Fuses and circuit breakers

Fuses and circuit breakers perform the same function: to limit the amount of electricity that can flow through a circuit. Each size of wire has a maximum amount of current that it can safely handle. The more current a wire carries, the hotter it gets. If a wire chronically carries too much, the insulation can become brittle, crack and eventually fail. Fuses and circuit breakers also guard against shorts, either in a device plugged into the protected circuit or in a problem in the electrical system itself.

The power going through the circuit breaker is quickly shut off and is doing so minimizes the risk of fire or damage to equipment or people. While there is not much inherent difference in the protective value of fuses versus circuit breakers, there are some valid reasons to consider upgrading from fuses to circuit breakers. First is financial: often, insurance companies either require circuit breakers for new policies and will give better rates if a house is converted. This is because of the second reason: it is all too common for misguided people to over-fuse in response to blown fuses. A standard fuse socket will accommodate 15, 20 or 30 amp fuses, so over-fusing is easy.

If insurance companies are not involved, the simple fix for this problem is to insert "S" type fuse holders into existing sockets. These adapters cannot be removed without destroying the socket and have unique threads for each size of "S" type fuse.  This is best left to a qualified electrician so the wire size can be identified and the proper adapter installed. The only exception to preventing over-fusing is a fairly uncommon situation where the neutral wire is fused. This is a potentially dangerous situation and can be eliminated by bypassing the fuse altogether or mitigated by deliberately placing a fuse of larger amp rating than the corresponding 'hot' wire. An electrician should determine this as well.

Another reason for upgrading to circuit breakers is convenience. If a circuit breaker opens, it is fast and cost-free to reset it once the offending problem has been corrected. No emergency trips to the hardware store. No fussing with boxes of replacements. As a side note, a surprising number of people don't know how to reset circuit breakers. When they trip, the handle doesn't move all the way to the off position. To restore power, it must first be turned completely off and then returned to the 'on' setting.

The most common reason for upgrading is to provide more capacity. This is either the total amount of load available in a house or to provide for additional circuits even if the overall service is sufficient. Adding circuits to an existing fuse panel is often difficult or impossible.

Even if your house has circuit breakers, financial considerations or safety concerns may warrant a replacement. There are several brands of circuit breakers that are no longer in production. Even if there is sufficient capacity, it might prove to be more expensive to get re-manufactured circuit breakers to add to an existing panel than to replace the panel with a modern one, especially if a major remodel is undertaken.

There is one brand, Federal Pacific Electric, or FPE, which had its Underwriters' Laboratories listing revoked because their circuit breakers proved to be unreliable. According to a web site that compiles information on them, up to 60% may fail to trip under fault conditions. The company has long since gone out of business, but it was locally the most common brand installed for many years.

Ground Fault Interrupters

A complementary safety device is the GFI. To understand their function, it helps to have a very basic understanding of how electricity works. When electricity is generated, an unstable situation is created. Electrons are separated from atoms, and they really don't like this state. We get them to do our work, from lighting our homes to powering our electronics, by giving them a return path back to 'ground' (the electrically continuous part of it) through light bulbs, motors, etc. Merely connecting an appliance to a 'hot' wire will do nothing. It needs a completed path back to ground through a wire called a neutral.

Unfortunately, alternate paths will do just as well, including paths through people. Equally unfortunate is the fact that it may take considerably less electricity than is needed for a fuse or circuit breaker to trip off to seriously injure or kill you.  What a GFI does is constantly monitor for any difference between the amount of current being delivered to a device and the amount returning through the neutral wire. If anything more than 5/1000 of an amp is detected, it will shut off. So if you are using a defective hair dryer while holding on to a water faucet (a very good ground path), the GFI can keep you from being electrocuted.

Arc Fault Circuit Breakers

There is a relatively new technology called arc fault protection. The arc fault circuit breaker, in addition to acting as a standard circuit breaker, monitors for any sparks. If a lamp cord is run under a rug (never a good idea) and the cord becomes frayed, it may generate sparks. A regular fuse or circuit breaker doesn't see any unusual amount of current, and even a GFI won't trip, as the sparking electricity all returns through the proper neutral path. In the mean time, your house has caught on fire. These new devices are required in all new outlets in bedrooms, and will soon be required in bedroom lights as well. They can't be used where sparks are purposefully created, such as electronic igniters for stoves and furnaces. They can be problematic to install in existing wiring, and are only available for circuit breaker panels that are still in production.


The ground system consists of two basic parts. The service ground connects the metal portions of the electrical system to a solid ground path. Should anything fail in any of these parts, the unwanted electricity is quickly returned to ground, causing the fuse to blow or the circuit breaker to trip. If the fault occurs before the main, the ground conductor is large enough that PG&E's switches are supposed to open before the ground wire melts.

The secondary grounding system bonds to this system, and consists of a backup path to protect the house wiring and the people in the house from accidental shorts.  In addition, jumpers connect the gas and water pipes, so that if a live wire comes into contact with any metal pipe within the house, it immediately shorts out. Without this safeguard, an arcing wire may cause a fire or explosion, or shock someone touching an energized pipe.

Should you rewire your house?

Most homes built before 1950 had knob and tube wiring installed, a technique so called because individual wires were suspended on porcelain posts (knobs), and run through wood framing with porcelain tubes. The wire itself is insulated with rubber and cloth. There is no secondary ground wire installed. It is usually visible in unfinished attics and crawlspaces under houses. To replace all of this wiring is a huge expense, which doesn't stop when the electricians are done. Your house will look like a war zone and will need to be extensively patched and repainted. With some exceptions, the adage "if it ain't broke, don't fix it" usually applies. Copper wire, if used properly, doesn't age. Even the old insulation will probably outlast the house it is installed in.

The two most common reasons to replace it are chronic overloading and illegal modifications. As noted above in the section on fuses and circuit breakers, each size of wire has a rating for how much electricity it can safely carry. Even small amounts of current will result in a wire heating up, and the more current that a wire carries, the hotter it gets. The insulation on wires will eventually break down if too much current chronically goes through them. This condition is easily determined by physically examining the wire where it is accessible.  The covering on wires can also deteriorate where it is directly buried in insulation, with no provision for the heat it normally generates to dissipate. Another common source of problems is where a light fixture mounted directly to the ceiling is over-lamped, with the resultant heat from the bulb 'cooking' the wires contained above it.

When do-it-yourselfers attempt to add to or move knob and tube wiring, they never do it correctly. The requirements and techniques for doing it right are difficult to master and comprise a dying art. Often the original installation is compromised and requires repair or replacement beyond just removing the illegal extensions.

The lack of a secondary ground is often cited as a reason to rewire. This can be remedied by using the aforementioned GFI technology. It is legal to replace outlets that don't have a ground wire available with a ground fault outlet. While there still isn't a true ground, the device accomplishes the same protection electronically. The only disadvantage is that the test button doesn't work because it functions by intentionally shunting a small amount of current to the (otherwise present) ground wire, bypassing the correct return path through the neutral. GFI outlets in ungrounded locations can be tested by an electrician, and should be every 5 years or so, more often if they have tripped open more than a couple of times or if they appear to be nuisance tripping.

There is one circumstance under which perfectly good knob and tube wiring may warrant replacement. Some cities require that a licensed contractor 'certify' knob and tube installations, either as a requirement in conjunction with a major remodel, or prior to insulating an attic. There is a potentially huge liability problem in doing so, which must be added to the cost. It may be cheaper for the contractor to replace the accessible parts of the wiring.


Last update: Thursday 03-18-2010 - David Gerrard