Is your wiring safe? BoatsExplained.com explains how everything works and shows what to look for

ABOAT'S DC electrical distribution system exists in a harsh environment, so it must be inherently robust. The wiring and the various devices should be electrically and mechanically sound, and also well protected so they remain reliable over a period of years. Above all, the installation has to be safe, for where there's electricity, there's also the risk of fIre. Many people believe that because a voltage is low - usually 12V on all but the largest yachts - it poses no threat. This just isn't so. The lower the voltage, the higher the current (in amperes) must be to do any given amount of work. And it's lots of amps which can cause a component to overheat, perhaps igniting surrounding materials.

Building on basics Let's start developing a typical boat circuit, beginning with the simplest possible arrangement. Figure 1 shows a battery supplying current to a single load or appliance - here represented by a light bulb. The current flows from the +ve terminal, through the circuit, and back to the -ve terminal.
When more than one appliance is being served, these are best connect ed in parallel  so that each receives an equal voltage. But to wire a whole circuit that way would be both expensive and untidy. Luckily, we can start to make economies by sharing some of the return leads ), bringing them together with the help of junction boxes. In this type of system the returning wire gets larger as you approach the battery. This is because it must handle the collective current of the various circuits feeding into it - rather like minor roads joining a motorway, adding to its traffIc flow. For safety, we need an isolating switch on the battery's positive side and fuses or circuit breakers (CBs) to protect the individual load circuits

By mounting the CBs on a common bus bar rail, we can use just one large cable to convey the power from the battery.
For convenience, the bus bar and protection devices are commonly mounted behind a labelled panel  from where the distribution system is controlled. Individual circuits reach out to serve the boat's appliances, each of which usually has its own on/off switch for local control. Overall circuit protection is provided by having an additional CB (or fuse) between the battery and distribution panel - a very important function. And, as this is a sort of focal point where both sides of the primary circuit can be brought together, it's also a convenient spot to add monitoring instruments, such as a voltmeter and ammeter, the latter connected to a shunt.
The capacity of a battery to store (and then deliver) electricity is measured in ampere/hours, and depends on the type of battery and its size. Capacity can be increased by connecting a number of batteries in parallel. This is known as a 'bank' of batteries and, for all practical purposes, can be thought of as a single large battery having a capacity equal to the sum of them all. Each battery should have its own isolator mounted as
close to its positive terminal as possible. Isolators come in two types: key operated or combined isolator/selector. The fIrst of these will serve a single battery (or bank of them) and has the advantage that the key can be removed to disable that battery. As the name suggests, the isolator/selector type can choose between batteries, isolate them, or connect them together to combine their power - perhaps to start the engine. As we saw last month, this often brings its own problems, because if you connect a fully charged battery to one that's flat, they could equalise so neither has enough charge to be of much use. However, some form of emergency start facility is certainly desirable.
You'll notice in that the negative leads from both the engine start and domestic batteries go to one arm of the horseshoe shunt, thereby connecting them together. The volt meter and ammeter also collect information from the shunt and, by using selector switches on the gauges you can monitor either system independently. As a further refmement, indicator lights or LEDs have been added to the appliance circuits to show which CDs are 'made' and their circuits therefore live.

Don't switch off
It shows our completed schematic circuit. A second bus bar with its own separate protected feed adds the fmal touch. This is know as a 'maintained supply' and is there to provide an uninterrupted supply for essential services - for example, automatic bilge pumps, emergency radio, burglar alarms, and forced draft combustion heaters. The latter are rather like miniature jet engines and it's vital that they complete their pre-programmed cooling cycle before being shut down. Failure to do so might result in unburned fuel remaining in a hot and unventilated combustion chamber, which could lead to a rather nasty explosion!
You'll see that the maintained supply's CB is what's known as a 'reset only' type, which means it can't be used as a manual switch like the others, but can be reset if the CB trips in service.