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After removing the old fan, inspect the existing electrical box. It must be marked 'acceptable for fan support' and securely fastened to a ceiling joist. Standard plastic electrical boxes or those not properly secured are insufficient for a ceiling fan's weight and vibration. If your box doesn't meet these requirements, you must install an expandable ceiling fan/fixture brace. Insert the brace through the ceiling hole and turn the arms until they securely grip the ceiling joists. Also, ensure the electrical box itself is grounded, or that a separate ground wire (usually green or bare copper) is present in the ceiling.