Protection stray currents

In very built up areas, telephone cables are laid in ducts that mostly consist of shaped duct blocks. This means that cables running parallel to tramway tracks cannot be avoided. Metal-sheathed cables with poor coating, or with none at all, are then heavily exposed to stray currents from the tramway [5,6]. 

Measurement of the cable sheathing/soil potential can be used to assess the corrosion danger from stray current interference (see Section 15.5.1). Since the measured values vary widely and the stray currents cannot be switched off, 71 -free potential measurements are only possible with great effort. In order to keep the IR term of the potential measurement low, the reference electrode must be placed as close as possible to the measured object. With measurements in cable ducts (e.g., underneath tramway tracks), the reference electrodes can be introduced in an open duct. 

Direct current installations that are grounded in several places cause stray currents in the soil which can interfere with other installations (see Section 9.2). All dc railways are sources of stray currents. Protection methods that can be applied in the same way to cables are described in Chapter 15. 

All protection measures must be checked and monitored at regular intervals to guarantee their effectiveness. To keep the labor cost as low as possible, the time interval between inspections should be as long as possible. This requires that the components of the installation, such as transformer-rectifiers, which can often fail 

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Where there are stray currents, the switching method described in Section 3.3.1 cannot be used. Stray current protection stations are usually installed where the pipeline has the most positive pipe/soil potential. When the stray current drainage is cut off, a too-positive stray current exit potential that is not 7/ -free is quickly established. In distant areas a too-negative stray current entry potential that is not 71 -free will be measured. The determination of the 71 -free pipe/soil potential is only possible in stray current areas when the origin of the stray current is not oper-... 

According to Ref. 32, the functioning of impressed current cathodic protection stations should be monitored every 2 months, and the stray current protection station every 1 month. If protection installations are provided with measuring instruments for current and potential, this supervision can be carried out by operating staff, so that the readings are recorded and sent to the technical department for... 

Stray Current Interference and Stray Current Protection... 

Combined Stray Current Protective Measures in Urban Areas... 

The cathodic protection of reinforcing steel and stray current protection measures assume an extended electrical continuity through the reinforcing steel. This is mostly the case with rod-reinforced concrete structures however it should be verified by resistance measurements of the reinforcing network. To accomplish this, measuring cables should be connected to the reinforcing steel after removal of the concrete at different points widely separated from each other. To avoid contact resistances, the steel must be completely cleaned of rust at the contact points. 

Since stray current corrosion damage can occur after only a few years, the economy of stray current protection measures is obviously not questionable [12], In Fig. 22-3 the effect of stray currents is shown by curve 2 [14]. Without there being firm evidence, it is apparent that the shape of the corrosion damage curve in steel-reinforced concrete (see Sections 10.3.6 and 4.3) is similar to that for stray current corrosion [15]. 

Further chapters cover in detail the characteristics and applications of galvanic anodes and of cathodic protection rectifiers, including specialized instruments for stray current protection and impressed current anodes. The fields of application discussed are buried pipelines storage tanks tank farms telephone, power and gas-pressurized cables ships harbor installations and the internal protection of water tanks and industrial plants. A separate chapter deals with the problems of high-tension effects on pipelines and cables. A study of costs and economic factors concludes the discussion. The appendix contains those tables and mathematical derivations which appeared appropriate for practical purposes and for rounding off the subject. 

Static electricity, lightning, and stray current protection... 

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