25.1 INTRODUCTION

This chapter considers those aspects of power quality related to disturbance of the voltage level. This includes short-term interruptions and long-term deviations from the declared voltage level. With the increasing dependence of industry and commerce on electronic equipment and processes with sophisticated controls, this is an area of increasing concern to power supply utilities and to their contractors and consultants. Some standards relating to this subject are given in Table 25.1. The IEC approach is as explained in Chapter 24 (Section 24.1) i.e. to set limits for equipment connected to utility systems but the thresholds of irritability provided are equally valid for users already connected to an existing supply system, and indicative planning limits provided are quality targets for a supplying utility. Standards of supply system quality to be expected in Europe (for systems up to 35 kV) are set out in EN 50160.