I spoke about grounding and bonding at a recent meeting of the International Association of Electrical Inspectors a few weeks ago. One attendee asked about the requirements in 250.50, 250.52 and 250.53 in the 2023 National Electrical Code. His question dealt with using underground metal water pipes as grounding electrodes and whether this applied to all underground metal water piping systems supplying a building.
My answer was no. Not all underground metal water piping should be used as part of the grounding electrode system, just as underground gas piping is specifically prohibited from that use. There was a lively discussion about what the NEC requires and what is not permissible based on other NFPA codes and standards.
There is an actual restriction in 250.52(B) about using metal underground gas piping systems as grounding electrodes. That restriction is related to similar installations for underground metal piping used to supply building sprinklers, private fire service mains and other similar critical water supplies.
Why is there a restriction on metal underground gas lines and metal water piping for sprinkler systems and fire service main water supplies being used as grounding electrodes? A small amount of direct current can cause cathodic action (galvanic corrosion) that affects the metal piping system.
Cathodic action is an electrochemical process, where one dissimilar metal with a different electrical potential reacts to another dissimilar metal with another potential. One of the metals becomes an anode and the other a cathode, and there is an electrolyte in between them. The anode metal dissolves its molecules (ions) into the electrolyte, the electrolyte provides the medium for the ion migration and the cathode metal (or earth) collects the molecules or ions from the anode metal. The anode metal corrodes more rapidly than normal due to the electrically conductive path from the anode metal through the electrolyte to the cathode metal.
The effects of cathodic action are evident in the way the anode metal quickly develops rust and deteriorates. In other words, the metal in the underground piping becomes sacrificial and migrates into the soil, causing leaks in the water piping system.
Codes and standards
Section 250.6(E), Isolation of Objectionable Direct-Current from Cathodic Protection Systems, provides a remedy for this action: “If isolation of objectionable direct currents from a cathodic protection system is required, a listed isolator device shall be permitted in the equipment grounding conductor path to provide an effective return path for AC ground-fault current while blocking the flow of direct currents.”
The NEC Handbook further explains: “The listed AC coupling/DC isolating device allowed by this section blocks the DC current on grounding and bonding conductors and allows the ground-fault return path to function properly. These devices are evaluated by a recognized qualified electrical testing laboratory for proper performance under ground-fault conditions.”
A simple fix
One easy remedy is to install a dielectric union that has a plastic insulator between the metal piping emerging from earth and the metal piping distribution system throughout the building. These dielectric unions are often located at the natural gas underground riser to help block the DC and counter the effects of cathodic action.
Section 250.50 states “All grounding electrodes as described in 250.52(A)(1) through (A)(7) that are present at each building or structure served shall be bonded together to form the grounding electrode system.” Section 250.52(A) states “A metal underground water pipe in direct contact with the earth for 3.0 m (10 ft.) or more (including any metal well casing bonded to the pipe) and electrically continuous (or made electrically continuous by bonding around insulating joints or insulating pipe) to the points of connection of the grounding electrode conductor and the bonding conductor(s) or jumper(s), if installed.”
However, Section 6.5.1 in NFPA 13, Standard for the Installation of Sprinkler Systems, states “in no case shall the underground piping be used as a grounding electrode for electrical systems.” The standard gives the following explanation: “The use of underground fire protection piping for electrical grounding increases the potential for stray ground currents and increased galvanic corrosion,” which is why such use is prohibited by 6.5.1.
These same statements are also located in 10.5.1 in NFPA 24, Standard for the Installation of Private Fire Service Mains and Their Appurtenances. Additionally in 10.5.1, “If a large-diameter copper cable runs from the electrical service panel attached to the sprinkler system’s service main, as it enters the building, it is a telltale sign that the underground service is being used as a grounding electrode. If a larger diameter copper cable is attached to the fire service main as it enters the building, the electrical inspector will need to examine it to make sure the sprinkler system’s service main is not being used as a grounding electrode.”
The result is not all underground water piping must be a grounding electrode.
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