BS 7197:1990 pdf free.Specification for Performance of bondsfor electric power cableterminations and jointsfor system voltages upto 36 kV.
BS 7197  Power cables used for the distribution of electricity aregenerally provided with a metallic envelope(s [protective conductor(s)]. The purpose of this envelope is:
a) to contain the electric field around the cores where there isno dielectric screen;
b) to ensure that in the event of a failure of the insulation ofthe cable, a circuit is provided to enable current to flow
through the fault to the neutral point of the system supplyingthe cable which is usually connected to the mass of earth;c) to carry fault current, derived from a failure of insulation inelectrical plant that is connected to the cable, to the neutralpoint of the system supplying the cable;
d) for use, in certain designs of low voltage cable, as a neutralconductor which will carry the neutral current of the circuitand is rated accordingly.
The metallic envelope may be made from steel,copper oraluminium wires that are equally spaced around the
circumference of the cable or cores and wound in a helix or waveform along the length of the cable or cores.Screens can alsobe made from lead or aluminium that has been extruded over thecable to form a metallic tube. In addition tapes of copper or aluminium may be wound in a helix around the cable or cores, orapplied longitudinally along the cable or cores, and folded aroundthe circumference of the cable to provide a complete screen.
The metallic envelope is designed to have an adequate currentrating to enable it to carry, without damage, any fault currentthat may flow during the time taken for the protective devices tooperate.
Provision shall be made in a cable joint to connecttogether the metallic envelope(s) of the cables in thejoint by means of bonds.Collectively the bonds needto have the same fault current rating as the
envelope which in turn shall be not less than theearth fault rating of the cable.
At cable terminations, connections shall be includedas part of the termination to provide means wherebythe envelope(s) of the cable can be connected to:
1) the metallic enclosure of an item of electricalplant;
2) any other electrical circuit that will enablefault current to return to the neutral point of thesupply system.
The connections shall be rated to carry a current noless than the earth fault rating of the cable.BS 7197 pdf download.