Porcelain Insulators Used in a House

Porcelain Insulators

• Porcelain insulators appear as knob- and tube-shaped casings found on a home’s electrical wiring. Because of their shape, porcelain insulators also go by the name of “knob-and-tube wiring." These shapes are designed to fit inside specific areas within a home’s wiring system. Knob-shaped pieces insulate wires found in open space areas while the tube-shaped pieces cover wires that run through wood joists and studs. Since the majority of homes constructed before 1960 used the old plug-in type fuse boxes, porcelain insulators are not designed to work with circuit breaker systems.

Insulator Components

• In terms of design, porcelain insulators were well suited for plug-in fuse boxes in terms of durability. Porcelain components include a glazed coating that acts as a sealant for porcelain materials. This glaze helps increase the insulator's capacity to contain electrical current and protect porcelain materials from the effects of heat. Both knob and tube pieces include two individual wires that run through the porcelain and connect with the home’s wiring. Over time, the glazed coating found on insulators deteriorates, which weakens the insulator’s ability to contain electricity and resist heat.

Benefits

• The knob and tube designs used in porcelain insulators serve specific purposes within open spaces and wood joist areas. The knob designs found in open space areas allow heat to dissipate from electrical wires. The tube designs used inside wood joists and studs protected wiring from possible nail punctures made during floor construction or repair. The two wires that extend from a porcelain insulator keep hot and neutral wires separated by a minimum of 3 inches, except around the fuse box area. Insulators located near the fuse box use a woven sleeve casing to protect the wires.

Considerations

• After 80 years or so, worn porcelain components may cause electrical problems and become safety hazards in homes that have porcelain insulators within their electrical systems. Electrical problems can result in cases where alterations or additional electrical fixtures are added to the home’s electrical system. Adding or modifying existing wiring tends to overload the system, which results in blown fuses on a frequent basis. Since porcelain insulators operate from the plug-in fuse box design, these systems lack the protective ground wire found in circuit breaker systems. These conditions can create a potential fire hazard in cases where a system overloads.