As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
(CAN) 1. ON TAKEOFF, FLIGHT CREW HEARD CB'S POP WITH SMELL OF BURNING PLASTIC.2. EMERGENCY DECLARED & AIRCRAFT RETURNED TO YYZ. AIRCRAFT LANDED WITHOUT INCIDENT. 3. UPON INSPECTION CB'S J71J74 & K73 WERE FOUND POPPED WITH VISIBLE SIGNS OF ARCING. ALL CB'S REPLACED & RELECTED SYSTEMS CHECKED SERVICEABLE IAW TSM 24-00-00. 4. NO FURTHER INCIDENT DURING SUBSEQUENT FLIGHT.