Electrical – Why are ground wires sometimes not insulated

dimmer-switchelectricalelectrical-panelgroundingswitch

I recently replaced a single AC dimmer light switch that was one of two light switches in a larger box. The ground wire leading to the switch was raw copper without any insulation.

After replacing the switch, and turning on the circuit breaker, the breaker sparked and tripped. I'm thinking one of two things explain why:

  1. The new FEIT switch was defective (which would be my 4th defective switch from the big carton of FEIT switches I bought).
  2. The naked ground wire touched the contacts of the other switch in the box.

I turned off the breaker, put it a completely different switch, and did my best to get that naked ground wire as far away as possible from the other switch. Then I flipped the breaker on (shielding my eyes, and standing away from the breaker box), and everything worked.

So my question: Why are ground wires sometimes installed without any insulation? The cost of insulated wire is almost identical to that of non-insulated wire, so I tend to think cost is not the primary driver of this decision.

Best Answer

Cost, mostly

Whether your grounding conductors are bare, insulated, or not even a wire to begin with depends on what wiring method you are working with. NM, because it's made in large quantities, uses a bare grounding wire to save cost (likewise with UF and most SE cables, as those pennies add up at industrial scales). On the other hand, if your house is wired using armored cable (AC), there are no ground wires, because the metal cable armor serves as the ground with the aid of a thin "bonding strip" to prevent "choking" of fault currents caused by stray inductive interactions. That lack of grounding wires is also a notable feature of many installs done in metal conduit, as the conduit itself then becomes the grounding conductor.

Furthermore, some wiring methods aren't uniform in how they handle this; it's legal (and occasionally required) to run a separate equipment grounding conductor in metal conduit, and in most (albeit not all!) cases, equipment grounding conductors inside conduits can be either bare or insulated. Likewise, metal-clad cable usually uses an insulated grounding conductor, but can sometimes be found with a bare grounding conductor, or in configurations where the armor is the grounding means.