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Mitigation measures for telecommunication installations
PART 2: CASE STUDIES
| Case study # |
2.2 |
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| Title |
Lightning surge troubles on CSM
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| Type of trouble |
Damage.
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| Source of trouble |
Lightning surge.
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| System affected |
Transmission equipment.
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| Location |
Telecom centre.
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| Keywords |
Damage, lightning surge.
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| Version date |
2004-01-01 |
| System configuration |
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The system configuration and estimated lightning surge
current flows are shown in Figure 2.2-1. The figure shows two
telecom buildings in the same area. One telecom building has
a high-altitude antenna tower with coaxial cables connecting
it to building B. The two buildings' earthing systems were
separated from each other, and the power mains were fed by
different routes. However, a clock line was located between
the two buildings, as shown in Figure 2.2-1. The antenna
tower was not used at this installation.
The investigation determined that a lightning surge
entered via the power mains of building A and that a power
fault had occurred near the buildings. The surge current
entered building A, flowing to its earthing. The increase in
potential was caused by the lightning surge current.
Buildings A and B were connected by the 64‑kHz clock line;
however, the earthing systems of the two buildings were
independent from each other. Therefore, there was potential
difference between a clock unit of the telecom equipment (CLK
Unit), in Building A, and a clock interface of the CSM (Clock
Supply Module) in Building B. As a result of surge current
flows, the two equipments were damaged. It was estimated that
the surge current went through the CSM to earthing E0.
Figure 2.2-1 – System configuration and estimated surge
current flows |
| Mitigation method/Results/Conclusion |
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The investigation revealed that this damage was due to the
potential difference between the two buildings. Therefore,
bonding of earthing systems should be used to reduce the
potential difference, although the residual voltage
difference might still remain. Therefore, it was recommended
to insert an isolation transformer in the clock line, as
shown in Figure 2.2-2. Additional mitigation measures were
taken: the antenna tower that is not in use was removed; and
to achieve an equipotential earthing system, earthings E0,
E1, E3, and EMDF are connected to each other.
More detailed consideration should be given to connecting
earthing networks when dealing with power systems of IT or
other systems. The earthing resistance of IT systems is too
high to absorb power fault current. If the earthing networks
of several buildings are connected to each other, the total
earthing resistance is lower than the original individual
one. Therefore, national regulations or the relative
Recommendations should be referred to.
Figure 2.2-2 – Mitigation configuration |
| References |
Recs ITU-T K.27, ITU-T K.35, ITU-T K.40, ITU-T K.56; Annex C.
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