As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ON TAKEOFF ROLL, RECEIVED "AC ESS BUS" CAUTION & "EMER LIGHTS ON" STATUS MESSAGE. RAN QRH, MESSAGE REMAINED. RETURNED TO DCA. NOTED NO ACCESS, BUS SYSTEMS APPEARED AFFECTED. INSTALLED NEW A/C ESSENTIAL BUS RELAY (K1XD) AND COMPLIED WITH OPS CHECKS. NO DISCREPANCIES AT THIS TIME, PER CRJ 200 AMM 24.51.27.