THE WEAK LINK Fuses and circuit breakers prevent too much amperage from traveling through a wire, building heat through resistance that may damage electrical and electronic equipment, and more important, cause an electrical fire. You can avoid potentially dangerous problems by reviewing ABYC standards to make sure adequately sized circuit protection devices are installed properly. "Electrical circuit” refers to a complete path for electrical current flowing through wires. A boat’s power system is comprised of various electrical circuits formed by connecting a voltage source such as a battery bank or AC power source to one or more electrical loads (equipment that uses electricity) by means of a conductor (wire or cable). In a DC (direct current) circuit, battery voltage pushes electrical current through various electrical loads when they are switched on, using some of the energy stored in the battery to perform useful work. In an AC (alternating current) circuit the voltage source (shorepower, inverter or gen-set) pushes electrical current in a similar manner, again using energy in the process. There are primary or main circuits, which typically include the main power sources, either DC or AC (Figure 1). There are also secondary or branch circuits that are complete electrical paths within the primary circuits. All electrical loads on the boat, for example, are individual secondary circuits. Finally, there are internal circuits within each electrical or electronic device. All electrical circuits must be protected from too much current flow, which can cause wire conductors to overheat and wire insulation to burn, and can damage internal circuits in individual appliances. Protection of an electrical circuit is in the form of an intentional weak link, typically a fuse or circuit breaker, which the industry refers to as a overcurrent circuit protection device (CPD). Circuit protection on board a boat must be taken seriously. A potentially devastating electrical fire can result when too much amperage travels through a wire and enough heat is generated to melt and burn the wire insulation and surrounding materials, causing a fire. CPDs are meant to protect against unexpected problems, and shouldn’t preclude proper wire sizing and adherence to proper electrical system design and installation practices. Sizing wire is relatively easy. You simply match the maximum sustained amperage in a given circuit and the total length of the circuit wiring with a proper wire size that will be safe and also prevent an excessive voltage drop for the type of appliance used (Figure 2). [Ed: For specifics on wire sizes, factoring in voltage drop, length of run and whether or not the wire passes through a heated space (i.e. engine compartment), refer to DIY 1998-#4 issue, or “DC Electrical Systems” CD-ROM, or sections E-8 and E-9 of ABYC Standards.] Sizing wire correctly can’t protect against accidental grounding through wire chafe, equipment failure or grounding a circuit while performing system maintenance that temporarily allows a dangerous amount of current to flow. CPDs handle unsafe levels of current by opening the circuit, either through thermal devices or devices that sense a magnetic field created by excess amperage.
Types of CPDs
Fuses are strictly thermal devices that melt at a predetermined amperage. They are reliable and relatively inexpensive, although total cost includes the purchase of a fuse mounting block and a protective cover of some type, spares, since fuses must be replaced after each overcurrent condition, and some form of circuit disconnect. Circuit breakers can be thermal or magnetic devices, or a combination of the two. Circuit breakers are typically more expensive than fuses, especially for high load circuits, but they also serve as circuit disconnects and since they are resettable the need to carry spares is not as critical. (Figure 3 illustrates a quick-reference comparison of CPDs.) Class T Fuse: Recommended by most inverter manufacturers, it has an extremely fast short-circuit response, a 20,000 ampere DC interrupt capacity, and is rated for up to 160 volts DC (VDC). ANL Fuse: With a 6,000 ampere DC interrupt capacity, it meets ABYC requirements for main DC circuit protection on large battery banks with a voltage up to 32DC. Sea Fuse: An economical choice for circuit protection between 100 and 300 amperes. It has a 2,000 amperes DC interrupt capacity and is rated for up to 32VDC. Automotive Style Fuse: Inexpensive and widely available through automotive stores, it’s the most economical choice for between 30- and 80-ampere circuit protection. It has 1,000-ampere DC interrupt capacity and a 32DC voltage rating. Glass Fuses: Available in current ratings from less than 1 up to 50 amperes, these inexpensive fuses are used for branch circuits in a variety of applications. AGC models are fast-acting fuses, while MDL models are time-delay fuses for high inrush motor type loads. Thermal Circuit Breakers: The T-1 series CPD from Blue Sea is thermally responsive bi-metal breakers combining switching and breaker function in one unit. They are available with ampere ratings from 25 to 150 amperes, a voltage rating of 48VDC, and 5,000 amperes at 24VDC interrupt rating. Blue Sea’s standard thermal circuit breakers are similar to the T-1 Series but have a 3,000-ampere DC interrupt rating, a 30VDC voltage rating. Magnetic Circuit Breakers: Available in a wide range of styles and ratings, there are standard DC and AC single pole circuit breakers used for protecting branch circuits in electrical distribution panels. Some low ampere models, rated as “quick trip” are designed specifically for electronics. Double pole AC breakers are available to switch both hot and neutral legs of a 120VAC circuit or two hot legs of a 240VAC circuit. Standard magnetic circuit breakers typically have a 2,000- to 3,000-ampere interrupt rating, although some models are available with a 5,000-ampere interrupt rating. It used to be that only fuses could handle heavy DC loads, but high load circuits can now be protected with single, double or triple pole breakers, such as those from Blue Sea Systems and Paneltronics. In these devices breakers rated up to 100 amperes each are ganged to provide various levels of protection. Sizes range from 50-ampere single pole to 300-ampere triple pole models.
Sizing and Selecting
When choosing CPDs, take it one circuit at a
time. First, choose whether you want a fuse or circuit breaker
for each circuit. Circuits in an explosive vapor area, such as
gasoline engine rooms, battery compartments and propane lockers,
must be protected by a vapor-proof circuit breaker. Then, check
to see what ampere interrupt rating is required for the application
(see Figure 4). Next, make sure the CPD is rated to open at an
amperage greater than the maximum circuit load and less than the
rated amperage capacity of the smallest wire in the circuit. It’s also useful to know the maximum momentary
or surge current experienced in the circuit. Choosing CPDs that can
withstand this surge and still offer the required protection means
you’ll avoid nuisance tripping. As a final check, make sure
the CPD’s voltage rating meets or exceeds the circuit voltage.
Before locating CPDs in your electrical system, it’s important to understand the concept that CPDs are installed to protect individual wires in a circuit, and that they should be sized specifically for those wires. Secondary and small branch circuits use smaller (in number only, larger in size) gauge wires, so smaller CPDs should be used (Figure 5). Ultimately every wire on board would be protected, but that is impractical. In the DC side of an electrical power system, ABYC standards take a reasonable approach to circuit protection by exempting mandatory CPDs from wires between batteries, battery switch and engine starter motor (Figure 6). CPDs are required within 18cm (7") of the battery switch and starter motor on wires leading to various loads, and within 183cm (72") on wires leading to loads directly from a battery. The 18cm (7") dimension can be extended to 101cm (40") if wires are enclosed in a sheath or other enclosure in addition to the wire insulation (Figure 7). In the AC side, there should be a main circuit breaker at each AC power source, i.e., at the shorepower inlet(s), generator output and inverter output (Figure 8). In addition, there should be branch circuit breakers on every branch circuit. On boats with multiple sources of AC power, the main circuit breakers can be conveniently located in an AC source selector panel (Figure 9). This type of panel allows only one AC power source to be capable of supplying power at any given time. In this case the main circuit breakers also are serving as manual circuit disconnect switches.
(1) Single-phase 120V shorepower with shore-grounded (white) neutral conductor and grounding (green) conductor. (2) A 120VAC generator included as an additional AC power source. In both diagrams the ungrounded conductor and the grounded neutral are protected with a single overcurrent protection device that simultaneously opens both current-carrying conductors. ABYC does not recommend fuses to serve this function. 120VAC branch circuits are permitted to be single pole in the ungrounded current carrying conductors. (3) Single-phase 120/240VAC system with shore-grounded (white) neutral conductor and grounding (green) conductor. Each ungrounded shore conductor connects through the shorepower inlet to the boat’s AC electrical system through a single overcurrent protection device that simultaneously opens both ungrounded conductors. The shore-grounded neutral connects to the boat’s AC electrical system without overcurrent protection. It may be used provided the overcurrent protection device opens all current carrying conductors in the circuits (in this case a 3-pole switch is needed). (4) An isolation transformer system with single phase, 240VAC shorepower input and 120/240VAC output from the transformer. Circuit protection is provided by a main shorepower disconnect on the shore side of the transformer and secondary overcurrent protection on the boat system side of the transformer. Each ungrounded shore current carrying conductor connects from the shorepower inlet to the primary winding of the isolation transformer through an overcurrent protection device that simultaneously opens both ungrounded conductors. 120VAC branch circuit breakers are permitted to be single pole in the ungrounded current carrying conductors. 240VAC branch circuit breakers must be two-pole and simultaneously open all current carrying conductors. About the author: Kevin Jeffrey works as an independent electrical power consultant and is the author of the “Independent Energy Guide” and publisher of “Sailor’s Multihull Guide” available soon in its third edition. Resources Ratings Stats Amperage Rating: The amperage used to calculate the opening speed of the device, not the actual amperage at which the CPD will trip or “blow.” It usually takes an additional 20% or so of amperage above the rated value for the CPD to trip. Opening Speed or Delay: The relationship between the percentage a CPD is operating over its amperage rating and the length of time required for it to open. The higher the percent of current flow to amperage rating, the faster the circuit protector opens. Interrupt Rating: How much current the fuse can safely handle in short circuit situations. Refer to Figure 4 on page 13 to determine what minimum interrupt rating is required. Voltage Rating: The maximum voltage for the circuit in which the fuse is used. |
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