Earthing does not guarantee that humans will be safe when they come into contact with live conductors. Although relays may be installed to detect earth leakages, typically their settings are high. Therefore, earth leakage circuit breakers (ELCB) or residual current circuit breakers (RCCB) are used wherever there is a possibility of human contact with a live conductor. These breakers work on the core balance current principle.
Sensitive earth leakage protection
Figure illustrates the operation of the core balance leakage device. When the system is under normal conditions, the phase current and neutral current will be equal and in phase. Thus, the CT will not detect any current under normal circumstances since IL + IN = 0 (vector sum).
The ELCB consisting of core balance CT is mounted at the source end. Any time a human comes in contact with any part of the line, a portion of the current will flow through the body. This will result in an imbalance in the currents entering and returning to the CBCT of the ELCB. When the fault current flows through the human body, IN is reduced by the amount of IF. A relay is triggered by this unbalanced quantity, and the ELCB is immediately tripped.
It is normal that ELCBs are moulded breakers, similar to miniature circuit breakers, which are equipped with CBCT inside. It is also common for the CBCT to be mounted outside, with the unbalanced current being taken to trip a separate relay, the earth leakage relay.
In the above example, a single-phase system was considered. It is nevertheless the same principle for three-phase systems with neutral in which the vector sum of the three phase currents (IR + IY + IB) and the return neutral current IN will also be zero. As all phase and neutral conductors are taken in through the CBCT, the CBCT does not detect any current under normal conditions. As soon as any leakage occurs in any phase, the CBCT detects unbalance and trips the breaker.
There are three levels of sensitivity for the ELCBs: 30 mA, 100 mA, and 300 mA. ELCBs with 30 mA ratings are recommended for human protection, since currents flowing above 30 mA can cause serious damage, including death.
