What is Knob and Tube Wiring?

Knob and tube was one of the most hazardous wiring methods ever used because it is an ungrounded system. What does this mean? Electricity begins at the circuit breaker where it flows through the hot or “ungrounded” conductor and comes back on the neutral or “grounded” conductor completing its path. This system works well in theory until you become a path to ground.

Recently, an alternate path to ground is required by either using the outer metal sheathing of wire or conduit or by placing an actual ground or “grounding” conductor within the wire assembly. This provides an alternate path to ground tripping the circuit breaker disallowing YOU to become that path.

This might not sound very likely, but it is actually not hard to accomplish. If you are in your basement and pull the pull chain of an ungrounded light fixture, you can easily become the path to ground. The same can happen with any ungrounded light switch or receptacle.

The National Electrical Code permits ungrounded receptacles to be two-pronged instead of three-pronged. The reason the code allows this is because the absence of a third prong or “grounding” prong is meant to prevent people from plugging in equipment that needs to be grounded but instead most people break off the grounding prong of an extension cord or power tool making it extremely unsafe.

Another way this may be corrected is by installing a ground fault receptacle, which measures the current going through the plug itself and will trip if there is an unbalanced current, protecting the end user. While this is the safer “quick fix”, the potential homeowner is still left with a knob and tube system.

Some may think that they are safe because they have installed GFCI receptacles but another problem exists. When knob and tube circuits run, they would pull a neutral wire from whichever circuit was closest. This means is that there is an abundance of current overloading the neutral conductor that is not connected to a circuit breaker allowing that conductor to overheat and not open the circuit.

Excessive heat to a conductor will cause the insulation to breakdown. Over time, the outer coating will become frail and brittle exposing the bare copper conductor. Once this happens, it is only a matter of time before you need an experienced electrician.