Here are some of the effects that swer power lines can have on adjacent telecommunication lines from, a S African document. Easy to see why in some situations it could cause huge problems for detectors!
Background to SWER and coupling it with telecommunication linesSWER is a power delivery method in electrical distribution systems whereby the current to a load is supplied by a single overhead high-voltage conductor with an earth return path.
This method was pioneered in Australia and New Zealand in the 1940s and 1950s, and is widely used there and in other countries [3,6,9]. The voltages used in Australia are 12,7 kV and 19,1 kV, with full load operating earth return currents of the order of
15 A to 20 A. (Until the 1970s, a limit of 8 A was enforced, but this has now been relaxed.)
The first SWER schemes built in South Africa since 1988 were all built to meet the full load earth current limit of 8 A, in compliance with the earlier Australian practice. However, it has become desirable to increase this limit to the range of 15 A to 30 A maximum, to allow larger, more cost effective SWER networks to be operated in the future. A technical study has recently been done to see if this is feasible [5]; the finding from this is that maximum currents of 25 A to 30 A are feasible, provided, inter alia, that the levels of
electrostatic, electromagnetic and conductive coupling with adjacent telephone lines are kept to safe and compatible levels.
(As part of the safety assessments in any SWER scheme, step and touch potentials also have to be evaluated, but this aspect is outside the scope of this guide - see ref [18].)
The basic reason why SWER lines can cause proportionately more interference than conventional three-phase lines is that the 50 Hz full load and harmonic currents (line charging , transformer magnetizing and load distortion currents) penetrate hundreds of metres into the earth, depending on the soil resistivity and frequency; this in turn creates an inducing loop of large area that causes higher levels of steady-state electromagnetic coupling with adjacent telephone lines (detector coils as well?) than is the case with conventional three-phase lines. The resulting steady-state longitudinal (common mode) and transverse (differential mode) voltages induced in the telephone lines may then cause interference in the relevant voice-band circuits. During earth faults on the SWER line, the increased levels of earth return current cause a temporary increase in the 50 Hz longitudinal induced voltage in the telephone line, in the same way that happens during faults on three-phase lines.doug
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