EARTHING SYSTEM
By Engineer Ahmed Mahdy
EFFECT OF AN AC OR DC CURRENT ON HUMAN
BODY
• There are three basic factors that determine what kind of shock you
experience are
1. Amplitude of the current.
2. Duration of the current passing through the body.
3. Frequency.
4. Direction of current(path).
EFFECT OF ELECTRIC HAZARDS ON HUMANS
AC Current
Body Effect
1 mA
Slight sensation of current (threshold).
5 mA
Maximum current which is still harmless.
10-20 mA
Loss of muscle control, can’t let go.
50 mA
Difficulty of respiration.
100-300 mA
Breathing stops and is often fatal.
1000-6000 mA
Internal organs and tissues start to burn
EFFECT OF CURRENT TIME
Current
Time
Effect
0-0.5 mA
Continuous
No effect
0.5-5 mA
Continuous
Involuntary muscle
contraction but no harmful
effect on human
5-30 mA
Minutes
Muscle contraction and
difficulty of separation from
circuit
30-50 mA
Seconds
Difficulty in breathing and loss
of consciousness
More than 100 mA
More than 20 milliseconds
Death
TO CALCULATE MAX CURRENT WITHSTAND BY
HUMAN FOR TIME
For Example for time of 10 second, the current which human can
withstand during this time without any damage is 36.68 mA.
FREQUENCY OF CURRENT
• Either AC or DC currents can cause damage of the heart at high enough levels.
• This typically takes place at 30 mA of AC (rms, 60 Hz) or 300 – 500 mA of DC.
• This means more DC current required to produce same effect as AC.
• Low frequency AC is more dangerous than high frequency AC.
DIRECTION OF CURRENT
1. Foot to foot.
2. Hand to hand.
3. Left hand to feet.
4. Right hand to feet.
• Worst case is hand to hand and left hand to feet as both of them pass
through the heart
TYPES OF ELECTRICAL HAZARDS
1. Direct hazards as result of direct contact.
2. Indirect hazards as result of insulation failure and leakage current.
PROTECTION AGAINST SHOCKS
PROTECTION AGAINST DIRECT SHOCKS
1. Insulation of live parts.
2. Protection by barriers or enclosures.
3. Protection against direct contact by residual current device.
PROTECTION AGAINST INDIRECT SHOCKS
1. Earthing
WHAT IS EARTHING ?
• The process of transferring the immediate discharge of the electrical energy directly
to the earth by the help of the low resistance wire.
• The electrical earthing is done by connecting the non-current carrying part of the
equipment or neutral of supply system to the ground.
• Metallic frame of the machines is connected to the earth.
COMPARISON BETWEEN WITH AND WITHOUT
EARTHING
CLASSIFICATION OF EARTHING SYSTEMS
• TN system.
• TT system.
• IT system.
• First letter for source relation with earthing, if it is “T” , it is connected to earthing.
• “I” means no connection with earthing.
• Second letter for load relation with earthing, if it is “T”, enclosure is connected to
earthing.
• If “N”, it means that enclosure has relation with neutral.
TT SYSTEM
• Simplest solution to design and install.
• Requirement of RCD.
TN SYSTEM
• No RCD required unless the cable length very long.
• Circuit breaker will trip the circuit.
TN-C System
TN-S System
IT SYSTEM
• Solution offering the best continuity of service during operation.
• Best used in hospitals so the machine will operate despite the insulation failure.
• Insulation monitoring device (IMD) to detect insulation failure.
• Expensive.
COMPONENTS OF EARTHING SYSTEM
1. Type of soil.
2. Earthing electrodes.
3. Earthing conductors.
4. Accessories and termination fittings, bonding, welding kits and other materials
FACTORS AFFECTING EARTHING RESISTANCE
1.
2.
3.
4.
5.
Composition of soil as clay,sand,stone,…..
Moisture content of soil.
Temperature of soil.
Depth of soil.
Number of electrodes.
SOIL RESISTIVITY
HOW TO DECREASE SOIL RESISTIVITY IN POOR
SOIL?
1. By using chemical treatment of soil by adding salts.
2. We form holes at distance of 10 cm from the electrode and at depth
of 30 cm.
3. We fill these holes with copper sulfate, magnesium sulfate, or sodium
chloride.
EARTHING ELECTRODE
1. Material can be galvanized iron or copper or
any highly conducting material.
2. Material of the earthing electrode should be
the same as earthing conductor.
EARTHING CONDUCTOR
1. Earthing conductor material can be copper, aluminium or steel.
2. Galvanized iron or galvanized steel is a good choice since buried pipelines and
building structures are also steel.
3. This combination will not cause corrosion due to dissimilar metals.
SELECTION OF EARTHING CONDUCTOR
1. I is the worst case current
or the three phase short
circuit current.
2. t is the time which
conductor can sustain the
short circuit current before
circuit breaker operation.
3. K or alpha can be
obtained from tables.
SELECTION OF EARTHING CONDUCTOR
Example
Calculate the cross-sectional area for earthing conductor used for earthing 1.5 MVA
transformer, knowing that the Xp.u for the transformer=0.05, line voltage=380V.
T1=52. T2=240, alpha=226, Beta= 234.5.
Solution
MVAsc=MVAbase/Xpu=1.5/0.05=30 MVA.
Isc=MVAsc/(√3x0.38)=30/(√3x0.38)=45.58 KA.
Cross-sectional area=6*√𝑡xIsc=6* √1x45.58=273.48 mm2.
So Area required=300 mm2 nearest standard.
RESISTANCE OF EARTHING CONDUCTOR
CALCULATION OF EARTHING RESISTANCE
PARALLEL ELECTRODES ARRANGED HOLLOW
SQUARE
PARALLEL ELECTRODES ARRANGED IN LINE
PARALLEL ELECTRODES ARRANGED EQUILATERAL
TRIANGLE
OVERALL SYSTEM RESISTANCE
IN CASE THE RESISTANCE IS GREATER THAN
REQUIREMENT
1. Increase length of electrode.
2. Increase diameter of electrode.
3. Increase number of electrode.
4. Adding salts to soil
MEASUREMENT OF EARTH RESISTIVITY
BY MEGGER OR AUTO EARTH TESTER
MEASUREMENT OF EARTH RESISTIVITY
BY 3 POINT METHOD