Product Update August 2019 Issue

Preventing Electric Shock at the Dock

Alarms, no matter how accurate, are no substitute for strict adherence to safe practices.

The human body runs on electricity and if you overload the nervous system with an external field, everything goes haywire. Every year several people die because they go swimming near a dock, a wiring fault creates an electric field in the water, and their muscles freeze. It is called Electric Shock Drowning (ESD).

A victim of ESD doesn’t always appear to struggle, because they physically can’t. Their diaphragm is paralyzed, their swimming muscles incapacitated, and they simply sink. This danger is regarded as almost exclusively limited to freshwater—although no one should ever consider a saltwater marina to be free of electrocution risk. The human body is more conductive than freshwater, but less conductive than salt water. Because current seeks the least resistant path, the preferred electrical path in freshwater is your body, while in saltwater—except in unusual circumstances (metal prosthetics, for example)—it is water.

Drew Frye

Non marine-grade connectors on a wet dock create a recipe for disaster

How to Avoid ESD

Never swim near docks with AC power. Encourage your marina to test periodically for leakage.

• When you go for a sail, switch off the AC power breakers onboard and at the shore power pedestal and unplug the power cord.
• Move your boat at least 150 yards from any marina before entering the water to perform maintenance or to scrub the bottom. In freshwater, the power gradient can extend as much as 150 feet from the fault.
• If the dock is used by swimmers, install only low voltage lighting and no power outlets.
• Never use 120 volt power tools from a tender or float, or even in a way where wet conditions can create a ground through you. Using power tools near freshwater or saltwater can be lethal. The seawater becomes an excellent coupler from the sailor to ground, and wet hands make a good connection to the power tool.
• Always use GFI protection. If there’s none in the circuit, install a GFI pigtail.
• If you must swim in a saltwater marina, unplug yours and nearby boats and turn off pedestal breakers and battery switches. This should eliminate the
remote risk of a severe fault.
• Although swimming pool electrocutions are extremely rare (all pools equipment must meet strict safety codes),
accidents can happen. Never swim if a portable 120-volt pump is in use.
• Never swim in or near freshwater marinas.
• Do not swim around freshwater docks that have electrical equipment.
• Install galvanic isolator in all shore power systems. This provides a barrier between the shore ground and hull ground.
• Install a solar system on your boat, eliminating the need for shore power.

Field Detectors

While the best policy is to stay out of the water, a lakeside dock is a kid magnet. There are electric field detectors, that with various levels of rigor, detect dangerous faults.

These units simply measure the difference in voltage between the shore power ground and the water where the sensor is located. If it is located too far from the fault or located close to a grounded, metal portion of the dock, it may not measure a voltage differential. There is no UL listing for this category of products. Although several units claim to be UL certified, this appears to be in reference to their UL-compliant components. There is no universal standard for this type of equipment.

What We Tested

Ordinarily Practical Sailor tests all of the equipment we review, but for this preliminary market scan, we interviewed the manufacturers and several users, and reviewed the specifications. 

Fixed Mount Alarms

For fixed units, professional installation is strongly recommended, both to ensure that the unit is properly installed and to inspect all wiring for compliance with current electrical code (compliance with original code at time of construction means nothing). Installation cost varies, typically from $150 to $350.

Dock Lifeguard A single sensor, properly placed, is said to protect an area about 40 feet on a side. Up to four sensors may be used to increase the area of coverage. The panel includes both LED indicating the relative strength of the field, and loud alarm, and a flashing light, from $950 (
Safe Water Systems Shock Guard 24/7 Up to four sensors, each protecting a 26-foot radius. Detects as little as 1.7 volts differential from ground. Powered by a 9 volt battery, the alarm, which is not very loud, emits six second chirps for 24 hours, after which the battery is dead. Annual replacement is required. The battery can be tested by placing a supplied magnet against the panel is a specific location, which lights a light and causes a chirp. Panel includes 105 dB buzzer and contacts to trip up to four GFI circuits, $695 (

The professionally installed Dock Lifeguard detects a hot grounded dock.

Portable Floating

Targeted at the swimming pool market, these float on the surface and measure the electrical gradient between closely spaced sensors. As a result, range is limited. They are best used by walking them around the edge of a pool before use. As such, they may not detect changes after the swimmers enter the water. Additionally, they provide very little or no protection against hot electrical equipment near the water, such as lifts, railings, and dock frames.

Shock Alert This free-floating sensor detects tiny potential differences across its 5-inch width. A green flashing light is displayed if no gradient is detected, and a beeper and flashing red light activate if a gradient is present. $150 online and through Home Depot.
Shock Alarm Another free-floating sensor, the Shock Alarm measures the gradient in a small area, and flashes and beeps if any current is detected. $129.00 online (

The Shock Alert is easily deployed. In general, these systems can give swimmers a false sense of security.


We do not recommend the floating units. While they may provide some protection, failure to detect hot boat lifts and limited range allows too much room for error. We stand by the conclusions of the Electric Shock Drowning Organization, which does not support the use of what they refer to as “green light” devices. According to the ESO, these devices can encourage swimming in potentially hazardous areas, placing more people at risk. And, of course, the devices can fail.

Additionally, fault could occur after people have entered the water, with a new circuit introduced or activated. Given the prevalence of failures of boat electrical equipment and in marinas, the potential for failure is a valid point.

It is also important to remember that electricity is not the only hazard for swimmers in marinas. Moving boats, turning propellers, and entanglement hazards are just a few of the risks.

Does the equipment work? The most adamant insist that you can never be safe enough, since current detectors have their limitations. On the other hand, if you strictly adhere to these safety tips around the water and only use such devices as a means of fault or failure detection, we see value here.

Comments (9)

So I'm looking at the excellent diagram cited by Luceastman above. In all of these scenarios, it shows the AC circuit appliance grounding wire, not neutral, connected to the engine or DC negative bus. Maybe I'm missing something in basic marine electrical design but why would you ever want to connect the AC circuit grounding wire to the DC circuit negative bus? This seems totally unnecessary and potentially dangerous. Clearly a boat's DC circuit can fully operate without any AC system. And the AC circuit can operate without connection to a DC system. I know that ABYC requires this connection but what is it buying you other than the potential ESD and lots of various equipment to provide galvanic isolation?

Posted by: PeggyAnn | October 29, 2019 12:15 PM    Report this comment

The text has been changed to clarify the sequence when unplugging the boat: AC loads and breakers (aboard and ashore) should be turned off before any unplugging.

Posted by: sailordn | July 24, 2019 12:28 PM    Report this comment

I have a problem understanding your comment that "Isolation transformers are no help".
Electrical current in a circuit has to return to it's source. that is an undisputed fact. It cannot return to some other current source.
So if I have an isolation transformer, the current source is the secondary of the transformer. That is on the boat, so any shorts to ground powered by this source will NOT pass through the water to return to it's source, it can only return to the transformer on the boat.

The only ground path back to the shore source is at the primary. That should be grounded to the shore ground only. It never connects to anything on the boat.

So, with this configuration, how could I ever create a current path through the water back to shore ground?

Posted by: sailing Jack | July 24, 2019 8:24 AM    Report this comment

Captain Ed, this industrial electrician is very interested in the difference between proper shore and marine grounding. The NEC keeps changing the terminology, but the concept has remained the same, the ground is for safety, it's only job is carrying fault current, ideally tripping the overcurrent device, and should never carry any current otherwise. Grounding for lightning protection should never be carrying neutral, or "grounded conductor" current. I would consider putting an amprobe on the shore power ground and on the grounding from the mast to keel. Any current there indicates a problem, at least by my land electrician standards.

Electronics manufacturers often use chassis grounds (as VCC, my fellow nerds) for current carrying. I don't know if this is happening on boat electronics. We electricians hate that and it's why hospital electrical standards distinguish bonding and grounding, making the equipment ground an effective second neutral for shoddily designed, but extremely expensive, electronics.

Anyway, grounded DC returns should not be placing current into the water because the batteries are separately derived systems and the current paths should be confined to the boat. If I had my way, they would only be grounded at a single place, as we do with an electrical service with the neutral. Engines use case grounding and that's out of the control of those of us who work on our boats.

Summing it up, while AC neutrals are "grounded conductors" they are never to be used as what we call, "grounding conductors". Neither they nor bonding conductors should be used as DC returns, except where engine manufacturers do it.

Posted by: Seaman Recruit | July 24, 2019 8:02 AM    Report this comment

A picture worth a thousand words to be viewed at

Posted by: Luceastman | July 23, 2019 11:27 PM    Report this comment

Seems readers are confused about the source of current. It is only occasionally from the boat. Just as often it is from the shore. I am an ABYC certified marine electrician and am OCD about ESD. In my experience, the current is just as likely to originate ashore as aboard the boat. Isolation transformers are no help. It is not the current coming in to the boat that is the problem - it is current leaking out. This can be via thru hulls, air conditioner discharges, or running gear. This is because the DC and AC grounding circuits connect. The sources aboard can be household appliances not grounded to marine standards, DC wire touching energized AC wire, wire in bilge water, defect in water heater or AC pump, as examples. Lousy wiring on docks, lifts, dock lights, etc are a prime cause of ESD. I have also found fatal levels of current emanating from marina fixtures or through the ground. When I find these defects they are always the result of poor wiring practices and lack of knowledge of the differences between shore installations and marine installations. No surprise, they usually, but not always, are the result of owner installations afloat or ashore or terrestrial electricians working on boats.

BEWARE: Changes in salinity can make a non-lethal situation become lethal with a heavy rainstorm, heavy waterflow from upstream, etc. We have at least one situation in Punta Gorda where a marina is pumping lots of current into the water. No problem during the dry winters but an invitation to death when bottom cleaning during our very wet summers.

Posted by: Captain Ed | July 23, 2019 4:38 PM    Report this comment

How about everyone with any amount of electrical usage on the boat install a isolation transformer. then you can't create a path through the water, because ground in now your boat.

This problem only occurs when using direct shore power and you have a ground fault that goes to your underwater metals. Since the power is sourced at the grid, the shorted power has to get back there, causing the issue. with an isolation transformer, your power originated in the secondary of the transformer. There is now no path to ground from your boat.

Posted by: sailing Jack | July 23, 2019 1:14 PM    Report this comment

Shore and boat breakers off first, then unplug.

Posted by: bcboomer | July 23, 2019 12:12 PM    Report this comment

Shouldn't you turn off the breaker and then disconnect power cable? The text says to disconnect and then turn off the breaker.

Posted by: jlb | July 23, 2019 11:28 AM    Report this comment

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