Knob and Tube
Wiring Knob and tube wiring, and in some cases, even aluminum wiring
have been labeled unsafe. Does this mean these types of components
or systems are categorically dangerous? No. Like all electrical components,
they can be dangerous if they do not meet certain minimum requirements,
but they can also be as safe as any others.
Knob & tube wiring is a type of wiring which was in common use until
the 1940's and sometimes used as late as the 1950's. The nick-name
is derived from the ceramic knobs that are employed to insulate and
secure the wiring runs and the ceramic tubes employed to protect the
wires where they pass thru potentially abrasive materials (primarily
wood joists, studs etc.) Unlike subsequent wiring systems where all
the wires in a run are enclosed in a cable, the two wires (black/hot
and white/neutral) run separately and only come together at a terminal
(switch, receptacle, fixture, junction box etc.).
Knob & tube wiring does not provide a third wire for grounding and
is therefore considered unsafe in kitchens, bathrooms, laundry rooms
and outdoors. In other areas, knob & tube wiring that is in good condition
with sheathing intact, properly protected from damage, and that hasn't
been subjected to extended periods of overloading which can cause
it to become brittle, should not pose an increased safety risk.
Aside from the preceding concerns, the primary risk with knob & tube
wiring, it seems, is its relative accessibility for amateur home repairs.
It is not uncommon to see a system with knob & tube wiring that has
a history of work (heated loose and poorly joined connections, unfastened
runs, relocated neutrals, unprotected wires,etc.). Many older homes
with originally installed knob & tube wiring have had some of the
wiring upgraded. While modern wiring is visible in many areas, much
of the knob & tube wiring may still be in place and concealed beneath
floors, above ceilings and behind walls.
If a house does have knob & tube wiring it should be inspected
to ensure that it is properly installed and in good condition.
