Hit a wire while drilling a hole on a concrete wall. Didn't feel like hitting anything, except there was a short, spark, and tripped the breaker.

With the breaker back on, haven't noticed any problems. If the wires stay apart, can any issues arise from this? Other than the breaker tripping again, in case the wires touch, what else can happen?

  • Do you mean a concrete block wall? Wouldn't the wiring be in conduit? Are you sure you didn't just have a tool malfunction?
    – isherwood
    Mar 19, 2019 at 19:56
  • @isherwood Yes I'm sure, it tripped the breaker as mentioned. Mar 20, 2019 at 0:25
  • That alone isn't proof. You haven't answered all my questions.
    – isherwood
    Mar 20, 2019 at 2:40
  • @isherwood I'd be curious to know how. I don't have the answers to the other questions. Mar 20, 2019 at 11:20
  • You can't tell us whether it's a hollow block or solid concrete wall?
    – isherwood
    Mar 20, 2019 at 11:37

3 Answers 3


That can not be left in service that way. If it's cable, it's done for, and being set in concrete, the whole run is now unusable. If it's conduit, all the wires need to be pulled out of that conduit for inspection. You cannot mend a wire (not least, the conductor's width may be damaged and cause a hotspot), so the entire run of wire needs to be replaced.

One does not rely on breakers tripping to protect from a known defect. One fixes the defect.

It's hard for me to keep a straight face when warning you about fire, because I can't name too many concrete fires. However, if the PVC plastic conduit caught on fire, it would fill the space with some toxic stink, especially if the PVC pipe was not rated for electrical use. (Lots of people grab cheap plumbing PVC because it's cheaper).

  • Emphasis 'toxic' PVC fumes are double plus ungood for human health, as in instant emphasema. (Please don't ask me for a source. Decades ago...) Mar 20, 2019 at 15:44

You can't just leave damaged wires in the wall. The circuit might seem to work for now, but when a higher load is placed on it or anything moves the wires just a little bit, they can arc, short out against the conduit and melt/burn/damage other nearby wires. This could happen years down the road, or the next time you plug something in.

I mentioned conduit, because in a concrete wall, the wires are going to be running in some. This should make repair easier since the damaged wire can be pulled out and replaced. You will want to make a larger hole in the wall to inspect your damage and fix any sharp edges that would damage new wire or prevent you from pulling new wire in the first place.

Also, circuit breakers will trip when the wire is shorted completely, but some arcing and burning can happen at a low enough current that the breaker won't trip immediately. You can't rely on a breaker when dealing with physically damaged wires.


It would be wise to investigate further and make an intentional repair. We can only speculate what sort of damage might have been done to the cable: partial removal of insulation from one conductor or from more than one conductor, nicking or other removal of metal from one or more conductors, etc.

A nick in a conductor could make it more susceptible to breaking with vibration and thermal changes over time, and a break could lead to arcing.

Removal of insulation leaves a possibly energized conductor open to contact with whatever might exist or get into the wall, including contact with other conductors in the same cable, again with unpredictable effects or timing.

A "short" doesn't necessarily cause a circuit breaker to trip immediately (or even at all), so when the damaged wire is left energized, there's a risk of unchecked arcing which can lead to fire. Fire concealed inside a wall is a particularly bad thing.

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