Faults are instances of something happening when it should not. Electrical faults are when electric current flows where it should not be flowing. Electric current flowing from the live wire to the ground in place of the customary neutral wire constitutes a ground fault.
There are two major problems that a ground fault may cause. One is excessive current may cause overheating and fire may break out. The other is a ground fault could be fatal for any person being a part of the ground circuit. That is why it is important to protect against ground faults occurring.
Earlier to the 1970s, people did not realize the necessity of grounding electrical systems. As a result, most industrial and commercial systems remained ungrounded. Although ungrounded systems do not result in significant damage, the numerous disadvantages that they present led to a change to grounded systems. Grounded systems also help in protection against lightning, and reduction of shock hazards.
In electrical supply and distribution systems, faults are mainly of two types—phase-to-phase faults, and ground faults—with ground faults being 98% of them. While fuses form the main methods of protection in case of phase-to-phase faults, protecting against ground faults requires the additional use of protective relays.
For instance, a toaster may have the hot wire shorted to its metal casing. Turning on the toaster causes all or a part of the current to pass through the toaster frame and then on through the green ground wire. If the current is high enough, the circuit breaker will trip. Adding a ground protection relay would have detected the current flow at a significantly lower level and opened the circuit much quicker than the circuit breaker.
Ground faults occur for different reasons. These could be due to inclement weather, causing a tree to fall over and rest on power lines during a storm. Insulation degraded by age can also cause ground faults—heat from a current flow can break down old insulation. Moisture from high humidity can break down insulation. Excessive overvoltage and puncture the insulation and cause ground faults.
Protecting against ground faults means isolating the circuit with the fault so that there is no power to that part of the circuit. However, to clear the fault, it is necessary to first establish the presence of a fault, and then determine the source of the fault. System designers use a ground fault protection relay for this purpose.
In normal operation, electric current flows from the phase or hot wire into the appliance and returns via the neutral or the cold wire. As the two currents are equal, their resultant electromagnetic fields cancel out. A current transformer placed across the phase and neutral wires will yield zero output while the two wires carry equal currents.
In case of a ground fault, part or all the current from the phase wire bypasses the neutral wire, since it now flows through the ground wire. As the two currents through the CT are now unequal, there is a resultant output from the CT, tripping the associated circuit breaker.