UNINTERRUPTABLE A.C. POWER SUPPLY SYSTEMS AND STABILIZED POWER SUPPLY UNITS COMPRISING A RECTIFIER/BATTERY CHARGER, A BATTERY, A STATIC INVERTER AND A STATIC TRANSFER SWITCH FOR USE WITH COMPUTERS, TELECOMMUNICATIONS APPARATUS, INSTRUMENTATION APPARATUS AND FIRE AND GAS DETECTION SAFETY APPARATUS; STATIC INVERTERS; STATIC SWITCHES; AND STATIC FREQUENCY CHANGERS