IEEE Std 575-2014 pdf free download – IEEE Guide for the Application of Sheath-Bonding Methods for Single-Conductor Cables and the Calculation of Induced Voltages and Currents in Cable Sheaths.
4. Guidelines: Summary
4.1 Safety precautions and practices in design, construction, operation, and maintenance should be based on the principle that the voltage on the insulated sheath of a transmission cable may be considered analogous to the voltage on the con- ductor of nonshielded secondary cables.
4.2 Solidly bonded and grounded sheaths are the simplest solution to the problem of sheath volt- ages, and special bonding methods, by which are meant systems other than solidly bonded and grounded sheaths, are only justified on the basis of economics (usually at loads above 500 A) or at extremely heavy loads where all heat generation has to be minimized.
4.3 The simplest and most effective method of special sheath bonding is single-point bonding; a major disadvantage is that maximum cable length is governed by the permissible sheath standing voltage. As a result, when very low limiting values of sheath voltage are specified, this method will not be applicable. Single-point bonding may require a separate ground-return conductor whereas cross bonding does not.
4.4 Of the other bonding systems in use, cross bonding is the most widely used.
This system is especially suitable for long cable lengths.
4.5 Cable sheaths are usually expected to be nominally at ground potential but in a specially bonded system they may have appreciable volt- ages with respect to ground. Under some cir- cumstances, even solidly bonded and grounded sheaths can be well above ground potential. Some utilities allow sheath standing voltages as high as 300 V. With present-day jacket materials, sheath voltages of 600 V are possible.
4.6 Complete suppression of circulating sheath currents may not always be possible because of practical difficulties in the choice of cable lengths and spacings. It may then be necessary to calcu- late these residual sheath currents and assess their effect on the cable rating.
4.7 The use of special bonding gives rise to sheath overvoltages during system transients and faults and the values of these overvoltages must be considered. For higher voltage systems a sheath- voltage-limiting device is needed, and in all cases consideration must be given to the coordination of the sheath insulation levels in relation to the overvoltages to which this insulation will be subjected.
4.8 Failure of a part of the sheath insulation or of a sheath voltage limiter may result in considera- ble sheath currents and losses and hence “may cause overheating of the cables. Consideration must therefore be given to the duty imposed on the sheath-voltage-limiting device and to the moni- toring and maintenance of the complete systems in operation.
5. Bonding Methods
5.1 Introduction. The sheath of a single-conduc- tor cable for ac service acts as a secondary of a transformer; the current in the conductor induces a voltage in the sheath. When the sheaths of single-conductor cables are bonded to each other, as is common practice for multiconductor cables, the induced voltage causes current to flow in the completed circuit. This current causes losses in the sheaths. Various methods of bonding may be used for the purpose of minimizing sheath losses. Formerly, where special bonding was employed for the prevention of sheath losses on lead- sheathed cables without an insulating jacket, the sheaths were subjected to ac voltages, and the bonding was designed to keep the magnitude of the induced voltages within small limits so as to prevent the possibility of sheath corrosion due to ac electrolysis.
Various levels of permissible sheath voltage to ground were proposed at certain times, ranging from 12 V to 17 V, to prevent corrosion due to electrolysis. At the present time, cables are usually manufactured with an insulating jacket, so that induced voltages no longer constitute a corrosion problem, and voltages comparable to secondary cable voltages may be acceptable. The problem of sheath losses becomes particu- larly important when large, single-conductor cables comprising a circuit are placed in separate ducts, or spacing between directly buried cables is increased to reduce the effects of mutual heating, as significantly higher voltages are induced in the cable sheaths. The major purpose of special sheath bonding for single-conductor cables is the preven- tion or reduction of sheath losses.IEEE Std 575 pdf download.