How to troubleshoot wiring that trips AFCI breakers?

Question

Here's the first part of the story. Summing up, existing wiring was put on AFCI breakers. Some outlets had hot/neutral reversed. Electrician corrected this.

Now, the front porch light, not part of work that electrician performed, trips two AFCI breakers! The lights go off in the two of the bedrooms and the hallway outside the bedrooms. The two bedrooms are where the outlets had hot/neutral reversed. I didn't test the all receptacles to see if they have power. Each room had one receptacle that didn't have power.

What could be happening? How do I go about debugging this wiring? What are the steps?

Update Found this Siemens guide for troubleshooting. I have the Siemens QA115AFCP single-pole 15-amp Combination AFCI Breaker.

Update The house was built in 1939. There are metal junction boxes in various places (I've seen them in the attic). There is metal conduit that runs between junction boxes. I don't believe they are grounded.

Update 1/15/17 I tested all of the outlets in the house. Here's more info. All of the outlets in the living room (and one outlet in the adjacent room that shares a wall with the living room) trips the two AFCI breakers when I plug in a light and switch it on! (Is it basically one offending wire that feeds all of those outlets?)

Update 1/16 The electrician checked it out and said that the wiring consisted of two hot lines and a shared neutral. He's rewriting two of the rooms that we're tripping to have their own home run.

Update 1/17 The electrician fixed the situation and ran a new home run with an AFCI breaker to the two bedrooms. He also discovered that the forced air heating unit was wired with a shared neutral so he also ran a new home run to that (with a normal breaker). The living room with outlets that tripped an AFCI breaker, they wired to a normal breaker.

Answer 1

You don't need to decrypt breaker indicators. The fact that two breakers trip is all you need to know.

Why would one thing trip 2 breakers? These two circuits have their neutrals crossed, tied together, or interacting in some way. Perhaps one device which takes its hot from circuit 1 is taking its neutral from circuit 2.

Hots and neutrals are supposed to be monogamous -- they're only supposed to partner with each other, ever. That's always been in the code. But until recently, crossing neutrals "worked" - it didn't cause any detectable problems.

What happened "recently" was adoption of GFCI and AFCI. Those absolutely require hots and neutrals be monogamous. If current comes back on the neutral in an unexpected way, that makes it trip.

So you have to go through each circuit, look closely at every box, and find the place where a neutral doesn't match its hot. It's a tedious job, and I've done it. If it helps, get gray tape and put it on the neutral from one of the circuits. Gray is an allowable color for marking neutrals.