BASIC ELECTRICITY
Page 10 - AC Circuits
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 firs, 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.
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 is
currently looking at whether or not it would be good to have both GFCIs to
protect people and an RCD to protect the boat. There was an excellent
article in the December/January 2007 Professional Boat Builder Number
104, by Nigel Calder which explains this very well. Select Contents, and
Tools of the Trade 2. Another
Take on Trip Thresholds.
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 of 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 read the sections on Carbon Monoxide .
Then read my Hot Topics on Portable
Generators
Inverters
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.
Converter/chargers
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.
http://newboatbuilders.com/ Copyright
2007
|
