20 years ago we just cleaned the grounds once a year but didn't actually test them. Then after 1910.269 came out in the 90's we started testing. Found out real quick that all the grounds that had been made without using ferrells weren't worth much. Reworked all grounds using ferrells and rarely have a ground fail now. When we do it's usually the bolt on the ferrell has loosened and allowed grit to get between the cable and the head.