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Disconnectors on MV applications are often considered as providing a safe separation from a live network. This paper shows that the purpose of a disconnector is different, that its specification is not actually focused on safety, and that the installation conditions are highly influent on the actual safety level downstream. It seems relevant to consider these devices for what they are, and to deal with safety matters through a more global analysis of the physical installation and operation procedures. Awareness of all the players is needed in order to overcome many years of drifts and twists around the function provided by disconnectors, and to come back to an unbiased appreciation of these devices for the benefit of all users.

Relevancy of overvoltage protection on MV networks is discussed according to the network structure and the switching equipment installed. Over-voltages can be generated by natural phenomena like lightning, by network faults like earth fault or by interaction of the switchgear with the network components. Simulation of a distribution network is presented to show the amplitude of the over-voltages and their rate of occurrence. Based on the results of the simulation guidelines for protection are proposed. Although the best place to install protection is closest to the protected equipment, protection is often installed on the switchboard. Installation examples are given for a new MV distribution switchboard design. Keywords: Overvoltage protection, distribution networks, vacuum switching . i ntrod ucti on Medium Voltage Distribution networks are sometimes subject to stresses. An important part of these stresses comes from over-voltages. Over-voltages can have multiple origins. They can be induced by external natural phenomena such as lightning involving propagation of a current surge along lines and cables [1]. They can be caused by a faulty network condition such as an earth fault [2,3]. They can also be a consequence of network operation such as load and fault switching operations [4]. As an example, a simplified electrical distribution network is studied [3,5]. The simulated distribution network includes most of the usual elements that can be found in an MV distribution network: a primary substation with two power transformers, a medium size industrial substation fed by the primary substation and including motors, an MV distribution ring with significant number of MV/LV substations and a small industrial site fed by an MVI LV substation. The behavior of these elements and the proposed protective measures for over-voltage protection are discussed. A guide to protect the different network components is presented. Finally an example is given how such protections can be installed even though the best place to install is on the load terminals Over-voltage protection Normative considerations The effect of an overvoltage depends on the values of voltage, frequency and rate of rise, as well as on the withstand levels of the individual components in the distribution equipment and of the equipment connected to it. International Standard lEG 60071-1 sets out the main rules on the necessary withstand levels for the coordination between different types of equipment. However, it addresses only the most common kind of over-voltages. It is generally considered that most of the public distribution networks, consisting of cables and overhead lines are affected by rather low switching over-voltages, the effect of which is covered by the withstand demonstrated through the conventional Basic Impulse Level (BIL) [6]. Economic considerations During the de sign stage of new di stribution equipment, certain applications require special consideration either to assure the protection of the distribution equipment or the protection of the equ...

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Electrotechnical switchgear used for network operation has high stakes in term of safety and power availability. For this purpose, it is essential they rely on high level standards such as IEC, IEEE, GB or GOST standards. Using a marketing approach, the main expectations from standardization are to meet customer values. For the best efficiency, standards and consequently customer specifications should be expressed to look for the optimization of the products. In that sense, open functional approach in standardization rather than fixed technological approach is preferable. Optimized technical specification rather than over-specification allows for lower cost and is more environmental friendly. Because of its international nature, IEC functional approach, allows for the best optimization of MV switchgear designs. It is seen that other major standardization bodies now have close links with IEC.