If the wire is properly sized why is a CPD
Even though the correct wire size is used for the amperage that is to flow through
the wire normally, the circuit may be accidentally grounded, allowing a dangerous amount
of amperage to flow. A circuit can be grounded by a wire's insulation chafing through and
contacting a grounded conductor, by the failure of equipment in the circuit, or by
accidental grounding of the circuit during maintenance.
How does the CPD stop the amperage now?
There are two primary methods that CPD's use to determine that excess amperage is
flowing in a circuit. Thermal devices open to break the circuit and stop the current flow
in response to heat generated by the excess amperage. Magnetic devices react to a magnetic
field created by excess amperage.
How do fuses and circuit breakers differ?
Fuses are thermal devices that open the circuit by utilizing a 'fusible link"
that melts at a known amperage. Circuit breakers can be either thermal or magnetic devices
or a combination of the two.
Are fuses and circuit breakers interchangeable?
The short answer is yes. The longer and more accurate answer is more complex and beyond
the scope of this discussion. As amperage ratings increase circuit breakers become
relatively more expensive than fuses. Generally, fuses are less expensive for a given
rating, but circuit breakers are re-settable and don't require the purchase of spares, as
do fuses. Also, circuit breakers can be used as switches.
Are thermal circuit breakers appropriate for marine use?
Yes, it is sometimes argued that this circuit breaker type is inappropriate for marine use
because it is affected by temperature. The hotter the ambient environment (such as an
engine room) the lower the amperage at which the device will open; leading to undesired
trips. The percentage by which the rated amperage lowers in normal operating environments
is usually in the 10 to 20% range and when properly sized the risk of "nuisance
trips" is remote.
What wires need to have CPD's installed?
The ideal answer is that every wire in the boat needs to be protected by a fuse or a
circuit breaker. The CPD must be correctly sized to the wire it protects. As wires branch
away from the batteries or other power source and become progressively lighter, smaller
CPD's must be installed at the beginning of each wiring run.
ABYC standards exempt wiring between the batteries, the main battery switch, and the
starter motor. It is not that these wires do not require CPD`s, but that it is often not
practical to provide such protection. The diagram below shows the ABYC standards for CPD
placement. Measurements are maximum lengths between the point of connection and the CPD.
All 7" dimensions may be increased to 40" if the conductor is enclosed in a
sheath or enclosure in addition to the wire insulation.