What could cause conduit to arc where it touches another conduit? How
can this happen without the conduit being charged with 110V?

A few months back, I added two AFCI breakers and a new house ground
wire in a home improvement project. A licensed electrician installed
the breakers and made the box connections. I assume he did it
correctly.

Here’s the problem:

I installed a basement HVAC duct touching an old metal flexible
conduit. A few weeks later, I noticed arcing between the conduit and
HVAC duct where they touched. Neither of the two shocked me when I
touched them, but sure enough – an orange arc was making a little
noise. Upon examination, the arcing had welded holes in the HVAC duct
and the conduit.

I snipped & dissected the conduit. The wire sheathing inside were fine
– one was a bit darkened. Over a few weeks, I expected much worse. Oh
well, problem solved. Maybe it was HVAC static discharge.

But then yesterday, I heard the same noise inside a kitchen light
fixture on a different circuit. The box was hot to the touch. Upon
dissection, however, nothing in the box was damaged.

HOWEVER – that box’s conduit was touching another conduit, and sure
enough – a little hole had been welded where they had been touching.

What could be causing this? I am a little spooked.