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Mitigation measures for telecommunication installations
PART 2: CASE STUDIES
| Case study # |
1.3 |
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| Title |
Acoustic noise troubles caused by switching noise of an
elevator (lift) |
| Type of trouble |
Acoustic noise. |
| Source of trouble |
Elevator. |
| System affected |
Customer's equipment. |
| Location |
Customer premises. |
| Keywords |
Elevator, mutual coupling, switching noise.
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| Version date |
2004-01-01 |
| System configuration |
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Acoustic noise trouble occurred at a customer premises. The
customer complained that the acoustic noise could sometimes
be heard; however, it was not clear what the trouble source
was. The system configuration of this issue is shown in
Figure 1.3-1. At the customer premises, the telecom lines and
power mains cable for the elevator (lift) were parallel in
the cable duct. The motor of the elevator used a switching
power source. The telecom line connected to the telephone set
was also connected to the MDF where the power transformer was
located. The telecommunication line was changed; however, the
acoustic problem continued.
Figure 1.3-1 – System configuration
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| Measurement/Searching techniques/Experiment |
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To solve this acoustic problem, the
current in the telecommunication line was measured (Figure
1.3‑2). The common mode current wave shape is shown in Figure
1.3-3. A typical period 200‑300 μs periodic characteristic
can be seen in the figure. Its major frequency is about 5 kHz
and it was estimated that the noise source must be a
switching power source. The investigations showed that the
power mains cable was wired parallel to the telecommunication
line, and that the telecommunication line was connected to
the main distribution frame (MDF) room, where the mains power
transformer was located.
Figure 1.3-2 – Measurement of noise current
Figure 1.3-3 – Current wave shape on the line connected to
the telephone
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| Mitigation method/Results/Conclusion |
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The acoustic problem originated in the
mutual coupling of the switching noise of the motor. To solve
this problem, it was decided to apply a common mode filter to
the telecommunication line and an isolation transformer to
the mains power line (see Figure 1.3-4). The isolation
transformer with a filter was attached to the mains power
line, and the filter was located as near as possible to the
motor, so as to prevent the switching noise from travelling
along the cable. The common mode filter at the MDF was
attached to the telecommunication line.
Figure 1.3-4 – Applied mitigation using
common mode filter |
| Referencess |
Rec. ITU-T K.37; Annexes A and B. |
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