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One of the Hot wire (#10/3) going to my oven appear to have experienced a sudden very high in rise temperature inside the junction box, melted the connector, creating a contact with the face plate metal junction box, creating an arc fault, return to ground, making a crater in the face plate, yet the breaker (square D 30 A) did not trip. No arc fault detector nor GFCI on the breaker. The oven went on working, and I had to close the breaker manually. All other wires are fine. Followed all the wiring of all devices connected to the sub-breaker box and checked inside the kitchen sub-breaker box fed by a #6/3, everything appears fine. Tested the wires for temperature. The box has 2 double twin 15 A breakers, and 2 30 A breakers. The box is rated for 100 A. None of the other wires shows over heating damage, nor does the one dedicated to the oven. All other circuits work fine with their dedicated appliances or outlets working fine.

The oven went on working with only the digital display showing short mild intensity irregularities just when the flashing was occurring in the junction box leading to the oven. The whole event lasted about 20 seconds with 3 waves of flashing and noisy low frequency sparks/flames before I shut down the breaker. The connector melted and has a whole which allowed the tip of the hot wire to get in contact with the face plate of the junction box creating the cascade of events.

What could have triggered such a rise in temperature in the wire while this connection has been working fine for about 5 years?

Thank you.

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    \$\begingroup\$ That connector was loose, high resitance, got hot & melted… \$\endgroup\$
    – Solar Mike
    Apr 13, 2022 at 12:36
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    \$\begingroup\$ Yes, most likely a loose connection. What happens is that the power gets dropped across the high resistance connection and not the oven. Things can work loose, especially if you don't tighten connections according to the datasheet with a calibrated torque screwdriver or use wirenuts rather than more precision stuff like Wagos. \$\endgroup\$
    – Dannie
    Apr 13, 2022 at 12:41
  • \$\begingroup\$ Thank you for the cue. As you explain, it does seem like the most plausible explanation. I was not familiar with Wago connectors for big wiring, only for inside appliances like washing machines, electronics to electronics, I will try to find some. But effectively, the corrosion build up must have been recent, it worked perfectly until last night. The Marr was burn through out (it was calibrated for 8 to 12 awg. \$\endgroup\$ Apr 13, 2022 at 16:19
  • \$\begingroup\$ IDC crimp outlets are just as bad but tend to just go silently open ( hole for wire rather than wrap around screw) But Marr connectors are fine if done right. \$\endgroup\$ Apr 13, 2022 at 23:19

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Who installed this? Thermal runaway on loose oxidized connectors is likely. Highest resistance in series dissipates the most heat yet , current limited by heater element. Clock runs off 120 V only so it would be prone to massive EMI from arc both conducted and radiated.

When contacts are hot and not a gas-tight fitting, oxidation occurs faster thus increasing contact resistance with insulation carbon+oxides causing more heat. There is a critical point where this escalates into a potential run-away condition causing;

  • fusing of conductor with lower resistance with arcs then cool-down from reduced losses
  • fusing of conductor with higher resistance and thermal runaway glowing hot >3000'K then possibly fusing open like a burnt light bulb
  • potential fire condition

Since any added resistance in series with the > 10kW heater element will not trip the breaker, it is critical that the connections are torqued sufficiently that air cannot reach the contact surfaces and cause oxidation. Although a GFCI would likely trip from the unbalanced arc noise leakage currents, an over-current breaker will not trip.

Your situation might have been avoided by proper torque of suitable size Marr connectors. Tight enough not to break the wires or strip the spring but fairly tight such that tension increases rapidly for the last 10 deg. or so. ( Sparkies may have a better spec)

For all connectors I make I use the 5lb (2kg) pull test

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  • \$\begingroup\$ Thank you for the cue. I just read your colleague's answer just up. I might effectively try his wago connectors. I used Marr connectors calibrated for 8 to 12 awg. Worked fine for 5 years until last night. Good thing I was next to the applicance when it occured. But effectively, the only sound explanation has probably occidation, because thejunction box is large enough, in no way are the wires stress at any place. The whole connection wiring has never been disturbed. TY \$\endgroup\$ Apr 13, 2022 at 16:23

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