able many versions of this type of plant, including some which have been (Nightingale, 1986). completely 'cubiclised' to enable factory assembled 'packages' to 24/8 Telecommunication power systems Emergency By-Pass Mains/ Standby Supply Rectifier Figure 24.4 Two machine continuity set be produced, with the resulting reduced on site installation time, as as such plant must have a limit in the current they can supply. Hence well as reducing acoustic noise and adding to their aesthetic appear- in specifying design criteria for a UPS it is essential that non-linear ance in equipment rooms (Woolley, 1986). loads are defined as being present, and that the ability of a UPS is therefore designed to cope with such a load. Some inverter UPS systems are not designed to work on load in 24.2.10.3 Static inverters the normal mode of operation. Such systems are sometimes termed as being on 'hot' standby, and are usually found in lower power Static inverters (Henderson, 1986) are very prominent in installa- ratings of plant. This arrangement, shown in Figure 24.6, means that tions requiring no-break power supplies, and vary in rating from a the load is normally fed from, say, the mains, via a load conditioner few VA to hundreds of kilowatts. The basic plant schematic is such as a CVT. The inverter is only brought into service if a mains relatively straightforward, and is shown in Figure 24.5. failure is detected. Such a system is usually cheaper in terms of capital cost, and can have a higher operating efficiency. However the load in these conditions does not necessarily have the advantage of any form of voltage and frequency control, and requires the off load inverter to suddenly switch from no to full load on mains Contactor or Static Switch failure. Also, there is the possibility of a short break as the inverter is switched into use. Some plant designs also use various combinations of rotating Contactor or Static Switch Rectifier Inverter A.C. Supply A.C. Supply Regulator or Conditioner Rectifier Inverter Figure 24.5 On line 