Reverse Polarity (AC)

[GordMay]

Reverse Polarity

This common problem is one that should be thoroughly understood by all boaters. Reversed polarity can exist on the dock or within your own boat, which should be equipped with a Reverse Polarity Indicator (RPI) on your main AC electric panel. Regardless of whether the reversing point is on the dock or in you own boat, this is going to energize the neutral ground and create an electrical shock hazard. It should not find its way into your bonding system because these circuits should never be joined.

Keeping track of which wire is the Neutral (white) is necessary in order to prevent potentially dangerous voltage differences from existing between one boat and another, connected to the same or separated shore-power systems. In a properly wired AC system, there should be no significant voltage difference between the white Neutral wire and the green safety Ground wire. Therefore, an incorrect connection may be detected by sensing a current flow between the neutral wire from the shore power system and the electrical system's safety ground wire.

Upon detection of a reverse-polarity indication on any boat the shore power should be immediately disconnected and the shore-power system wiring on the dock inspected. It may be that the polarity of the shore power is reversed from the normal or the neutral is "floating" and not properly referenced to ground. It is also possible that the shore power safety ground wire (the green wire) may not be connected to proper earth ground. Connecting to an improperly wired shore-power system can create potentially harmful conditions for personnel as well as damaging galvanic currents.

Perhaps the easiest way to visualize the working of a Reverse Polarity Indicator (RPI) is to first visualize the relationship of the Safety Ground and Neutral wires. In a normal marine installation, when connected to shore power, these two wires are are not connected on the boat, but are connected together on shore at the system grounding point.

The job of the RPI is to determine if there is voltage potential between Safety Ground and Neutral. The Safety Ground and Neutral are effectively two ends of the same wire and therefore should have the same voltage potential and not be capable of lighting the RPI.

One further ABYC requirement for RPI's is that they contain a minimum 25,000 Ohm Resistor. A full treatment of the reason for this is beyond the scope of this article. It is sufficient, however, to say that because the Safety Green wire is connected to devices aboard the boat that may contain stray currents, the link created by an RPI could provide a path for stray currents via the grounded Neutral wire. (See also Note 3)

It is possible for such a circuit to faintly illuminate the Reverse Polarity light even though the circuit is properly wired. It is useful for boaters to understand how this can occur.

Because voltage is always consumed pushing amperage through a resistance (wire), the voltage is different at points A and B on the Neutral wire when there is current flowing through it. This is called “voltage drop”. When high amperage loads are operated in the circuit, enough voltage drop in the length of the Neutral wire can be created to overcome the resistance in the 25K resistor required by ABYC that sufficient current is driven through the LED to cause faint illumination. This situation is not inherently dangerous, however, it can indicate undersized wiring in the dock, shorepower cord or ship’s wiring portion of the AC circuit.


From ABYC E-8.10 SHORE POWER POLARITY DEVICES

E-8.10.1 Reverse polarity indicating devices providing a continuous visible or audible signal shall be installed in 120 V AC shore power systems and must respond to the reversal of the ungrounded (black) and the grounded (white) conductors (See E-8.23.1, Diagram 3,)
if
E-8.10.1.1 the polarity of the system must be maintained for the proper operation of the electrical devices in the system,
or
E-8.10.1.2 a branch circuit is provided with overcurrent protection in only the ungrounded current-carrying conductors per ABYC E-8.11.6.1

E-8.10.2 Reverse polarity indicating devices are not required in systems employing polarization or isolation transformers that establish the polarity on the boat.

E-8.10.3 The total impedance of polarity indicating and protection devices connected between normal current carrying conductors (grounded [white] conductor and ungrounded [black] conductor) and the grounding conductor shall not be less than 25,000 ohms at 120 volts, 60 hertz at all times.

NOTES:
1. Reverse polarity indicating devices respond to the reversal of an ungrounded conductor and the grounded (white) conductor only when there is continuity of the grounding (green) conductor to shore.

2. Reverse polarity indicating devices might not respond to reversals of an ungrounded conductor and the grounding (green) conductor, the grounded (white) conductor and the grounding (green) conductor, or three phase conductors.

3. Another strategy (NOT recognized by the ABYC) is to install a Momentary “Push to Test” Pushbutton in series with the RPI. The has the effect of entirely disconnecting the RPI from the circuit, until you test for polarity (by pushing button when you first connect to shore power).

4. I highly recommend the use of an AC Receptacle Tester* (circuit analizer), which will also indicate reversed Neutral & Ground, and open grounds etc. Test the Marina’s Shore Power System prior to (immediately upon) connecting your shore power cable.

A few examples:

*GB Electrical GFI-501A (And GB #GRT-800) Ground Fault Receptacle Tester & Circuit Analyzer - Tests for seven conditions: GFI interruption, open ground, open neutral, open hot, hot/ground reverse, hot/neutral reverse, and correct wiring.

*Ideal Model 61-035 tests for open ground, reverse polarity, open hot, open neutral, hot and ground reversed, hot on neutral, and hot open.
Ideal Model 61-051 also checks GFI-protected outlets against trip at minimal leakage current of 2mA, and tests their mechanical operation (with the push of a button) by intentionally overloading the GFI breaker with a 6.8mA current through the ground blade.

Regards,
Gord

[GordMay]

From Electrical Construction & Maintenance Magazine (EC&M)

The Case of the Hot Marina
By
Jim Shafer

(The author, Jim Shafer is the president of CAM Components/Harbor Marine Consultants in Stuart, Fla. Contact him at kp2r@bellsouth.net with any information on other electric shock drowning cases.)

An inadequate grounding system on a docked boat leads to a rare form of electrocution that disguises itself as drowning

While enjoying the Memorial Day weather on the sundeck of their moored houseboat on a lake in the Southeast, a mother and her adult daugher decided to go for a brief swim to cool off. Aside from a couple splashes and a shout that the water was cold, neither woman gave any indication that anything was wrong, but as a second daughter prepared to follow them a short time later, she looked down and saw her mother floating face down near the swim ladder; her sister was nowhere to be seen.

The subsequent frantic efforts to resuscitate the mother were to no avail. Despite administering CPR, a witness was unable to save her. Rescuers found the daughter several minutes later more than 50 feet below the surface of the water, but it was too late to save her. The post mortem suggested both had drowned because neither body had suffered any physical trauma, but the surviving daughter reported that they were both good swimmers. What could have possibly happened?

As the investigation into their deaths continued, it became increasingly evident that the women were the victims of a phenomenon that has become known as electric shock drowning.

Combining electricty and water always creates the potential for danger. If the grounding system at this marina had been working properly, these deaths could have been avoided.

Dangerous currents:
Electric shock drowning is often the result of a situation similar to a hair dryer falling into a bathtub; in these cases the hair dryer is a boat and the bathtub is a lake. The cause is often an undetected ground fault that energizes the hull and causes a low-level current to flow through the swimmers, thereby disabling muscle function. It's referred to as electric shock drowning and not electrocution because there is no physical injury. The victims either lose muscle control if the current level is in the 0.01A to 0.02A range or suffer ventricular fibrillation at 0.05A to 0.06A current levels. Because victims typically show no sign of injury, many electric shock drownings are mislabeled as deaths attributable to alcohol intoxication or heart attack. Oftentimes those drownings that are attributed to electric shock are classified that way because of circumstantial evidence like great distress, multiple deaths, and a tingling sensation reported by the survivors.

How does the hull become energized? What happens to the safety bonding system? American Boat and Yacht Council (ABYC) recommended practices require that the AC shore cord's green bonding wire be joined on a boat to the DC negative bus and the underwater gear bonding system. Should a fault develop on the boat, the fault current in the ground wire will initiate a breaker trip or at least prevent a potential (voltage) rise on the hull or underwater gear. However, there's no way to know if the ground wire is OK under normal operating conditions.

Current takes all paths back to the source, so even with a good ground system there still may be a small voltage rise on the hull as a fault establishes a parallel current path in the water. The boat may become lethal, however, if the ground return is damaged and located in fresh water.

The grounding system in this dockside service panel was called into question in the investigation.

Regardless of the size of the AC fault, the potential may rise to lethal levels as low as 15VAC. Even with a poor ground, a boat in salt water won't develop enough potential to cause a problem for a swimmer, making this an unheard of phenomenon with boats in the ocean. However, lakes are a different story. Fresh water is a very poor conductor by comparison, so an ungrounded fault will raise the potential on the hull as it attempts to enter the water. A swimmer represents a much lower resistance fault path, even if only in the electric field and not touching anything.

The common elements in all of the accidents for which information is available always include a fault to ground below the breaker trip point, a high resistance or open ground, fresh water, and a swimmer near the faulted boat. Possibilities include:

- Neutral ground connections, open ground, reverse polarity

- Motor or heating element insulation failure and open ground

- Metal conduit on dock, not bonded and water soaked

The investigation revealed that the boat owner failed to connect the grounding (bonding) wire to the female plug, which ultimately led to an ineffective ground-fault return path.

A preventable disaster. As more evidence was uncovered, the investigation into the deaths of the two women began to focus on the electrical system. On the day of the incident, the resort at which the boat was docked had put into service new power pedestals that required boat owners to use new shore power cords. The owner of the boat in question — and the husband and father of the two victims — had been working that morning on converting his existing Type SOW 600V power cord, which required nothing more than discarding the pig-tail adapter that had been used for the old service and plugging the cord into the new pedestal.

However, in making the conversion, he also altered the female plug connector at the boat. In doing so, he miswired the ground and hot wire, thus energizing the boat's aluminum hull, railing, and ladder. In addition, the wiring within the boat had been altered. Additional circuits had been added, and the incoming power had been changed from 125V (as designed) to 125/250V, which had exceeded the listed rating of the houseboat's female receptacle.

Without complete GFCI or isolation transformer protection, the safety grounding system had to be intact to protect the boat. Closer inspection showed that the boat owner connected a hot conductor to the ground connection.

The underwater metal hull of every boat in a marina is electrically connected through the shore power grounding system while moored. So while most of these accidents occur when both the fault and missing ground are on the boat, as was the case in this incident, the marina operator must ensure that his dock power system is in good condition. Sec. 3.21 of the NFPA 303 Fire Protection Standard for Marinas and Boatyards describes a visual inspection and a ground integrity test the marina operator should be aware of. Many don't even know this document exists. Lawsuits initiated in response to electric shock drownings have involved the marina operator at least to some extent if they weren't making the proper effort to comply with existing standards. It goes without saying that ignoring NFPA recommended practices increases the marina's exposure to liability.

The NFPA 303 ground integrity test should be conducted under load. One major manufacturer's test instrument locks the circuit momentarily to 15A and displays the ground impedance. Dock personnel can then log the readings for later review by an electrician.

Since most of the problems that cause these drownings originate on the boat and many are generated by nonqualified workmanship, it may be necessary to implement around-the-clock monitoring of the marina shore power system to detect ground faults. Such a system is available and currently in use in nine marinas.

[Alan Wheeler]

Great reading Gord. Without being nerdish, or pedantic or just a pain in teh proverbial, I would just like to mention that with AC power, it is called Phase reversal, not Polarity. Polarity is for a DC circuit. However, in this case, I think Polarity is an OK use of the term to describe the reversing of wires in an otherwise complicated situation. However, it would be good for those reading to have in the back of their mind that the "proper" term is being used incorrectly. Otherwise, it can lead to misunderstandings in more nitty gritty terminology if they dig deeper into other area's later on in life.
I think it important to ensure the use of a GFCI or RCD(NZ/Aus term) be used at all times. It is also important that a boaty have in their toolbox a multimeter and be able to use it. Alway's test, never assume is the rule. You can also obtain a plug in device that measures between phases and lights up in certain orders to tell you if the plug is safe or not. However, these also have limits and will not tell you how well a connection is bonded.
To add another story to the list, I also know of someone electrocuted on a boat. The guy was useing an Electric sander,
and had his feet dangling in the water at the same time.
Another thing that can happen, is a poorly bonded circuit can lead to electrolosis between one vessel and another next to it. So if you ever have a problem with electrolosis, but can't find why, check the boat next door or close by.

[GordMay]

Wheels:
Thanks for reading “critically”, and discerning the difference between DC “Polarity” (Positive vs Negative) and AC Terminal Identification (Current-Carying, Neutral, Ground).
Semantics can be interesting.

Actually, “polarity” is the commonly used term, in North America, indicating the identification of terminals (ungrounded current carrying = black hot, grounded = white neutral, & grounding = green ground) in Single-Phase AC Systems. Other parts of the world may use differing terminology (apparently NZ?).

DC Systems are identified (polarized) Positive and Negatice Return (NOT ground).

ABYC references “Reverse Polarity Indicators”.

The National Electrical Code (NFPA-70 Articles 200.10, 406.9 & etc.) references Polarized receptacles etc.

"Polarized" circuits, equipment, and devices are those that only permit connection in a prescribed manner (the AC black hot to a certain terminal, the white neutral to another, and the ground to a third - or DC positive to positive & Neg. to Neg.).

Phase Reversal is also a legitimate term, more often (in North America) indicating the reversal of any one of two current-carrying phases on a three-phase AC system (will drive a 3-phase motor in reverse).

The common Receptacle Tester/Circuit Analyzer ($5 - $15) will test for Ground Connection etc (as indicated in examples above). Testing Bonding & other “Quality” issues is generally beyond the capabilities of the average boatowner - and not something even I would do at every “Plug-In” to shore power.

Obviously, the referenced testers will only work on the configurations of receptacles (convenience outlets) for which they were designed. European devices (for instance) are configured differently, and operate at different voltages etc.

A multi-meter (as wheels alludes) will have universal utility, but requires some knowledge to use.

Regards,
Gord

[Alan Wheeler]

As long as you realise I was not trying to be criticle I am a sound engineer and frequent another BB with other sound pros from around the world. So yes I am aware of the loose way these terms are given, but over the years have seen the interesting situations aise when these terms are used incorrectly. So we have all endevoured to correct the use of this term and as a result have seen changes how manufacturers are using the term in equipment and some of your legal papers like NEC being corrected.
As I have quickly discovered, in the world of yachting, that many sailors are very skilled people, learning many new things along their journey. SSB will be most likely one of them along with wiring systems. And so in their learning, this term is going to become more and more criticle to exactness as individuals may climb the ladder of knowledge. Understanding the difference early on, could help greatly understanding other things later on.