Cathodic protection from cables

In the cathodic protection of asymmetrically connected communication cables, distortions are coupled into the transmission lines coming from the ripple of the sheath current. In this case also, limiting the residual ripple to 5% is usually sufficient. 

A direct current flows in the installations during operation of cathodic protection stations therefore, the transformer-rectifier must be switched off when pipes are out or other work on the fuel installation is carried out, and the separated areas must be bridged with large cross-section cables before the work is started in order to avoid sparking that could come from the current network. 

For several years now, cable ducts have been manufactured from plastic pipes, which are watertight and form a continuous run of piping. In laying the ducts, low points can occur in which condensed water or water penetrating from the ends can collect. In many cases this water has led to corrosion damage in lead-sheathed cables. Lead-sheathed cables must therefore only be used in such ducts with an additional PE sheath of type A-PM2Y. Cathodic protection of these cables is not possible because of their complete insulation by the plastic pipe. 

The switching-off method for 7/ -free potential measurement is, according to the data in Fig. 3-5, subject to error with lead-sheathed cables. For a rough survey, measurements of potential can be used to set up and control the cathodic protection. This means that no information can be gathered on the complete corrosion protection, but only on the protection current entry and the elimination of cell activity from contacts with foreign cathodic structures. The reverse switching method in Section 3.3.1 can be used to obtain an accurate potential measurement. Rest and protection potentials for buried cables are listed in Table 13-1 as an appendix to Section 2.4. The protection potential region lies within U[[Pg.326]

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. 

Test coupons of steel of a specified size are buried near the pipeline and connected by cable at the test point with the cathodically protected pipeline. They simulate artificial defects in the coating. The protection current taken from the test coupon can be measured via the cable connection and the true potential determined from a reference electrode in front of the test coupon by momentarily interrupting the cable connection [28]. Ohmic potential drops between the reference electrode and the test coupon are obtained from a measuring test probe that has a built-in reference electrode on the back of it to measure the 7/ -free potential directly [see Eq. (2-34) with 5 0] without having to switch off the protection... 

Lead-sheathed cable near cathodically protected pipelines must be connected to the cathodic protection of the pipeline since they will otherwise suffer interference from the protection current (see Section 9.2). Figure 13-5 shows the current requirement for a 10-in length of cable and the potential distribution of such cables. 

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