Protecting People from ESD

Person sinking to bottom of dark space
Published On
Sep 20, 2019

For the past few national electrical code cycles, there has been a growing concern in the electrical industry with a phenomenon called electric shock drowning (ESD) occurring in and around marinas, boatyards, floating buildings, fixed or floating piers, wharves, docks and similar areas. Recognizing that swimming in and around these areas is common, especially where people occupy and live in the boats and watercraft, the NEC and the electrical industry started investigating electrical hazards in these areas.

Electrical leakage current in fresh or salt water can inhibit or incapacitate muscle reactions of individuals swimming in the water with severe consequences. With this understanding, the electrical industry and other interested parties started studying and addressing this very serious issue. The following information chronicles the effort to address ESD.

The U.S. Coast Guard, the NFPA Fire Protection Research Foundation, the American Boat and Yacht Council and Foundation, UL, NEMA, Attwood Marine, Eaton Corp., Hubbell, Intertek and Leviton Manufacturing sponsored a study to help address ESD. With help from the International Association of Electrical Inspectors, NECA and other electrical industry members, the groups teamed up to gather data, map potential electrical fields in and around water (both fresh and salt water) and create swimming accident scenarios to determine possible solutions to the ESD phenomenon. The study identified many concerns, such as the harsh environmental conditions that result in the deterioration of the electrical components and installations in and around water as well as the electrical current and voltage potential magnitude in fresh and salt water. The industry study resulted in a report issued in November 2014 prepared by John Adey with the ABYC Foundation and Bill Daley and Ryan Kelly of CED Technologies for the NFPA Fire Protection Research Foundation.

The NFPA research foundation report overview states, “ESD begins with an electric fault on the dock or on board a boat when a voltage source comes into contact with the body of water. The voltage radiates throughout the water in a hemispherical field. As a swimmer approaches the electric field, electrical current begins to flow through their body. The human body has a much lower resistance than fresh water and is the better conductor of electricity. In the presence of an electric field, the human body, not the surrounding fresh water, conducts the majority of the electric current. As little as 10 milliamperes [mA] of current through the human body can cause loss of muscular control, which may result in drowning.

“The victim may not be exposed to the stray voltage field upon initially entering the water. This leads the victim to believe the water is safe for swimming until they unintentionally enter the voltage field and become shocked. Further, the voltage source may be intermittent as a function of when a particular AC device is automatically or manually cycled on or off, or when a fault is intermittent in nature. Although ESD has been mostly observed in fresh water, the incidence of ESD in brackish water cannot be ignored, since the conductivity varies based on the numerous environmental conditions.”

The study determined the human body in fresh water was more affected because it has more conductivity due to salt in the body. The same human body in salt water was not exposed to the same amount of shock hazard because of the salt in the environment.

In the 2011 NEC , Section 555.3 was added to require the main power supply to the marina or boatyard to have a service main ground-fault protective device not to exceed 100 mA. This device would limit the potential voltage and current supplying boats, watercraft and houseboats at the service main where the power originated.

Where installing a 100 mA at the service main was not feasible for any reason, this ground-fault protection requirement could be installed at each individual feeder or branch circuit supplying the watercraft. Ground-fault protection of equipment usually applies to the protection of equipment; however, the purpose of this 100 mA or smaller ground-fault device was to try and limit the leakage current in the water.

Two major changes have occurred in the 2017 NEC in Article 555, dealing with the safety requirements in marinas and boatyards. The first is in 555.3, where the 100 mA ground-fault protective main device was changed to not exceed 30 mA, thus making it more responsive to electric leakage. Section 555.24 was added to require swimming areas to have signage that states “WARNING—POTENTIAL SHOCK HAZARD—ELECTRIC CURRENTS MAY BE PRESENT IN THE WATER.” Look for more changes in the 2020 NEC .

About the Author

Mark C. Ode

Fire/Life Safety, Residential and Code Contributor

Mark C. Ode is a lead engineering associate for Energy & Power Technologies at Underwriters Laboratories Inc. and can be reached at 919.949.2576 and

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