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Basic Electricity - Page 10 - AC Circuits

Basic electricity for boat builders, boat repairers and owners. What you need to know about the electrical systems on your boat. Ground Faults, Generators, Inverters, Chargers.

Ground Fault Protection (shocks).

Ground fault protection, that is, protection against accidental leakage of current to ground, is extremely important. It protects the wiring on the boat from overheating and causing fires, and protects the people from shock.  Most ground fault protection is accomplished by use of fuse or circuit breakers.  But for protecting people on the boat there are better solutions.

For a video on Grounding see Basic Electrical Practices

Ground Fault Circuit Interrupters (GFCI)

Protecting the people on a boat is probably more important than anything else. One of the best ways to provide protection against shock is the use of a GFCI.  Most of us are familiar with these now because they are very common in our homes and at work. What a GFCI does is monitor the current in both the black or hot wire and the white neutral wire. As long as the currents are the same everything is ok. But if a ground fault, that is leakage of current to ground, occurs then the current in one wire is greater than the wire with the leakage. The GFCI sees this imbalance between these two wires and shuts the circuit off. They are extremely fast, tripping in milliseconds. In the US, GFCI's are required to trip at 5ma. That's 5/1000 of an amp.

GFCIs are required by ABYC on boats, in any wet areas, such as the head, and the galley, or any where you have an outlet and water. They are so effective that when they trip many people do not know anything has happened, unless the lights go out.

Residual Current Device (RCD)

In Europe where ISO is the standard, they do it differently. There they protect the whole boat rather than just certain areas on the boat.  They use what is called a Residual Current Device, or RCD.  This device is in the shore power inlet on the boat. An RCD works the same way as a GFCI but the current at which they trip is much higher than for GFCI. An RCD will trip at 30 ma, and they are slower than a CFCI but still trip within 100 milliseconds. If it were lower there would be a lot of nuisance or false tripping. But they do protect all of the wires and equipment on the boat although they are probably not adequate to protect the people from shock. In the US, ABYC has included a device called an ELCI, that is similar to an RCD.

ELCI: Electrical Leakage Circuit Interrupter

Much like an RCD. This protects the whole boat. This is the latest requirement from ABYC. An ELCI is designed to detect current leaking into the water surrounding the boat. It does this by measuring the current in the black (hot) wire and the return current in the white wire (neutral).  If everything is ok the difference between the two should be zero. If there is an imbalance, that is less coming back than is going, and this imbalance is 30 ma or more for 100 milliseconds then the ELCI trips and shuts off all power.  This is to protect people in the water or touching metal that is in contact with the water. Every year in the USA there are a small number of deaths from electro shock drowning.  This is designed to prevent that.  You can then isolate the problem by turning off all of the equipment on the boat, turning the ELCI back on, and turning on one device at a time until the ELCI trips again. The last item turned on has the problem. Usually this is due to the green wire having been cut or disconnected so that there is no return path for a ground fault.

Onboard Generators, and transfer switches.

So now you have power while at the dock.  But what about while you are underway, or anchored in a nice bay somewhere?  You need an alternative source of AC power. A generator can supply your needs. It can also recharge your batteries so you can continue to run all of your DC equipment. However, to properly use a generators power you need a power transfer switch.

 A transfer switch shuts off the power from shore and redirects the source of power to the generator. This is absolutely necessary for safe operation of electrical equipment on board. Most of these transfer switches are automatic, so once it's installed and tested, you do not have to do anything else. No switches to turn on and off.

If you choose to have an onboard generator it is absolutely imperative you install a marine generator, not a portable generator designed for land use. Marine generators are definitely more expensive, but they are designed to withstand the marine environment, they are ignition protected, and they are designed to mitigate Carbon Monoxide problems. Most are diesel powered, but gasoline powered sets are also available. In recent years the manufacturers of the gasoline powered generators have made serious reductions in the amount of CO these generators produce.  They have also made them much quieter. There is nothing more disturbing in a quiet anchorage than when someone lights off a noisy generator. Go to Ike's List and find the links on Carbon Monoxide . Then read my Hot Topics on Portable Generators


Inverters take DC power and change it into AC power. With an inverter you can run some AC equipment, if it does not require too much power. Air Conditioners and microwave ovens are two appliances that should not be run off of an inverter because of the large amounts of power they need. However, televisions, computers, VCRs and other small appliances can be run off of an inverter.

 You need to evaluate what you want to use, and how much power you need, and convert that to how large your house battery bank should be. These devices will rapidly deplete a single battery. Select an inverter designed for marine use, that is ignition protected.


A converter is the opposite of an inverter.  It takes AC power and converts it to 12V DC. Most of these have the dual function of supplying DC to power your DC equipment, and charging your batteries.  The converter can be run off of the shore power, or off your onboard generator.  Select a converter designed for marine use, that is ignition protected.

Links to Offsite References:
Wiring Your Boat 2007 All rights reserved. revised 12/05/2015


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