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Fuse Circuit Protection Practices


technical article from Blue Sea Systems

What is circuit protection?

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 resultant 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:



Is "Allowable Amperage" the only thing to be considered in sizing a wire?

No. Voltage Drop must also be considered. Voltage Drop is the amount of voltage "consumed" as the voltage "pushes" the amperage through the resistance of the wire. Sometimes Allowable Amperage will be the determining factor in sizing a wire and in other cases Voltage Drop will dictate the wire size. The wire must be the larger of either the size required by the Allowable Amperage or the Voltage Drop.

How is Voltage Drop determined?

There are tables published by ABYC that use the formula below to calculate Voltage Drop. If the tables are not available, Voltage Drop can be calculated using the formula:

Voltage Drop = Amperage x Resistance (this is Ohm's law, V=IR or I=V/R or R=V/I)

Voltage Drop = (Amperage) x (Circuit Length / 1000) x (Ohm's per 1000 ft) (from Allowable Amperage chart)

(Voltage Drop / Nominal Circuit Voltage) / 100 = % Voltage Drop

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 flow?

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 in a known length of time. 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-that is, 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. Typically, wires branch away from the batteries or other power sources and become progressively lighter. Smaller CPD's must be installed at the beginning of a smaller wiring run, if the wire ampacity is lower than the rating of the last CPD.

ABYC Standards exempt wiring between the batteries, the main battery switch and the starter motor. These wires can also burn if too much current flows through them, however, 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.

ABYC Required Fuse Postions



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" is a term used to describe a mechanical action by which the fuse or circuit breaker stops the current flow by "opening" the circuit. Fuses are described as opening by "blowing" and circuit breakers are described as "tripping." 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 of a fuse or circuit breaker 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.

Why is opening speed important?

When a circuit is activated, there is an initial surge of current (amperage) that the CPD must allow to pass without tripping the CPD. The two screen reprints below from the Blue Sea Systems' testing system illustrate the difference in inrush currents between inductive (like motors) and resistive (like light bulbs) loads.

Notice that the ratio of normal running current (represented by the flat portion of the amperage line) between the inductive and the resistive graphs varies dramatically. The 17A inductive load initially drew 80 A or 470%, whereas the 44A resistive load initially drew only 126A or 286% of its normal operating current. Such inrush currents must be ­considered when sizing CPD's. Each Blue Sea Systems CPD has a time/current chart shown on its catalog page.

We recommend sizing fuses and circuit breakers for 5X multiples for inductive loads and 3X multiples for resistive loads and assuming this inrush for approximately .5 seconds.

Inductive Load

Windlass pulling 20' 3/8" chain and 33 lb. anchor



Resistive Load

Bank of 100W DC Light Bulbs



What is Interrupt Rating?

The interrupt rating specifies how much current the CPD can safely handle in short circuit situations. See the ABYC tables I and II (combined) below for determining what minimum interrupt rating is required.



Interrupt Rating Comparison

Blue Sea Systems' Fuse and Circuit Breakers



What is Voltage Rating?

The voltage rating specifies the maximum voltage for the circuit in which the fuse is used.


Original article from Blue Sea Systems

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