In the case of all onboard electrical systems you will notice that designers refer to the "main bus bar" or simply "main bus"; essentially this is the spinal chord of the system, like a big cable unto which other electrical subsystems are connected. In reality it is not a cable but a metallic bar, and hence the name, but the idea is clear: everything connects to it, you have a master key to open or close that circuit and possibly several keys or fuses that protect each electrical subsystem.
A typical electrical panel, with its battery master key (yellow), avionics master (red) and several fuses.
That's exactly what those fuses that you will find on many a cockpit are. They protect various instruments and subsystems from spikes, short circuits and other problems, so that in the even of some sort of electrical failure, the pilot may isolate relatively the culprit but continue working with the rest of the equipment. Now it is perhaps easier to see why the pilot must know perfectly the electrical system of his aircraft.
However, it is interesting to say that Russian aircraft in this regard are generally the exception because the fuse panel is seldom accessible to the pilot in flight. Aircraft made in Russia have their fuse panels located elsewhere in the fuselage, generally accessible only by technicians on the ground. The idea is that only those duly qualified should put their hands on the circuits and when and if the plane experiences an electrical problem, the pilot should interrupt the flight as soon as practically possible.
The AN-1 flight training device showing a simulated electrical failure prior to takeoff, plus another in the attitude indicator... Can you fly like this?
As we said, electrical systems are complex and they differ among makes and models, but they just might even vary from serial number to serial number so that they should be considered as airplane specific, meaning that for each airplane that you fly, even if they have been made by the same manufacturer, you should study their electrical systems.