If that happens, it can cause damage to aircraft circuits due to overheating — Page 126, Lesson 124

I want to walk you through the overvoltage protection unit and the generator cut-out, or reverse current relay, which are two critical protection components in a DC electrical power system. Let's start with the overvoltage protection unit. This unit is fitted to protect against the output voltage of the generator rising dangerously high. If that happens, it can cause damage to aircraft circuits due to overheating. The relationship that explains that heating effect is given by the formula \( W = I^2R \), where \( W \) is power dissipated as heat, \( I \) is current, and \( R \) is resistance. So if voltage rises, current can rise, and the heat goes up with the square of the current — that's why overvoltage is dangerous. The overvoltage protection unit specifically protects against voltage regulator failure. If the voltage regulator fails and lets the generator output climb too high, the overvoltage protection circuit will automatically disconnect the field circuit. In a 14 volt system, it will trip when the voltage rises to typically 16.5 volts. Once it disconnects the field circuit, the generator output drops to zero, safeguarding the system. Additionally, the overvoltage protection unit may also open the generator cut-out to prevent reverse current flow — that's a secondary action to further isolate the generator. Now let's move to the generator cut-out, also called the reverse current relay. This device has two main jobs. First, it permits the generator voltage to build up to a preset figure before its contacts close and put the generator on line — meaning it connects the generator to the aircraft's electrical bus. Second, it will open to prevent the battery from feeding current back into the generator when the generator voltage is below that of the battery voltage. That reverse current could damage the generator, so the cut-out protects against it. The contacts of a cut-out are closed by rising voltage and opened by reverse current. Importantly, a cut-out is not fitted in an alternator system because the rectifiers in an alternator system already provide reverse current protection. So this component is specific to DC generator systems. Let me describe how the reverse current cut-out relay works in detail. It may be an integral part of the voltage regulator, or it may be a separate unit. Before the generator is started, a spring holds the contacts open. As the generator builds up voltage, that voltage is applied to the shunt coil — also called the voltage coil. This shunt coil has many turns of thin wire and is connected in parallel with the generator output. When the voltage has built up above the battery voltage, the current through the voltage coil causes a magnetic influence that closes the contacts, connecting the generator to the bus bar. Once the contacts close, current flows through the current coil, which has a few turns of thick wire, and then through the contacts to the bus bar and the aircraft loads. The current flow through the current coil increases the magnetic effect and helps to keep the contacts closed against the spring. So you have two coils working together: the voltage coil initially pulls the contacts closed, and the current coil then helps hold them closed as long as current is flowing in the correct direction from the generator to the bus bar. That diagram shows the reverse current cut-out relay and how these components are arranged.