Code Stories · why the rule exists
Gas: CSST Bonding and Lightning
The rule in Massachusetts today
Not in facts.json. Verbatim, 248 CMR 5.07(5): "Delete sub-section 7.13.2 and replace with the following: 7.13.2 CSST. CSST systems shall be bonded in accordance with subsection 7.13.1 unless such bonding would be in conflict with manufacturer installation instructions. However, to the extent the manufacturer's installation instructions require additional bonding which falls within the scope of electrical licensure, this code shall not govern such requirements. In such instances, the installation may not be approved by a plumbing or gas inspector unless there is proof that an electrical permit has been obtained to perform such bonding."
Why it exists
Lightning that strikes a building, or strikes near it, can raise nearby metal systems such as wiring, ducts, water pipe and the gas line to very different voltages. CSST has a thin wall. When the voltage between it and a nearby metal object gets high enough, an arc jumps the gap and can burn a pinhole through the tubing, and the leaking gas can ignite. Bonding ties the CSST to the building's grounding system so its voltage rises and falls with the other metal, which reduces the chance of an arc. Bonding lowers the risk but does not eliminate it. Research in 2011 concluded that a more complete equipotential approach is needed. Manufacturers responded with arc-resistant jackets.
How it came to be
- 2009 — Fire Engineering describes the field reports that drove the code change. fireengineering.com
- 2009 — The 2009 edition of NFPA 54 adds CSST bonding requirements. inspectapedia.com
- 2011 — Fire Protection Research Foundation report 'Validation of Installation Methods for CSST Gas Piping to Mitigate Lightning Related Damage' (April 2011). subrogationrecoverylawblog.com
- 2012 — NFPA 54 2012 moves the bonding connection point. inspectapedia.com
- 2017 — International Association of Fire Chiefs position on the mechanism. iafc.org
Make it stick
Mnemonic: 'Bond it so it rises together.' Lightning damage is not about the voltage itself but about the difference between two pieces of metal. Tie them together and the arc has less reason to jump. Picture the result without it: a spark-welded pinhole in a gas line.
On the exam
Expect: why CSST is bonded (lightning arcing through the thin wall), that it is bonded to the electrical grounding system (bank q102), and the MA twist. 248 CMR 5.07(5) replaces NFPA 54-2012 7.13.2: bond per 7.13.1 unless that conflicts with the manufacturer's instructions, and manufacturer-required bonding that falls under electrical licensure needs an electrical permit before the gas inspector approves. National details such as a 6 AWG copper jumper at the first downstream CSST fitting (NFPA 54-2012 7.13.2), or a jumper of 75 ft or less (a later NFPA 54 edition), are NOT the MA rule as written. In MA they apply only through the manufacturer's instructions. Arc-resistant jacketed CSST follows its own listing.
What people get wrong
- The appliance's electrical ground already bonds the gas line. Do not assume it. MA's rule (248 CMR 5.07(5)) defers to the CSST manufacturer's instructions, and for standard yellow CSST those call for a direct bond to the grounding electrode system. That bonding is electrical work, so the gas inspector may not approve the job without proof of an electrical permit.
- Bonded CSST is lightning-proof. The 2011 FPRF study found end-point bonding alone insufficient for a complete solution. Bonding reduces risk.
- Clamp the bond anywhere on the yellow tubing. No. Manufacturers forbid clamping the corrugated tubing; the clamp goes on rigid pipe or a CSST brass fitting as their instructions specify. The 2012 NFPA 54 wording (metallic pipe or fitting between the point of delivery and the first CSST fitting) is the national rule. MA replaced it with 'follow the manufacturer' (248 CMR 5.07(5)).
Try it: 2 real practice questions
Q1. CSST (corrugated stainless steel tubing) gas piping systems must be:
A nearby lightning strike can put high voltage on a building's metal systems. Thin-walled CSST can be perforated by arcing between it and other grounded metal, starting a gas fire. Bonding keeps the tubing at the same electrical potential as other metal in the building, which reduces that arcing.
248 CMR 4.00Q2. CSST must be bonded because:
CSST's corrugated stainless wall is very thin, and when a nearby lightning strike energizes the building, a voltage difference between the CSST and other grounded metal can arc across it and burn a hole, starting a gas fire. Bonding it directly to the grounding electrode system keeps it at the same potential as other metal so no arc forms.
248 CMR 5.00These come from the PressureTest bank. Start 7 days free for the full bank, timed exam simulations and drawn isometric problems.