Most aircraft require some form of electrical power to operate navigation-, taxi-, landing-, strobe lights, one or more COM and NAV radio's, transponder, intercom and other advanced electronic system of your choice. The electrical system consist of a battery and an alternator (or generator on older type aircraft) to recharge the battery, fuses and switches and lights for indication purposes.
All of this is connected through several meters (kilometers/ miles in large aircraft) of wire and connectors, attached to the airframe with insulation material as cushion clamps, ty-wraps and what not. Even for the private pilot it pays to have some basic knowledge of the electrical systems of his or her aircraft, which could be a life saver in case of an electrical emergency.
A typical electrical system consists of switches, fuses, wiring and some voltage and a current flowing to and from the device. Monitoring by the pilot is normally done through a volt, ammeter and the state of the fuses or even a dedicated interface into a EFIS.
The remainder of the basic electrical system consists of a master switch, relays, one or more bus bars, ammeter and or voltmeter and fuses or circuit breakers and switches to control the radio's, lights and other electrical devices. Modern installations may use a power distribution system which consists of solid state relay's/ FETs for a much simplified installation.
Switches the electrical system on/off through a heavy duty relay. Usually a split red switch with one side for the alternator and the other for the battery. When switched to ON, electrical power is supplied via the one or more bus bars to each of the devices. Each bus bar should have its own circuit breaker so that a short circuit with, for example, a flap motor doesn't take out the radio's or the EFIS.
Each device is attached to a bus bar through a circuit breaker or fuse and a switch (which can be the same device). Again making sure that one faulty radio doesn't short circuit the other devices on the same bus bar. As a fault in COM1 should not have any consequences for either COM2 or any other device on the radio bus bar.
The bus bar is normally a thick copper strip or wire connected to group of switches / circuit breakers with the same basic function. Think of an engine instruments, radio/communication, lights and auxiliary bus bar. In a multi-engine or an engine with dual alternators, bus bars can be interconnected should an alternator fail for some reason. So that electrical power is always available to several essential devices.
To measure how the system is operating an ammeter is usually installed and sometimes even a voltmeter. The ammeter indicates the amount of current (flow of electrons) from the alternator to the attached load (left zero ammeter). There are two types in use: the center zero ammeter measures the current to or from the battery. The left zero model indicates only when the alternator is delivering a load. The center zero model has a '+' side, indicating a charging battery and a '-' side indicating that the battery is supplying electrical current.
The voltmeter indicates the voltage of the battery which, when the alternator is not operating, should be just over 12 or 24 volt. This voltage, however, says nothing about the capacity left in the battery, for that you will need to measure the specific gravity of the acid inside the battery. With the aircraft engine / alternator running, the indicated voltage should be around 13,8 volt (or 28 volts for aircraft with those systems).
If one of the cells (there are six in a 12 volt battery) has failed or short circuited then this will be indicated in the unloaded battery voltage, it will read about 2 volts lower than normal. This will not show if the alternator/engine is running (13,8 regulated voltage) but the charging amps to the battery will be somewhat higher than normal.
Every device in the system will need two wires to connect to the DC electrical system. One is connected to the positive terminal and the other to the minus or return of the system. Sometimes the metal aircraft frame is used as ground. Which is not a perfect conductor due to the resistance of paint, corrosion and such.
These irregularities will induce a lot of problems with interference in radio's, intercoms, transponders and EFIS by high voltage strobe systems, high power COM transmitters or even the engine ignition (wires and airframe will act as pickup antenna, in fact any conductor can as an antenna).
It is wise to have a common ground point to which all return wires are connected and that ground point on the airframe should have one thick wire connected to the battery. Connecting everything this way makes sure that no high current pulses flow through the airframe which can cause unexpected interference in other devices like intercoms and could result in GPS receiver failures.
And of course, make sure that you use aviation grade wiring, switches, fuses and wire terminals. These have the durability to withstand this tough environment in terms of vibration, temperature changes, heavy use and all of that. A good example of this is shown by Viking Aircraft Engines in their video about: The Importance of Wiring and Trusting Your Crimps".
In short: use proper crimps, wiring and tools to get a reliable end result regarding the wiring loom, and eventually the safety of your aircraft and the lives of those aboard.