What is the difference between line to line, line to ground and double line to ground fault?
A line to line fault is one where shortcircuiting occurs between two phases of a system.
A line to groung fault is one where shortcircuit occurs between one phase of the system and the earth.
A double line to ground fault is one where shortcircuiting occurs between two phases along with the earth at the same time.
The differences are as stated in the answer above. Calculating the actual fault current in each situation can be a bit tricky, for the following reason: when any one of these fault types occurs, the 3-phase system is no longer balanced i.e. angles and magnitudes change dramatically, making calculations rather difficult. The standard method of analysis is by way of positive, negative and zero sequence theory. This basically involves converting the three unbalanced phase vectors into an equivalent “balanced” vector system. It’s not as hectic as it sounds and for more info, consult any good power systems textbook. Googling on “positive negative zero sequence fault current calculation” returns some useful info as well.
