Using a 117V AC appliance on a 127V AC mains in Brazil can be safe, depending on the actual voltage fluctuations. The mains voltage can vary between 114V and 139V, which may exceed the appliance's tolerance. If the appliance is rated for 117V with a tolerance of ±10%, it can safely operate between 105V and 128V. However, to ensure safety and protect the appliance from potential overvoltage, using an auto-transformer is recommended. Overall, checking the equipment ratings and considering voltage variations is crucial for safe operation.