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Mitigation measures for telecommunication installations
PART 2: CASE STUDIES
| Case study # |
2.5 |
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| Title |
Damage of 2-Mbit/s HDSL system
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| Type of trouble |
Damage.
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| Source of trouble |
Mistakes in bonding.
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| System affected |
2-Mbit/s HDSL system using copper pair cable.
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| Location |
Telecom centre.
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| Keywords |
Damage, short circuit, mistakes in bonding.
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| Version date |
2004-01-01 |
| System configuration |
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In 1998, a new 2-Mbit/s HDSL (high bit rate digital
subscriber line) system using paired copper cables was put
into use by an operator after a pilot-use period. In the
summer of 1999, several damages to it occurred. Analysis
revealed that all the damaged systems had remote power
feeding, and that the damage was caused by a short circuit in
a protection component. The immediate action was to change
existing equipment in exposed areas to local power feeding
when possible, and start discussion with the manufacturer on
future action.
Tests revealed that the origin of the damages was 50-Hz
induction or earth potential rise voltages rather than
lightning transients. The manufacturer decided to re-design
the protection inside of the equipment to fulfil enhanced
level power induction and earth potential rise requirements
of Rec. ITU‑T K.20. Systems having this new design were
installed for pilot use in spring 2000 and they worked
properly all of summer 2000. However, too many systems of the
original type were again damaged in 2000. Maybe there were
cases where the instruction to change to local power supply
was followed. A field survey also revealed another reason:
improper earthing of the equipment. A practical example is
shown in Figure 2.5-1.
Figure 2.5-1 – System configuration (An
example of mistakes in bonding) |
| Mitigation method/Results/Conclusion |
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As can be seen from Figure 2.5-1, the equipment is not
installed according to the manufacturer's instructions. The
idea of the manufacturer was that the equipment, provided
with high-current inherent protection, would be earthed via
the backside of the front panel to the installation frame
that is further connected to the bonding system of the
telecom centre. In the damaged system, however, the
installation frame was painted with non-conducting paint, and
this prevented the intended contact from the backside of the
panel. Moreover, the manufacturer of the installation frame
had installed the bonding bar of the frame on insulating
supports, and although the installation personnel had
connected the bar to the bonding system, the frame was not
earthed.
This example contains several general principles of
problem solving methodology for the operator:
- When a problem arises, react immediately.
- Try to find the real reason of the problem by analysing
the condition in the field and by laboratory testing.
- Cooperate effectively with the manufacturer.
- Check whether earthing instructions have been followed
properly.
- Check that the instructions are understood and taken
into account throughout the country.
- Arrange courses for installation personnel on practical
problems and protection practices.
It also shows one particular problem of bonding: paint may
sometimes cause discontinuities. |
| References |
Recs ITU-T K.27, ITU-T K.35, ITU-T K.40, ITU-T K.56; Annex C.
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