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Mitigation measures for telecommunication installations
PART 2: CASE STUDIES
| Case study # |
3.1 |
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| Title |
Power and telecommunication systems grounding
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| Type of trouble |
Abnormal operation, damage.
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| Source of trouble |
Intermittently connected power neutral.
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| System affected |
Customer's equipment – occasional continuous ring tone (no cadence) sometimes resulting in damaged customer apparatus.
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| Location |
Customer premises.
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| Keywords |
Safety, power transmission line, GDT, SPD, earth electrode
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| Version date |
2004-01-01 |
| System configuration |
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Normal PSTN line, isolated/exposed location on very high
resistivity soil. Power feed and telecoms line to premises
via aerial cables. The area was prone to lightning activity,
and surge protective devices (SPDs) were fitted at the
customers' premises to the telecom line (bonded to the power
system earth). |
| Searching techniques
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All measurements on the telephone line
indicated that the line was well within specification and
that there were no faults with it. The line-card at the
exchange was replaced, just in case the fault was due to a
'sticky' relay or faulty subscriber line interface circuit (SLIC).
The line was no better. Speaking with the customer revealed
that the power to the premises appeared to fluctuate at
times, sometimes just dimming, and other times going out
completely for a few seconds. The power company had been
asked to check the supply quality and claimed it was all OK.
The last few instances of continuous ringing caused the SPD
mains fail-safe to operate, resulting in an engineer
call-out. The engineer put this down to lightning activity,
but was later told that this was unlikely, so the
investigation continued. Measurements of the earth electrode
resistance at the customer premises revealed it to be higher
than expected (note, the power supply necessitated that an
earth electrode be used). |
| Mitigation method/Results/Conclusion |
|
Due to the power fluctuations and the SPD going S/C, it
was thought that the problem was likely to be due to system
supply. The only possibility was if the neutral became
temporarily disconnected, and as the earth electrode
resistance was not particularly good, the return current
route was via the telephone line.
As the power company could not immediately find any fault,
the customer SPD was moved out of the premises, to a telecom
pole about 25 m away, and a suitable earth electrode made.
This still gave a good level of protection for lightning, and
also removed the continuous ringing problem. The increase in
resistance between the power earth and the telecom earth was
enough to avoid the power return current flowing into the
telecom line. Whilst the solution was not ideal (possibly
giving rise to unwanted EPR during lightning) it has not
resulted in any further damage due to the power of lightning.
The power companies were informed of our findings and
subsequently renewed part of the route. |
| References |
Rec. ITU-T K.37, Annexes A and B.
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