Low Resistance grounding involves connecting a low resistance in series with the grounded neutral of the three phase system. Low Resistance grounding limits permits a fault current above 10A but limits it to around 50A

Low resistance Grounding is used in situations where quick operation of ground fault relay is required. This form of earthing is used when faults need to be cleared quickly.

Low resistance grounding resistors have a time rating beyond which they may not be able to maintain thermal stability due to the heat generated by the fault currents.

Low resistance grounding ensures that equipment and conductors are not exposed to the electric and mechanical stresses during an earth fault.

However, the downside of low resistance grounding is that the system needs to be de-energized after a ground fault.

These are usually used in medium and high voltage Systems

Resistance Grounding

In Resistance Grounding, the three phase power system is grounded through a series resistance.  This resistance is intended to limit the fault current when there is an earth fault.  Resistance grounded systems are ideal as they enable quick identification of a fault and clearance.  The series resistors used to limit current are designed for thermal stability during fault conditions.  The resistors also have a time rating.  They are designed to be in circuit for a particular period of time till the fault is cleared.  Resistance grounded systems can be classified into

  • High Resistance Grounding and
  • Low Resistance Grounding

High Resistance Grounding restricts the ground fault current to less than 10A.  These systems are advantageous because the system can continue to run when there is a fault between a phase and the earth.  This ensures the system reliability and the system continues to run while the fault can be identified and rectified.  However, care must be taken to ensure that the permitted ground fault current is greater than the charging current of the line capacitances.

This is essential to ensure that there are not transient overvoltages during intermittent earth faults.  The series resistors used in High resistance grounding are designed for longer time rating as they may have to be in circuit as long as the system is running with the fault still present.

High Resistance grounding Systems are not permitted in systems which feed single phase loads.

Modern High Resistance Grounding Systems are equipped with a pulser circuit which is activated when a ground fault is detected.  This pulser circuits generates a pulsating current which can be used to identify the exact location of the ground fault with a handheld device.  This is extremely useful in identifying the fault within a short period and restoring the system.

See Also : 

Types of Earthing

A proper grounding scheme is vital component of any power system. Improperly grounded systems can result in equipment failures, overvoltages, and flashovers.  Grounding uses the earth as a return conductor in the event of a fault.  This helps to identify the fault.  Resistances can be used to limit the fault current to desired levels.  Grounding ensures system stability and prompt identification and clearing of faults.

In three phase systems, the neutral of the Star Point is usually grounded. In the case of delta connected systems, a special grounding arrangement such as Earthing Transformers or Zig-zag transformers are used.

On the basis of the grounding used, Power Systems can be classified into
  • Ungrounded Systems
  • Solidly Grounded Systems
  • Low Resistance Grounded Systems
  • High Resistance Grounded Systems

Ungrounded Systems

Ungrounded Systems can function normally in the healthy condition.  In the fault condition, as one phase gets earthed, the voltage between the other two phases and the ground increases to the line voltage(phase to phase voltage).  This places the insulation of the equipment connected to the system under excessive electrostatic stress.  Ungrounded systems are the most expensive for this reason.

Electric Equipment connected to ungrounded systems need to have insulation rated for the line voltage.  In the event of a fault on one phase, the fault current is fed by the capacitance charging current flowing the other two un-faulted phases.

This current is usually less and power can continue to flow in the other two phases.  However, if the fault is intermittent and the contact with the ground is of the make-break type.  The capacitances which form in the other two phases may charge and discharge into the system causing high overvoltages, sometimes 5 to 7 times the normal voltage.  This can cause extensive damage to other devices connected elsewhere in the system. 

While the ungrounded system can run with the other two phases even when one phase is faulty, a fault in any of the other two phases can cause a phase-to-phase short circuit via the ground.

See Also

Earthing in Three Phase Systems          Part II

Three phase High Voltage electric systems connected to transformers or generators that are solidly grounded may experience transient overvoltages during fault conditions due to the line capacitances getting charged and discharged.

Besides, the intensity of a ground fault will be greater and will be accompanied by a flashover. This may be dangerous to personnel who are in the vicinity.

The damage to the equipment is also extensive as a higher current flows in solidly grounded system. The conductors carrying the current are subjected to extensive electrical and mechanical stresses.

The ground current which flows through the soil can pose a danger to people if the step potential exceeds the safe limits.

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Neutral Grounding Resistors

Grounding Resistances - An introduction

Grounding Resistors are used to limit the fault current in Transformers and Alternators. When a phase to ground fault occurs, the fault current is limited only by the soil resistance. This current, which can be very high, can damage the windings.

Grounding resistances can be classified into high and low resistances.

In high resistance grounding, the fault current is limited to less than 10 amperes. While, in low resistance grounding, the current is limited to a value from 25 amperes or more.

The resistances are also categorized on the basis of time they can withstand the fault current. Typical durations are 1 second, 10 second, one minute and 10 minute rating.

The Extended Time rating resistor is used in systems where the reliability of the system is critical. This is true in petroleum industries, mines etc. In these situations, a high resistance which can sustain the fault for a long period is used. When an earth fault occurs of one phase, an alarm is generated. However, the system continues to run until the next scheduled shutdown.

Resistance grounding is not used in systems where the phase voltage exceeds 15kV for cost reasons.

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Disadvantages of Solid Grounded Systems

Grounding Resistances - An introduction

The magnetic field in a motor or a generator is ideally conducted along paths that are symmetrical. However, sometimes the magnetic fields within an electric machine such as a motor or a generator is asymmetrical. This asymmetry can be caused usually by variation in the quality of the steel used in the motor construction or in some cases to shaft eccentricity.

These asymmetrical magnetic fields which are varying over time can induce currents in the shaft of the motor or generator. These currents which are induced in the shaft tend to flow from one end of the shaft to the other through the bearings and then through the earth.

When these currents flow through the bearings, the tend to cause arcing and consequent pitting in the bearings. This can lead to failure of the bearings.

Shaft currents can be prevented by insulating one of the bearings. A Teflon layer is usually placed between the shell of the bearing and the bearing housing. This ensures that the shaft voltage induced does not have a return path. This prevents shaft currents from flowing.

Flame Proof motors are used in hazardous locations usually in chemical, petroleum and gas industries, where the environment may contain gases or inflammable vapours. Electric equipment are notorious for being the source of sparks during switching. This could be catastrophic in areas where inflammable gases are present.

Hazardous areas classified into zones depending on the possibility of the presence of gases and combustible dust-air mixtures.

The following are the classification for hazardous areas

Zone 0, An Explosive atmosphere is continuously

Zone 1, An Explosive atmosphere is present for 1000 hours in a year.

Zone 2, An Explosive atmosphere is present for 10 hours in a year.

Flameproof motors are specially designed so that any explosion which occurs within the motor is contained within the motor frame itself. This prevents any secondary explosion in the surrounding area. The flame-proof motor is specially enclosed within a protective shield which covers the entire motor including the bearings.

Flame proof motors are heavier that motors of similar rating due to the increased weight of the reinforced design of the end shields so as to withstand the impact of an explosion.

Flame proof motors have special arrangements for cooling. Since conventional air circulation is not possible with these motors, special arrangements such as using the internal air to carry heat to the surface of the motor where the heat is transferred to the external air by means of special tubes have been devised.

Another method of cooling is the use of a coolant such as water which circulate in the stator and rotor of the motor. This water is brought to the near the inside of surface of the motor where it is cooled by means of the atmospheric air.

The cooling system ensures the isolation of the internal cooling medium from the external atmospheric air.

Single phase motors have two windings, the main winding and the auxilliary winding. The auxiliary winding is used to start the motor and may be disconnected once the motor picks up sufficient speed.

Reversing a single phase motors cannot be done by reversing the polarity of the supply to the entire motor. To reverse the single phase motor, the polarity of the supply to only one of the windings needs to be changed.

This can be done by reconfiguring special links which may be provided in the terminal box of the motor.

Shielding in Power Cables involves covering the insulation of a single cable or a group of cables by a conducting material usually a sheath which is grounded.

The objective of a shield is to ensure that the cable insulation is subjected to a uniform electric stress. The shielding also prevents transient overvoltages which are induced along the cable by ensuring a eliminating surge potentials and ensuring a uniform surge impedance.

Shielding also protects personnel from dangerous shocks which may be caused by intense electric fields.

The Energy Star Classification is a system of classification introduced by the United States Environmental Protection Agency in 1992. The Program aims at reducing energy consumption by developing more efficient electric gadgets and devices.

The program has been adopted by other countries such as New Zealand, Japan, Taiwan, the European Union and Australia.

The labelling program is voluntary. Devices which bear the Energy Star Logo consume about 20-30% less energy that normal ones.

The Energy Star labelling System is now used in numerous electric appliances such as heaters, televisions, computer monitors, lighting and even LED traffic lights.

Energy Star certified refrigerators use about 20% less power than the minimum standard. Fluorescent lighting which is standardized by Energy Star use about 75% less energy than conventional incandescent lighting.

The Energy Star standard is also being extended to commercial building which use less energy and to industrial facilities.

A Zero Watts Bulb is a bulb with low light output.  It is frequently used as a night lamp and other applications requiring low intensity lights such as.  It is called a zero watts bulb as it consumes relatively low power, around 15 W.

When all the appliances were switched off at night and only the zero watts bulb was switched on, the power consumed in the house was too small to be measured by the older electro-magnetic meters.  Hence, the moniker "zero watts".

Zero watts bulbs are increasingly being replaced by more efficient CFL bulbs which are also last longer.

Triplen Harmonics are harmonics whose orders are multiples of three.  Thus the 3rd, 6th and 9th Harmonics are known as Triplen Harmonics.  These harmonics flow through the neutral conductor of Star connected transformers, causing heating.  This heating also affects cables and causes nuisance trippings of relays.

These are usually caused by power supply units for electronic appliances.  The power supply units are usually connected to only one phase.

Triplen Harmonics can be avoided by use of the delta connection for transformers and three phase loads.  Increasing the sizing of the neutral conductor is also a means of attenuating the effects of these harmonics.