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I live in a country where the formal electrical code is more of a light suggestion than requirement, at least for old buildings. My lights began flickering when the shower was on so I went to the breaker box (outside my apartment, in the public hall of the building) and saw sparks.

The electrician showed me that the wire feeding one of the breakers melted / charred. He spliced in a new wire and changed the breaker. He told me that the breaker was old and this is why the cable insulation melted before the breaker interrupted current. I think that the breaker never.... broke?... while this was going on.

I have 3 10-amp breakers, Siemens brand. Why would the wire melt before the breaker broke? I think the excuse that the breaker is old is questionable - it seems like they should fail open by that point. And why'd the insulation melt near the breaker, and not some other random point from the building's mains input to my breaker box?

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  • Electrically-heated shower? 10A won't be enough for one of those (even on 230V I have a 30A breaker for mine), so how's it connected? Breakers, like most components, can fail in various ways, some more likely than others. An old design in a country where electrical standards are weak could easily have failed closed, even though Siemens know how to make things properly.
    – Chris H
    Commented Jul 4 at 13:46
  • Insulation melting on a wire can be from a loose connection. The loose connection causes heat, melting the insulation. It might not even trip a new breaker, if the amps do not go much above the breaker rating.
    – crip659
    Commented Jul 4 at 13:58
  • " My lights began flickering when the shower was on" is pretty scary.
    – Wastrel
    Commented Jul 5 at 14:31
  • For interest, what country?
    – Russell McMahon
    Commented Jul 6 at 11:21
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    Because breakers are not "Everything Detectors". You had series arcing and you did not have an Arc Fault Detector installed. Eaton has taken a swing at making an Everything Detector, but only in the US market and only on 120V 15-20A circuits. However, we Yankees know why this happened. We have the science to prove that screw torque matters on all terminals. As such, we require torque to be set with a torque setting tool.
    – Harper - Reinstate Monica
    Commented 2 days ago

3 Answers 3

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And why'd the insulation melt near the breaker, and not some other random point from the building's mains input to my breaker box?

Poor connection to the breaker. Probably not tightened to the correct torque specification. Poor connection makes high resistance, high resistance makes heat, heat is localized to the area with high resistance so that's where the insulation melts. The problem can start small and grow as repeated thermal cycles cause the connection to get looser. Once you have actual arcing (you saw sparks) the heat is extremely intense at the point of arcing.

Does not necessarily mean the circuit was drawing more than the power the breaker is rated for.

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    What you've described is the most likely issue for the problem described. It's also possible that the wires used for the circuit were too small and too long, making it impossible to draw enough current to trip the breaker, even with a dead short at the end of the line. If a short forms in that case, all of the energy is dissipated in the resistance of the wires, resulting in them getting quite hot and, likely, a fire. In such case, even if there's not a fire, the entire wire run should be replaced, as there's likely damage throughout.
    – Makyen
    Commented Jul 4 at 22:15
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    @Makyen the question specifically mentions the insulation melting at the breaker connection, as well as sparking (so, arcs.) Both of which suggest a loose connection, rather than the entire wire overheating.
    – Ecnerwal
    Commented Jul 4 at 22:19
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    Yes ... Did you read the first sentence which I wrote? I'm not trying to be facetious. I'm just a bit confused as to what you're looking for as a response. I've literally already agreed with you that you've described the most probable issue. But, there are other possibilities, even if substantially less probable. What I'd point out is that the OP sounds quite inexperienced. I'd be uncomfortable, given the described damage, not strongly suggesting to them that they have someone local who is more experienced take a look at what's physically there.
    – Makyen
    Commented Jul 5 at 3:45
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While Ecnerwal his response is pretty good and explains the WHY is misses the fact that the shorting/arcing happened BEFORE the breaker, so from the main feed. The issue here is that from the breakers perspective everything after the breaker is just ok. It cannot detect issues before the breaker.

How to prevent this:

  • test your breakers "frequently". At least once a quarter.
  • check if the wires are still connected by trying to tightening the breaker screws. Very often these get loose over time. Especially if your circuit is dealing with high currents due to PV installations or use of many appliances simultaneously due to the wire expanding and contracting due to heat which can loosen the screws over time.

When checking the screws switch off everything, ideally check if everything is switched of by measuring the voltage when NOT using an insulated screw driver.

Checking if screws are tightened should really be done always after making changes to your breakers after e.g. a renovation.

I'm a voluntary firefighter and we have a related issues at least once a year on a population of only 35.000 so yeah, check those screws and test your breakers!

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  • My breaker box is built into metal, it seems like they all would be to try and prevent fires.
    – Ken - Enough about Monica
    Commented Jul 5 at 14:39
  • @Ken-EnoughaboutMonica does not really matter if the box is from metal or plastic. If you have a short in your box there are enough plastics to use for fuel from the breakers which can create enough heat to ignite something else nearby. Still, even from only having a fire in the breaker box you will have a lot of toxic smoke.
    – Ramon Smits
    Commented 2 days ago
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Because the fault was before the breaker

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