Strange burning smell coming during charging and look....

[tga]

That's what is called a high-resistance "glowing" connection, and likely wouldn't have tripped AFCI, either. AFCI's search for an arc signature. In this case, this is just a high current going through a high resistance connection.

OK, I was kind of hung up on all the discussion of arcing above.

Looking back at the first page, the 14-50R clearly got pretty hot (deformed plastic), but the blackened pin on the 14-50P make me think there was some arcing involved. No? Would glowing give you that charring without arcing?

 

[mknox]

Would glowing give you that charring without arcing?

Sure. Any impurities on the metal prongs would blacken like that from the heat. Even some molten plastic may have run on to the hot metal surfaces.

 

[kennybobby]

One thing... leads to Another

Series Arcing and the Glowing Connection

The glowing connection usually begins its life as an intermittent connection, or a series
arc. But then something very strange happens. The localized heating caused by the series
arc makes the connected metals begin to oxidize. Once the contact points are oxidized,
electrical resistance begins to increase. This increase in resistance, because it is in series
with the normal current flow, causes resistance heating just like in an electric heater. The
heating that occurs is very localized, but it begins to cause an alloying of the dissimilar
metals. These alloys have additional, and unexpected, electrical properties. That is, they
are semiconductors, and behave in a non-linear fashion to electric current flow. Because
of this non-linear behavior, heating can occur at very low currents.

from Frequently Asked Questions about Glowing Connections