Earthing Systems1

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LV Earthing Systems.

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Transcript of Earthing Systems1

LV Earthing Systems. Presentation to Dar Al Handasah Consulting Engineers, Cairo, Egypt. December 29, 2003Definition
“Contact of persons or livestock with live parts which may result in electric shock”
Division - Name - Date - Language
Low Voltage Earthing Systems
Definition:
“Contact of persons or livestock with exposed conductive parts made live
by a fault and which may result in electric shock”
Protection of people, indirect contact
Low Voltage Earthing Systems
Division - Name - Date - Language
with automatic disconnection of supply
Earthing of all the exposed conductive parts of electrical equipment and all accessible conductive parts
2 simultaneously accessible exposed conductive parts must be connected to the same earth electrode
Automatic disconnection by a protective device of the part in which a dangerous insulation fault occurs
The protective device must operate within a time that is compatible with "Maximum Touch Voltage & Time-Safety requirements"
Protection of people according to IEC 364
Low Voltage Earthing Systems
Division - Name - Date - Language
Time ms/current mA curve for AC current from
15 to 100 Hz
Low Voltage Earthing Systems
Body Impedance = 2000 Ohms.(Dry)
UL (MAX. TOUCH VOLTAGE) = 2000x 0.025 = 50 V (Dry Conditions)
= 1000x0.025 = 25 V (Wet Conditions)
Zone 1 : perception
c1-c2 :prob. increases by 5%
c2-c3 :prob. increases by 50%
> c3 :prob. more than50 %
Division - Name - Date - Language
Standard IEC 60479-1
Effect of frequency
The human body is most sensitive to frequencies in the 50 Hz/60 Hz range
Current-sensitivity thresholds
with Automatic Disconnection of Supply
Maximum Touch Voltage Time (Protection of people according to IEC 364)
Low Voltage Earthing Systems
prospective maximum protective device
< 50 5 5
50 5 5
75 0.60 5
90 0.45 5
120 0.34 5
150 0.27 1
220 0.17 0.4
280 0.12 0.3
350 0.08 0.2
500 0.04 0.1
Maximum touch voltage time in UL = 25 V conditions (sockets/wet areas)
prospective maximum protective device
25 5 5
50 0.48 5
75 0.30 2
90 0.25 0.80
110 0.18 0.50
150 0.12 0.25
230 0.05 0.06
280 0.02 0.02
The Three Earthing Systems
I = Neutral unearthed or
N = Frames connected to the supply
point which is earthed,
Or combined with the Neutral (C)
1.
2.
3.
Neutral conductor (N)
Protective conductor (PE)
Combined protective and
neutral conductor (PEN)
Definition
The neutral point of the LV transformer is directly connected to an earth electrode
Low Voltage Earthing Systems
Division - Name - Date - Language
Definition
The neutral point of the LV transformer is directly connected to an earth electrode
The exposed conductive parts of the installation are connected to an electrically separate earth electrode
Low Voltage Earthing Systems
Division - Name - Date - Language
"Deep" earth
1000 km
1 Ω
15 Ω
10 Ω
10 Ω
5 Ω
"Deep" earth
11 Ω
Well designed network

o equal to unbalanced load
currents and/or 3rd order

o equal to unbalanced load
currents and/or 3rd order
Measurement of current IPE can be used
o for protection of persons (values depend on the earthing arrangement)
o protection against fire hazards
However, it is necessary to detect the "true" IPE

The fault current generates
a dangerous touch voltage
The SCPD is usually not suitable for eliminating this type of fault
E95420
Load
L1
L2
L3
N
suitable for this type of fault
(ST setting at 25 A)
A residual current device
Tripping conditions:
Ru x IDn < UL
IDn = UL / Ru
standard IEC 60479-1 into tables presenting the disconnecting-time versus the nominal AC-voltage (Uo)
From table 41 A of standard IEC 60364
TT system
Maximum disconnecting times
50 V < Uo £ 120 V 120 V < Uo £ 230 V 230 V < Uo £ 400 V Uo > 400 V
Disconnecting time (s) AC DC AC DC AC DC AC DC
TT system 0.3 5 0.2 0.4 0.07 0.2 0.04 0.1
System earthing arrangements
– auxiliary supply required
RCDs are immune
Division - Name - Date - Language
Earth leakage protection
separate from the short-circuit protection device
– auxiliary supply
Operating principle of residual current devices requiring no auxiliary supply (electronic)
Detection
Measurement
Tripping
SEAs and devices
Application: protection of life and property in all sectors (industrial, commercial and residential)
Main characteristics: continuity of service and safe if neutral
conductor is cut
Selection of solutions
SEAs and devices
Electronic technology for power distribution
Application: general protection from the main low voltage switchboard to the secondary switchboard in industrial and large commercial buildings
Main characteristics:
o miniaturisation
E37541
Selection of solutions
SEAs and devices
SEAs and devices
– time-delay settings
RCD1 > RCD2
Caution. For an RCD not integrated in the SCPD, RCD2 disconnecting time = tripping time + time delay
horizontal discrimination
Dt (RCD1) > Dt (RCD2) + Dt (CB2)
(including the disconnecting time)
To implement condition , it is necessary to know the total breaking
time guaranteed for the CB2 + RCD2 combination or to run tests
on the combination
SEAs and devices
setting (RCD1) setting (RCD2) +1
Applications
is combined with a circuit breaker/switch disconnector from
the Multi 9 or Compact ranges
Division - Name - Date - Language
Discrimination rules with Vigirex upstream
Two conditions: IDn (RCD1) > 1.5 IDn (RCD2) setting (RCD1) setting (RCD2) +1
Applications
CB1
RCD1
CB2
RCD2
E94442
Vigi
RCDs
is combined with a circuit breaker/switch disconnector from
the Multi 9 or Compact ranges
Division - Name - Date - Language
Protection of persons:
o fault current is dangerous
o fault current is too weak to trigger the short-circuit protection devices
o protection must be practically instantaneous
It is provided by a specially designed RCD device
Fire protection:
o "naturally" managed by RCDs for the protection of persons
Continuity of service:
System earthing arrangements
transformer is directly connected
to an earth electrode
The exposed conductive parts
of the installation are connected by the PE to the same earth electrode
E95416
L1
L2
L3
N
PE
Rn
are separate
for both the PE and the neutral conductors (PEN)
L1
L2
L3
PEN
Rn
(with PEN)
(with PE and N)
Note. A TN-S system may not be used upstream of a TN-C system
E95424
Rn
L1
L2
L3
N
PE
L1
L2
L3
PEN
The fault current is equal to a Ph/N short-circuit
Consider the PH & PE Conductor are Copper, 50 m Long with a X-section of 35 mm2. The Fault Current
Id =U0/(RPE +RPH)
Id = 230/(2 x 0.3214) = 3578 A.
This Fault Current will generate a Touch Voltage
Uc = RPE x Id = 3578 x 0.03214 = 115 V.
Since the fault current depends on the Length of the Lines, it is necessary to check that that the Fault Current is more than the Protection Operating Threshold of the CB i.e Id > Ia
E95425
Id = 0.8.Uo. SPH
L < Lmax
r.(1 + m).Ia
If the length of the conductor is greater than Lmax., it is necessary to;
Reduce Ia.
Increase Spe.
Protection of the neutral
disconnected
and the exposed conductive
to the same earth electrode
E95427
L1
L2
L3
*
50 V < Uo £ 120 V 120 V < Uo £ 230 V 230 V < Uo £ 400 V Uo > 400 V
Disconnecting time (s) AC DC AC DC AC DC AC DC
TN system 0.8 5 0.4 5 0.2 0.4 0.1 0.1
The disconnecting time depends on the distribution-system voltage Uo
TN system
System earthing arrangements
aluminium), the fault
of the conductors
circuit breaker protection:
E95442
E95449
SEAs and devices
Low Voltage Earthing Systems
The max. Length of any circuit of a TN-earthed installation is
0.8.Uo.Sph
Sph= Cross-Sectional area of Ph. Cond.
=resistivity in Ohm-mmsq/metre (22.5 mohm for Cu)
m = Ratio between Sph and SPE
Ia = Trip Current setting for Inst. Operation of CB.
If the condition is not met
reduce the magnetic setting
Increase Cross-Sectional area of the Cond.
see pg. G20 for Tables.
Division - Name - Date - Language
or
or
Protection by short-circuit protection devices
Devices for the TN-S system
SEAs and devices
o a low insulation fault can cause a short-circuit
o an RCD with a current setting between 3 and 30 A avoids this risk
Applications
The fault current generates a dangerous touch voltage
The circuit breaker trips
Check the loop impedance
Earth-leakage protection set to 300 mA is recommended if there is a risk of fire
E95428
Rn
400 V/230 V
and ST upstream protection
SEAs and devices
Division - Name - Date - Language
Devices for the TN-S system
TN-S / 4P 3D, 4P 4D
SEAs and devices
Division - Name - Date - Language
fault current is usually high enough to trip the SCPDs
tripping must be practically instantaneous
It is ensured by the magnetic settings on the SCPDs
if the fault current is not high enough, RCDs may be used to ensure
protection
Continuity of service:
E37542
fault current is dangerous
fault current is usually high enough to be tripped by the SCPDs
tripping must be practically instantaneous = same as TN-S
It is ensured by the magnetic settings on the SCPDs
if the fault current is not high enough, the installation must be resized
Fire protection:
cannot be provided (TN-C not allowed where there is a risk of fire)
Continuity of service:
TN-C
TN system
transformer is Isolated, not connected to an earth electrode
The exposed conductive parts
by the PE conductor
E95429
Definition
transformer is Isolated and not connected to an earth electrode
The exposed conductive parts
by the PE conductor
L1
L2
L3
N
PE
PE
PE
*
Under Normal operation, the System is earthed by its System Leakage Impedance.
E95431
L1
L2
L3
PE
Uc=10 x0.065= 0.6V
The fault does not cause
tripping but it must be indicated
E95432
L1
L2
L3
PE
sequence signal
sequence signal
Fault-clearance principle:
outgoer
E95434
L1
L2
L3
PE
System earthing arrangements
(*) Insulation Resistance
E95435
L1
L2
L3
N
PE
RI
e
SEAs and devices
Merlin Gerin range
Devices for first fault
1 70 mA
5 360 mA
30 2.17 A
SEAs and devices
Standardised rule
IEC 60364-5-53: The RCD current settings must be greater than twice the first-fault current
IDn setting
300 mA
1 A
5 A
E95437
Study of the 2nd earth fault
The SCPD protection trips
the same circuit breaker
as for TN-S, mais
– 4P 4t is compulsory
Check the loop impedance
Merlin Gerin circuit breakers
are appropriate for protection
(length of conductors)
E95438
L1
L2
L3
N
CPI
Maximum disconnecting times for IT systems for the 2nd fault
Protection plan for second fault
SEAs and devices
50 V < Uo £ 120 V 120 V < Uo £ 230 V 230 V < Uo £ 400 V Uo > 400 V
Disconnecting time (s) AC DC AC DC AC DC AC DC
IT system 0.8 5 0.4 5 0.2 0.4 0.1 0.1
Drawn from table 41 A of standard IEC 60364
IT system
Protection of persons:
Protection is ensured by the IT system itself, however
a maintenance strategy is required
A second fault is dangerous and protection must be ensured
by the magnetic setting of the SCPD ’s or the RCDs
Fire protection: the fault current is close to zero
Continuity of service is total
SEAs and devices
First-fault touch voltage is very weak
Dangerous touch voltage in the event of a double fault
Tripping after the second fault
Optimal safety when first fault occurs
Continuity of service when first fault occurs
Use of IMD for fault tracking
Check on tripping conditions
Calculations necessary for extensions
First fault touch voltage very weak
Dangerous touch voltage if there is a double fault
Second fault tripping
continuity of service when first fault occurs
use of PIM for fault tracking
checking of tripping conditions
calculations necessary for extensions
Low Voltage Earthing Systems
Division - Name - Date - Language
Protection of people XXXX XXX XX XXXX
Protection against Fire XXXX XXX X XX
Ease of Implementation XXX X X X Continuity of service XX XX XX XXXX
Upgradable installation XXXX XX XX XX
Cost Saving XX XXX XXXX X
XXXX=Excellent XXX=Good XX=Average X=Caution
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