TimR brought next idea :
On Tuesday, September 6, 2016 at 2:46:31 PM UTC-4, FromTheRafters wrote:
TimR submitted this idea :
Does an AFCI need a ground to work? I think I should know that but I
don't.

No, it only needs to compare hot and neutral.

I don't think it compares hot and neutral. GFCIs do that. AFCIs look for a
wave form change.

The Leviton site says they're outlets work in older homes with two wire
circuits so that answers my question.

They work the same as GFCI with the addition of simulating a ground
fault to the GFCI detector when waveform detectors detect a specific
signal of arcing on the line. It is still a comparator creating the
actual trip signal.

I suppose there could be some that don't work that way, but that is the
way I was given to believe.