Protective Multiple Earthed (PME) systems which use Combined Neutral and Earth (CNE) cables have become the preferred choice in the UK public supply network, both for new installations and for extensions to existing circuits.
This is primarily because of the elimination of one conductor by the use of a common concentric neutral and earth, together with the introduction of new designs which use aluminum for all phase conductors.
Before CNE types became established, 4-core paper-insulated sheathed and armoured cable was commonly used. The four conductors were the three phases and neutral, and the lead sheath provided the path to the substation earth.
The incentive for PME was the need to retain good earthing for the protection of consumers. With the paper cables, while the lead sheath itself could adequately carry prospective earth fault currents back to the supply transformer, the integrity of the circuit was often jeopardized by poor and vulnerable connections in joints and at terminations.
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By using the neutral conductor of the supply cable for this purpose the need for a separate earth conductor was avoided. The adoption of 0.6/1 kV cables with extruded insulation for underground public supply in the UK awaited the development of cross-linked insulation systems with a performance similar to paper-insulated systems in overload conditions.
An example of the cables which have been developed is the Waveform CNE type which is XLPE-insulated and has the neutral/earth conductor applied concentrically in a sinusoidal form. Insulated solid aluminum phase conductors are laid up to form a three-phase cable and the CNE conductor consists of a concentric layer of either aluminum or copper wires.
If the wires in the CNE conductor are of aluminum, they are sandwiched between layers of unvulcanized synthetic rubber compound to give maximum protection against corrosion. This construction is known as Waveconal and is illustrated in Fig.1.1.
Where the CNE conductors are of copper, they are partially embedded in the rubber compound without a rubber layer over the wires. This is termed Wavecon and is illustrated in Fig. 1.2.
Some electricity companies initially adopted Wavecon types because of concern over excessive corrosion in the aluminum CNE conductor, but through standardization all companies had moved to the use of copper wire design by 2001.
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Waveform cables are manufactured in accordance with BS 7870-3.4. Both waveform types are compact, with cost benefits. The aluminum conductors and synthetic insulation result in a cable that is light and easy to handle. In addition, the waveform lay of the CNE conductors enables service joints to be readily made without cutting the neutral wires, as they can be formed into a bunch on each side of the phase conductors.
Source: Electrical Power Engineers
