Fuses and Circuit Breakers

(continued from page 43) through with several tables to help decipher these secret codes. From ABYC section E-11 we see temperature-rating codes for flexible cords in figure 4-5, and insulation characteristics and temperature ratings for typical wiring conductors in figure 4-6.

Another table, shown in figure 4-7 on page 48, shows the allowable amperage for various sizes of wires used inside and outside engine-room spaces. Notice that the amperage decreases inside engine rooms where expected temperatures are greater than outside temperatures. Use this table to determine the size of conductors for low-voltage DC systems only; AC wiring uses different criteria. The information on this chart must be compared to the information in the charts shown in figures 4-3 and 4-4. Always use the largest wire gauge indicated by the two charts for a given amperage based on the length of the wire run.

All wire used in DC circuitry should have a minimum rating of 50 volts stamped on the insulation. Most quality wire will have the AWG gauge embossed on the insulation as well. In general, wire sold as marine grade at the major supply houses will have a 105°C rating for the insulation, but it pays to check.

Depending upon when your boat was built and who built it, you may have any combination of circuit breakers or fuses of different types used as circuitprotection devices. All such devices used on pleasure boats work on one of three basic principles, two of which depend on heat generated by resistance and a third that works on current-induced magnetism. Since fuses are easier than circuit breakers to understand, let’s take them first.

SO, SOW

ST, STW

STO, STOW, SEO, SEOW

SJO, SJOW

SJT, SJTW

SJTO, SJTOW

Fig. 4-5. Table VIII from section E-11, showing insulation markings and temperature ratings for insulation on flexible cords as used on board. (© ABYC)

Fuses

If you were ever foolish enough to connect the positive and negative terminals of any 12-volt battery with a length of wire with no intervening load to slow the flow of electrons, the wire would immediately become red-hot and melt. This is how an unfused short circuit can quickly destroy a boat, and it’s how a fuse works. If you interpose a short piece of smaller-gauge wire into this ultrasimple circuit, the smaller wire will self-destruct and stop the flow of electrons before the heat builds up enough to damage the larger wire. You have, in effect, built yourself a fuse in the form of the smaller wire.

You might recall that in chapter 1 I noted that a by-product of resistance is heat. Well, a fuse is designed with an element that will carry a certain amount of current and then heat up to a point where it melts, opening the circuit it’s designed to protect. The trick is to select a fuse that will allow sufficient amperage to flow freely through the circuit to run all the equipment on the circuit but will self-destruct just before the current reaches the point where it can

THW

TW

HWN

XHHW

MTW

AWM

UL 1426

NOTE:Some ofthe listed types are not commonly available in stranded construction for sizes smaller than 8 AWG.

Fig. 4-6. Table IX from section E-11, showing insulation markings for typical wiring conductors found on board (exclusive of flexible cables). (© ABYC)

NOTE:Cross reference with voltage drop tables in figures 4-3 and 4-4.

Fig. 4-7. Table IV from section E-11, showing the allowable amperage of conductors for DC systems under 50 volts. (© ABYC)

damage anything. Fuses are simple devices, but to avoid problems with them there are some important things you should know.

Fuse Types and Ratings

Popular fuse types shown in figure 4-8 include the cylindrical glass bus-type fuse designated as AGC, the newer and increasingly popular blade-type fuse designated as ATO, and a heavy-duty slow-blow fuse designated MDL, or type T. There are many fuse designations other than AGC, ATO, and MDL, but these three are the most common and will do for our purposes.

The most important specification to look for when selecting a fuse is the amperage. When selecting a fuse for a circuit, base the size of the fuse on the current-carrying capability of the smallest wire in the circuit. For example, if you select a 14-gauge AWG wire to get power from your distribution panel to a new depth-sounder but the leads built into the sounder are only 16 AWG, the fuse must be rated for the 16 AWG leads, even if they are only a few inches long.

Bus Fuse

ATO Fuse

Fig. 4-8. Bus-type glass fuse, slow-blow cylindrical fuse, ATO blade-type fuse.

AGC and MDL Fuses

AGC fuses (often generically called bus fuses) are the popular 1.25-inch glass-barrel fuses with tinned-copper end caps that have been around since the beginning of time. They are by far the most common type of fuse used to protect individual pieces of equipment and circuits on boats today, and are the type most often supplied with a piece of equipment when you buy it. Many bilge pumps, just to name one item, will have an AGC fuse in an in-line fuse holder wired right into the positive lead of the pump. At least one manufacturer (PAR) voids the warranty on the pump if this fuse is tampered with.

As you examine an AGC fuse, you may notice that it has a voltage rating embossed on the metal end cap. Usually this will be a fairly high number like 250 volts. Since you’re using these fuses for DC circuits, this rating is completely irrelevant and can be disregarded. If you blow a 10-volt fuse, for example, it’s perfectly OK to replace it with a 250-volt fuse, or vice versa. However, you would never replace a 10amp fuse with anything other than another 10-amp fuse. We aren’t quite done yet, though. Some motors and inverters require what is called a slow-blow fuse.

Motors and certain other devices require as much as five or six times more amperage to get started as they will use when they are up and running. Inverters that supply AC loads to motorized equipment like electric drills will experience a surge of current whenever the AC motor starts up because AC output is proportional to DC input. Obviously, if you tried to protect any of these circuits with a regular AGC fuse, you would stay pretty busy changing fuses. A slowblow (MDL) fuse allows a substantial amount of extra current to flow for a specified length of time before it blows and opens the circuit, and is just the thing for this situation.

Never make assumptions about the amperage of an AGC or MDL fuse based on a visual inspection of the element. Manufacturers use different materials for these elements with varying sizes and currenthandling capabilities. Two fuses with elements that appear to be the same size could have entirely different ratings. You need to read the amperage specification embossed into the end caps of the fuse itself to be sure.

ATO Fuses

An increasingly popular fuse being used by many of the large production boatbuilders is the ATO type, commonly referred to as a blade-type fuse. ATO fuses work in exactly the same way as the barreltype AGC fuses mentioned above (the heat from excessive amperage melts the element and opens the circuit), but they offer several advantages over AGC fuses. They are color coded, for one thing, and the colors match the amperage ratings, as shown in the table in figure 4-9 on page 50. With AGC fuses you must remove the fuse to read the amperage; however, ATO fuses have the amperage embossed on the end of the fuse where it’s always visible, even when the fuse is installed. ATO fuses have transparent plastic cases that allow you to see the entire element, so there is never a question about whether it’s burned out or not. ATO fuses are also slightly easier to remove and replace than are AGC fuses.

A disadvantage to ATO fuses is that many of them use aluminum elements and blades. Using these fuses flies in the face of the ABYC directive for not using aluminum wiring or connections on boats. I have seen aluminum ATO fuses literally rot from corrosion when they were exposed to salt spray. Alwaysuse

ATO in-line-blade fuse holders with rubber covers as shown in figure 4-10.

If you find any aluminum fuses or fuse holders of any type on your boat, you should immediately remove them and replace them with tinned copper or brass fuses and holders. If there is a question of what material an ATC fuse is made from, simply scrape one of the fuse blades with your pocketknife. Aluminum easily flakes away and reveals a shiny silver coloration right through the blade. Tinned copper or brass will show a yellow or pink coloration as the tin coating gets scraped away.

ATO Fuses, Color and Amperage Rating

Amp Rating . . . . . . . . . . . . . . . . . . . . . . . .Color 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Black 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Gray 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Violet 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Pink 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Tan 71⁄2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Brown 10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Red 11 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Blue 12 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Yellow 13 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Clear 14 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Green 15 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Orange

Fig. 4-9. ATO fuses, color versus amperage rating.

Fig. 4-10. In-line-blade fuse holder with rubber waterproof cap attached.

Circuit Breakers

Fuses are an inexpensive and effective way to protect most circuits from too much amperage, and they are practically fool-proof if you pay attention and match the fuse to the circuit you want to protect. They do have several important disadvantages, though. For one thing, once they blow they are useless and must be replaced. This isn’t an economic problem because both AGC and ATO fuses cost only a few cents apiece, but it can be a nuisance if you blow a fuse and find that you don’t have a replacement. If this should happen with a critical piece of navigation equipment when you’re caught offshore in a storm, it could even be dangerous. Another danger is that many of us faced with not having the correct size of replacement fuse will be tempted to use a fuse of a higher amperage or, even worse, to jump the holder with a piece of wire. (Actually, a chewing-gum wrapper is the fuse jumper’s tool of choice.)

Another, less dangerous drawback to fuses is that they tend to be awkward to remove so that deactivating a circuit is sometimes inconvenient. This is especially true when the boatbuilder has located the fuse holder in the back of some obscure locker, as often happens. Here again, those among us who are easily led astray might be tempted to work on a circuit while it’s live instead of taking the trouble of removing the fuse. The answer to all these problems is to have a reusable fuse that incorporates a switch so the circuit can be easily shut down, and this is just what a circuit breaker does.

Circuit breakers are available in only a limited range of sizes, the most common being 5, 10, 15, 20, 25, 30, 40, and 50 amps. AGC, MDL, and ATO fuses, however, are available in increments of 1-amp, and even fractions of an amp. This means that nearly all

circuits on a boat need a combination of circuit breakers and fuses. In most cases a circuit breaker will be used to protect the circuit, and fuses will be used to protect each piece of equipment on the circuit. A typical lighting circuit, for example, might have five separate 25-watt light fixtures wired in parallel, with a 15-amp circuit breaker protecting the circuit and a separate 3-amp fuse protecting each of the five lights. If a problem were to develop causing a current surge at any one of the light fixtures, the fuse for that fixture would blow, leaving the others still working. A dead short in the main circuit, however, would trip the circuit breaker and extinguish all the lights.

Ampere-Interrupting Capacity

The amperage rating of a circuit breaker is calculated as a percentage of the current-carrying capability of the smallest wire in the circuit. There is an additional rating, known as the ampere-interrupting capacity (AIC), that is a peak rating taking extreme situations such as catastrophic short-circuits and other surges into account. As a user, you need not worry about the AIC rating, as all the reputable producers of marine circuit breakers take these values into account.

Trip-Free Breakers

ABYC specifications state that all circuit breakers used in pleasure craft be of the trip-free type, defined as “a resettable overcurrent-protection device, designed so that the means of resetting cannot override the overcurrent protection mechanism.” This is an elaborate way of saying that they don’t want you to be able to override the breaker by holding it closed once it has tripped. With trip-free breakers, the problem that caused the breaker to trip in the first place must be located and repaired before the circuit can be reactivated.

Types of Circuit Breakers

There are two kinds of circuit breakers commonly used on today’s boats. Bimetallic breakers sense resistance-generated heat, and magnetic breakers sense the magnetism induced by current flow. Let’s look at them one at a time.

Bimetallic Circuit Breakers

Bimetallic circuit breakers use two dissimilar metals, such as copper and stainless steel, fused together into a thin strip. In a normal state this metal strip connects the circuit inside the breaker. As current flows through the breaker, heat is generated, and since the two metals have different rates of thermal expansion, one metal expands more than the other, bending the strip. When the bending reaches a preselected critical point, the contacts inside the breaker separate and open the circuit.

Magnetic Circuit Breakers

We already know, from our reading in chapter 3, that any conductor with current flowing through it will be surrounded by a magnetic field. Our second type of circuit breaker senses this magnetism, and when the magnetism reaches a critical magnitude, the circuit breaker trips and opens the circuit.

Either of these two types of circuit breaker works just fine in most marine circuit-breaker panels, and it’s perfectly OK to mix them. In fact, since they both function in exactly the same manner with no significant external differences, it isn’t important that you know which type you have. Just be certain that whatever breaker you select is designed for marine use, and you’ll be all set.

Removing Circuit Breakers

A big problem with many of the circuit breakers I’ve seen lately is in their labeling. Circuit breakers essentially all look alike, and the only way to be certain of their ratings is to read the label affixed to them. The problem is, some of these labels are located on the side of the breaker, requiring removal of the breaker from the switch panel to be certain of the amperage rating. These labels are typically glued-on paper labels that after a few years in the marine environment dry up and fall off or become very hard if not impossible to read. Fortunately, removing a circuit breaker from its holder is a straightforward procedure. First be sure to turn off the master switch feeding the distribution panel to avoid arcing any wiring as you remove the breaker. Next, back out the one or two screws adjacent to the switch handle on the face of the panel and