DC Electrics - Circuit Protection and Capacitors — Page 42, Lesson 35

I want to walk you through the start of DC Electrics, specifically circuit protection and capacitors. We're beginning with electrical faults and the devices that protect against them. In any electrical circuit, abnormal conditions can arise for many reasons. These conditions create either an overcurrent — too much current flowing — or an overvoltage — too much voltage being applied. If we let these faults persist, they will damage or destroy equipment, and in extreme cases, cause loss of life. The essential power supplies will certainly fail. So we must protect circuits against all such faults using fuses and circuit breakers. Now, there are several protection devices used in aircraft electrical systems, but we're focusing on just two basic types here: fuses and circuit breakers. The fundamental difference between them is in their time of operation relative to when the maximum fault current is reached. A fuse normally opens the circuit before the full fault current is reached. A circuit breaker, on the other hand, opens after the full fault current is reached. That's a critical distinction. Because the circuit breaker lets the full fault current flow for a short time, both the circuit breaker itself and the component it protects must be capable of withstanding that full fault current during that brief period. The circuit breaker has one major advantage over a fuse: it can open and close the circuit repeatedly. A fuse can only operate once — it blows and must be replaced. A circuit breaker can perform many open-close operations before it needs replacement. It can also be used as a circuit isolation switch, meaning you can manually open it to disconnect power for maintenance. Let's move to fuses. There are three basic types currently used on aircraft: the cartridge fuse, the high rupture capacity fuse — often called an HRC fuse — and the current limiter fuse. The cartridge fuse is the one I want to describe in detail. It consists of a tubular body made of glass or ceramic, two brass end caps, and a fuse element inside. That element can be one of several materials: tinned copper wire, silver wire, or a strip of pure zinc that has been electro-tinned — meaning it's coated with tin through an electroplating process. That figure shows you what typical fuses look like, including the cartridge type I just described. The glass body lets you see if the element has blown, while ceramic bodies are used for higher current ratings where the fuse needs to withstand more heat. So to summarise what we've covered: faults cause overcurrent or overvoltage, we protect against them with fuses or circuit breakers, fuses open before full fault current while circuit breakers open after it, and the cartridge fuse is one of three types with a glass or ceramic body, brass end caps, and an element made from tinned copper, silver, or electro-tinned zinc.