Power-line interference

Resistivity can be measured using a four-point probe [26] placed directly on the concrete surface. The probe consists of four equally spaced point electrodes that are pressed on the surface (Figure 16.12). Usually a small amount of conducting liquid is applied locally to improve the contact between electrode tips and the concrete surface. The two outer point electrodes induce the measuring current (usually with an AC current at a frequency of between about 100 and 1000 Hz (avoiding the exact values 100, 150 Hz, etc., which may induce power line interference) and the two inner electrodes measure the resulting potential drop of the electric field. The measured resistance R is given by AE/AI and the resistivity of the concrete, p, is calculated as ... 

In the preceding example, the line-induced interference is more than 10,000 times greater than could be tolerated in a recording. It is an artifact with a sinusoidal waveform characteristic. Figure 17.28 provides an example of this type of power-line interference in an ECG recording. 

ECG 1-5 0.05-100 Ag-AgCl disposable Motion artifect, 50/60 Hz power-line interference Diagnosis of ischemia, arrhythmia, conduction defects... 

CMmeno MF, PaUas-Areny R (2000) A comprehensive model for power line interference in biopotential measurements. IEEE Trans Inst Meas 49(3) 535-540, June 2000. 

In earlier years, to reach a remote area, where. separate telephone lines had not been laid it was normal practice to rttn them through the same poles as the HT power distribution lines (generally 11-33 kV). This was particularly true of internal communications of the electricity companies for ease of operation and to save costs and time. This commitnication was known as the magneto-telephone system. But the proximity of telephone lines to power lines adversely affected the performance of the telephone lines due to generation of overvoltages (Chapter 20) and eleetrical interferences (conductive and inductive interferences, discussed later) on the telephone lines by the power lines.. Some of these interferenees, particularly system harmonics, had the same frequency as the audio frequency of the telephone lines and alTected their audio quality. 

The running of telephone lines through power lines is long discontinued. They are now run on separate structures within a city and nearby areas at audio frequency (— 0.3-3.4 kHz), and maintain enough distance from HT power distribution lines. They are therefore almost unaffected from such disturbances. Nevertheless, interferences must be kept in mind when installing these lines so that they are out of the inductive interference zone of the power lines. The latest method in the field of communications to avoid disturbances is to use underground optical fibre cables, where possible, as discussed later. Optical fibre cables are totally immune to such disturbances. 

Central Board of Irrigation and Power. India, Interference of power lines with telephone lines Technical Report No. 29. Research Scheme on Power, March (I9S2). 

Figure 15-2a shows the stray current interference by a bipolar high-voltage dc power line [7]. When the system breaks down, large voltage cones occur in the soil at the grounding installation. A few kilometers away, the current density in the soil is relatively low. 

Inductive interference the production of electrical potentials in conductors due to the induction from alternating magnetic fields arising from short-circuit currents or operational currents in high-voltage power lines. 

Capacitive interference of pipelines is of minor importance. It arises in the immediate vicinity of overhead power lines or railway power lines in the construction of pipelines where the pipe is laid on a foundation that is well insulated from soil (e.g., on dry wood). The pipeline picks up a voltage with respect to the soil. The value of this voltage depends on the voltage of the interfering conductor at the time as well as the capacities between the conductors and the pipeline. 

With aboveground, welded lengths of pipes that are insulated from the ground and that are within 10 m of overhead power lines, measures have to be taken against inadmissible capacitive interference if the following length limits are exceeded [2] ... 

Fig, 23-1 Limiting length of a parallel insulated stretch of pipe with capacitive interference by a 50-Hz, three-phase overhead power line with a nominal voltage >110 kV [2]. 

Inductive interference on pipelines is usually only to be expected with extended and close or parallel routing with three-phase, high-voltage overhead power lines, as well as with conductors and supply lines of ac railways. 

With long-term interference from operational currents of high-voltage power lines or railway power and supply lines, the contact voltage must not exceed 65 V [9]. The relevant operational current has to be stated by the operator responsible for the basic calculation. If the calculated pipeline potentials are above 65 V, connection with grounds is always necessary. 

Electromagnetic (EM) Conductivity Measures the electrical conductivity of materials in microohms over a range of depths determined by the spacing and orientation of the transmitter and receiver coils, and the nature of the earth materials. Delineates areas of soil and groundwater contamination and the depth to bedrock or buried objects. Surveys to depths of SO to 100 ft are possible. Power lines, underground cables, transformers and other electrical sources severely distort the measurements. Low resistivities of surficial materials makes interpretation difficult. The top layers act as a shunt to the introduction of energy info lower layers. Capabilities for defining the variation of resistivity with depth are limited. In cases where the desired result is to map a contaminated plume in a sand layer beneath a surficial clayey soil in an area of cultural interference, or where chemicals have been spilled on the surface, or where clay soils are present it is probably not worth the effort to conduct the survey. 

Noise can be divided into three categories (1) radiofrequency interference, microphonics, unstable main power lines and other disturbances in the environment (2) oscillator source noise, both AM and FM (3) the overall receiver noise figure. In well-designed spectrometer systems, the third category is always the limiting noise source. 

Tables VIII, IX and X demonstrate that the accuracy of the SPD is generally similar to the accuracy of the PMT. In the case of some spectral lines the SPD results were less accurate than the PMT results. This is caused by the lower resolving power of the SPD causing it to be more subject to specific spectral interference. The adverse effects of spectral line interferences resulted in relatively large concentration errors for the low analyte concentration sample. The effect of these spectral interferences is twofold. First, there is the analyte signal intensity error caused by coincident spectral line interferences from unresolved matrix lines. Second, there is the background interpolation...

The field application of electrical resistivity techniques can be affected by the presence of nearby power lines, fences, railroad tracks, and buried pipes and cables. These cultural features may create electrical interference or alter the subsurface pattern of current flow distribution. In addition, in order to complete electrical resistivity surveys you must be able to "seat" the electrodes in the ground to establish electrical continuity with the subsurface materials to be studied. 

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