THE SERVICE HOIST TRIPPED OUT ON AN OVERCURRENT AT 10:14 AM. UPON INVESTIGATION, A SECTION OF THE CONCRETE LINING OF THE SHAFT HAD SEPERATED AND WEDGED ALONG THE COUNTERWEIGHT GUIDE. CONCRETE SECTION WAS APPROX 18 INCHES BY 24 INCHES IN SIZE. MINE WAS EVACUATED EXCEPT FOR THOSE DIRECTLY INVOLVED IN REPAIR WORK. HOIST WAS OPERATIONAL.