AFCI Not Tripping on Divided Neutral?

[brycenesbitt]

Do AFCI breakers now tolerate weird neural setups?

I diagnosed a new subpanel install that was showing no problems, but in the attic there was an odd connection to the neutral.
Turns out some prior worker noted that half the knob & tube had no neutral, so just patched in a neutral from the main.
Thus hot from an AFCI breaker, neutral to half the circuit, a different neutral to the rest of the circuit.

The ACFI breaker did not complain.

 

[ramsy]

If you want breakers to trip on crossed neutrals, use SQ-D AFCI's, a GFCI, or Dual-Function breakers.

 

[Fred B]

There are now mfg's that have AFCI that are useable with MWBCs. They presumably eliminated the internal CT that would trip on ground fault (an imbalance on neutral).
There may be a different more hazardous situation with what was done. They in effect created a MWBC. The concern would be: Are these two on same leg in panel? Are there handle ties to simultaneously disconnect both circuits? Has parallel neutral been created? K&T with receptacles, do any have grounded outlets present without GFCI protection?