Generator Plant Control Logic Plant Control Logic Mains Direct or Mobile Mains Direct or Mobile Output Switching Output Switching Cubicle Cubicle Medium Voltage Supply Figure 24.2 Multiple a.c. standby plant for major installations. (All switches are mechanically interlocked to prevent supplies being paralleled or overloaded.) A.C. systems 24/7 Mains/ Standby Supply Trickle Charger Mains Supply Float Charger Figure 24.3 Three machine continuity set between these supplies, (normally at MV), and even with inter- 24.2.10 No-break or uninterruptible power locked electro-mechanical contactors, automatic change-over times supplies of the order of 250 milli-seconds can be achieved. Where equipment design is such that any interruption of the power 24.2.9 Interconnected a.c. systems supply, however short, causes malfunctions in the switching or computer system, (and increasingly it is becoming difficult to dif- The reliability merits of feeding discrete load blocks from multiple ferentiate between such systems), then additional power plant has supply distribution sections, via appropriate engine generator plant, to be provided to guard against these breaks. Such plant is by is irrefutable, but in practical terms this is often very difficult to definition called a 'no break' power system, although popular jargon achieve at acceptable cost. Also such systems can be relatively is that of 'uninterruptible power systems', or more shortly UPS's. inflexible once designed, and more difficult to adapt to changing switching equipment designs. This is even more evident when the introduction of new equipment technology is particularly volatile. 24.2.10.1 Three machine continuity set In these circumstances there are attractions to providing multiple standby plant which is capable of being operated in parallel, the The three machine continuity set principle (Spurgin, 1970) consists second and subsequent plants being brought into service on demand of an induction motor operating from the mains a.c. supply, driving (Nightingale, 1985). T