If the section of starter is burnt or melted.

PROBABLY NOT THE "GROUND" CONNECTED TO THE STARTER. IT GETS GROUND BY BEING BOLTED TO THE ENGINE

I CAN THINK OF A FEW REASONS FOR THE BURNT OR MELTED PLASTIC

1) THE POSITIVE CABLE MAY HAVE GOTTEN EXTREMELY HOT DUE TO EXTENDED/ OR EXCESSIVE CRANKING

2) A LOOSE CONNECTION WILL HEAT UP FAST. POSSIBLE ARCING. IT DOESN'T MATTER IF IT WERE THE POSITIVE OR NEGATIVE CABLE WITH THE LOOSE CONNECTION, THE WHOLE CIRCUIT WOULD GET HOT

3) A DEFECTIVE STARTER OR ENGINE DRAG MIGHT MAKE THE STARTER WORK HARDER---DRAW MORE FROM THE BATTERY, HEATING THE CABLES

THE MEDIC