In Relays Earth fault current can be calculated in two ways.
- Derived Earth Fault:
Earth Fault current is equal to the vector sum of the three line current values.
It is calculated using formula:
IE: Earth Fault current
IR: Current flowing through R phase
IY: Current flowing through Y phase
IB: Current flowing through B phase
- Measured Earth Fault:
Earth fault current can be measured in any one of the following ways.
- Direct Ground Current Measuring:
CBCT is used for earth leakage and sensitive Earth Fault conditions. CBCT encircles the three-phase conductors and measures zero sequence (ground) current directly. This is used as an input by the Relay to measure Earth Fault current.
Direct Ground Current Measuring
This method is well suited for use on low resistance grounded systems, but not well suited for use on solidly grounded systems
- Residual Ground Current Measuring:
Measuring of ground current by means of an overcurrent relay located in the common return conductor of the CT secondary current.
Residual Ground Current Measuring
This method is appropriate for solidly grounded power systems, but not well suited for low-resistance grounded systems.