A review on grounding and safety practices, how a system should be grounded etc.

1. STUDY ON GROUNDING SYSTEM AND
SAFETY PRACTICES
MOHAMMED FAIZ M
S7 EE
ROLL NO. 25
DEPARTMENT OF ELECTRICALAND ELECTRONICS
COLLEGE OF ENGINEERING KIDANGOOR
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2. CONTENTS
1. INTRODUCTION
2. EFFECT OF ELECTRIC FAULT CURRENT
3. TYPES OF GROUNDING
4. FAULT RETURN CONDUCTOR
5. METHODS OF SENSING AND CLEARING A FAULT TO EQUIPMENT
6. OTHER PATHWAYS
7. CONCLUSIONS
8. REFERENCES
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3. INTRODUCTION
Grounding, generally means providing a connection from one conductor of the
system to an electrode that is buried in the earth.
Grounding is generally accepted as an operation to make systems safe.
The fault return conductor brings the potential at equipment enclosures to ground
but only when there is no fault current.
The potential rise due to the fault current flow can raise the potential to a
hazardous level.
Simple grounding does not provide for safe systems.
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4. The fault return conductor and the overcurrent protective device, in combination,
may prevent ventricular fibrillation and provide safer systems.
Grounding or connection to earth, is an action to minimise the consequence on
electrical faults.
If the enclosure is at earth potential and an individual is in contact with the earth,
there will be no difference in the potential to result in a shock.
For a fault current flows in the connection to the earth, the enclosure is raised in
potential equal to the product of the fault current and the impedance to the earth.
Now there will be a difference of potential between the enclosure and the body in
contact with the earth, and a shock potential is established.
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INTRODUCTION(Contd… )

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Non-normal electric current flow results in electric shock and fire.
Fires are started by raising the temperature of a combustible material above its
ignition point.
The power generated by a current flowing through a resistive element is
determined by the i2R function.
The power dissipated in the resistive element can raise the temperature.
Minimizing the total time that the fault current flows will minimize the total energy
developed.
INTRODUCTION(Contd… )

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EFFECTS OF ELECTRIC FAULT
CURRENT
The effects of an electric current through the body (1000ohm) from 5 to 95
percentile have been measured as follows (independent of time):
Threshold of feeling: 1 mA
Let-go current: 6-14 mA for women and 9-22 mA for men
Arrest respiration: 20 - 40 mA across chest.
Thus, maximum fault voltage of 6 V is desirable.
Obtaining this low level of fault voltage may not be practical.
The two factors which defines the level of safety of an electrical system:
magnitude
duration

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The relatively safer system is one that minimizes the magnitude of the fault voltage
and/or the duration that the fault current exists.
The fault magnitude is determined by:
 the impedance of the fault path
 the fault voltage.
The impedance of the body, including the contact impedance where the current
enters and where it leaves the body is the limiting factor.
The fault duration is determined by:
 how long the body is subjected to the fault current.
EFFECTS OF ELECTRIC FAULT
CURRENT(Contd… )

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TYPES OF GROUNDING
System Grounding
Equipment Grounding

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System Grounding
NEEDS OF SYSTEM GROUNDING:
1. to limit the voltage imposed by lightning
2. to limit the voltage due to unintentional contact with higher-voltage lines
3. to limit the voltage due to line surges
4. to stabilize the voltage to the earth during normal operation.

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Fig 1. System Grounding

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The supply system has a connection to ground and is "system" grounded.
When lightning strike to a load side conductor, system grounding provides a path
for the lightning current to travel to the earth.
There is no electrical connection from the faulted supply conductor and the human
in contact with the enclosure.
Line surges, if they are voltage, cannot be reduced by system grounding which is a
current path.
Only surge protective devices can provide a means of short circuiting an
overvoltage to ground.
System Grounding (Contd… )

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Sudden opening of single conductor in a 3 phase long distribution system leads to
unstable voltage.
The capacitance to ground and the inductance of the line react to result in ferro-
resonance and an overvoltage can occur.
System grounding changes the capacitance vs inductance relationship and prevents
the overvoltage from occurring.
System grounding provides little assistance in minimizing the amplitude or duration
of downstream faults at equipment.
 The fault current must return through the earth with its relatively high resistance
compared to fault supply current conductors.
System Grounding (Contd… )

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Equipment Grounding
NEEDS OF EQUIPMENT GROUNDING:
1. to facilitate fault current clearing
2. to minimize the hazardous fault voltage

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Fig 2. Equipment Grounding

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The supply system has a connection to the earth and is "grounded".
There is a fault at load equipment between the supply ungrounded conductor and
the metal enclosure of the load.
There is a connection to the earth from the equipment enclosure and the equipment
is "grounded". This conductor is truly an "equipment grounding conductor"
Equipment Grounding (Contd…)

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The fault voltage at the enclosure with respect to local ground is:
𝑉𝑓𝑎𝑢𝑙𝑡 = 𝐼𝑓𝑎𝑢𝑙𝑡 × 𝑍 𝑟𝑒𝑡𝑢𝑟𝑛
𝐼𝑓𝑎𝑢𝑙𝑡 =
𝑉 𝑠𝑢𝑝𝑝𝑙𝑦
𝑍 𝑝ℎ𝑎𝑠𝑒+𝑍 𝑟𝑒𝑡𝑢𝑟𝑛
𝑉𝑓𝑎𝑢𝑙𝑡 =
𝑉 𝑠𝑢𝑝𝑝𝑙𝑦
𝑍 𝑝ℎ𝑎𝑠𝑒+𝑍 𝑟𝑒𝑡𝑢𝑟𝑛
× 𝑍 𝑟𝑒𝑡𝑢𝑟𝑛
Equipment Grounding (Contd…)

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The fault voltage will be very close to the supply voltage by Ohm's Law for series
circuits.
Any increase in conductor size or reduction of circuit length will merely make the
fault voltage higher.
Fault current magnitude is minimal due to the relatively high resistance through the
earth back to the system grounding connection and finally to the source.
Thus equipment grounding, there fore, does not minimize the fault voltage.
Equipment Grounding (Contd…)

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Fig 3. Fault Return Conductor
FAULT RETURN CONDUCTOR

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The conductor connecting the enclosure directly to the earth has been removed.
A separate conductor has been added between the enclosure and the source.
The added conductor provides a path for the fault current to the source without
passing through the earth.
 This conductor is known as “fault return conductor”.
FAULT RETURN CONDUCTOR (Contd…)

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The magnitude of the fault voltage can be calculated because the resistance of the
return path is a known quantity.
To provide a high level of fault current, a low-impedance electrically conductive
path from potentially faulted piece of equipment to the system source is necessary.
If that path is through the earth, the impedance of the earth will generally limit the
fault current to low levels that are insufficient for fault clearing.
FAULT RETURN CONDUCTOR (Contd…)

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Zero-sequence current sensing:
 Use a current transformer to sense the current going and leave the load.
 If the currents are not equal, there is a fault current returning to the source by some other
path.
 The current transformer output is used to operate the overcurrent device.
 Commonly found in kitchens, bathrooms etc.
 These devices are capable of opening the circuit in under 1 s.
METHODS OF SENSING AND CLEARING
A FAULT TO EQUIPMENT

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By the use of a current transformer surrounding all of the circuit conductors.
 If the net current is not zero, the current transformer secondary output is used to trip the
supplying overcurrent device.
Circuit overcurrent protection device:
 Circuit breaker, fuse, or relay can provide fault current clearing if the fault current is in
excess of the normal device rating.
 Most of these devices have an inverse time-current characteristics.
 To operate this device quickly it is necessary to provide a high overcurrent.
METHODS OF SENSING AND CLEARING A FAULT
TO EQUIPMENT (Contd…)

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OTHER PATHWAYS
There may be one or more fault return paths in parallel with the fault return wire
conductor.
The other paths are metallic raceways and building steel.
Metallic raceways are considered as a fault return conductor without regard to
material or size.
Raceway encircles the phase conductor, the net magnetic field is near zero inside
raceway, resulting in minimum impedance.
In buildings that have a structural steel metal frame, electrical equipment is often
fastened to the structural steel.
The structural steel is generally bonded to the earth and is a parallel path for the
fault current to return to the source.

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Issues that reduce the effectiveness of the raceway as a conductor:
 High fault currents flow on the inner surface of the raceway due to eddy currents on
the outer surface.
 Reduces the effective cross-sectional area and thus increases the impedance of the
raceway.
 High fault currents will saturate the steel and hence increasing the impedance of the
raceway.
OTHER PATHWAYS (Contd…)

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The parallel paths make the system safe in following ways:
 It reduces the effective fault return impedance.
 It reduces the fault voltage.
 It reduces the fault current through the human.
 It increases the fault current through the overcurrent protective device.
 It fasten fault clearing.
OTHER PATHWAYS (Contd…)

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CONCLUSIONS
The act of grounding does not make systems safer.
In fact, grounding minimizes the hazardous voltage due to an enclosure fault is a
myth.
The relatively safer system is one that minimizes the magnitude of the fault voltage
and/or the duration of the fault current.
This is accomplished by a combination of an adequately sized fault return
conductor and an overcurrent device with a current versus time characteristic
adequate to operate rapidly.

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REFERENCES
1. E. Rappaport, “Grounding vs. bonding—What the National Electrical Code
Does Not Explain”, IEEE Transactions on Industry Applications, Year-2014,
Volume: 50, Issue: 4, Pages: 2776-2779.
2. E.Rappaport, “Does Grounding Make A System Safe? Analyzing the factors
that contribute to electrical safety”, IEEE Industry Applications Magazine •
May|june 2015 Pages:48-57.
3. John P. Nelson ,” Improved Electrical Safety Through High Resistance
Grounding ” IEEE Transactions on Industry Applications, Year: 2015, Volume:
51, Issue: 6 Pages: 5198 – 5203.
4. S. K. Kaul; Jai Kishore , Electrical safety in india - a perspective , Electrical
Safety In Industry, 2000. Proceedings of 2000 IEEE IAS Workshop ,Year: 2000,
Pages: 42 – 47.

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5. https://en.wikipedia.org/wiki/Ground_(electricity)
6. http://electrical-engineering-portal.com/good-grounding-system
7. http://electrical-engineering-portal.com/what-is-the-difference-between-bonding-
grounding-and-earthing
8. http://ecmweb.com/power-quality/ground
REFERENCES (Contd…)

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THANK YOU