Section III, Note 4:

During electroporation, there is a possibility of the sample "arcing". This is characterized by a loud "pop" sound. Arcing occurs when the sample is too conductive. There are several reasons that a sample will arc including: not washing all of the salt from the growth medium out of the bacterial suspension; placing too much DNA in the electroporation mix; using DNA dissolved in a high salt buffer; using a bacterial suspension that is too concentrated; using a bacterial suspension containing lysed bacteria; and trying to electroporate using electroporation cuvettes that are too warm. If arcing occurs, dilute the DNA and try again, this is usually the most common problem. If arcing continues, perform an electroporation without DNA to see if the cells are too concentrated. Expect a time constant over 4 seconds; electroporation at time constants below 4 seconds will not produce good efficiencies of transformation and should be repeated with less DNA.