Limitation of the Sheath Voltage

As mentioned in the previous section, the limitation of the sheath voltage is an important factor that decides sheath bonding and other cable system designs related to the sheath. There are two types of limitations in the sheath voltage (1) the continuous voltage limitation and (2) the short-term voltage limitation. 

The continuous voltage limitation is the limitation of the sheath voltage induced by the normal load flow in phase conductors without any faults. It is enforced by government or district regulations in many countries and differs from country to country or district to district. The limitation is enforced for safety reasons as a maintenance crew may touch the sheath circuit. Even when the limitation is not enforced by these rules, utilities follow their own standards for the continuous voltage limitation. 

The short-term voltage limitation is specified in lEC 62067 Annex G (informative) as impulse levels [14]. Considering the short-term voltage limitation, the following phenomena are studied  

Single-line-to-ground (SLG) faults (external to the targeted major section) 

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If one of the structures to be bonded is the sheath or metallic armouring of an electric supply cable, special precautions will be necessary to ensure that the voltage rise at the bond in the event of an instantaneous earth fault on the power-supply system does not endanger personnel or equipment associated with other buried structures. The bond and any associated current-limiting device should be suitably insulated and of adequate current-carrying capacity. 

One of the simplest sheath models is that of a DC high voltage ( T ) sheath that contains no electrons and in which ion flow is collisionless (space-charge limited current). The resulting sheath equation is called the Child-Langmuir law [165]. 

Numerically, the matrix sheath thickness is large and exceeds the Debye radius by 10-50 times at high voltages. A more accmate approach, the so-called Child law sheath, takes into account a decrease in the ion density due to their acceleration across the sheath. In frameworks of the model, the ion current density Jq = n eu is taken from the Child law of space-charge-limited current in a plane diode ... 

The intention of SELV is to minimise the shock hazard by voltage limitation. Subsection 411-02 sets out the parameters, which include a safe source of supply such as a safety transformer to BS 3535 and other precautions to avoid the SELV circuit becoming live at a higher voltage. If the SELV does not exceed 25 V a.c. or 60 V ripple-free d.c., i.e. not more than 10% ripple, the direct contact shock risk is regarded as negligible and exposed live parts are allowed except in locations of enhanced shock risk, such as most of those in Part 6 where the conductors have to be protected against direct contact by a barrier, enclosure or insulation. The SELV circuit is not earthed and the cables are not metal-sheathed. As conductive parts of the installation are not deliberately or fortuitously earthed, it will usually be more practicable to use insulated rather than metallic conduit and ducts. 

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