POWERBOATER’S GUIDE TO ELECTRICAL SYSTEMS An easy way to isolate RFI is with a small portable transistor radio. Simply tune the radio between two AM stations (FM stations don’t work as well), and you’re ready to go. Turn on the electrical component you want to check, and listen for a loud hum from the radio. Move the radio alternately closer, then farther away from the component you’re checking, and listen for a change in the humming noise. A good device to try this with for the first time is a battery charger. I have not found one yet that did not emit some RFI; it’s quite normal for these devices. The problem with RFI is that it can affect things like electronic compasses, autopilots, and Loran-C systems, and you’ll never hear a sound. However, you could end up in Timbuktu instead of your favorite fishing hot spot! Like the magnetism problem already discussed, you’ll need to ensure that a good zone of separation is kept between devices that cause RFI and devices sensitive to it. For ignition systems and alternators, a variety of suppression devices and filters are available, and they are easy to install. All come with simple instructions. Check your local NAPA Auto Parts store or a good marine electronics distributor to get the parts for this job. With inverters and battery chargers, you can establish the zone of separation you need using the transistor-radio method described above. Simply move the radio away from the activated device in question until no noise is heard, and your zone of separation will have been established. If you’re doing this test with a battery charger or inverter, make sure the device is not only on but also under full load or output, whichever the case may be. RFI emission will vary proportionally with the amount of electrical activity within the device. In all cases it’s advisable to consult the manufacturer of any electronic device you intend to install and find out about its sensitivity to RFI. While you’re at it, get any recommendations they may have regarding separation zones and RFI suppression methods.
Power Supply 182
Virtually all small-boat electronic equipment available today requires only a positive and a negative lead
to get the basic unit running. Some sensitive electronic equipment housed in metal cases may require a chassis ground (green wire) in addition to the negative lead (yellow or black wire). Still other equipment may have an additional hot lead (red wire) to power an internal memory. This extra lead must get power at all times and must not be switched in any way. If you have any doubt about how these leads should be connected, refer to the installation instructions for the particular equipment in question. Remember, unlike things such as incandescent cabin lights, most new electronic gear is polarity sensitive. This means if you inadvertently switch the positive and the negative wires when you connect them to your power source, you could severely damage the equipment. Most manufacturers supply a red and a black lead with their equipment, indicating DC positive and negative, although white and black wires are still widely used. Always refer to the equipment installation instructions to be absolutely certain! With the exception of VHF and single-sideband (SSB) radios and radar units, the amperage requirements for most new electronics are comparatively low, so wire gauge is not too much of an issue. Generally, on small boats, the length of the wire between the distribution panel and the equipment is not that long, so sticking with the same gauge wire supplied with the equipment (usually 16 AWG) will suffice. Figure 12-2 shows a typical wire harness with an in-line fuse installed, as supplied with a Loran-C unit. As for the connection to your boat, it’s best to dedicate a circuit breaker at your DC distribution panel for instruments and designate separate positive and negative bus bars to connect your various pieces of equipment. Not only can direct connections to your batteries be messy, but they will add to the length of wire needed to power-up your electronic gear. Figure 12-3 on page 183 illustrates the best method to supply your equipment, by creating a dedicated branch bus from your main distribution panel. This way you can use bus bars that are adequately sized, and the addition of more equipment later on will be a much simpler task. Once your new gear is mounted where you want it,
