Without knowing the model number or where it's arcing I will not be able to provide any model specific information. Generally speaking, the arcing is caused when two pieces of metal are touching or nearly touching each other or burnt carbonized food. Look for two pieces of metal that is touching or nearly touching each other. I must assume that you do not leave the wire rack inside the microwave when it's not needed. The wire rack is only to be put in and used when you are cooking something on the lower turntable and on the wire rack as well, otherwise the rack should be removed when not needed for cooking. Leaving the wire rack inside the microwave when not be used can cause the microwave to arc between the rack and the walls of the microwave.
: I do not recommend using the microwave until it has been repaired or it could damage the magnetron or cause a thermal event.
I recommend resubmitting your question with the model number and additional details for model specific information.