Tag Archives: current

Protecting Against Ground Faults

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.

AC vs DC – What is the Difference?

AC vs. DC

Electric current is the flow of electrons carrying electric charge. There are 2 types of electric current – direct (DC) and alternating (AC). In Direct Current the electron flow takes place only in one direction. A battery is a source of direct current. DC is widely used in many electronic circuits operating in low voltage levels.

In Alternating Current, both voltage and current alternate in direction back and forth following a sine wave pattern. The number of cycles per second, called the frequency, varies from 50 or 60 depending on the power system in a country. Alternating current is produced universally in power stations using AC generators. The AC theory is briefly described below.

A rotating coil in a magnetic field cuts the magnetic lines of force in two different directions during each half rotation in an AC generator. Thus the current produced travels alternately from left to right and then from right to left. When the coil is parallel to the magnetic lines of force, no current is generated. The alternating current so generated is collected by slip rings attached to the ends of the rotating coil and then transferred to an external circuit through metallic brushes.

Alternating current can be readily transmitted over long distances with minimum loss unlike DC. Any voltage drop along the way can be easily boosted using transformers. Also motors with high power can be designed using AC. Eddy current and radiation losses are the principal disadvantages of AC. 3 phase AC is generated in power stations, with each current out of phase by 120 deg to each other.

For a simple explanation about converters and inverters, visit this web page.