DC Electrics - Circuit Protection and Capacitors — Page 42, Lesson 41

I want to walk you through circuit breakers and capacitors. Let's start with circuit breakers. A circuit breaker combines the function of a fuse and a switch. That means it can protect a circuit like a fuse does, but you can also use it to manually switch circuits on and off in certain circumstances. Circuit breakers are fitted to protect equipment from damage resulting from overload or fault conditions. The design and construction of circuit breakers is wide and varied, so there are many different types. Generally, a circuit breaker incorporates an automatic thermo-sensitive tripping device — that's a device that senses heat and automatically opens the circuit when it gets too hot — and a manually or electrically operated switch. Some electrically operated circuit breakers may also include electromagnetic and reverse current tripping devices. Let's talk about sizes. The smaller type, single-button circuit breakers, range from 5 amps to 45 amps. The larger reverse current circuit breakers can be rated up to 600 amps. Looking at typical circuit breakers, the single push-pull button type has a white marker band. That white band helps you identify a 'tripped' circuit breaker amongst a panel of many — when it trips, the band becomes visible, so you can spot it quickly. The circuit breaker at (b) in the diagram is fitted with a "manual trip" button and is more usually associated with a heavy duty circuit. Circuit breakers are common on the flight deck of modern aircraft. They can be categorized as either a non-trip free circuit breaker or a trip free circuit breaker. Here's the critical difference. A non-trip free circuit breaker may be held in under fault conditions, and the circuit will be made — meaning if you physically hold the button in, it will complete the circuit even though there's a fault. This is clearly dangerous. A trip free circuit breaker, if held in under the same circumstances, the circuit cannot be made. No matter how hard you hold the button, the internal mechanism prevents the circuit from closing while the fault exists. That's the safer design. Pressing the reset button will reset either type of circuit breaker if the fault has been cleared. Now let's move to reverse current circuit breakers. These are designed to protect power supply systems and associated circuits against fault currents reversing against the normal current direction of flow — that is, current flowing backwards — of a magnitude greater than those at which cut-outs normally operate. They are furthermore designed to remain in a "locked-out" condition to ensure complete isolation of a circuit until a fault has been cleared. So once they trip, they lock out and stay open until someone physically addresses the fault. Now let's talk about capacitors. A capacitor can perform three basic functions. First, it stores an electrical charge by creating an electrical field between its plates. Second, it will act as if it passes alternating current — I'll explain what that means in a moment. Third, it blocks direct current flow. In its simplest form, a capacitor consists of two metal plates separated by an insulator called a dielectric. Wires connected to the plates allow the capacitor to be connected into the circuit. That's the basic construction — two conductive plates with an insulating material between them.