What is circuit protection?  

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  Circuit protection is the intentional installation of a "weak link' in an electrical circuit. This is a fuse or circuit breaker, referred to here as a circuit protection device or CPD.
What is the CPD protection against?
   Prevention of wire conductor overheating and burning of the wire insulation is the primary reason to install a fuse or circuit breaker. In some cases they are also installed to protect electrical or electronic equipment from damage.
How does fire start in an electrical circuit?
   Fire results when too much amperage travels through a wire. Amperage is electron flow through a conductor. If too much amperage flows through a wire, enough heat can be generated to melt and burn the wire insulation or surrounding materials.
How much amperage can a wire safely conduct?
  The American Boat and Yacht Council (ABYC) publishes the following table showing how much amperage each size wire can carry:

If the wire is properly sized why is a CPD necessary?
  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.

What size CPD is required?
The short answer is that the CPD should be rated to open at an amperage that is greater than the maximum load the circuit will carry and smaller than the rated amperage capacity of the wire in the circuit. We recommend choosing a size as close to, but not greater than the amperage capacity of the wire.

What is Amperage Rating?
The amperage rating is the amperage on which the opening speed of the fuse is based. This is the number that is usually printed on the fuse and the most common way in which fuses are referenced. Most fuses will operate indefinitely at their amperage rating. Only when the amperage rating is exceeded by some significant percentage (usually at least 20%) will the fuse open or "blow."

What is Opening Speed?
Opening speed defines the relationship between the percentage by which the fuse is operating over its amperage rating and the length of time that will be required for it to open. The opening (blowing) of a fuse is determined not just by the amperage rating, but by the amount of time and the percentage over its amperage rating at which it is being operated. There are other factors, such as ambient temperature, that influence a fuse's opening, but they are not significant enough to be included in this discussion.
What is interrupt Rating?
The interrupt rating specifies how much current the fuse can safely handle in short circuit situations. See the ABYC Tables I and II (combined) below for determining what minimum interrupt rating is required.

What is Voltage Rating?
The Voltage rating specifies the maximum voltage for the circuit in which the fuse is used.

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