|
Index to this Page
Basic Electricity - Page 8 - AC Circuits
Basic electricity for boat builders,
boat repairers and owners. What you need to know about the electrical
systems on your boat. Overcurrent Protection, Distribution, Wire,
Isolation, transformers.
On Board The Boat.
Overcurrent Protection:
Now that you have the power on the boat, where do we go
from here?
The first thing that should come after the shore
power inlet is the
main circuit breaker for all AC power on the boat. This should
be a double pole circuit breaker, that is, it breaks the connection
in both the black and white, the hot and neutral wires, at the same
time. Marine, Ignition Protected trip free circuit breakers are
required. Do not use household circuit breakers. see
https://newboatbuilders.com/pages/elect_a.html#Over Current Protection
for a description of a marine ignition protected trip free circuit
breaker.
In DC circuits, single pole breakers are used that only break the
positive side of the circuit, but they should not be used in AC circuits.
The reason for this is if there is a fault in the circuit and only one
wire is broken, you could still have a hot circuit.
The main breaker should be as close to the
power inlet as possible. The power inlet is considered the source of
power and as we learned in DC circuits the overcurrent protection is
required to be within seven inches of the source of power. This protects
the wire and prevents fires due to overheating. Remember, these should
be marine, ignition protected, trip free circuit breakers..
Sizing The Circuit Breaker:
The rating for the circuit breaker in an AC circuit should not be any greater than the maximum
current rating of the conductor. If it's a 20 amp circuit, it should
have a 20 amp circuit breaker. A 30 amp circuit should have a 30 amp
circuit breaker, and so on.
DO NOT size them to 150%
like in DC Circuits.
DO NOT use fuses in AC
circuits.
All parts of the electrical systems should be marine rated or UL - Marine
Listed. This is for your protection. Household devices are not
designed to operate in the damp marine environment, and circuit breakers
used in homes are not ignition protected and are not trip-free. Metal
parts will quickly corrode. This results in bad, high resistance
connections, and heat. Corrosion is a real problem with electrical
equipment on boats.
AC circuit connections, plugs and contacts, should
be inspected and cleaned on a regular basis. Many boat fires over
the winter are caused by the shore power inlet connection on the boat
getting too hot due to bad or high resistance connections from
corrosion.
ELCI: Electrical Leakage Circuit Interrupter
Combined with the main circuit breaker should be an
ELCI (Electrical
Leakage Circuit Interrupter). An ELCI detects current
leaking into the water. 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. See
https://newboatbuilders.com/pages/electricity10.html for more on ELCIs
Distribution Panels:
The wire leaving the main breaker should go to a
distribution panel, where it is distributed into separate branch circuits.
There should be a circuit breaker for each branch
circuit. The main breaker can be combined with this distribution
panel. This panel should be rated for marine use. In 1995 a
houseboat manufacturer who used a distribution panel designed for
household use, had to recall several thousand houseboats and replace the
panels. Several people died from being shocked, some while swimming near a
houseboat. So as well as resulting in deaths, this cost the manufacturer
so much he ended up having to sell the company. So do it right the first
time. Use marine rated equipment.
Wire:
The wire should be triplex marine rated boat
cable. This is an example of
Triplex boat cable
https://www.ancorproducts.com/en/131370 (this is not an endorsement of
Ancor products. It is cited only as an example) This cable contains three wires, the black, white and green.
This looks very much like
standard romex cable used in homes but is much better suited for the
marine environment. In addition to meeting all the chemical and oil
resistance requirements, it is stranded tinned copper, which is very
corrosion resistant. It is rated for 600V and meets or exceeds the
UL 1426 standard. Additionally, the outer
plastic sheathing is abrasion resistant. It can be used without grommets or other abrasion protection
where it passes through holes in bulkheads or other structure. However,
you should still provide this protection for safety.
Before you start wiring the boat you should be aware of several problems
that can occur. If the green wire is connected to the ground on the
engine block, as it should be, along with the DC ground, then it is possible for
stray DC currents to exist on the metal fittings in the boat. These
DC currents are not enough to cause a shock or fire hazard, or trip any
breakers, but they will result in galvanic corrosion.
If you recall
my discussion of how a battery works, you will remember that a battery is
two dissimilar metals in an electrolyte. If you use the grounding
bus and connected all the metal together and then accidentally introduce a
DC electrical current, then you start the current flowing between the
dissimilar metals. One of them is going to be eaten away. The number one
victim is aluminum lower casings of stern drives and outboards. So, by
protecting yourself and everyone on your boat from shock you have
aggravated the problem of galvanic corrosion. See
Corrosion On Boats. https://newboatbuilders.com/pages/corrosion.html
Do not under any
circumstances cut the green wire, there is a solution! It is called
isolation and can be done several ways.
ISOLATION
Galvanic Isolators:
One way of isolating the AC on the boat is a galvanic isolator. In the ABYC standards the green wire is
not allowed to be broken by any device, except a galvanic isolator. The
galvanic isolator prevents stray DC currents from passing through the
green wire, while at the same time it will pass AC, if a ground fault
occurs. So, you still have ground fault protection and have now
added protection against galvanic corrosion. A galvanic isolator is a
small electronic device made up of diodes. It is inexpensive, but the
isolator can fail and even with a monitor for the isolator you won't know
it has failed. But this is better than no isolation.
See
ABYC Standard A-28 Galvanic Isolators. A current version of A-28 can be purchased from
ABYC.
https://abycinc.org/?
There is another solution, but it's a more expensive. It's an
isolation transformer.
Transformers
Near the beginning of Basic Electricity I talked about
magnetic
induction. That was how we generated electricity, both
DC and AC. To recap, if you pass a wire through a magnetic field it
induces an electric current in the wire, and if you spin a magnet
surrounded by a coil of wire, you generate electricity in the coil.
If you have a coil of wire and pass AC through it, it generates an
electromagnetic field around the coil. If you then place another coil
of wire near the first coil, electric current is induced
in the second coil. If both coils have the same number of turns around the
central core, then the voltage induced in the second is the same as the
first. So if the first has 120 volts, then you get 120 volts off the
second coil. This is exactly how a transformer works.
The benefit is that there is no physical connection between the two coils.
So the second coil is effectively isolated from the rest of the power
grid. This principle can be used to isolate the AC circuits on a boat from
the AC circuits ashore.
The green
wire on the boat is connected to the white wire on the boat side of the
transformer, at the transformer terminal. This is the only exception
to the rule about not connecting the green and white wire on the boat!
The following diagram from ABYC E-11 illustrates this connection. Click
on the image for full size.
References
Wikipedia
on Isolation Transformers.
https://en.wikipedia.org/wiki/Isolation_transformer
BoatUS on Galvanic Corrosion And
Isolation Transformers:
https://www.boatus.com/expert-advice/expert-advice-archive/2020/january/the-current-scoop-on-galvanic-corrosion
Steve D'Antonio on Isolation transformers and Galvanic isolators:
https://www.passagemaker.com/technical/galvanic-isolators-and-isolation-transformers
Steve D’Antonio is
PassageMaker Magazine's
https://www.passagemaker.com/ Technical Editor and the VP of operations for
Zimmerman Marine, a custom boatbuilder and full-service repair yard in
Mathews, Virginia. Published 2006
Here is another article
that compares the Galvanic Isolator and the Isolation Transformer.
From SmartGauge Electronics.
http://www.smartgauge.co.uk/galv_tran.html
Some people do not agree that DC Currents are the only cause of Galvanic
Corrosion. They believe that stray AC current can also cause significant
Galvanic Corrosion. There is an article by Dick Troberg that appeared
in the February/March, 2007 Professional Boat Builder, Number 105 called
The
Other Stray Current. Unfortunatley I do not have a link to this
article but you should be able to get a reprint from Professional Boat
Builder. Tests that he has
conducted indicate that AC current can result in galvanic corrosion.
Links to Offsite References:
Wiring Your Boat https://newboatbuilders.com/docs/WiringYourBoat.pdf
© newboatbuilders.com 2007 All rights reserved. Revised 02/025/2022
|
Everything Boat Building
