks caused by piercing. For example, it should not be installed where things such as cabinet screws, picture hanger nails, or drywall screws might pierce it.

References

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Is Gas Pipe Grounding Legal?

You must not use an underground metal gas piping system as a grounding electrode. So, what's the confusion all about?

The controversy behind bonding and grounding metal gas piping systems arose when many electrical professionals questioned the rules and regulations of the National Fuel Gas Code (NFPA) 54. For example, Sec. 3.14(a) in NFPA 54 requires each aboveground portion of a gas piping system upstream from the equipment shutoff valve to be electrically continuous and bonded to any grounding electrode. Sec. 250-104(b) of the National Electrical Code (NEC) agrees with this mandate. On the other hand, NFPA 54 [Sec. 3.14(b)] parallels Sec. 250-52(a) of the NEC, which states: "You must not use an underground metal gas piping system as a grounding electrode" No wonder so many people are uncertain.

Let's take a closer look at the rules to help clarify the position of both Codes.

Metal gas piping: NEC Sec. 250-104(b) and NFPA 54 [Sec. 3.14(a)]. Sec. 250-104(b) of the NEC and NFPA 54, Sec. 3.14(a) requires you to bond the aboveground portion of a metal gas piping system to a grounding electrode system for safety reasons. NFPA 54, Sec. 3.15 does not allow you to use aboveground portions of a metal gas piping system or its components as a conductor in electrical circuits. However, NFPA 54, Sec. 3.15, Ex. recognizes under certain conditions, which applies when you are using low-voltage (50V or less) control circuits, ignition circuits, and electronic flame detection device circuits as piping or components of an electric circuit.

You should size the bonding jumper connecting and bonding the metal gas piping to the grounding electrode system as noted in Table 250-66 of the NEC; based on the largest ungrounded service phase conductor. You should size the conductor just as you would size it if it were a metal water pipe isolated from earth ground because of a nonmetallic underground water supply system.

However, some disagree. They believe you should size it from Table 250-122, determined by the size of the overcurrent protection device ahead of the circuit. The only problem we have with this concept and method of sizing is which protective device should you use: the service device, feeder device, or branch-circuit device? Due to this uncertainty, we feel the need for a proposal for the 2002 NEC to clarify which Table to use when sizing this conductor. Depending on where you're working, you should check with the AHJ for their official interpretation.

Other metal piping: NEC Sec. 250-104(c). Sec. 250-104(c) in the 1999 NEC (previously Sec. 250-80 in the 1971 NEC) mandates you must bond all interior metal piping (subjected to energization) to the service equipment ground and the common grounding electrode conductor at the service enclosure (if it's of sufficient size) or to one or more of the grounding electrodes.

You must size this bonding jumper according to Table 250-122. As a means for bonding, you may use the equipment-grounding conductor for the circuit that may energize the piping.

For example, you may use the equipment-grounding conductor in a circuit supplying a gas range to bond and ground the metal gas piping as well as the enclosure of the range.

Nonmetallic piping concerns. Nonmetallic piping has come into use in recent years in many different piping systems. We use it in new installations as a complete replacement to traditional metallic piping systems or as a replacement component in older metallic systems. When used as a replacement component (i.e. sections of nonmetallic piping inserted in a metallic piping system), it breaks the continuity of the metal-to-metal contact throughout the piping system. The use of nonmetallic unions or joints has the same effect.

If an isolated section of metallic water (or gas) piping accidentally becomes charged, a shock or fire hazard may occur. Grounding all isolated sections removes this hazard and provides safety for the user. Where there's a possibility that any metallic piping on a premises might come in contact with a "hot" wire or become energized, you must bridge all nonmetallic breaks with a bonding jumper and ground the entire piping system to the grounding electrode system. You must size this bonding jumper based on the rating of the overcurrent protection device ahead of the circuit that could make contact with the metallic piping.

In summary, you cannot use a metal underground gas piping system as a grounding electrode. However, you must bond the piping system to the grounding electrode system for safety reasons.

Sidebar: Terms to Know.

Bond: The permanent joining of metallic parts to form an electrically conductive path that will ensure electrical continuity and the capacity to conduct safely any potential current.

Bonding jumper: A reliable conductor to ensure the required electrical conductivity between metal parts required to electrically connect the equipment.

Ground: A grounding connection between an electrical circuit or equipment and the earth, or to some conducting body that serves in place of the earth.

Grounded: Connected to earth or to some conducting body that serves in place of the earth.

Grounding conductor: A conductor used to connect equipment or the grounded circuit of a wiring system to a grounding electrode or electrodes.

Grounding conductor, equipment: The conductor used to connect the noncurrent-carrying metal parts of equipment, raceways, and other enclosures to the system grounded conductor, grounding electrode conductor, or both; at the service equipment or the source of a separately derived system.

Grounding electrode conductor: The conductor used to connect the grounding electrode to the equipment-grounding conductor, to the grounded conductor, or to both; of the circuit at the service equipment or at the source of a separately derived system.

Piping: As used in codes, it can be either pipe or tubing, or both.

(a) Pipe (rigid conduit of iron, steel, copper, brass, aluminum, or plastic)

(b) Tubing (semi-rigid conduit of copper, steel, aluminum, or plastic)

Piping system: All piping, valves and fittings from the outlet of the point of delivery from the supplier to the outlets of the equipment shutoff valves.