Electrical safety - Protection against earth faults
Electrical safety tutorial
Electrical safety - Earth fault protection
We will now understand what earth fault protection is. We are going to be focusing on the earth fault.
Earth fault is an inadvertent fault between the live conductor and the earth. When a ground fault occurs, the electrical system is shorted and the shorted current flows through the system. The fault current returns through the earth or other electrical equipment, damaging the equipment. It also interrupts the continuity of the power supply and may shock the user. To protect the equipment and for the safety of persons, fault protection devices are used in the installation.
Protective devices against earth faults
The devices give the trip order to cut the circuitwhen grounding fault occurs. The fault current is limited and the fault is dispersed by the restricted earth fault protection scheme (REFP). Normally, the earth fault relay, earth leakage circuit breaker and earth fault circuit breaker, etc. are used to limit the fault current.
Earth fault relay (EFR)
It 'sa safety device used in electrical installations with high earth impedance. It detects small parasitic voltages on the l housings of electrical equipment. The result is to interrupt the circuit if a dangerous voltage is detected. The EFR is protected against the triggering of transients and avoids shocks.
The following figure shows the earth fault relay -
Earth leakage circuit breaker
The earth leakage circuit breaker detects the leak current directly and prevent humans and animals from injuring themselves due to electric shock. It is a voltage sensing device and has recently been replaced by a Residual Current Circuit Breaker (RCCB), which is a current sensing device. It is a special type of latch relay which is connected to the main power supply. When the fault current flows from the live wire to the earth wire in the installation, the coil of the ELCB detects the voltage and cuts off the power supply. It requires a manual reset process to work again. The RCCB detects the leakage current and sends a signal to trigger the system.
Earth leakage circuit breaker
The earth leakage circuit breaker is a safety device to prevent un electrical accident in the event of a breakdown. The tool is plugged in. This is a fast acting circuit breaker to cut off power when the ground fault occurs within 1 / 40th of a second. It compares the current entering and leaving the equipment along the circuit conductor. If there is a difference as small as 5mA, the GFCI will limit the current and trip quickly. GFCI doesn't help much with line contact hazards, but protects against fire, overheating, and destruction of wire insulation.
Restricted earth fault protection scheme
Consider a star-wound transformer, which is protected by Restricted earth fault protection with EFR protection device as shown in the figure below.
The following image shows earth fault protection with EFR -
When an external fault F1 occurs in the network, I1 and I2 cross the secondary side of the CTs. The result of I1 and I2 will be zero. However, if an internal fault F2 occurs at l Within the protection zone, only I2 flows and I1 is neglected.The resulting current I2 flows through the earth fault relay, which detects the fault current and protects the restricted part of the winding. The fault current is about 15% greater than the rated current of the winding. To avoid magnetizing inrush current, the stabilization current must be in series with the relay.