Category Archives: Ground Faults and Ground Fault Sensors

Ground Faults and Dockside Ground Fault Sensors

Major addition: “Test Tools,” incorporated 12/13/2015.
Major addition: “Isolation Transformers,” incorporated 1/18/2016.


The rollout of 30mA Equipment Protective ground fault sensors (EPDs) at marinas and boatyards is having a big impact on many boats and boat owners. In some places, more sensitive Ground Fault Protection devices (GFP) have been installed. The more sensitive the ground fault sensor, the more likely the conditions discussed below are to cause both real and nuisance power interruption  problems for cruising  boat.

Man-made wiring errors (circuits mis-wired by unqualified personnel) are fairly easy to isolate and identify because they are constantly present and detectable until they are corrected. Some ground faults are “transient” and can appear to “come and go.” Transient ground faults can be difficult, time-consuming and expensive to isolate.

Based on recent experience on several boats, I am more confident than ever that a substantial percentage (20%-50%) of the [pleasure craft] fleet does have man-made wiring errors aboard. In the past, the majority of man-made wiring errors have been hidden, silent and non-symptomatic aboard boats. These same errors will cause ground fault sensing circuit protection devices on docks and at boatyards to trip AC power “off.”   Some of these problems originate with non-complying OEM component design and device selection, so even newly purchased, “straight-from-the-factory” boats are not necessarily free from the possibility of denial of power. Boats manufactured “offshore” are not necessarily designed to be compatible with North American standards.

This article will highlight some ground fault causes that might require service attention from a boat owner.   These conditions will NOT affect all boats. Transient conditions will NOT necessarily affect boats that also have  man-made wiring errors. But for those that are affected, awareness of these possibilities will help with problem isolation and correction. Certainly, transient conditions – if present – will complicate troubleshooting of man-made wiring issues on boats.


The Fort Pierce, Florida, City Marina has recently completed a multi-million dollar major expansion project.   The new floating docks at FPCM are equipped with Square D 125V and 250V, 30mA Equipment Protective Device (EPD) ground fault sensing circuit breakers located at the slip-side pedestal.

When we arrived at FPCM, the dock attendant who landed us suggested we have all AC branch circuits on both panels (“house” panel and “heat pump” panel) set to “off.” Sanctuary has two 125V, 30A shore power circuits, but that is the right advice for all boats in all cases. It is particularly useful for connecting the first few times to docks with ground fault sensors on their pedestals. After attaching to the pedestal, set individual branch circuits “on” one-at-a-time. If the pedestal breaker should trip while powering up, take note of which branch circuit breaker caused the trip. It will become obvious in reading this article why that information is very valuable to have.


TOPIC: Inverters and inverter/chargers

One major governing concept for all AC distribution systems in North America is that the neutral conductor of the system must be bonded to the safety ground AT THE SOURCE POINT of the AC power.   For shore power (AC power source ashore), the neutral and ground are bonded together in the shore power infrastructure. An ABYC corollary is, for boats operating on shore power, the neutral and the safety ground MUST NOT be connected together aboard the boat.   However, for inverters operating   in “invert” mode (AC power source onboard), the neutral and safety ground MUST be connected together at the inverter, aboard the boat. In one case, the bond can’t be on the boat. In the other, the bond must be on the boat. Contradiction? No, it’s entirely consistent. In all cases, the neutral-to-ground bond is at the AC power source.

An onboard inverter integrated into the boat’s electrical system must comply with the neutral-to-ground bonding requirements in the manner described by ABYC, E-11. To accomplish that, modern inverter devices have a relay inside the device. When operating in “invert” mode, the relay joins the boat’s onboard neutral (white) and ground (green) electrically together to create the required bonding connection. The relay disconnects the onboard bonding connection when the device has shore power and is operating in “passthru” mode. The operation of the relay maintains compliance with North American electrical standards.

Some inverter devices may not have automatic transfer relays, and instead accomplish the transfer back and forth from shore power “passthru” operation to “invert” operation with manual switching.   That is OK, as long as the transfer switching is wired correctly.

The operation of the inverter’s internal transfer relay has implications that boat owners should understand. Inverters (or inverter/chargers) that are fully integrated into the boat’s electrical system will cause a short duration transient ground fault when shore power is first applied to the boat.   At the exact moment – the very instant – the boat is connected to shore power, an inverter operating in “invert” mode will have the boat’s onboard neutral and safety ground connected together through the bonding relay. As viewed from the pedestal ground fault sensor, that condition is a true ground fault.   In normal operation, when the inverter “sees” shore power, it transfers out of “invert” mode into “passthru” mode. The internal ground transfer relay removes the neutral-to-ground bond, and thus clears the ground fault.   The ground fault sensor at the pedestal will not trip unless the ground fault exceeds 30mA and persists longer than the trip-time of the pedestal ground fault sensor.   So for boaters and service technicians, the specification and operation of an inverter’s transfer interval is important. That relay transfer-time should be in a range less than 25mS.   If the transfer relay is slow (due to equipment specification or environmental contamination), or if manual switching for the inverter is required, the transient ground fault may/will persist long enough to trip the pedestal ground fault sensor.

TOPIC: isolation Transformers

The input side of an isolation transformer (primary winding) connects to the shore power pedestal. The output of the transformer (secondary winding) supplies the AC loads on the boat. The “connection” between the primary and secondary windings of the transformer is via an alternating magnetic field that rises and falls with the rise and fall of the shore power primary voltage waveform. There is no continuous electrical connection between the shore power ground and the boat’s ground system. Like an onboard inverter or a onboard generator, an isolation transformer is treated by electrical codes as an onboard AC source, and so the neutral and ground of the transformer output (secondary winding) are bonded together on the boat.

When a boat with an isolation transformer arrives at a dock after an outing, there is no alternating magnetic field present in the transformer.  At the instant that shore power is applied to the transformer, there is a very high instantaneous “inrush,” or power-on surge, of current. The inrush current can be 10 to 15 times higher than the rated working current of the transformer. For large transformers with low winding resistance and high inductance, inrush currents can last for several seconds until nominal operating equilibrium is reached.

All magnetically coupled devices (transformers, motors, generators) experience small, naturally occurring internal leakage currents. These leakage currents are proportional to current flow, and exhibit inrush-related transient spikes.

One docks, pedestal circuit breakers with ground fault sensors provide two functions. First, they protect against overload. The nominal overload set-point is 30A for 125V circuits and 50A for 240V circuits. Second, they protect against leakage currents. The nominal leakage current set point is 30mA. So the question becomes, does the inrush transient of the transformer fall to a level that is within both the overload and leakage current trip criteria of the breaker in a sufficiently short time to avoid having the breaker trip power “off?” Emerging evidence seems to suggest that there are some instances where this does not occur.

A friend has a boat with a 50A, 240V shore power input to a Charles Industries IsoBoost Isolation Transformer. The IsoBoost never trips the 50A over-current set-point of conventional pedestal breakers. Not anywhere; not ever. So, the overload transient does fall within breaker tolerances sufficiently quickly. However, even with all onboard load circuit breakers set to “off,” that transformer routinely trips pedestal breakers containing 30mA, 100mS ground fault sensors. On that boat, the Charles IsoBoost inrush spike does not resolve itself within the trip interval of the ground fault sensor, so successful connection to shore power is not possible. The overload tolerance of the breaker is longer than the ground fault trip tolerance.  Charles Industries has developed a “SoftStart” module that clamps and limits the magnitude of the inrush current. That “SoftStart” module is the solution that Charles recommends for tripping ground fault sensing breakers.

It is highly likely that isolation transformers from other manufacturers may also be affected by this inrush spike phenomena. The “ground fault” in this scenario may result from capacitive coupling between the transformer windings and/or ground, or of inductive coupling through the electrostatic shield to ground, or a mix of factors. Whatever, it really doesn’t matter to an affected boat owner. It is something that can be mitigated by design improvements in the future, but those with affected transformers today will have to find work-arounds such as the Charles “SlowStart” module.

TOPIC: Galvanic Isolators

Galvanic Isolators are devices that are installed aboard the boat.   They are electrically located in series with the boat’s safety ground (green wire). Galvanic Isolators contain a diode pack (full-wave bridge rectifier) that blocks small DC galvanic currents but allows AC fault currents to flow.

There are three “generations” of Galvanic Isolator technology. The first generation device consisted of a passive diode pack. That passive diode pack was subject to damage by an overload or surge, and that damage could leave the boat’s safety ground conductor electrically non-conductive to AC fault currents. Many of these 1st generation GI devices are still in service. Unknown to their owners, some percentage of those 1st generation devices are inoperative. That is a potentially serious safety risk. In response to an ABYC standards revision, GI equipment manufacturers developed a “second generation” device. The 2nd generation device utilized an electronic control module to periodically “test and verify” the electrical integrity of the diode pack. This 2nd generation equipment created a transient ground fault, and is incompatible with the emerging presence of shore power ground fault sensing equipment on marina docks. The newest third generation GIs are of the “failsafe” design. They have both a diode pack and a large capacitor, and no longer have electronically active test modules. Third generation devices are designed so that they will not fail in an electrically non-conductive state. If they fail, they fail in an electrically conductive state. In that state, the boat may lose DC galvanic protection, but WILL NOT lose AC safety ground continuity.

Many boats are still fit with second generation Galvanic Isolators manufactured between approximately 2002 through approximately 2008.   One such unit is the Professional Mariner (ProMariner) Prosafe 1, which is the device I installed on Sanctuary. At the time these devices were developed, impressing a small ground fault current on the ground conductor was not a concern in marine shore shore power systems, because marine shore power services did not have ground fault sensors. Thus, using an intentional ground fault was a viable approach.

The Prosafe 1 tests the ship’s ground connection when the device is first connected to shore power, and periodically thereafter in regular operation.   The Prosafe 1 monitor detects the presence of the impressed ground fault, thereby confirming the integrity of the boat’s ground connection through the Galvanic Isolator diode pack and into the shore power infrastructure. The Prosafe 1 ground fault current is specified at 30mA, and it can last a variable period up to several seconds.  The net is, that ground fault can cause a shore power pedestal’s ground fault sensor to trip. In my personal experience aboard Sanctuary, the symptoms have been variable.   At Chesapeake City, MD, I was able to connect to the two 30A receptacles but not to the 50A receptacle through my splitter.   At Ft. Pierce, FL, I was not able to connect to a 30A receptacle, but was able to connect to a 50A receptacle through a splitter.   HOWEVER, at both Chesapeake City and Ft. Pierce, Sanctuary would trip the pedestal breaker at random time intervals.   Sometimes the monitor’s “test pulse” would not cause a trip, and sometimes it would.   During the overnight period, Sanctuary tripped the shore power breaker, on average, twice.   But I was, for some reason I can’t explain, able to reset the breakers.

ProMariner Technical Support has confirmed the above description.   ProMariner acknowledges the problem.   The Prosafe 1 GI device is now discontinued and obsolete, so the company’s advice is, “upgrade the Galvanic Isolator to a ‘failsafe’ design.”   It’s unfortunate that the investment in the 2nd generation device is lost, but ProMariner’s advice is the right technical advice. The important thing is to be aware that this problem exists.   This issue can cause unexpected results and RANDOM loss of power on an otherwise properly wired boat at docks equipped with ground fault sensing circuit breakers.

TOPIC: Equipment Aging (esp: Hot Water Heater)

The heating element of a hot water heater, by design, lives in a pool of stored water.   That water provides a path for an electric current to flow from the heater element, through the water, to ground (via the plumbing connections) to the frame of the device.   As water heater elements age, and through many years of heating/cooling cycles, micro-cracks develop in the ceramic insulation of the heating element. Electrical contact between the live conductor of the heating element and the water in the heater’s tank will cause transient ground faults as the water heater cycles “on” and “off.”   The physical size of the contact area, the voltage present at the point of contact and the conductivity of the tank’s water (mineral content) will affect the magnitude of ground fault currents. This can be an elusive problem to isolate. If the water is also heated by a propulsion engine hot water heat exchanger, the water in the hot water tank will be hot enough at the end of a day’s outing that the water heater will not cycle when the boat is first connected to shore power. In that case, the ground fault will appear at some miscellaneous and random later time; maybe the middle of the night, maybe the next day, maybe at shower time.

Random equipment aging problems are common in battery charging equipment, household appliances like refrigerators, freezers and ice makers, and other motor-driven appliances like washer/dryers.   If a shore power ground fault sensor trips at random intervals, try cycling one piece of equipment “on” and “off” at a time to isolate the cause.

SOMEWHAT OBSCURE GROUND FAULTS: (Transient or continuous)


TOPIC: Reverse Polarity indicator

Boats fit with 125V, 30A marine shore power services require a Reverse Polarity indicator.   A Reverse Polarity device detects, and warns the boat owner of, reversal of the incoming hot (black) and neutral (white) shore power conductors.   This is a very rare but very dangerous condition. AC power distribution panels and several aftermarket devices are built with reverse polarity detectors, so some boats may actually have several such RP detectors aboard. Electrically, they are all connected in parallel.   Both of Sanctuary’s factory-installed power distribution panels have them, our aftermarket Galvanic Isolator (Prosafe 1) has them, and our aftermarket Bluesea Systems Generator Transfer Switch has them.  The ABYC E-11 Standard calls for these devices to present at least 25KΩ of electrical impedance, but of course, several of these devices in parallel can result in much lower net impedance.   Since by definition, these devices are a “ground fault,” their net effective resistance, if too low, can cause random trips of ground fault sensors on docks. And, especially so in combination with other conditions.

TOPIC: Cable TV and Wired Ethernet

Dockside cable wiring for TV and Internet services, and wiring for Internet service via DSL telephone lines, can cause transient ground faults on connected boats. Normally, cable services and telephone services at marinas are grounded at their point-of-entry to the marina property. HOWEVER, that point IS VIRTUALLY NEVER the same physical point where the shore power ground bond is established. That difference in connection point leads to a phenomena called “ground loops” between and among the dockside services. Ground loop currents can cause transient or continuous ground faults. These ground faults are prone to appear when AC electric demands are highest (hot summer days, cold winter days) on docks. Also, TV and telephone cables are less resistant to corrosion and environmental conditions than the heavy conductors of the AC shore power system. Boaters who use cable TV and/or wired Internet services and experience random trips of ground fault sensors should try disconnecting these services for problem diagnosis and isolation. The long term fix will most likely require the marina to get the various system grounds tied together at the bonding point of the AC power system ashore, but disconnecting those small signal wires from the boat will interrupt the ground fault path and may temporarily alleviate random nuisance tripping of pedestal shore power ground fault sensors.

TOPIC: Surge Suppression Devices

With the advent of the computer age, surge suppressors have become ubiquitous in homes and on boats. There are whole-house surge suppressors that attach to the building’s incoming power line, and there are supplementary surge suppressors in many forms that attach to residential 125VAC power outlets. Computers, TVs, home routers and any number of electronic toys can be protected from transient spikes on power lines by these devices.

The way surge suppressors work is by dumping surge energy “spikes” to ground. There are special diodes in the surge suppressor that bridge the hot current carrying conductor to ground. Recall here that the neutral current carrying conductor is already grounded in the shore power infrastructure. When a high-energy “spike” occurs, the diode is intended to conduct that transient energy to ground. Imagine that a boat is in the vicinity of a “near-miss” lightening strike. That lightening strike causes a large but short-lived energy spike on the electric power line. The onboard surge suppressor acts to ground that spike, thus “saving” attached computer and entertainment equipment from damage. This now becomes the same discussion about damaged diodes that applied to the diode pack of a galvanic isolator. The diodes in the surge suppressor can be damaged in a way that leaves them partially-conductive. That is a true ground fault. In this scenario, the ground fault sensor at the dock pedestal will trip when the circuit breaker with the defective surge suppressor is powered “on.” If a utility outlet circuit causes a dockside pedestal ground fault sensor to trip, consider the possibility of a defective surge suppressor.

GROUND FAULTS CAUSED BY INAPPROPRIATE EQUIPMENT CHOICES: (permanent fault; requires wiring fix/equipment replacement)

TOPIC: Generator Transfer Switch

Every boat with an onboard generator integrated into its electrical system in a manner prescribed by ABYC E-11 will have a selector switch (Generator Transfer Switch) to transfer the boat’s distribution panel(s) between shore power or generator power.   That switch MUST transfer BOTH the hot wires (red, black) AND the neutral wire (white).   The reason for the need to switch the neutral conductor lies with the grounding requirements for AC circuits, described above.   The neutral is bonded to the safety ground AT ITS SOURCE.   For shore power (source ashore), the neutral and ground are bonded in the shore power infrastructure ashore, and MUST NOT be connected together on the boat.   For generators, the neutral and safety ground MUST be connected together at the frame of the generator (source onboard).   To comply with the AC bonding requirement, the neutral, as well as the hot wires, must be switched by the Generator Transfer Switch.   Some boat manufacturers have used switches that do not transfer the neutral.   Some aftermarket installers, to save cost, have used switches that do not transfer the neutral. In the past, that was a hidden, silent, non-symptomatic wiring error. Now, the permanent leakage fault that it creates will trip pedestal ground fault sensors.

TOPIC: Heat Pump Raw Water Circulator

Many boats with two 125V, 30A shore power inlets are wired so that “house” circuits are fed from one of the inlets and heat pump circuits are fed from the other inlet.   However, some boat manufacturers have heat pumps wired to both of the incoming shore power circuits.   In many cases, regardless of how their compressors are wired, heat pumps share a single 125VAC raw water circulator pump across multiple incoming shore power services. In normal operation, any time any of the multiple heat pumps aboard comes “on,” the shared raw water circulator also comes “on.”

The wiring to a shared circulator pump needs to be handled in a way that does not interconnect (bridge, commingle) the two shore power neutral circuits on the boat. A mechanical relay or electronic controls are needed to avoid bridging the two incoming shore power neutrals. If the neutrals are bridged together aboard, that will cause a leakage fault that will trip shore power ground fault sensors.

TOPIC: Neutral-to-Ground Connection Incorrectly Made At A Household Appliance

Some household appliance manufacturers,  and some residential electricians, connect the neutral wire of the appliance to the green safety ground wire at the appliance.   That practice is generally obsolete today.   It has it’s roots in older (1940s and 1950s) residential systems where there was no safety ground in the residential wiring.   So, grounding the neutral at the appliance provided some protection from some kinds of faults.   Today, that condition is called a “phantom ground.”   In modern residential  systems, it is an NEC code violation and on a boat, it is a violation of ABYC E-11, in both cases because it results in a man-made ground fault.   If the affected  appliance  is permanently wired into the boat’s electrical system, this condition will always and continually trip a pedestal ground fault sensor.   It does not matter if the appliance  is powered on, nor does it matter if the circuit breaker feeding the device is set to “on.”   If the appliance is pluggable, physically removing the plug from the receptacle will clear the fault.


After I put this article up as a post on my website, several readers  asked if I could recommend tools that can test for ground faults and leakage faults. I would only suggest a DIY approach to ground and leakage fault diagnosis to people who self-describe their personal electrical skills as “high” or “advanced.” This work requires the technician to have contact with energized AC electrical circuits containing dangerous, life-threatening voltages. ONLY THOSE WHO – BY TRAINING AND EXPERIENCE – CAN SAFELY PERFORM WORK ON AND AROUND ENERGIZED AC CIRCUITS SHOULD ATTEMPT TO DO SO.   PERIOD.

Indeed, there are some test tools that allow technicians (and appropriately skilled boat owners) to detect the presence of faults on their boats. However, identifying the presence of faults aboard a  boat is only the first step, and actually one of the easiest steps, in overall remediation. The follow-on activities of 1) diagnosing cause conditions, 2) sorting out multiple simultaneous causation conditions, and 3) applying corrective actions require a thorough knowledge of boat AC electrical systems.   Many corrective actions for ground and leakage faults can lead to  re-wiring AC circuits; in some cases, re-designing AND re-wiring of systems may be required.   And as I’ve said before, diagnosing ground faults and leakage faults is a  “high” to “advanced” skill.

For those who are confident in their skills and ability to work safely, my first suggestion is to look at the “AC Safety Test” article on this site, located here:   These tests will expose the presence of wiring conditions that result in faults due to wiring errors made by unqualified  personnel. If performed  as I have described them, they require minimal to no exposure to energized electrical circuits.

Three “test-tool” options that come to mind for follow-on self-diagnosis:

1.  There is a local group of businesses in Georgia (Marine Surveyors of North Georgia) that is making and selling a device they call a “Stray Current Sensor” (SCS).  It is a versatile test tool that can be used to track down ground faults.  The tool can accommodate EITHER 50A boats or boats with two 30A inlets.  It sounds an alarm, but DOES NOT trip off the electric service to the boat, when a ground fault is detected.  I like that as a test tool approach, because as a technician, I can keep working without having to reset the whole boat each and every time a fault is detected.  The tool is built upon one of the ABYC-compliant Equipment Leakage Circuit Interrupter (ELCI) current transformers (North Shore Safety Systems, PGFM Control Module, which by the way is also a solution I really like).  Here’s a link to the device:  As I read the MSNG website, it looks like boat owners could rent one of these tools to use for diagnosis and troubleshooting.  That would be a great solution for boaters possessing appropriate electrical skills.  The tool is really only needed on a one-time-use basis.  Once the boat is “cleaned up,” the tool isn’t needed any longer.  So, if the rental charge is reasonable, I’d seriously consider this option.

2.  Home Depot offers an electric panel that’s intended to provide electric service to home Spa pools:  It would be ideal for a test tool for a 50A boat.  It would be possible to wire this box as a tester for two 30A boat circuits, but that would require changing the double pole 50A GFCI breaker to two 125V GFCI breakers.  In either configuration, it would be necessary to add the necessary wiring and connectors, as this is just the raw box.  Hubbell and Marinco marine-rated 50A plugs and receptacles cost about $80 – $100 each. NEMA SS-2 50A male plugs and female receptacle fittings are also available at Home Depot that would be suitable for an OCCASIONAL USE, FAIR-WEATHER-ONLY USE, test tool, for a lot less than that.   Likewise, NEMA L5-30 male plugs and female receptacles are also available at Home Depot.  The breaker that comes installed in this box is a GFCI Personal Protective breaker with trip sensitivity of 5mA.  The ground fault sensors on docks are Equipment Protective Devices (EPD) with a 30mA trip setting.  The implication is, if you have BOTH 1) one or more significant ground faults AND ALSO 2) one or more of the transient types of ground faults, the sensitivity of this breaker could complicate using it as a diagnostic tool.  “Clean” boats will probably operate OK on 5mA breakers; there is some (as yet unpublished) experience that leads to that conclusion.  However, a completely safe boat may also have nuisance trips with 5mA GFCI breakers.

3.  As above in item 2, buy the Home Depot Spa box for the enclosure itself, and then replace the 5mA GFCI breaker with a 30mA EPD breaker.  However, the 30mA breakers are very pricey, and probably not available at big box stores.  MSRP prices for two pole, 50A, 30mA EPDs are in the range of $500.  Go to an electrical supply house to get one.  Electrical supply house counter prices would certainly be better than MSRP.  But, certainly not inexpensive. Most supply houses will sell to the public, but some may not; still worth investigating.  Also contact Ward’s Marine in Ft. Lauderdale, FL, for availability and price for a 50A, double pole, 30mA EPD.  Ward’s has – literally – “all things electrical,” including Euro and Asia form-factor stuff…


Man-made wiring errors can be hidden, silent and non-symptomatic. While they may remain non-symptomatic for many years, they should not be regarded as fundamentally safe. Electrical codes are intended to protect us when ABNORMAL things happen in electric circuits; when connections get corroded, when insulation fails or is abraded to expose the metal conductor, when wires get disconnected, when short circuits occur. Bridged (commingled) neutrals work when everything is in perfect order with good connections, but if one of those conductors fails, the other can be severely overloaded and becomes a fire risk. AN open ground conductor means protection from electric shock is absent or compromised. Corrective action should be taken with any boat found to have wiring issues. It’s just the safe thing to do!

Reference: An excellent reference article on GFCI, ELCI and GFPE technologies can be found here: