Interference with Communication Circuits- Electromagnetic Compatibility (EMC)
When power and telephone lines run in parallel, under certain conditions voltages sufficient to cause high noise levels may be induced into the communication circuits. This may be caused by electromagnetic and electrostatic unbalance in the power lines, especially if harmonics are present. The major problem, however, is due to faults to ground producing large zero-sequence currents in the power line, which inductively induce voltage into the neighbouring circuits. The value of induced voltage depends on the spacing, resistivity of the earth immediately below, and the frequency. If the telephone wires are a twisted pair or are situated close together and transposed, no voltage is induced between the communication conductors. However, a voltage can exist between the pair of wires and ground. These induced longitudinal voltages can be controlled by connecting the communication circuits to ground through an inductance which produces little attenuation at communication frequencies of 400-3500 Hz.
Capacitive coupling can occur if open communication circuits are run along the same route as power lines. Interference from underground cable circuits is much less (10%) than that from overhead lines.
Because of right-of-way constraints, telephone and power distribution lines run parallel along the same street in many urban areas. However, the interference in rural areas is often greater because communication lines and plant may be unshielded or have higher shield resistances, and unlike urban areas there is no extensive network of water and gas pipes to share the ground return currents.
Resistive coupling between power and communication circuits can exist:
- • because of physical contact between them;
- • via paths through the soil between telephone and power grounds.
Various formulae exist to calculate the value of mutual inductance-in H/m between circuits with earth return. These assume that er (soil) is unity, displacement currents are much less than conduction currents, and the length of conductor is infinite.
During line-to-ground faults, induced voltages into communication circuits may be sufficient to be a shock hazard to personnel. Although in transmission circuits, equal current loading may be assumed in the phases, this is not the case in the lower voltage distribution circuits where significant residual currents may flow. Most telephone circuit standards now require up to 15 kV isolation if communication circuits are to be connected into substations for monitoring, control and communication purposes. Particular care must be taken with bonding the sheath of communication circuits brought into buildings where the power distribution system is also bonded to earth and to the building structure.