I mounted a fused service disconnect
on my house when I rewired it. A few people have asked me why I didn't just use a circuit breaker
. A 100 Amp Square D
breaker in a box, really wouldn't have cost much more than the disconnect I set, and it could have been reset.
Score one for circuit breakers, they can be reset once tripped. And I like them, my panelboard is full of them. But fuses have their advantages too.
The primary advantage of a circuit breaker is that it can be reset. That really matters when you're running small circuits, like in your kitchen. Older homes really weren't designed with microwaves and the many modern conveniences we have today. Which is why the 2002 National Electric Code (or NEC) calls for two 20 ampere general appliance circuits, plus dedicated circuits for refrigerators and the like in new, or remodeled home kitchens. That's a minimum folks. A lot of older homes, like mine, started out with four circuits total. Here fuses would be at great disadvantage, because you'd blow them periodically.
But for bigger power levels, say above 50 amps, the advantages become less clear, particularly when the house electrical system has been properly designed for the expected load. Fuses are more reliable than circuit breakers. When a fuse rated at 20 amps exceeds twenty amps it blows. Period. So if you buy a house with a fusebox don't panic and decide you need circuit breakers right away. Even an old fuse will not betray you-- provided it's not too big for the wire it protects (and people are notorious for upsizing screw-in fuses when they have one blow). A fuse is nothing more than a wire that melts when faced with a known level of current. Fuses don't age. But if you need more circuits you'll need to put in a new panelboard, as nobody makes fuseboxes these days.
Remember the purpose of a fuse is to prevent circuit from overheating and starting a fire. Fuses and circuit breakers are overcurrent protection devices in the NEC. You chose your breaker or fuse size after you've picked your wire size. Of if you've picked a fuse size, then the wire must match it. A lot of modern circuit breakers are quite reliable: Square D, Siemens and newer Cutler-Hammer gear I'd trust, among others. But some breakers are worthless. I've seen people weld with Federal Pacific Electric gear. They're useful only as switches. If you own a house with an Federal Pacific panel (or FPE) pay someone to change it. Unless you'd like the insurance money.
Older breakers can 'freeze' in place. Breakers need to be exercised, turned on and off, so corrosion doesn't weld the contacts together enough to make the breaker slow, or reluctant to blow. Old Pushamatic panels are notorious for this problem, but it's a good idea for everyone.. A local rep I know suggests that you flip all your breakers on and off whenever Daylight Savings Time changes, that will prevent such seizures. Best to turn everything off first, or risk pitting the contacts.
Fuses are fast. They open in one fifth of a cycle, breakers 1.5 cycles. That's not much time at 60Hz, but it matters when you're trying to protect computer equipment. A fuse might make the difference if you get nailed by lightning or something else. It particularly matters during fault current situations, but that's something most people will only have to worry about if they're in a downtown high rise or an industrial plant. High fault current rated breakers are available, for mucho dinero. Mucho, mucho dinero.
Fuses are more flexible. In my rather small house I installed a 100 Amp service. That's it. Of course, the panel is rated at 150 Amps, and the conductors at 110. My fuses are rated at 100. Only, they are what is called a dual element time delay fuses. In a dead short, they'll break in that fifth of a cycle, but a slight, temporary overcurrent, say 200 Amps will make them blow in a few seconds. Long enough for an air conditioning compressor to get up to speed. Or a dryer. But short enough not to overheat anything. You can get breakers to do that too, also for mucho dinero.
Of course, fuses must be changed, while breakers need reset. To change a fuse, turn the circuit to off and use an insulated tool, like linesman's pliers to pull the fuse, and push in the new one. Screw-in fuses (Type H) can be safely unscrewed if you resist the temptation to stick your finger in the socket. And remember to put exactly the same rating fuses back in. Remember an uprated fuse does NOT uprate the wire behind it. Also they do make adustable circuit breakers, something you can't get in a fuse. However, those are used only in commercial and industrial applications, often for motors. They are specialized devices, and also quite expensive. Homeowners need not worry about them.
But breakers sometimes have to be changed too. Square D, a manufacturer renowned for quality switchgear, only warrantees its breakers for one trip. The reason is that every trip involves an arc, and potential damage to the breaker's contacts. They're good for a lot more than one trip, but a lot less than infinity. A breaker that trips often needs changed. Fuses are generally cheaper, but at the household current level, good breakers are quite affordable.
In my house I use both circuit breakers and fuses, each in their proper place. They're both useful overcurrent devices, but they're not exactly interchangeable.
Noders wishing further information are encouraged to read article 240 of the National Electric Code (NFPA-72) a publication of the National Fire Protection Association. I also admit that my fused disconnect looks a lot cooler. Table 310-16 will show the proper wire sizes for each current rating. Be sure to read the fine print.
Pushamatic panels are were very common during the sixties and seventies, as everyone thought pushbuttons were the wave of the future. When you move into a place, it's a good idea to check out the panel, just so you know what you're dealing with. Exercise those breakers, have spare fuses already on hand. With a flashlight, thank you!