Power cross events can have a number of causes. Some are accidental contact of the protected line with power lines. One example is construction crews digging up and rupturing telephone cables and power cables, causing them to be connected together. Traffic accidents involving power and telephone lines is another. Wind and ice storms have been know to break lines and have them fall into contact with others. Power cross events can also be caused by inductive coupling between power and telephone cables sharing a common duct. Improper grounding can also cause problems. Power cross events usually cause excessive current to flow in the protected equipment. The main protection devices used are overcurrent protectors such as fuses or PTC resistors. However, the overvoltage protector must survive long enough to allow the overcurrent device to operate. In some cases, the requirement is only that the overvoltage protector must not cause a fire. In other cases the requirement is that the overvoltage protector survives undamaged. To simulate a power cross event, the AC source open circuit voltage and short circuit current are defined as well as the duration of the test.