What is the concern?
Recent fires in residential properties have brought to light a possible concern with Corrugated Stainless Steel Tubing, otherwise known as CSST. The concern is that lightning, striking on or near a property can cause an electrical arc to puncture the CSST tube and subsequently ignite the gas causing a fire. A professional Home Inspection from a qualified Home Inspector will help identify safety concerns such as this.
What is CSST?
It is produced in coils and air-tested for leaks and has standard gas fittings attached to either end to allow it to be connected to rigid piping or other CSST lengths.
It is used in residential properties primarily to install gas appliances such as boilers, furnaces, water heaters, gas stoves and barbeques for example, and to provide longer gas runs in areas difficult to install the more common rigid black steel piping.
Is the concern a real one?
In a word, yes. The CSST steel pipe is about 1/15th of the thickness of a rigid black steel pipe. This makes it easier to puncture both from mechanical damage and from things like high current arcs, such as found in a lightning strike.
The PVC coating is typically thought to protect the steel tube inner from electrical current, but the voltages and amps in a lightning strike can be vast, and the thin coating of PVC no match for the ability of the electricity produce in a storm to penetrate to where it needs to go.
Although the holes in the CSST seen produced from these storms is small, coupled with the ability of an electric spark to ignite the gas under pressure, the devastation can be catastrophic.
How does lightning cause this?
Most people think lightning comes from out of the sky to the ground, or jumps between clouds in the sky, but this is not the case. Lightning is created inside a single cloud formation. Lightning is just static, and it is created by particles rubbing against each other to create an electric potential. In clouds this potential is built up between the tops and the bottoms of a cloud. The top of a thundercloud is normally at such a height that the water molecules inside the cloud turn to ice and rub against each other creating a positive charge. This positive charge actually starts to create a negative charge at the bottom of the cloud. When the charges are sufficiently great, a spark is created between the top of the cloud and the bottom of the cloud and lightning is created. At the same time, because the clouds are negatively charged at the bottom it causes a positive charge to be created at ground level. Again when the potential is sufficiently different a spark is created, this time from the ground UP to the clouds, and it meets with a charge coming down from the clouds.
So why should lightning outside cause a problem inside?
The main objective of Lightning, or more importantly the electrical energy of lightning, is to find the path of least resistance to and/or from the cloud to the ground. This energy does not always use a direct path to the ground, it can sometimes jump sideways (known as a side-flash) enter a house and then ‘branch’ and utilise more than one path to ground at a time. It can also jump through the air from one conductive path to another. For example, the arc flash of lightning may first connect to electric lines in the attic of a house, then jump to better-grounded water pipes on the first or second floor. Lightning can travel via metallic gutters, then jump to a window frame as a ‘stepping stone’ to the electrical system or water pipes. The arc flash of Lightning has been seen jumping from wall outlets to sink faucets and even across rooms!
The important things to remember here is the electrical discharge from lightning will try to find the path of least resistance to ground, and that the electrical discharge can be both the ground up, the clouds down and sideways. This means that anything that provides a grounding point (or in other words provides an electrical connection to the earth) provides a possible flow path for lightning.
Why does CSST pose more of a problem than other gas piping?
The first generation of CSST had very thin steel walls, around 0.2mm thick, modern CSST pipes are not much thicker. This thickness is around that of a sheet of paper. A Rigid Black Iron pipe by contrasts is around 15 times that thickness. Apart from the electromagnetic energy created by an arc flash, such as in a side flash in lightning, intense heat is created. Even a small arc entering a sheet of metal this thin will be sufficient to burn a hole through it similar to a spark plasma cutter.
How do you reduce the risk.
Much has been said on the internet and on the CSST safety sites about “bonding” as a technique to reducing the risk a CSST failure in a lightning strike, but it is difficult to find out what the pipe should be bonded to, and how. So let’s look at what is the likely cause of the side-flash arc in the first place and then discuss ways of reducing the risk.
As I said above, an arc flash is caused by the electrical potential of lightning trying to reach ground. In order to do this it will travel between conductive objects of different electrical potential.
For a lightning arc flash to burn a hole in a CSST there must be an object that offers either a higher or lower potential of electricity than where the Lightning is coming from or going to in close enough proximity to the CSST to cause a side-flash. In the majority of cases where a side-flash arc has found to puncture CSST is has been from adjacent communications cables, water pipes or ground connected components such as water heaters, furnaces etc.
The manufacturers of modern CSST have realised that this is a problem and go to great length to try to reduce their liability by putting the issue into the hands of the installer and usually include the following wording in their installation procedures:
“In accordance with the NFPA 54 Section 7.13, [Insert manufacturer name here] requires proper bonding of the [insert trademark of the CSST pipe here] gas-piping systems in a structure to the structure’s electrical grounding system. This must be performed by a qualified person recognized by the local jurisdiction as capable of performing such work.”
The problem is, this does not necessarily solve the issue. If the reason for any electrical arc is the movement of the lightning from one component to another of lower potential, bonding the CSST to the grounding of a building just changes the electrical potential of the CSST to the same as the buildings grounding system. It does not resolve the problem of components adjacent to the CSST that may have a higher potential, and yet still be part of the lightnings path to ground. Indeed in an electrical sense, it could even be a reason for lightning to choose to go to the CSST for it’s path to ground.
What is needed is to ensure that ALL components that CAN carry and electrical charge, an are in close proximity to the CSST are ALL bonded to the same ground. This would mean that all the components are at the same potential which will inhibit the lightning from hopping from one to the other.
This means connecting, via a series of bonding jumpers, the CSST to the water pipes, which should be connected to any component that has a metal-ground connection and in-turn to the electrical ground of the home (e.g. Grounding rod and panel ground). By doing this, it allows any electrical current from a lightning strike to run along a continuous electrical path to ground and reduce the likelihood of side flash inside the building. The Gas Codes, Electrical Codes and Manufacturers Codes are all heading along the right lines, but once again in my opinion, the distinction between the trades and the protectionist nature of the manufacturers who don’t want to be seen selling products that could possibly be seen as dangerous, do not tell the whole story, and do not provide people with the absolute protection that could make their products safer.
Here’s an NBC news clip on the story.