International Telecommunication Union
ITU-T Study Group 05
Principles of Protection
Célio F. Barbosa
Brazil
Technical Session, Protection
Buenos Aires, 12/04/2010
Introduction
ITU-T
Study Group 05
12.04.2010
The lack of adequate protection may cause
extensive
damage
and
malfunction
in
telecommunication equipment, and may even
jeopardize human life.
TECHNICAL SESSION – PROTECTION - BUENOS AIRES
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Sources of Surges
ITU-T
Study Group 05
The threats on equipment and human beings are
caused by overvoltages and overcurrents imposed
on power and telecommunication lines, which are
generally referred as surges.
The three most important sources of surges are:
• Lightning discharges;
• Induction from power lines;
• Direct contact with power lines.
12.04.2010
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Lightning
ITU-T
Study Group 05
Lightning discharges may impose surges on
metallic lines by three distinct mechanisms:
• Direct strikes;
• Coupling due to earth potential rise;
• Electromagnetic induction.
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ITU-T
Direct
strikes
Study Group 05
12.04.2010
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ITU-T
Study Group 05
Coupling through
the earth
12.04.2010
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ITU-T
Study Group 05
12.04.2010
Coupling through
electromagnetic fields
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Power induction
ITU-T
Study Group 05
12.04.2010
Power induction is caused by power lines that run in
parallel with telecommunication lines. High values of
power induced surges are related to power lines with
earth return, such as power distribution lines based on
single-phase with earth return, and earthed three
phase lines during a phase-to-earth fault.
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Power induction
ITU-T
Study Group 05
Example of line-to-earth induced voltage in a 12 km
telecom cable in joint-use with a power distribution
line. Note the high content of 3rd harmonic.
300
200
Voltage (V)
100
0
-100
-200
-300
0
5
10
15
20
25
30
35
Time (ms)
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Power contact
ITU-T
Study Group 05
12.04.2010
Power contact is usually caused by physical failures
of power lines that share the same poles with
telecommunication lines (joint-use) or at crossings.
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Power contact
ITU-T
Study Group 05
12.04.2010
Example of telecommunication indoor cabinet
burned-down due to power contact in joint-use
outside plant.
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Principles of protection
ITU-T
Study Group 05
Surges S(u) : depend on environment and installation
Resistibility R(u) : depends on equipment design

Risk =
 S u  Ru du
Cumulative Probability (p.u.)
0
1
S(u)
R(u)
Surges
Resistibility
Risk
Risk
0
0
1
2
3
4
5
6
Overvoltages (kV)
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Principles of protection
ITU-T
To increase the equipment resistibility reduces the risk.
Study Group 05
Cumulative Probability (p.u.)
1
Surges
New resistibility
New risk
Old resistibility
Old risk
0
0
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1
2
3
Overvoltages (kV)
4
5
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13
Principles of protection
To improve the installation reduces the risk.
ITU-T
(e.g., equipotential bonding, earthing, shielding,
use of SPD, etc …)
Study Group 05
Cumulative Probability (p.u.)
1
New surges
New risk
Old surges
Resistibility
Old risk
0
0
1
2
3
4
5
6
Overvoltages (kV)
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Equipotential bonding
ITU-T
Study Group 05
12.04.2010
Although the potential of a high-voltage
conductor may be at several hundred kV above
earth potential, it is harmless to a bird …
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Equipotential bonding
ITU-T
Study Group 05
12.04.2010
… provided the bird doesn’t touch and
earthed structure!
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Equipotential bonding
ITU-T
Fortunately, aircraft manufacturers apply the
equipotential bonding concept in airplane design …
Study Group 05
As they can’t use earthing anyway!
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Earthing
ITU-T
Study Group 05
12.04.2010
The main functions of earthing are:
• Provide a safe path to the flow of surge current to
earth (e.g., from lightning);
• Reduce the current and voltage propagating along a
transmission line (e.g., a telecom cable);
• Reduce the voltage between the telecom line and
local earth;
• Provide sufficient current for the tripping of circuit
breakers in case of a power to earth fault.
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Use of SPD
ITU-T
Study Group 05
12.04.2010
The use of SPD limits the surge voltage and / or the
surge current at its location. Typical voltage limiting
devices are gas discharge tubes (GDT), varistors
(MOV), diodes and thyristors. Typical current
limiting devices are termistors (PTC), fusible
resistors, fuses and electronic current limiters.
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Next presentations
ITU-T
Study Group 05
12.04.2010
The next presentations will address in detail the
following issues:
• Equipment resistibility
• Earthing and bonding
• Surge protective devices
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Thank you!
ITU-T
Study Group 05
12.04.2010
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