A friend asked me the other day to take a look at the wiring in the farmhouse he purchased. He was telling me that his wiring was unlike anything he's ever seen before. A lot of the wire was exposed and he was worried about safety issues.
I finally made it over to his house and discovered that he had a mess of different applications going on throughout his house. The most common wiring was knob-and-tube. The name comes from the use of porcelain "knobs" which hold the wire in place when they were run throughout the house, and the porcelain "tubes" which are used to protect the wire as it passes through a joist or stud.
Knob-and-tube was an innovative way to wire your home in the pre-1930's era. Essentially, wires are strung through the rafters, usually along the center joist and held in place by porcelain knobs. When wires needed to be spliced in knob-and-tube, they were soldered together, making a permanent connection. Porcelain knobs are usually installed within 6 inches of the splice to prevent stress on the wires.
On a knob-and-tube wiring system there is no ground wire included and the only insulation that protects the wire is the insulation on the wire itself. There is no plastic sheathing covering the wires like there is in modern applications.
One of the major pitfalls of knob-and-tube is the fact that usually the neutral wire is the line that gets turned of and on in a switch. That means that voltage still flows through the wires, but the circuit isn't complete.
I call knob-and-tube "innovative" because the wire is actually run through open-air spaces and not stapled to boards. The open-air aspect results in less heat build-up throughout the wire. The trouble with knob-and-tube is usually not with the wiring system itself, but with what unknowing homeowners have done with it. Many times, homeowners make repairs or additions to the system without understanding how it works and that can be dangerous for the next owner of the house.
Another common problem with knob-and-tube wiring is the fact that mice, squirrels and other small creatures love to chew the insulation off the wire. Having exposed wire with voltage flowing through it is never a good situation. Also, it's important to realize that the wire is meant to dissipate heat via the open-air installation, so it's dangerous to use insulation in the areas where the wire is run. With the insulation in place, there is no where for the heat to go and this could be a fire hazard. In fact, in 1987, it was included in the National Electric Code that no thermal insulation can come in contact with knob-and-tube wiring.
Some things to keep in mind with older wiring is that it may not be a grounded system. Also, even though your home's electric may be working fine, the insulation on the older wire can become very brittle and eventually fall off in places. These can be very dangerous situations for the homeowner.
Some older home electrical distribution panels utilize fuses rather than more modern circuit breakers. A fuse is a round screw-in type of circuit protector. They’re mainly comprised of porcelain, glass and metal.
A small piece of metal inside the fuse allows voltage to flow through to your outlets. On the face of the fuse, the glass window allows you to actually see the metal band for easy inspection. If an overload or short occurs on the line, the metal band breaks and the flow of electricity is interrupted on that circuit.
Once a fuse is blown, it should be unscrewed and thrown away. Since a tripped circuit breaker can be reset, it’s easy to see how they can be more convenient than fused systems. It is easy, however, to change the blown fuse. Simply unscrew the fuse and screw a new one in its place. It can be dangerous if the blown fuse controls the lighting where the fuse box is located. Bring a flashlight with you if you need to change a fuse in a dark area. It is also imperative to replace the bad fuse with an identical one. Fuses are usually color-coded depending on their amperage.
While fused systems still offer safe protection, the quality of the wires is many times the trouble causing culprit. Fused systems are usually very old and the wire may have turned brittle or there may be weak connections within the fuse box.
If you have purchased, or are looking to purchase an older home, it's wise to get a certified electrical inspector to thoroughly check the wiring in your home. Finding a problem early and getting it repaired quickly could save a lot of trouble down the road.
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Dave Donovan is a freelance copywriter living in Atco, N.J. An electrician for 15 years, an injury forced him to pursue his true passion - writing.