## What are the points to be covered?

- What is the line to line fault?
- How line to line fault occurs?
- Fault current in line to a line fault.
- Calculate line to line fault current.

## What is Meant by Line to Line Fault?

When two conductors of a 3 phase system are the short-circuited line to line fault or unsymmetrical fault occurs. This fault is severe compared to symmetrical faults in the power system. In order to safeguard power system network analysis of the unsymmetrical faults (in this case line to line fault) must be done.

## Fault Current Calculation in Line to Line Fault

Consider a 3-phase system with an earthed neutral. Assume a line-to-line fault between the blue (B) and yellow (Y) lines as shown in the figure. The conditions created by this fault lead to :

Again taking R-phase as the reference, we have,

Take the above result as equation (i)

Take the above result as equation (ii)

**Note:** From the above steps, it is clear that the line-to-line fault the zero sequence component of current I0 is equal to zero in line to a line fault.

**Fault Current in Line-to-Line Fault:** Examination of exp. (i) and exp (ii) reveals that sequence impedance should be connected as shown in the above figure. It is clear from the figure that :

**Phase Voltages.** Since the generated e.m.f. the system is of positive phase sequence only, the sequence

components of e.m.f. in R-phase are :

**Summary:** For the line-to-line fault (Blue and Yellow lines) :

Above what is the line to line fault and how to find the line to line current fault current are discussed.