DC Electrics - Circuit Protection and Capacitors — Page 42, Lesson 38

I want to walk you through circuit protection, starting with the fuse — the simplest and most fundamental device for protecting an aircraft's electrical system. Let's look at Figure 3.1, which shows two types of fuses. On one side we have a light duty circuit fuse, and on the other a high rupturing capacity fuse, often abbreviated as HRC. The HRC type is generally used in heavy duty circuits. Its element is a zinc strip cut to a specified width — that width determines the current at which it will blow. So how does a fuse actually work? A fuse operates when the current flowing through it is sufficient to melt the wire or strip element. The time it takes to melt varies inversely with the current — meaning the higher the current, the faster it blows. This is an important relationship: double the fault current, and the fuse blows in roughly half the time. Every fuse is rated at a specific current value. That rating tells you the current the fuse can carry continuously, or intermittently, without unduly heating up or deteriorating. So the rating is not the blow point — it's the safe continuous carrying capacity. When we select a fuse for a particular circuit, the rating must satisfy two conditions. First, it must not be less than the normal current flowing in the circuit — so it won't blow during normal operation. Second, it must operate — blow — at a current level below the safety limit of the equipment or the cable being protected. That's the whole point: the fuse is the weak link that sacrifices itself before the wiring or component gets damaged. For this reason, only the specified fuse should ever be used in a particular circuit. Never substitute a higher rating because the wire or equipment downstream might not survive a fault that the higher-rated fuse would allow through. Looking at typical aircraft fuses, the ratings can vary between 0.5 and 500 amps. The higher ratings — above a certain point — are limited to the HRC or current limiter types. So a 200-amp fuse, for example, would almost certainly be an HRC type, not a light duty wire fuse. What are fuses made of? They use a type of wire that has a low melting point. The fuse is placed in series with the electrical load — meaning all the current for that load must flow through the fuse. When a current higher than its ampere rating is placed upon it, the element melts, blows, or ruptures, opening the circuit. Fuses are rated in amps — that's the unit we use. Now, a very important operational point. A blown fuse may be replaced with another of the correct rating — but only once. If it blows again when you switch the circuit on, that tells you there is a defect in the system. In that case, the fuse must not be changed again until the circuit has been investigated. You don't just keep replacing fuses hoping the problem goes away — that could lead to a fire or equipment damage. Finally, spare fuses. The carriage of spare fuses is mandatory — you must carry them on the aircraft. The quantity of spares must be at least 10% of the number of each rating installed, with a minimum of three of each rating. So if you have ten 5-amp fuses installed, you need at least one spare 5-amp fuse — but because of the minimum of three, you'd actually carry three. If you have thirty 5-amp fuses installed, 10% is three, so you'd carry three. If you have fifty, 10% is five, so you'd carry five.