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Mitigation measures for telecommunication installations
PART 2: CASE STUDIES
| Case study # |
2.9 |
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| Title |
Malfunction of circuit breaker due to lightning surges
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| Type of trouble |
Abnormal operation.
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| Source of trouble |
Lightning.
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| System affected |
Access system.
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| Location |
Outdoors.
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| Keywords |
Safety, immunity, lightning, power transmission line, circuit breaker, MOV.
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| Version date |
2004-01-01 |
| System configuration |
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The circuit breakers in the power units of an access network
system located outdoors malfunctioned due to lightning. The
access network system had an optical fibre cable, which was
connected to centre equipment, a metallic subscriber cable,
and an AC mains line (Figure 2.9-1). The system had four
power units. Each unit had a circuit breaker, with a
capacitance of 5 A at the AC mains frequency. Figure 2.9-2
shows the system's protection circuits. The power units are
produced by two manufacturers. Their circuits were almost the
same, but they had different malfunction occurrence
probabilities.
Figure 2.9-1 – Access network system
Figure 2.9-2 – Structure of overvoltage
and overcurrent protection |
| Measurement/Experiment |
1) Observation
The lightning surge currents in common mode and
differential mode were observed in ten systems where
failure often occurred. Two types of waveform and
occurrence probability were obtained. The data are
summarized in Table 2.9-1 and Figure 2.9-3. Malfunctions
occurred with both waveforms. The peak current values were
about 30 and 300 A, respectively.
Table 2.9-1 – Observed waveforms
Figure 2.9-3 – Cumulative occurrence
probability for differential mode currentt
Noise immunity and lightning surge were tested.
Malfunction did not occur in the immunity test. Also,
several lightning surge generators (e.g., 10/700 ITU-T and
1.2/50 combination) and coupling/decoupling methods were
tested (Figure 2.9-4). It was necessary to test the
coupling and decoupling units to confirm the induced
waveform and current value, which depended on them (Figure
2.9-5). The results are shown in Figure 2.9‑6.
Figure 2.9-4 – Example of experimental
set-up
Figure 2.9-5 – Waveform difference by
coupling methods
Figure 2.9-6 – Experimental results |
| Mitigation method |
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This malfunction was caused by two mechanisms. One was the
lightning surge current flowing into the circuit breaker when
MOV3 operated. In the field, the 15-A fuse did not break
often, but the breaker did trip. The relationship between the
lightning and operation time is shown in Figure 2.9-7. A new
circuit breaker, whose characteristics at the AC mains
frequency were the same, was developed and the response
characteristics to lightning were improved.
The second mechanism was the malfunction of the pulse
width modulator (PWM) controller, which has an overcurrent
latch circuit consisting of a current transformer and filter
circuits, as shown in Figure 2.9-8. The filter circuits
(e.g., R114, R115, R34, RV1, C38) were from different
manufacturers. In particular, R114 and R144 determine the
sensitivity of CT1 and CT4, so their resistances were set
smaller, and this improved the immunity level against
lightning surges.
Figure 2.9-7 – Operation time and energy
Figure 2.9-8 – PWM controller and filter |
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